# NavList:

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

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Re: Exercise Lunar Distance with Mercury
From: Jeremy C
Date: 2009 Sep 23, 20:53 EDT
Venus Lunars:

I shot 15 of them.  The worst error was 1.2'  the best was 0.0.  Average error: 0.00667' arc.  Standard Deviation was 0.58.  Only Jupiter had a better standard deviation of 0.55.

I don't think I'd say that Venus should be excluded out of hand.

Jeremy

In a message dated 9/23/2009 7:12:32 P.M. Eastern Daylight Time, bmorris@tactronics.com writes:

Thanks Frank.

I don't have enough practical experience with lunars to answer questions like that.  There is a phase, I can see that phase with my 11x scope and I was just curious.

It should be interesting to see the outcome of the Venus Lunars that Jeremy shot.  Did the phase of Venus really affect his result?

Best Regards

________________________________________
From: navlist@fer3.com [navlist@fer3.com] On Behalf Of frankreed@HistoricalAtlas.com [frankreed@HistoricalAtlas.com]
Sent: Wednesday, September 23, 2009 3:59 AM
To: NavList@fer3.com
Subject: [NavList 9871] Re: Exercise Lunar Distance with Mercury

"Since Mercury is an inferior planet and very near to the Sun, Mercury always
shows a phase when we can see it.
Did you see the phase of Mercury when performing the observation?  What power scope?
When aligning the limb of Mercury to the limb of the Moon, were both curved
limbs in alignment or were caused to estimate the tangency, because the
phases of both objects were out of alignment."

Er... This is negligible. Mercury has a very small angular semi-diameter --about 5 seconds of arc when favorably placed. Now, in a lunar observation with a planet that has a discernible diameter, you have three choices: place the limb of the planet (only one available) on the Moon's limb, place the geometric center of the planet on the Moon's limb, or place the center of illumination on the Moon's limb. The illuminated limb of Mercury and its geometric center would be separated by about 5 seconds at a favorable appearance, while the center of illumination would fall in between. It would rarely amount to a difference greater than a few seconds of arc. In other words, this difference is safely negligible. So which choice should you actually use if the options are available? The historical recommendation was almost invariably to use the geometric center. A peek through a higher-powered telescope can help here if you're not certain about the phase at low power. In practice for modern lunarians, most will end up using the center of illumination, and for that the positions in the standard Nautical Almanac are appropriate. Personally, I don't recommend using any planet with a really substantial difference between center of illumination, geometric center, and illuminated limb. That rules out only one planet: the planet Venus. Don't shoot Moon-Venus lunars.

-FER

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