NavList:

Geoff Hitchcock,

I was saving a question about the disparity in lunar altitudes given by JPL Horizons and USNO for a potential ‘I give up’ post. I’d been using JPL Horizons for azimuth and altitude data and not seeing anything like what the NavList experts were posting for results. (In my case, I’m talking simple things, like seeing the difference in lunar and solar lower limb altitudes when their azimuths were close.)

The method I was using was an attempt at comparing apparent altitudes (Ha’s), subtracting the sum of altitude corrections (refraction, PA, and SD), taken from the USNO site, from altitudes taken from JPL Horizons. I had done this for the hours between 12:30 and 22:30 at 30-minute intervals for 14 October at 28N 90W—without paying any attention to what USNO reported for azimuths and altitudes (Zn and Hc).

A few days ago I did what’s shown in the attachment for solar noon at 28N 90W on 14 October.

The upper half shows the procedure I’d been following--taking azimuth and altitude data from JPL Horizons and subtracting Sum, taken from USNO, from the solar and lunar altitudes to get their apparent lower limb altitudes, Sun Ha LL and Moon Ha LL.  Their difference, shown at far right, is 43’ which is well in excess of the lunar diameter of 30.4’.  Since the difference in azimuths was 0.1’, I took the result to indicate the solar lower limb was in the clear and a solar noon sight could be taken.

I then checked USNO azimuth and altitude data for the first time in this project. The result is shown in the bottom half of the attachment. The USNO lunar altitude is 0.544 deg (32.6’ ) greater than that reported by Horizons. This gave an Ha LL difference of 10.4’, putting about 20’ of the moon diameter above the sun lower limb, precluding a lower limb noon sight.

A check of several other days and times showed a consistent disparity of 32+ arc minutes in lunar altitudes between JPL Horizons and USNO.

Chuck V

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