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Re: Real accuracy of the method of lunar distances
From: Frank Reed CT
Date: 2004 Jan 8, 18:02 EST
From: Frank Reed CT
Date: 2004 Jan 8, 18:02 EST
Fred you asked:
"> And of course Moon-Planet angles will sometimes change more slowly
> still.
Could this be the one exception, when a planet is in retrogradge motion?"
Fred, there is a small difference in the rate of change of a lunar distance because of the motion of a planet, but its practical significance is not that large. There's another problem (with two of the planets) that's more serious.
Jupiter and Saturn are fine for lunars. They have a disk which is just barely visible in a sextant with a high-power telescope. It's clear that you "split the disk" and place the center of the planet right on the Moon's limb. But Mars and especially Venus go through phases. As you know there are phase corrections included in the tables for altitudes in the Nautical Almanac. Those corrections would be quite a bit more difficult in the case of lunars. I know of no case where they have been taken into account. In the case of Venus, the error in the apparent center of the planet could be as large as perhaps 0.3 minutes of arc (in addition to the other ordinary errors). The simplest solution would probably be to omit Venus from lunar distance sights when the distance from the geometric center to the visual center is larger than, say, 0.1 minutes. The phase problem is less serious for Mars, but it's still something to consider.
Letcher suggests skipping Mars and Venus altogether, but that's partly because his method specifically omits corrections for parallax for the Sun, Venus, and Mars. He could achieve nearly the same end by skipping Venus and Mars whenever their parallaxes are greater than 0.15 minutes (the Sun's parallax).
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"> And of course Moon-Planet angles will sometimes change more slowly
> still.
Could this be the one exception, when a planet is in retrogradge motion?"
Fred, there is a small difference in the rate of change of a lunar distance because of the motion of a planet, but its practical significance is not that large. There's another problem (with two of the planets) that's more serious.
Jupiter and Saturn are fine for lunars. They have a disk which is just barely visible in a sextant with a high-power telescope. It's clear that you "split the disk" and place the center of the planet right on the Moon's limb. But Mars and especially Venus go through phases. As you know there are phase corrections included in the tables for altitudes in the Nautical Almanac. Those corrections would be quite a bit more difficult in the case of lunars. I know of no case where they have been taken into account. In the case of Venus, the error in the apparent center of the planet could be as large as perhaps 0.3 minutes of arc (in addition to the other ordinary errors). The simplest solution would probably be to omit Venus from lunar distance sights when the distance from the geometric center to the visual center is larger than, say, 0.1 minutes. The phase problem is less serious for Mars, but it's still something to consider.
Letcher suggests skipping Mars and Venus altogether, but that's partly because his method specifically omits corrections for parallax for the Sun, Venus, and Mars. He could achieve nearly the same end by skipping Venus and Mars whenever their parallaxes are greater than 0.15 minutes (the Sun's parallax).
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
