NavList:

Dear Jeremy,

Here you are again !

:-) + :-)) !!!

You just requested :

QUOTE

Is this the reason that we choose bodies close to the ecliptic for lunars?

UNQUOTE

I think it safe to first indicate that the Historical Interest of Lunars has been to attempt determining TIME with the "best" achievable accuracy.

To this effect the fastest moving body being the Moon, to achieve such "best accuracy" you certainly want to take the Moon Distance by reference to (second) bodies having the fastest varying distance from the Moon per unit of time, i.e. close to the Moon path in the sky. This is where the distance from any past/future point on this path will likely vary to the greatest extent per unit of time. Such path is simply ... close from the Ecliptic (up to +/- 5°).

If Moon + other body are (almost) 180° apart while both close to the Ecliptic, they will likely have almost opposite declinations.

Hint ! Do not take an object too close from the Moon (case of an Occultation for example). By the effect of Parallax, you might see both bodies "fast" closing or moving away while actually their Geocentric Centers Distance might remain almost constant !

Another more "classical" counter-example : just figure out the expected accuracy from a Lunar using Polaris ... Such Moon-Polaris distance might not vary very fast (absolute maximum distance variation per unit of time would be more or less the Moon declination hourly variation). It then explains why all Lunar stars are close to the Ecliptic. Planets do also fully this specific condition.

In the very best cases if you have the chance of getting a second body close from the Moon path, you would get a variation of their distance close from up to 1800" per hour. That is about the absolute maximum hourly distance variation which can be observed.

Hope this first "go" can help you. And, of course ...

... I leave to Frank and/or to George to get into further details on this.

Best Friedly Regards from

Antoine M. "Kermit" Couëtte

----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------