# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Lunar vs. Occult software test**

**From:**Paul Hirose

**Date:**2023 May 12, 22:47 -0700

I compared my Lunar software to the Occult program to test the effect of the lunar limb profile. Occult takes that into account. Lunar does not. I used occultations by the Moon in the near future: http://www.lunar-occultations.com/iota/iotandx.htm Gam Vir (Porrima), sig Sco (al Niyat), and del Sco (Dschubba) were excluded because they are double. (A close double is OK.) The choice of observer sites was random. Sometimes a site was selected because it was near the top of the list (Abu Dhabi) or because it was interesting (Easter Island). If I were doing this again, I would pick sites to get an approximately uniform occultation distribution on the Moon limb. The test method was to compute immersion and emersion times with Occult. For those times I calculated lunar distances with Lunar 4.4. In most cases the star was below the magnitude 3.0 cutoff in the Lunar catalog, so I got its data from the SIMBAD site. I assumed any failure to get zero lunar distance was entirely due to my software. The standard error (root of the mean squared error) was 1.07″ for the 10 events (5 occultations, immersion and emersion for each). The Moon position from Lunar is the position of its center of gravity. However, that point is a little different from the mean limb center of figure. In the Astronomical Almanac eclipse predictions, corrections in ecliptic latitude and longitude are applied to adjust for the difference. See page A79: https://archive.org/details/binder1_202003/page/n93/mode/2up?view=theater The main window of Lunar has a "solar eclipse corrections" check box to apply the corrections. In theory they should also improve occultation computations. In my test the standard error decreased a little, from 1.07″ to 0.87″. However, a lunar distance observation with a sextant is much different from an occultation. The observer oscillates the body and makes its motion tangent to the limb. A low power telescope in the hand cannot resolve limb irregularities that are obvious in an occultation. In lunar distance calculations I believe a typical limb error from my Lunar program is about 0.5″. That's a mix of calculation, observation of the limb profile display in Occult, and guess. First, how large is the arc that affects the perception of the limb in a sextant observation? I guessed 8°, that is, 4° either side of the true tangent point. Such an arc is 2.2′ in great circle measure and departs 2.3″ from a straight line. I analyzed the limb display in Occult for each of the 10 occultations in my test. At each occultation, I obtained the height of the visible limb with respect to the mean (circular) limb at five points: at the contact point, and 2 and 4 degrees (position angle) either side of contact. I computed the sample standard deviation of the five values. That gave a value for limb irregularity in the vicinity of each immersion or emersion. The 10 values were averaged (root mean square). Result was 0.50″ for the typical semidiameter variation in an 8° arc of the limb. It soon became obvious there was a great deal of luck in what limb details were sampled. Often I measured a limb height close beside a narrow peak or valley which would have made a large difference. But such small details would not be visible in a sextant. I have in mind a second test with more occultations and a more uniform distribution on the limb. -- Paul Hirose sofajpl.com