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Thread: New utility by Jeff Conrad: Sun/Moon calculator

  1. #31

    Join Date
    Apr 2004
    Location
    SF Bay Area, California, USA
    Posts
    285

    Re: New utility by Jeff Conrad: Sun/Moon calculator

    That's certainly an interesting idea, but it may prove a bit messy to give a reasonable implementation. In the default case, i.e., within the polar circles on days on which the Sun or Moon doesn't rise or set, it might be fairly straightforward. But in more special cases, e.g., at moderate latitudes on a day on which the Moon doesn't rise or set (there's one of each every month), results could be a bit confusing, especially if the user has specified something like 1-minute intervals for +/- 10 minutes on either side of the phenomenon that did not occur. Sun "rise" or "set" also may not occur at moderate latitudes if a nonzero altitude has been specified for rise or set; the greater the altitude, the further outside the polar circles this can happen.

    At the very least, the suggestion to manually select 0:00 and 24:00 for the display of Sun and Moon positions when rise or set do not occur should probably be added to the Help page. You obviously figured out the solution, but it may not be obvious to everyone.

    I recently found a couple of real bugs, though they won't affect most users:
    1. The latitude of Shanghai is wrong (it should be 31:14 rather than 39:14).
    2. Handling of an observer at a height greater than the surrounding terrain isn't always correct. If a nonzero Observer's Height and a nonzero rise/set altitude are both specified, the altitude is still adjusted for the observer's height--this should not happen. Furthermore, atmospheric refraction isn't included until the apparent altitude reaches zero. If an observer is substantially above the surrounding terrain, the Sun or Moon is visible at apparent altitudes less than zero, and the effect of refraction should be included. In most cases, this isn't an issue, but an observer watching the sunrise from Pikes Peak in Colorado can see the Sun at an apparent altitude of about -1.8 degrees.

    These problems will be fixed in a future version. In the meantime, there are simple workarounds for two of the issues:
    1. If the location is Shanghai, select it, then click Copy Place, select Custom Location, and edit the latitude.
    2. For an elevated observer, either ensure that a nonzero rise/set altitude is not also specified, or if a nonzero rise is required, set Observer's Height to zero. Again, this is a pretty unusual situation.

  2. #32

    Join Date
    Aug 2006
    Location
    Melbourne, Australia
    Posts
    38

    Re: New utility by Jeff Conrad: Sun/Moon calculator

    Quote Originally Posted by Jeff Conrad View Post
    Are you talking about today? If so, the rise and set azimuths for Melbourne are as they should be--you folks are in the middle of summer. The situation in the southern hemisphere is the reverse of what it is in sunny (er, rainy) San Francisco; the Sun still appears to move from east to west, but its azimuth decreases during the day, transiting to the north, and further decreasing until it sets. In the summer, rise and set are south of east and west, consistent with a longer day; in the winter, rise and set are north of east and west, with values essentially as you suggest. This is what happens, isn't it?

    Other calculators, including the U.S. Naval Observatory Data Services Sun and Moon positions page give essentially the same values as the LF calculator.
    Stop spouting and think for a minute. Melbourne is at 34 South latitude. There is NO time of the year at which the sun rises or sets south of East or West.

  3. #33
    All metric sizes to 24x30 Ole Tjugen's Avatar
    Join Date
    Mar 2002
    Location
    Norway
    Posts
    3,393

    Re: New utility by Jeff Conrad: Sun/Moon calculator

    Think for a minute:

    I'm at the opposite situation, far north.

    At midwinter, the sun rises (barely) in the south-southeast, and sets (almost immediately) in the south-southwest.

    At the equinoxes, it rises in the east and sets 12 hous later in the west.

    In the summer it rises in the north-northeast, and sets 22 hours later in the north-northwest. A little farther north than here it doesn't set at all, but circles around through due north.

    So in summer in the northern hemisphere it rises and sets north of due east/west.

    In the southern hemisphere it's just the same (only opposite) - in the summer it rises and sets south of due east/west.

    The calculator is correct.

  4. #34

    Join Date
    Apr 2004
    Location
    SF Bay Area, California, USA
    Posts
    285

    Re: New utility by Jeff Conrad: Sun/Moon calculator

    To an observer in space, the apparent motions of the Sun and Moon are quite simple. Governed primarily by Earth's rotation, the paths of the Sun and Moon are quite simple: parallels determined by the Sun's or Moon's declination (angle with the plane of the celestial and Earth's equator). To an observer at a particular point on Earth, the apparent paths are a bit more complicated.

    The attached file illustrates what's happening. The figures are for the northern hemisphere; the situation in the south is just the reverse, as Ole described (imagine everything mirrored about the vertical line through the zenith). The purple axis through P is the north-south axis of the celestial sphere, as well as the axis of Earth's rotation. The paths of the Sun and Moon are circles, as shown in yellow.

    To an earthbound observer, whose horizon is the plane NESW, the paths appear anything but circular, which is why an azimuth/altitude table seems so complex.

    The figures indicate the general characteristics of the Sun's path, for any geographic latitude. Ar and As are the azimuths of rise and set. The descriptions that follow are for the northern hemisphere.

    • Summer solstice: The Sun rises and sets north of east and west, and reaches its greatest maximum altitude at due south.
    • Spring/Fall equinoxes: The sun rises and sets due east and west; day length is approximately 12 hours, for all latitudes.
    • Winter solstice: The Sun rises and sets south of east and west, and reaches its least maximum altitude at due south.
    Attached Files Attached Files

  5. #35

    Re: New utility by Jeff Conrad: Sun/Moon calculator

    Great program, Jeff - thank you.

    Farzad

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