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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Exercise Lunar Distance with Mercury
From: Frank Reed
Date: 2009 Sep 23, 00:59 -0700
From: Frank Reed
Date: 2009 Sep 23, 00:59 -0700
Brad, you wrote: "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 --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
