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Subject: Re: Lunar mechanics and Double Alts.
From: Fred Hebard (Fred@XXX.XXX)
Date: Tue Apr 29 2003 - 23:39:37 EDT
Dear Mr. Royer,
The basic idea of a lunar is to find out where the moon is in its
orbit. This information is then used to determine the time of day.
It's feasible since the moon moves about 0.5' of arc in 1' of time.
One can measure to an absolute accuracy of about 0.1' of arc, or 12" of
time, which still introduces a 3' error in longitude. So the
measurements need to be as precise and accurate as possible. The
position in the orbit can be determined more accurately by measuring
the moon's distance from a star or planet than by measuring its
altitude.
Since you want to measure the moon's position along its orbit, the best
choice is a star or planet along the orbit. If you measure to one off
the orbit, it's not as accurate by the cosine of the angle of departure
from the line of the orbit. As the moon and object converge, even the
best star in the path probably will not intersect the moon.
Additionally, the farther away the other object is from the moon, the
more accurate the determination, as long as the object is fairly close
to the orbit. This is because the angle of departure from the line of
the orbit will decrease the farther the moon is from an object.
The moon's declination changes radically from day to day, so the
ecliptic is only a first approximation as to the best location for a
star. You could pick Regulus, which is smack dab on the ecliptic, and
still have it be 35 degrees of declination out of the moon's orbit.
In the old days, the Nautical Almanac displayed the distance between
the moon and four stars. This made it easier for mariners to determine
the equivalent of GMT, since they didn't have to calculate these
distances. Herbert Prinz was referring to the Almanac's choice of
these stars in his post. I wonder whether the Almanac's authors
limited the choice of stars to reduce the difficulty of calculation or
limited it to those with well known positions.
For George Huxtable, Karo Corn Syrup is similar to molasses or treacle,
but made from maize rather than sugar cane or sorghum.
My level of expertise is quite low, unlike George Huxtable's and
Herbert Prinz', so I thought my comments might be helpful, if not to
you, who seems to grasp it now, then to others.
Yours Truly,
Fred Hebard
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