# NavList:

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

**Re: Moon Venus Lunar - Interpretation of results**

**From:**Antoine Couëtte

**Date:**2020 Feb 7, 23:41 -0800

re : Moon-Venus-Lunar-Interpretation-results-FrankReed-feb-2020-g47027

Good Morning Frank,

Thank you for your reply and for having agreed to enter an *interesting debate*.

*If and when *for whatever reason the rates of change of observed topocentric distances happen to momentarily "slow down" - in terms of elapsed UT - a necessary and immediate consequence is that increasing [UT] time spans are required to cover one same given topocentric distance change value.

As an immediate result any given observational topocentric error angle will require a longer [UT] time span to be covered. Hence an unavoidable loss in accuracy in the final UT determination.

We do record and write down topocentric angular separation between limbs which *may be* affected by local adverse environment effects. Unfortunately we do *not* immediately measure and write down geocentric angular separation between body centers which are not affected by such local topocentric effects.

Having recourse to geocentric angular separations in the course of classical lunar clearing process is only a change of variable in order to ultimately recover UT1.

I fail to see how actual topocentric measures performed in a momentarily degraded topocentric environment - *if any such environment* - can become more accurate simply because we have performed a geocentric variable change.

Under non degraded environments, geocentric angular rates of change most generally stay linear except when bodies are quite close. And they are followed up by their topocentric angular rates of change counterparts which generally stay more or less nearly equal.

Hence the geocentric/topocentric change ratios generally stay more or less equal to 1. Under degraded environments, e.g. in our case, this ratio is already equal to 1.6 . It could reach 2.0 under extreme cases.

Good sense simply shows that if you are directly observing and recording the momentarily slowest changing variable - i.e. the topocentric angular separations - any given observation error on this observation variable will necessary be amplified by the above ratio if for any computational reason you end up using another faster changing variable i.e. the geocentric angular separations.

A ** reductio ad absurdum** immediately shows that -

*again if this could happen*- if the topocentric angular separation were to momentarily require one full hour of elapsed UT time to change by only one arc minute, simply switching into a different computation variable - i.e. into a geocentric angular separation variable even though not experimenting itself such drastic slow-down rate - can in no way erase the adverse consequences of such an utterly unfavorable initial observation environment as regards UT1 determination.

To recap : I fail to see how simply changing a computation variable could yield improved accuracies onto end results.

And also for the time being I still entirely fail to see any prohibition to “ *solve Lunars for UT *” which incidentally is exactly what Lunars have been ultimately designed for.

Hence I need additional light to be shed here.

Therefore please be so kind as to confirm that your here-after quoted " *observed* altitudes, shot with a sextant " are the *geocentric* altitudes i.e. the ones derived from Instrumental recordings of the UL/LL for the Moon and Sun, and center of light for other bodies, all transformed into their geocentric counterparts.

Thanks in advance.

Antoine