# NavList:

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

**Re: Regulus Occultation, unexpected results**

**From:**Paul Hirose

**Date:**2017 Sep 29, 21:04 -0700

On 2017-09-29 10:13, Antoine Couëtte wrote: > *18 sep 2017* > *TT-UT=+70,4 s* : although this value is not displayed to the Users here - which is a bit unfortunate because we are dealing with fast moving Moon here, and because it also prevents from super accurately checking other independent software - the actual value used by the NavList web based application is likely quite close from +70.4 s. > *UTC = 2h51m00.0s*, *N29°11'4* , *E 029°58'0*, *height of Eye 0* , *Observer's Altitude 0* , *Temperature 59°F (15° C)* and* Sea level */ Observer's Pressure *29.92" (1013.25 hPa)* TT = UTC + 37s + 32.184s = 02:52:09.18 UT1 = TT - 70.4s = 02:50:58.78 (Thus delta T = 70.4s implies UT1 - UTC = -1.22s, which should never happen. Nevertheless, I will continue the computation.) According to USNO MICA (version 2.2.2) delta T = 69.482, or .92s less than the value supplied by M. Couëtte. However, there's a way to force MICA to use a different delta T. You adjust UT1, such that TT is the desired value after applying the MICA delta T. In this case we want the TT computed above, and thus: UT1 = 02:52:09.18 - 69.482 = 02:50:59.70 With that UT1, MICA calculates with the correct TT. But the .92s adjustment to UT1 puts the observer 15 * .92 = 13.8″ too far east. (I neglect the difference between the sidereal and solar rates.) To correct, move the observer west the same amount, to E029°57′46.2″. (In this case the adjustment is insignificant since we are interested only in the separation between the objects. However, if accurate azimuths and altitudes are needed for an arbitrary delta T, this method is good to know.) At 02:50:59.7 UT1, N29°11.4′ E029°57′46.2″ 0 height, MICA computes unrefracted coordinates: 83.32804 77.40263 Moon azimuth, zenith distance 83.14933 77.66785 Regulus .31748 separation angle .26606 semidiameter .05141 lunar distance = 3.08′ My Lunar4 program agrees with the separation angle and semidiameter (within one unit in the last digit). It says the refracted lunar distance is 0.05123° (3.07′). As I said, a delta T of 70.4s leads to an impossible value for UT1-UTC. The correct value can be determined from IERS Bulletin A, which says UT1 - UTC = 0.32604 s at 0 h on that date. Thus delta T = 32.184 + 37 - 0.32604 = 68.9 s. With the correct UT1-UTC, Lunar4 gives a 0.31750° unrefracted separation angle from the Moon center and 0.05124° refracted lunar distance. The latter is only .00001° different from the angle computed with the "wrong" delta T. In fact, lunar distance is relatively insensitive to delta T, partly due to the eastward velocity of the observer. I believe this diminution of the lunar rate was dubbed "parallactic retardation" by the late George Huxtable.