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Discrepancy in Lunar Distance Value

Michael Teoh

April 06, 2020 08:03

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Over the years, I've noticed that the lunar distance as given by the app does not agree with other sources. Usually they are off by a few thousands kilometers. I wonder if this is due to the different database or calculation the app developer is using. 

For example, the closest full moon for this year is suppose to be the April's Pink Moon, but according to SkySafari, the March's Worm Moon was closer.

Below is the comparison between the lunar distance value as in SkySafari 6 Plus, TheSkyX, and Astropixels.com (by Fred Espenak, which is also reference source for Space.com). I am unable to obtain data from USNO (which is my primary data reference source) due to their current site upgrade.

 

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7 comments

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  Pedro Braganca April 08, 2020 16:11

  Hi Michael,

  I suspect the other programs are giving you the distance to the moon from the Earth's centre, whereas SkySafari and Starry Night give it to you from the surface of the Earth (observers current location).  I'll make a note of this for followup however.

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  Chris April 08, 2020 16:39

  Pedro, thanks for that explanation. If that ends up being the explanation, I think I would always prefer distance from my observing location versus the center of the earth. Curious if others agree or would want a center to earth measurement for some reason I haven't thought of?

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  Michael Teoh April 09, 2020 02:18

  Hi Pedro,

  I understand that for most cases in astronomy context, we always measure from center to center.

  Anyway, if the Earth’s radius is the reason, that still doesn’t explain why the full moon was closer in March as in SkySafari, when value between my geographical location to Earth’s center is the same. Whether or not I include the earth radius (which is a fixed value) in the calculation, if the April full moon is closer, it still should be closer.

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  Pedro Braganca April 11, 2020 21:16

  Your geographical location might be the same on the Earth but the geometry of your home location (think direct line) to the Moon would be different (on another date).

  I'll put a note to our dev team to include "Distance from Earth" and "Distance from Observer" in the Info panel.

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  Dean April 11, 2020 22:11

  I'm still somewhat confused as to which method SkySafari is using for the distance to the moon.

  Is it Center to Center, Surface to Surface, or the User's location to the Surface (or Center?) of the moon. (The latter would seem to be an overly complex calculation with dubious usefulness, since it would be constantly changing as the earth rotates.)

  See Diagram.

   

  

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  Michael Teoh April 12, 2020 04:40

  Dean, I believe it is the third case in your diagram.

  According to my rough calculation below, the lunar distance from my location between the March and April full moon is indeed around 10,000 km difference.

   

  

   

  Btw, I just found another program I have which the lunar distance matches those of SkySafari, which is the Virtual Moon Atlas 6.0. 
  
  Personally I would prefer the center to center measurement.
  
  From an observer's point of view, if I want to compare the distance or visual/photographic size between two full moons, I would observe at the same time (e.g. when the Moon transits) on the two different dates, where the April full moon would have been technically nearer and bigger.
  
  If according to SkySafari, at the exact times of the two full moons occurred, the March full moon would be technically bigger and closer, but I will not be able to observe the April full moon at that time as it was below horizon. So, no comparison can be done. By referring to the value given by SkySafari, I may misunderstood that the Moon was closer to Earth (center to center) during the March full moon.

  Nonetheless, value given by SkySafari is correct in term of distance between the particular observer and the Moon. 

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  Pedro Braganca April 12, 2020 14:12

  You guys are rockin' the analysis!  We'll look at the code tomorrow  to figure out the end points for the Moon distance measurements.  In either case, I also made a note to include center-center distances in a future update. In the meantime, you can use the measuring tool in SkySafari to measure centre-center distance (must be in orbit mode).