Using the Sun/Moon Calculator

Effective planning of most outdoor photography requires at least a rough idea of where the Sun (and sometimes the Moon) will be at a given time. The Sun/Moon Calculator provides the information most frequently needed—Sun and Moon rise and set times and azimuths for one or more days, or Sun and Moon positions throughout the day.

This tutorial gives brief illustrations of the calculator’s main features; complete descriptions of these features, as well as descriptions of many terms that may be unfamiliar, are given in the Sun/Moon Calculator Reference.

Contents

Introduction
Setting the Location
Basic Calculations: Rise/Set Times and Sun and Moon Positions
Getting the Times of Rise and Set for One Day
Getting the Times of Rise and Set for More Than One Day
Getting Sun and Moon Positions for Times Throughout the Day
Advanced Calculations: Rise/Set Criteria
Getting the Times of Rise and Set When the Horizon Is Not Level
Finding Dates on Which the Sun or Moon Rises or Sets in a Particular Direction
Finding Dates on Which the Moon Rises Close to Sunset
Finding Dates on Which the Sun or Moon Is Near a Natural or Man-Made Feature
True North vs. Magnetic North in Searches
Other Calculations
Observing Changes in the Times and Azimuths of Sun Rise and Set Throughout the Year
Observing Changes in the Times and Azimuths of Moon Rise and Set Throughout a Month
Height Above Horizon
User Preferences
Printing and Saving Results
Script Timeout
Running the Sun/Moon Calculator as a Local Application
Determining Azimuths and Altitudes of Natural and Man-Made Features
Field Measurements
Mapping Software
Geodetic Calculations
Heights of Man-Made Features

Introduction

Basic Operation

Using the calculator to get the basic information is simple: select or specify a location, select the type of calculation (rise and set times or Sun and Moon positions), select the desired date or dates (and the desired start and end times for positions), and click the Display or Print button.

Advanced Features

The calculator also has advanced features to accommodate special requirements. “Official” times of Sun and Moon rise and set are for a level horizon, as one would encounter on a seacoast. If the horizon consists of mountains rather than an ocean or a flat plain, the times when the Sun and Moon actually are visible can be quite different from the official times of rise and set. The Sun/Moon Calculator allows the rise and set to represent the times at which the Sun or Moon crosses a point at any angle above the horizontal plane. The calculator can find dates on which the Sun or Moon rises or sets in a particular direction, and again, rise and set are not limited to a level horizon. This feature can be helpful if you require that a landscape be illuminated from a specific direction, or want to have the Sun or Moon rising or setting near a natural or man-made feature.

Sun and Moon Coordinates

The positions of the Sun and Moon are given in terms of azimuth, the clockwise angle from north, and altitude, the angle above the horizon. These terms are explained in greater detail in the Glossary of the Sun/Moon Calculator Reference.

True North or Magnetic North

The calculator can display Sun and Moon azimuths relative to either true north or magnetic north; the former often is preferable when working with a map, but the latter may be more convenient for comparison with field measurements made with a compass. The type of display is selected on the User Preferences form.

Context-Sensitive Help

The calculator provides context-sensitive help for most inputs and output data—when the cursor is passed over an input label, or the column heading for most output data, the cursor changes to a pointer and a brief tooltip, such as

Select a location from the database,

is displayed. Clicking on the label or output column heading brings up the appropriate section of the Sun/Moon Calculator Reference.

User Preferences

User Preferences allow certain features to be enabled or disabled, and allow control over how the calculator displays information.

Setting the Location

Selecting a Location from the Database

If you are near one of the locations in the built-in database, selecting that location usually will suffice; differences of a few miles are insignificant. Alternatively, you can browse the database by clicking the List Locations button; when the cursor is passed over a location name, the cursor changes to a pointer, and the tooltip Set location to name is displayed; clicking on the location name selects that location and gives focus to the main form (if your browser is set to open pages in tabs, you may need to manually select the tab for the main form).

Searching the Database for a Location

If Show Location search is enabled via User Preferences, you can search the built-in database for a location that matches a pattern; if you know that a location is in the database, this often is the fastest way to select it. Enter the pattern in the text box after Search for; the search begins when the first character is entered, and is updated as each additional character is entered. A location matches if any part of its name contains the pattern; pressing Enter or clicking Next Match finds the next matching location. This can be convenient for rapidly scrolling through several locations that match the same pattern; for example, entering TX and pressing Enter several times finds all locations in Texas. A message is given after the last matching location; clicking New Search repeats the search, beginning with the first match. The search is case-sensitive if the pattern contains any uppercase letter; you can reduce the chances of a matching an unwanted location by observing capitalization. For example, enter AB to match locations in Alberta, Canada and avoid matching Moab, UT. If you use any capital letters, you must use them wherever they appear in a location name: either san fran or San Fran will match San Francisco, CA, but San fran will not. All entries are stored as ASCII text, so the pattern should not contain accented characters. To match Orléans, France, enter Orleans, F.

By default, the search pattern is ordinary text. If you are familiar with Perl-style regular expressions, you can enable Allow regular expressions in location searches to possibly save a few keystrokes when a long pattern is needed to match a location. For example, NP.*UT allows a quick search of all National Parks in Utah.

Specifying Location Properties

If your location is not near one of those in the database, you will need to enter the location properties in the Specify: area. The location Name is optional, but is used to determine the rules for daylight saving time, so it is a good idea to provide a name. The format that the calculator understands is place, country (for Canada and the United States, the two-letter abbreviation for province or state (including American Samoa, Puerto Rico, and the US Virgin Islands) is used rather than the country (e.g., Washington, DC). The Latitude and Longitude may be entered as DMS (degrees:minutes:seconds), decimal degrees, or combination thereof. You can either select a Time Zone from the list, or have it calculated from the longitude; if you know the time zone, it’s usually better to select a value. Finally, select Yes from the Uses Daylight Time? box if the location observes daylight saving time, or No if it does not.

The latitude and longitude for many locations can be found from one of the databases in the list following Look Up a Location; select the desired database from the list and click Go.

If you enter a location name, a query using the GeoNames database will automatically fill in the location’s properties, usually including time zone and elevation, if a matching location is found. Google Maps can show a topographical map of the location, using either the latitude and longitude or the name that you have entered.

The other databases provide only lookup, requiring you to transfer the values to the Sun/Moon Calculator. The GeoNet Names Server and Getty Thesaurus give only latitude and longitude; the USGS GNIS also gives elevation. timeanddate.com gives latitude, longitude, time zone, and information about the observance of daylight saving time.

The angle and time input formats are described in detail in the section DMS and HM Input in the Sun/Moon Calculator Reference. Location lookup is described in the section Look Up a Location.

Locations are Remembered

If cookies are enabled, the selected location and any user-specified location are remembered across sessions; this saves repeated entry if your normal location is not in the database. Unfortunately, only one user-specified location is remembered.

Basic Calculations: Rise/Set Times and Sun and Moon Positions

Getting the Times of Rise and Set for One Day

This is the simplest and most obvious application. Ensure that the Rise and Set Times radio button is checked; select the desired Start Date, set the End Date to the same date, and click the Display button. To print the results, click the Print button at the bottom of the results page.

Getting the Times of Rise and Set for More Than One Day

If you are planning a trip, you may need the rise and set times for more than one day. Ensure that the Rise and Set Times radio button is checked; select the desired Start Date, then click the appropriate radio button to specify either the desired End Date or a Date Offset, and click the Display button. It’s often more convenient to specify a date offset (e.g., two weeks) than a specific end date.

If you plan to visit several locations, repeat the procedure for each location. If you just want to print the results, it is faster and easier to click the Print button at the bottom of the main form rather than the Display button and then the Print button on the results page.

Getting Sun and Moon Positions for Times Throughout the Day

The default settings giving positions between sunrise and sunset often are what is needed. Ensure that the Sun and Moon Positions radio button is checked; select the desired calculation Date, and ensure that the default values of 0 min, before, and Sunrise for Start Time, 0 min, after, and Sunset for End Time, and a Time Interval of 30 minutes are selected. and click the Display button. To print the results, click the Print button at the bottom of the results page.

If you want Sun and Moon positions for several successive days, the + 1 day button makes it easy to advance the date. If you just want to print the results, it is faster and easier to click the Print button at the bottom of the main form rather than the Display button and then the Print button at the bottom of the results page.

If you plan to photograph a rising Moon, the default settings may not be the most appropriate. For example, if moonrise is close to the time of sunset, you may want to show positions at 5-minute intervals between moonrise and 20 minutes after sunset. For Start Time, enter 0, and select before and Moonrise; for End Time, enter 20, and select after and Sunset. Select a Time Interval of 5 minutes.

For a setting Moon near the time of sunrise, you might want to show positions between 20 minutes before sunrise and moonset. For Start Time, enter 20, and select before and Sunrise; for End Time, enter 0, and select after and Moonset. Select a Time Interval of 5 minutes.

Alternatively, you can show positions between two specified times, or between a Sun or Moon event and a specified time; with the default values of 00:00 and 24:00 (or 12:00 am and 12:00 am), clicking the radio button to the left of each time will show positions from midnight until the following midnight. Times may be entered in either 24-hour format or AM/PM format, using either decimal hours or hh:mm format; see DMS and HM Input in the Sun/Moon Calculator Reference for a more detailed description. A start time of 12:00 am is interpreted as midnight at the beginning of the date; an end time of 12:00  is interpreted as midnight at the end of the date.

Active and Inactive Inputs

Some pairs of inputs are mutually exclusive; when one is selected, the other is inactive. For example, when calculating Sun and Moon rise and set times, if End Date is selected, Date Offset is deselected and inactive. Similarly, when calculating Sun and Moon positions, if a time value is selected, giving the start or end time relative to a Sun or Moon event is deselected and inactive. By default, the inactive inputs are disabled; this makes it more obvious which inputs are active in the calculation but require that you click the appropriate radio button to reactivate a disabled input or change its value; if Enable inactive inputs in active area on User Preferences form is selected, you can simply enter a value in the desired field without having to first click the radio button. Similarly, when this option is selected, you can edit values on the Criteria for Sun and Moon Rise and Set form without checking the box to the left of the applicable range.

Data Entry and Validation

For some items you select from a list of options; for others, you enter a value. If you enter an invalid value (e.g., a Date Interval of 0), the value will often be changed to a reasonable valid value; you can either accept this value or enter another valid value. In other cases, you will get an error message explaining why the value is invalid; if a particular field (e.g., Time Offset) requires values to be within a certain range, that range will be indicated. With some browsers (e.g., Internet Explorer), you will need to correct the error before doing anything else; with others, such as Firefox, you are not forced to correct the error immediately but will be unable to perform a calculation until you enter a valid value.

In most cases, an entry that is blank or consists only of spaces will be changed to the default value for that field; this provides an easy way of resetting one field rather than the entire form.

Advanced Calculations: Rise/Set Criteria

Getting the Times of Rise and Set When the Horizon Is Not Level

Standard times of Sun and Moon rise and set assume a level horizon (i.e., zero altitude). This is fine if you are on a small, flat island in the middle of an ocean, but less useful if you are in a valley bounded by hills on the east and mountains on the west, and you want to know when the Sun or Moon will be visible. Suppose, for example, that the hills to the east have an altitude of 3° and the mountains to the west have an altitude of 8°. Click the Rise/Set Criteria... button to open the Criteria for Sun and Moon Rise and Set form (with some browsers, if the Use tabs for all pages User Preference is selected, and the Sun/Moon Rise/Set Criteria tab already exists, it may be necessary to manually select that tab). Check the boxes at the left of the Altitude column to allow entry of values, and enter 3 in the first text boxes for Sunrise and Moonrise, and 8 in the first text boxes for Sunset and Moonset; note that these values are automatically copied into the second text boxes when you click anywhere outside the first text boxes. Leave the default values of Top selected so that the times indicate when the tops of the Sun’s and Moon’ disks emerge from behind the hills or disappear behind the mountains. Click the Apply Settings button to apply these values, and, if you wish, the Close button to close the form. Calculate the rise and set times for the appropriate dates.

In practice, it will seldom be quite this simple, because the altitude of the horizon will usually vary with azimuth.

Finding Dates on Which the Sun or Moon Rises or Sets in a Particular Direction

You may envision a photograph in which the early morning light illuminates the scene from a particular direction, or perhaps you envision a composition in which the Moon rises in a certain spot. For example, suppose that you want to capture alpenglow on a nearby mountain peak at sunrise, and have determined that, because of intervening mountains, the first light from the Sun strikes the peak only when the Sun’s azimuth at rise is between 61° and 67°. Click the Rise/Set Criteria... button to open the Criteria for Sun and Moon Rise and Set form. If you have done a previous search, either clear the values individually, or click the Reset button to reset all values to default and click OK when prompted to confirm. In the row for Sunrise, check the box at the left of the Azimuth column, and enter the values 61 and 67 in the text boxes. In the Altitude column, check the box at the left and ensure that the Top radio button is checked. Click the Apply Settings button to apply these values, and, if you wish, the Close button to close the form. Ensure that the setting (True North or Magnetic North) under Azimuth Display on the User Preferences form matches how you obtained your azimuths. Select an appropriate range of dates, and click the Display button on the main form to find the dates on which alpenglow is possible.

Finding Dates on Which the Moon Rises Close to Sunset

The best days for a landscape photograph with a full moon are often those on which the Moon rises close to the time of sunset. Click the Rise/Set Criteria... button to open the Criteria for Sun and Moon Rise and Set form. If you have done a previous search, click the Reset button to reset all values to default and click OK when prompted to confirm. In the row for Moonrise, check the box in the Moon Rise/Set Time column and use the default range of 15 minutes before to 10 minutes after sunset. Click the Apply Settings button to apply these values, and click the Close button to close the form. Select a location, and 1 January of the current year and a date offset of 1 years to examine the current year. Click the Display button to show the dates.

The default values assume conventional rise and set for which the top of the Moon crosses zero altitude, and assume that the Moon would be photographed sometime after rise or before set at an altitude greater than zero; they usually give a rough idea of good dates on which to photograph the Moon during twilight. If you specify a nonzero altitude or an altitude range for Moon rise or set, you should also specify the time range for which you want the Moon within the specified altitude range. For example, in San Francisco, most east-facing locations have hills on the horizon, and the Moon may not be visible until it reaches an altitude between 1° and 2°. You might decide that you want to photograph a full moon at an altitude between 2° and 5°, at a time between 5 minutes after sunset and 25 minutes after sunset. Giving these criteria for 2014 yields nine dates, ranging from January to December, with azimuths ranging from 71° to 118°.

Seldom does every potential date prove worthwhile, but results like those above provide a good starting point. With such a list of dates, you would then need to find suitable locations for each date and determine whether one or more of them might provide a picture that you wanted to take. One method for finding locations is to find the Moon azimuth that corresponds to the middle of the desired altitude range, and draw lines on a map corresponding to that azimuth to see if the view from a potential camera position affords an interesting composition. For the last example, the mean altitude would be 3.5°; on 10 August, the corresponding azimuth would be 106°. This occurs at 15 minutes after sunset; the pink sky color would largely be gone, but building lights would be starting to come on, so a good view might feature several prominent buildings.

Finding Dates on Which the Sun or Moon Is Near a Natural or Man-Made Feature

You may envision a photograph in which the Sun or Moon rises or sets near a natural or man-made feature. The procedure for finding dates on which this may happen is straightforward, but it does involve some effort:

  1. Get a picture of the scene to aid visualization. If at all practical, visit the location well in advance, and preferably, get a photograph from each potential camera position—you won’t struggle to remember details that you may have overlooked at the time of your visit, such as a large building behind the feature. If an advance visit isn’t practical, a photograph taken by someone else may suffice if the camera position is accurately known. For a natural scene, software (such as Google™ Earth) that provides 3D visualization can be helpful in planning an image, though it may not include man-made features, vegetation, or small natural features, so it’s not really a substitute for a photograph. But if scouting isn’t practical and you don’t have a photograph, a 3D simulation is often much better than nothing; of course, you may encounter some surprises when you arrive at the location, and you should be prepared to adapt accordingly. In a location with hills in the distance, if at all possible generate an elevation profile that extends beyond the hills, and verify that the hills will not occlude the view from your desired feature to the Sun or Moon.
  2. Get some measurements of the scene. From each potential camera position, determine the direction (azimuth and altitude) of the feature, using field measurements with a compass and clinometer, topographic mapping software, or geodetic calculations. If you just want the Sun or Moon in the general vicinity of the feature, a single direction may suffice. But if you want the Sun or Moon in a more specific relationship to the feature, determine the direction of at least one other feature if possible; knowing the angular distances between two or more features allows determining the direction of any other point in the image, and also allows determining the Sun’s or Moon’s size in the image.

    If you use field measurements, and getting directions to other features is too much trouble, having a photograph of the scene and knowing the focal length (and hence the angle of view) of the lens can serve much the same purpose.

    If you use mapping software or geodetic calculations, a GPS reading can assist in accurately fixing a camera position; with accurate coordinates for a camera position, you can later determine directions to any feature that you can locate on a map.

  3. Choose an area of the image in which you want the Sun or Moon to appear; using image-editing software to draw a rectangle in a picture of the scene can be one means of visualizing this area. If you have the directions to two or more features, you can determine the angular distance between them and use the approximate ½° angular diameter of the Sun or Moon to estimate the body’s size in the image. Once you have made an aesthetic decision, you need to associate the area with ranges of azimuth and altitude; this will be relatively simple if you have the azimuths and altitudes of at least two features or have the azimuth and altitude of one feature and know the angle of view in a photograph of the scene.
  4. If your subject is the Moon, decide the phase that you want the Moon to have. Alternatively, and perhaps more usefully, choose a time range relative to sunrise or sunset within which you want the Moon to be in the desired position.
  5. Choose a range of dates and use the Sun/Moon Calculator to find the dates, if any, that match your criteria.

Example 1: An Urban Setting with a Fixed Camera Position

You know of a location in northwest San Francisco that affords a good view of the downtown skyline, or at least the top of it—the intervening Russian and Nob hills hide all but the tallest buildings. You think the setting might make for a good image, but that something additional is needed, such as the Moon silhouetted against the spire of the Transamerica Building. You want to find all dates between 1990 and 2010 on which such an alignment is possible. You have determined that from this location, the azimuth to the building is 101.5°, relative to true north. The altitude to the bottom of the spire is 1.7° and the altitude to the top of the spire is 2.3°; the difference between the top and bottom is 0.6°, slightly greater than the Moon’s mean angular diameter of ½°. You would like to capture the Moon approximately centered on the spire. You realize that if you insist on a perfect alignment, you aren’t likely to find many dates; you still want exact alignment in azimuth, but you will accept a variation in altitude of ±0.3°.

Click the Preferences... button to open the User Preferences form. To make examining individual dates easier, select the option to send the Sun and Moon positions to a different window than that for the rise and set times, so that the latter is not overwritten by the position results; to do this, check the box in front of Show rise/set times and positions in separate windows. On the same form, ensure that the setting under Azimuth Display is set to True North; save the settings using the Apply Settings button; close the form to avoid the clutter of too many open windows.

Click the Rise/Set Criteria... button to open the Criteria for Sun and Moon Rise and Set form. If you have previously specified rise or set criteria, either clear the values individually, or click the Reset button to reset all values to default and click OK when prompted to confirm. In the row for Moonrise, check the box at the left of the Altitude column, and enter 1.7 in first text box and 2.3 in the second text box; note that the Ctr radio button is automatically selected when you click anywhere outside the second text box. In the Azimuth column, check the box and enter 101.5 in the first and second text boxes. Click the Apply Settings button to apply these values; you may want to leave the form open in case you need to refine your criteria.

From the Select: drop-down box, select San Francisco, CA as the location. Set the Start Date to 1 January 1990, click the Date Offset radio button, enter 20, and select years. Ensure that the Date Interval is 1 days. Click the Display button on the main form to find the dates that meet your criteria. Depending on the computer and the browser, the calculation may require anywhere from a few seconds to more than a minute. With your browser’s default settings, you may get a warning about a script causing your browser to run slowly, with a prompt asking if you want to abort the script. If so, click No to allow the calculations to continue; you may need to do so several times. See the section on Script Timeout for information on how configure your browser so that you do not get this message.

You get more than 40 dates, and are pleased by the many opportunities to get your shot. Upon closer examination, however, you notice that on most of the dates, the Moon is far from full, and in many cases moonrise occurs during the middle of the day when it is uninteresting, or late at night when it is far too dark to capture the foreground. You now realize that you wanted the Moon close to full, but forgot to so indicate; you also want some of the city lights to be on, so you decide that the alignment should happen between 10 and 20 minutes after sunset. You could specify a phase range, but your timing requirement will ensure that the Moon is close to full, so that phase criteria are unnecessary; moreover, if phase criteria were specified too narrowly, they actually could exclude some dates that would be perfectly acceptable aesthetically. The timing restriction is what you really consider important, so that is what you decide to specify. On the Criteria for Sun and Moon Rise and Set form, go to the Moon Rise/Set Time column for Moonrise, check the box, enter 10 in the following box, click the after radio button, enter 20 in the box following to, click the after radio button, and finally, click the Sunset radio button. If you started with the default values, you actually can get by with less effort: check the box, enter 10, click the after radio button, and enter 20 in the second text box; and the default setting for the second radio button will be what you want. If Enable inactive inputs in active area on the User Preferences form is selected, you can simply click in the first field to enter the value, and the box in the Moon Rise/Set Time column will be checked automatically. Apply the settings, return to the main form, and click the Display button to repeat the calculations.

You see that the real possibilities are few and far between: you get two dates: 13 Aug 1992 and 20 Sep 2002. You now want to examine each date to see how well it might match what you had envisioned. Pass the cursor over 13 Aug 1992; note that the cursor changes to a pointer and the tooltip

Set Sun and Moon Positions date to 13 Aug 1992

is displayed. Click on the date to set the positions calculation Date to 13 August 1992 and return to the main form. You decide that you want to see the Moon positions at one-minute intervals between five minutes before and five minutes after the time at which the Moon is near the top of the Transamerica Building. Recall that “moonrise” is the time that the Moon crosses the specified altitude; when an altitude range is specified, the altitude crossing shown in the search results is the average of the minimum and maximum values, in this case 2.0°. You can specify your desired time range as follows: for the Start Time, enter 5 the text box before min, and select before and Moonrise. For the End Time, enter 5 the text box before min, and select after and Moonrise. Times entered in this manner will be automatically adjusted for different times of moonrise, and you will not need to change them for the different dates. Select a Time Interval of 1 minute, and click the Display button to show Sun and Moon positions for 13 March 1992.

You note that when the Moon is aligned with the Transamerica Building in azimuth (at 101.5°), its altitude is 1.7°, at the bottom of the spire. Although this meets your criteria, you now recognize that it’s lower on the building than you would prefer. Return to the times results window, click on 20 Sep 2002, and on the main form, click the Display button to show Sun and Moon positions for 20 September 2002.

On this date, when the Moon’s azimuth is 101.5°, its altitude is 2°, exactly in the middle of the spire. You mark the date on your calendar and hope that the weather will cooperate; further analysis will indicate that the next matching date is in 2108.

Example 2: A Natural Setting with Many Possible Camera Positions

For this example, you’ll use the location search feature to set the location. If the Search for field isn’t displayed below the Select: field, you’ll need to enable the feature. To do so, open the User Preferences form by clicking the Preferences... button at the bottom of the main form; if the User Preferences form is already open in a tab, you may need to manually select that tab. Check Show location search, click Apply Settings, and return to the main form.

You want to capture the Moon setting near California’s Mount Williamson from Manzanar, and you want to find all the dates between 2000 and 2010 on which this might happen. In many locations, potential camera positions are quite limited; this isn’t the case near Manzanar, which affords an almost continuous range of positions easily accessible from the Manzanar Cemetery road, with azimuths to Mount Williamson ranging from about 237° to 243° relative to true north. The altitudes from those locations range from about 11.6° to 11.9°; the section Geodetic Calculations describes one means of determining those values. Searching for dates for each possible location would be an almost endless task, so you decide to allow for the full range of azimuths and altitudes, and then examine each of the matching dates to see if they work for one or more specific locations.

You decide that you nominally want the top of the peak between the center and bottom of the Moon, and want the center of the Moon to be within two diameters (1°) on either side of the peak. With the center of the Moon at the top of the peak from the location with the lowest altitude to the peak, the altitude of the Moon’s center is 11.6°; with the bottom of the Moon at the top of the peak from the location with the greatest altitude, the altitude of the Moon’s center is approximately 11.9° + 0.25° = 12.15°. With your tolerance of the Moon being within ±1° of the center of the peak, the azimuth range is 236° to 244°. To ensure that you don’t miss any dates, it’s usually better to specify a slightly wider range than you think you need—it’s easier to eliminate a few dates than to find a date that the search barely missed. Accordingly, you choose an azimuth range of 235° to 245° and an altitude range of 11.5° to 12.2°

You also want the perfect image with alpenglow on Mount Williamson and accordingly want the Moon to be near the top of Mount Williamson between two minutes before and three minutes after sunrise. You could specify a phase range, but, as with the previous example, your timing requirement will ensure that the Moon is close to full, so that phase criteria are unnecessary. Once again, the timing restriction is what you really consider important, so that is what you decide to specify.

Click the Rise/Set Criteria... button to open the Criteria for Sun and Moon Rise and Set form. If you previously have specified rise or set criteria, either clear the values individually, or click the Reset button to reset all values to default and click OK when prompted to confirm. In the row for Moonset, check the box at the left of the Altitude column, and enter 11.5 in the first text box and 12.2 in the second text box; note that the Ctr radio button is automatically selected. In the Azimuth column, check the box at the left, and enter the 235 in the first text box and 245 in the second text box. The maximum azimuth value cannot be less than the minimum; if you have previously entered a value less than 235° for the maximum azimuth, that value is automatically changed to 235. If this happens, change the value for maximum azimuth to 245. Go to the Moon Rise/Set Time column for Moonset, check the box at the left, enter 2 in the first text box, click the before radio button, enter 3 in the text box following to, click the after radio button, and finally, click the Sunrise radio button. As in the previous example, if you started with the default values, you can get by with considerably less data entry. Click the Apply Settings button to apply these values; you may want to leave the form open in case you need to refine your criteria.

Manzanar isn’t in the locations database, but Lone Pine, 12 miles to the south, is close enough for a preliminary search—you can get the coordinates for Manzanar from the USGS GNIS if you wish to carefully examine the Moon’s position on one of the matching dates. Enter Lone in the box after Search for; this should set the location to Lone Pine, CA. Ensure that Azimuth Display on the User Preferences form is set to True North, and save this setting using the Apply Settings button if necessary. Set the Start Date to 1 January 2000, click the Date Offset radio button, enter 10, and select years. Ensure that the Date Interval is 1 days. Click the Display button on the main form to find the dates that meet your criteria.

You should get four dates: 5 Aug 2001, 28 Apr 2002, 17 Mar 2006, and 19 Jul 2008. You now want to examine each date to see if it might work from one of your favorite locations. Pass the cursor over 5 Aug 2001; the cursor changes to a pointer and the tooltip

Set Sun and Moon Positions date to 5 Aug 2001

is displayed. Click on the date to set the positions calculation Date to 5 Aug 2001 and return to the main form. You decide that you want to see the Moon positions at one-minute intervals between five minutes before and five minutes after sunrise. For the Start Time, enter 5 in the text box before min, and select before and Sunrise. For the End Time, enter 5 in the text box before min, and select after and Sunrise. Select Time Interval of 1 minute, and click the Display to show Sun and Moon positions for 5 Aug 2001. Return to the times results window and repeat the process for the other dates.

If you are willing to explore slightly less well maintained roads and hike short distances, many additional camera locations near Manzanar are available, including the location of Ansel Adams’s famous image of Mount Williamson. With suitable adjustments to criteria to cover the additional locations, a search of the same ranges of dates can yield a considerable number of dates. The task then reduces to finding suitable foregrounds within the range of potential locations, and examining potential dates to see if alignments from one or more locations might result in suitable images.

If you perform a search with overly restrictive criteria, you may get the message

**** No dates meet criteria ****

To find some matching dates, you will need to increase the date range of your search or compromise and relax your criteria, by expanding the moonset altitude range, the moonset azimuth range, the moonset time range, or combination thereof. You also may find that a slightly different location (with a different azimuth and altitude to the peak) will give better results. Continue the process until you find a suitable date or until your criteria are so compromised that you no longer want to capture the image, and decide to fake it with Photoshop.

True North vs. Magnetic North in Searches

In most cases, true north is the best choice when performing a search, even if that means converting compass measurements. Magnetic declinations change over time, so that as measured with a compass, natural and man-made features appear to move. Azimuths you specify as Rise/Set Criteria are static, however, so that the azimuth range appropriate for the end of a search may be slightly different from the range appropriate for the beginning. Using true north usually is much simpler than having to figure out how to adjust search criteria for changing magnetic declinations.

Other Calculations

Observing Changes in the Times and Azimuths of Sun Rise and Set Throughout the Year

The Earth makes one orbit of the Sun in a year. The Sun’s rise and set azimuths, as well as the day length and maximum altitude, vary throughout the year. Observing the values for each throughout the year may help in planning photographs of different locations. Select 21 December of the desired year as the Start Date, and select a Date Offset of 1 year. Select a Date Interval of 1 month (or 1 week if you want finer resolution). and click the Display button.

Observing Changes in the Times and Azimuths of Moon Rise and Set Throughout a Month

There are several measures of the Moon’s orbit; perhaps the most familiar is the synodic month, in which the Moon returns to the same position with respect to the Sun (and hence the same phase) approximately every 29.53 days. The Moon returns to the same position with respect to the fixed stars in the course of sidereal month, which on average, is approximately 27.32 days. In the course of a sidereal month, the Moon goes through its range of minimum and maximum rise and set azimuths, as well as its least and greatest maximum altitudes.

Using the default location of San Francisco, CA, click the Rise and Set Times radio button, set the date to 21 December, and set a date offset of 30 days. Observe that the approximate rise and set azimuths, and least and greatest maximum altitudes repeat roughly every 27 days. Note that the full Moon rises and sets approximately opposite the Sun, both in time and azimuth (more precisely, the Moon’s rise and set azimuths are approximately at minimum and maximum when the Sun’s are at maximum and minimum). Note also that full moon occurs close to the day of greatest maximum altitude (it “rides high”).

Set the date to 21 June and repeat the calculations. Observe that, again, the Moon rises and sets approximately opposite the Sun. Note, however, that now the full moon occurs close to the day of least maximum altitude (it “runs low”), the opposite of what happens in Winter. This behavior is important if you plan to photograph moonbows or make other photographs by the light of the full Moon.

Even these two examples do not illustrate the full range of the Moon’s behavior. Because its orbit is inclined to that of the Sun, its declination varies between approximately ±18.1° and ±28.9° over an 18.6-year period. The Moon’s phases and dates repeat approximately every 19 years (the Metonic cycle), but the repetition of the Moon’s position in a scene almost never is exactly the same. Stated otherwise: there are few second opportunities for scenes that include the Moon.

Height Above Horizon

If your location is substantially higher than the local horizon, enter your height above the local horizon. This value is the difference between your elevation and that of the local horizon rather than your elevation above sea level. For example, if you are at an elevation 6000 ft and the elevation at the horizon is 4500 ft, the value to enter is 1500.

You also should enter a value if you want to know the time at which first light will strike a nearby mountain; the value to enter is the difference between the elevation of the mountain and the elevation at the horizon. For example, if the horizon is at an elevation of 4000 ft, and you want to know when alpenglow first will appear on a 12,000-ft mountain, enter 8000.

The elevation at the horizon is not always easy to determine; see Height above Horizon in the Sun/Moon Calculator Reference for additional discussion.

If cookies are enabled, the setting of this option is retained across sessions.

User Preferences

The User Preferences form, accessed via the Preferences... button, allows you to set several options that enable or disable certain features or affect how the Sun/Moon Calculator displays information. A few of the options are briefly described below; the other options are explained in the Sun/Moon Calculator Reference.

Window Behavior

By default, results of times and positions calculations are sent to the same window, with each new result overwriting the previous one. If you don’t close the results window, it usually is possible to view previous results using the browser’s history. However, it usually is easier to select the Show rise/set times and positions in separate windows option to send times and positions calculations to separate windows This can be handy if you are searching for dates that meet specified Rise/Set Criteria and you plan to calculate Sun and Moon positions for several of the matching dates. You save the trouble of having to remember the list of matching dates, and you can save time by simply clicking on a matching date in the times results to set the positions calculation Date to that date.

Many browsers that support tabs can be set to allow the browser to choose whether to open pages in new windows or new tabs. With a browser so configured, by default, the Rise/Set Criteria and User Preferences dialogs open in popup windows, and positions or rise/set times and help open in new tabs. The Use tabs for all pages option usually causes all pages, including dialogs, to open in new tabs. If you are doing a search that may require several adjustments to the rise/set criteria, it may be easier to have this dialog in a tab rather than a popup window. This option should also be selected if the browser is set to open all pages in new tabs.

If either of these preferences is changed, any existing tabs or windows for results, Help, or Tutorial pages should be closed to avoid unexpected behavior.

Location Search

The Show location search feature allows searching the built-in database for a location that matches a specified pattern rather than selecting from the list. In many cases, this may be faster. For example, entering yellowst and clicking New Search (or pressing Enter) provides a fast way to select Yellowstone National Park. If the pattern contains any capital letter, the search is case sensitive; otherwise case is ignored. If you are familiar with Perl-style regular expressions and want to use them in patterns, select Allow regular expressions in location searches.

Show location search is enabled by default.

Time Display

Select AM/PM if you want the time displayed in AM/PM format (e.g., 12:25 am, 7:21 pm). Use the default selection of 24 hour if you want the time displayed in 24-hour format (e.g., 0:25, 19:21). Select Universal time (UT) if you want to show universal time; in most cases, you probably want local time. Universal time is incompatible with daylight saving time, and selecting UT will deselect daylight saving time.

Select Show daylight saving time if you want to show daylight saving time when it is in effect; deselect this option if you wish to show standard time throughout the year.

Azimuth Display

If you will using a compass to measure azimuths in the field, you may wish to select Magnetic North so that you don’t need to add or subtract the magnetic declination for comparison. If you will be comparing the results with directions on a map, use the default selection of True North.

Elevation and Azimuth Units

Choose either feet or meters for the input and display of the location’s elevation and the height above the horizon.

Miscellaneous Options

If you work with both maps and compass measurements, you often need to convert one type of value to the other unless your compass has a built-in declination adjustment. If you have trouble remembering whether to add or subtract the magnetic declination, checking the Show conversions between True and Magnetic North option will indicate the required conversion on the results page. You can, of course, calculate results using True North and again using Magnetic North, and print both, but you then continually need to ensure that you are referring to the correct printout.

The Use a font size of nn pt for printed output option, explained in the next section, allows you to specify a font size for printed output. With some browsers, doing so may be necessary to get acceptable printed output.

Printing and Saving Results

Printing

Although it is improving, printed output from most web browsers leaves much to be desired, especially when printing wide tables. Unfortunately, the behavior varies considerably among browsers.

Some browsers, such as Firefox 2 and 3, and Internet Explorer 7 and 8, do a reasonable job of shrinking the output to fit on a page, and Firefox even repeats table headings at the tops of new pages. With Firefox and similar browsers, best results are usually obtained by checking a “Shrink to Fit” box (or similar) on the browser’s Page Setup dialog and not specifying a font size for printing. In some cases, it may be necessary to manually select a scale rather than relying on automatic shrinking to fit.

With other browsers, such as Internet Explorer 6, shrinking to fit is less effective, and better results usually are obtained by specifying a font size, typically 9–10 point for printing in portrait orientation. If rise/set criteria restricting the time difference between sunrise or sunset and moonrise or moonset have been specified, the output includes an extra column, so the smaller font size may be needed; the default margins also may need adjustment. Some experimentation using the browser’s Print Preview may be needed to determine the best settings.

To specify a font size, click the Preferences... button to open the User Preferences form, check the box in front of Use a font size of nn pt for printed output, enter the desired font size, and click Apply Settings to save the values.

Saving Results to a File

Saving results to a file doesn’t work as expected with most browsers. With some browsers, the browser’s Save As dialog saves the main form rather than the results; with other browsers, the Save As feature for the results is disabled. If you have a PDF print driver, you can usually get good results by printing to it. If you have Microsoft Office or an equivalent, you can usually copy the contents of the results window to either a word processor or a spreadsheet. On Windows with Internet Explorer 8 and Firefox 3, you can use the View Source feature, copy everything, paste the result into an editor such as Notepad, and save the file using an ‘htm’ suffix.

Script Timeout

Astronomical calculations are very computationally intensive; a search extending over several years, such as those described in the examples in the Advanced Calculations section, requires the execution of millions of instructions, and is not really a task for which a scripting language such as JavaScript was intended. Many browsers are set to give a warning after a given number of instructions have been executed, assuming that at that point the script is a runaway. If you run long searches (or even a large number of calculations) with your browser’s default settings, you probably will get a message to the effect of

A script on this page is causing [your browser name] to run slowly. If it continues to run, your computer may become unresponsive. Do you want to abort the script?

If you get this message, click No to allow the script to continue to run; unfortunately, you may need to do this several times. If you conduct many searches, you probably will want to change the default settings so that you no longer get the message. Information about how to do this for Internet Explorer is given in the Microsoft Knowledge Base article ID 175500, and similar information about how to change the settings for Firefox is available in this article on John Walker’s Fourmilab web site. Be aware, of course, that if you change your browser’s default settings, you become more vulnerable to a truly runaway script.

As of version 8, Internet Explorer is by far the slowest of the major Web browsers, and accordingly, it is not recommended for date searches that span more than a few years.

Running the Sun/Moon Calculator as a Local Application

If desired, the Sun/Moon Calculator can be downloaded and run as a local application; this can be handy if you expect to be in an area without Internet access. Without Internet access, Web-based services such as location lookup and weather will be unavailable, but a location in the built-in database may be close enough, and as a last resort, the location properties can be entered manually.

All necessary files are included in a single zipped file; download it and extract the files to a single directory. To run the calculator, open the file SunMoonCalc.htm.

As of release 35 (and perhaps earlier), Google Chrome treats different files on a computer as coming from different domains, precluding use of the Sun/Moon Calculator as a local application.

Google Chrome for Windows, as of release 18, does not support file cookies by default, so calculator settings such as a custom location and User preferences are not saved between sessions. To enable cookies when running from a file on your computer, create a shortcut and include the switch -enable-file-cookies in the shortcut. For example, the Target of the shortcut might be something like

Chrome PATH\Chrome.exe -enable-file-cookies SunMoonCalc PATH\SunMoonCalc.htm

where Chrome PATH is the folder path to Google Chrome and SunMoonCalc PATH is the folder path to the Sun/Moon Calculator.

As of release 18, Google Chrome also sometimes has problems determining the program that opened a window, making the Rise/Set Criteria and User Preferences forms unusable when the Sun/Moon Calculator is run as a local application.

For personal use, you may do whatever you wish with the downloaded files, including modifications, but there are restrictions on commercial use, posting, redistribution, and claims made for derivative works; see the section Legal in the Sun/Moon Calculator Reference for more information. If you modify the JavaScript file, such as to add locations to the internal database, you should read the file Readme.txt included in the distribution on how to have these changes recognized.

Determining Azimuths and Altitudes of Natural and Man-Made Features

Relating the coordinates given by the Sun/Moon Calculator to features in an outdoor scene requires at least a rough idea of the coordinates of one or more features.

Field Measurements

Azimuths usually are measured with a compass, and altitudes usually are measured with a clinometer. Surveyors use more sophisticated instruments, but the cost, bulk, and complexity of those instruments usually makes them impractical for photographers. Because the magnetic and geographic poles do not coincide, a compass needle generally does not point to true north. The difference between true north and magnetic north is the magnetic declination. Some compasses include a provision to adjust for the magnetic declination so that they can indicate relative to true north; others do not, and require that measurements made with them be converted to true north. The Sun/Moon Calculator includes the option of showing azimuths relative to magnetic north so that no conversion is needed. Better compasses allow the user to sight the target while observing the pointer and scale, usually giving greater accuracy.

It’s important to recognize the accuracy limitations of measurements with compasses and clinometers. Under the best conditions, it usually is difficult to measure altitude to better than ±½°, and azimuths often cannot be measured closer than ±1°. Under less ideal conditions, the errors can be considerably greater. If buried ferromagnetic items are nearby (which is possible at almost any urban location), errors in measured azimuths can be substantial. For comparison, recall that the angular diameters of the Sun and Moon are approximately ½°.

Mapping Software

Topographic mapping software from several suppliers includes the capability of determining azimuths, often relative to either true north or magnetic north. If your position and the positions of the features of interest are accurately known, and your location is not extremely close to those features, the azimuths determined by the software usually are more accurate than those determined by compass measurement. At very small distances between camera positions and features, slight errors in positions result in considerable errors in azimuths; if the camera position is not easy to pinpoint on a map, GPS coordinates transferred to the software can improve accuracy.

The Sun/Moon Calculator az/alt tool uses Google Maps to determine the azimuth and altitude from one location to another, and it can generate an elevation profile of the terrain between the two locations, including a sight line to help determine visibility of one location from the other. It allows you to specify the additional heights of man-made structures.

The Photographer’s Ephemeris can determine the azimuth and altitude from one natural feature to another; as of this writing, it does not have provision to specify the height of a man-made structure above the base elevation, but this capability may be added in a future version.

Other mapping software, such as Delorme Topo North America and Maptech Terrain Navigator Pro can provide azimuth and distance, and can provide the elevations of features and camera positions. Unfortunately, it doesn’t include provision for determining altitude; however, the altitude can be calculated from the distance and elevation difference. If h is the altitude in degrees, Δy is the elevation difference in feet, and D is the distance in miles, the altitude can be calculated to good approximation by

h ≈ tan−1 [Δy/(D × 5280)] − 0.0062D .

National Geographic Maps TOPO! provided azimuths and distances, and allowed easy export of latitude, longitude, and elevation, but the product was discontinued in 2012.

Geodetic Calculations

If the latitudes, longitudes, and elevations of two points are known, the azimuth and altitude from one point to the other can be calculated, often to greater accuracy than possible with either field measurements or mapping software. The US National Oceanic and Atmospheric Administration’s National Geodetic Survey have an online calculator that will perform such a calculation, known as the geodetic inverse; a PC (Windows® and MS-DOS®) version of the program is available for download. Latitude, longitude, and elevation can often be obtained from topographic mapping software or a printed map; if the camera position is not easy to pinpoint on a map, GPS coordinates can improve accuracy, though the elevation from a GPS is usually not as accurate as that obtained from a map. For man-made features, the feature height must usually be added to the elevation obtained from the software.

At first glance, geodetic calculations may seem daunting. But the effort required isn’t much greater than that with mapping software if you need to calculate altitudes. And the accuracy is the best of all approaches if the positions of the camera and the features are accurately known.

Let’s use a geodetic inverse calculation to find the azimuth and altitude of California’s Mount Williamson from the cemetery at the Manzanar National Historical Site. For a well-defined feature such as a summit, the US Geological Survey GNIS database is a good source for coordinates; the search result for Mount Williamson gives

Lat: 363922N
Lon: 1181840W
Elev: 4384 m (14383 ft)

Note that by default, lat/lon are in packed DMS format; unfortunately, the GNIS packed DMS format differs slightly from that for the NGS calculator. The GNIS search page has an Advanced Search that allows latitude and longitude to be displayed in either decimal or DMS format; the former is easier to use in conjunction with the NGS INVERS3D calculator. With decimal format, the search yields

Lat: 36.6560456
Lon: −118.3112048
Elev: 4384 m (14383 ft)

The GNIS doesn’t have an entry for the Manzanar cemetery; get the coordinates from Google Earth, putting the camera position just to the east of the fenced area:

Lat: 36.725401
Lon: −118.162601
Elev: 1208 m

Here Google Earth is set to show lat/lon as decimal degrees; this is the easiest format to use with the NOAA NGS calculator.

Let’s use the NGS online calculator; go to the NGS INVERS3D Computation page, and ensure that the default Geodetic coordinates is selected. Acceptable input formats are indicated at the top of the form. Note that west longitude is positive; east longitude must be indicated by prepending an ‘E’ to the value. The calculator’s handling of the longitude sign is unusual; though it takes a minus sign as indicating south latitude, it ignores a minus sign for longitude, so that either 118.3112048 or -118.3112048 would indicate a west longitude. The NGS calculator is also fussy about its packed DMS format, which differs slightly from the USGS GNIS format. If you use DMS format, the lat/lon for Mount Williamson must be entered as ‘N363922.0’ and ‘W1181840.0’ (or minimally as ‘363922.’ and ‘1181840.’). For north latitude or west longitude, the prepended hemisphere indicator is optional; for south latitude or east longitude, the hemisphere indicator is required. The decimal point is required in all cases. It’s obviously easier to use the decimal format from the GNIS Advanced Search and directly copy and paste ‘36.6560456’ and ‘118.3112048’. Note also that elevations are in meters.

Enter the values for Manzanar as the FROM station and the values for Mount Williamson as the TO station; including names is helpful if you want save the results and know what the two stations are. Click the Compute Three-Dimensional Inverse button at the bottom of the form and get

Output from INVERS3D

First  Station : Manzanar cemetery             
---------------- 
X =   -2416156.5334 m  LAT =  36 43 31.44360 North 
Y =   -4513184.1151 m  LON = 118  9 45.36360 West  
Z =    3793734.3329 m  EHT =      1208.0000  Meters
   
Second Station : Mount Williamson              
---------------- 
X =   -2431234.1875 m  LAT =  36 39 22.00000 North 
Y =   -4513194.9405 m  LON = 118 18 40.00000 West  
Z =    3789463.4915 m  EHT =      4384.0000  Meters
   
Forward azimuth        FAZ = 239 57 32.7888 From North
Back azimuth           BAZ =  59 52 13.3457 From North
Ellipsoidal distance     S =     15338.9277 m
Delta height            dh =      3176.0000 m
Mark-to-mark distance    D =     15670.8601 m
   
DX =    -15077.6541 m   DN =     -7684.2298 m
DY =       -10.8254 m   DE =    -13287.5272 m
DZ =     -4270.8414 m   DU =      3157.5449 m
   
Zenith (mk-to-mk)       ZD =  78 22 32.91
Apparent zenith distance   =  78 21 57.49

Only two of these results are needed: the forward azimuth and the apparent zenith distance; unfortunately they’re not in the most convenient format. Convert the forward azimuth from DMS to decimal degrees to get 239.9591°. Convert the apparent zenith distance from DMS to decimal degrees to get 78.3660°; subtract this from 90° to get the altitude of 11.634°.

Heights of Man-Made Features

When calculating the altitude of a man-made feature, the height of the feature must be added to the base elevation. Heights and illustrations of many tall buildings and other structures in major cities can be obtained from the Skyscraper Page.