NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Moon coordinates from NASA Scientific Visualization Studio
From: Paul Hirose
Date: 2023 Jul 15, 13:50 -0700
From: Paul Hirose
Date: 2023 Jul 15, 13:50 -0700
On 7/9/2023 9:46 PM, NavList Community wrote:
> Go here https://svs.gsfc.nasa.gov/5048.
>
> There remains a more puzzling problem The RA and Dec even after
> factoring out the display problem (above) is still wrong by a
> significant amount.
The explanation lies in the "J2000 Right Ascension, Declination" label
on the coordinates. So they are in the J2000 system, which is almost
identical to the ICRS. The difference ("frame bias") is only a few
0.01″. Precession and nutation are not applied.
To avoid the sexagesimal conversion bug, I selected a time when minutes
and seconds were less than 30. Output from the NASA page:
Time (UTC) 7/5/2023, 12:00:00 AM
Diameter 1990.1 arcseconds
Distance 360140 km (28.23 Earth diameter(s))
J2000 Right Ascension, Declination 20h 29m 26s, -24° 11' 12"
My geocentric apparent ICRS coordinates agree with NASA:
20h 29m 26.26s -24° 11′ 12.2″.
I also agree with their diameter and distance:
1990.15" diameter
360139.8 km distance
That's the apparent distance (light time multiplied by speed of light).
The geometric (actual) distance at that instant is 12.7 km greater.
My diameter is calculated from apparent distance and the IAU adopted
Moon radius of 1738.4 km.
The NASA page uses the JPL DE421 ephemeris vs. DE441 in my calculation,
but at this precision the differences are not visible.
I don't like the AM/PM on the web page. That's ambiguous at midnight and
noon, which is why legal documents often say 12:01 AM to avoid confusion.
Another criticism is that the page displays RA and dec with 1-second
precision. But at the equator one second of RA is 15 times bigger than a
second of declination. A good rule of thumb is to give RA one more
significant digit than dec.
--
Paul Hirose
sofajpl.com
