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mcveyp@kingman.com wrote:
>
>         I finally got a copy of Mary Blewitt's "Celestial Navigation for
> Yachtsmen." The Sidereal Hour Angle section refers to 0h as the first point
> of Aries. Now I understand that this is not an actual star but an arbitrary
> point in the sky called zero hours. I assume this meridian has something to
> do with the constellation Aries. My astronomy book shows 0h closer to Omega
> Piscium. The most likely first point of Aries is 2 hours away (to the east).
> Is it me or is this off? Can any one guide me back to the path?

We measure longitude from an arbitrary reference point, which we call
the Greenwich Meridian. Similarly, Sidereal Hour Angle (SHA) is measured
from an arbitrary reference point, which we call the First Point of
Aries, or the Vernal Equinox. In the spring, on or about March 21, the
sun crosses the equator, at which instant its declination is 0. At that
instant, if you were to draw a line from the center of the earth through
the center of the sun and out into space, that line would be the First
Point of Aries, where SHA is 0. Why is it called the First Point of
Aries? When it was defined, many centuries ago, it did point to the
constellation Aries. Because the earth's axis has a slight "wobble"
(called precession), it no longer points to Aries.

When the First Point of Aries was first defined, astronomy and astrology
were one and the same. From about 4,000 B.C. to about 2,000 B.C., the
sun was in Taurus. That was called "the Age of Taurus" in astrological
terms. From about 2,000 B.C. to the year 0 or 1, the sun was in Aries,
and it was "the Age of Aries". Since then the sun has been in Pisces and
it is "the Age of Pisces". Soon the sun will enter Aquarius and it will
be "the dawning of the Age of Aquarius", as in the song.

The earth's axis completes one "wobble" in about 25,765 years. Besides
changing the location of the First Point of Aries with respect to the
stars, this phenomenon also has the effect of changing the pole star. In
3,000 B.C. the pole star was Thuban. In 10,000 A.D. it will be Deneb. In
14,000 A.D. it will be Vega, and in 23,000 A.D. it will be Thuban again.

There, now, aren't you sorry you asked? :-)

Chuck Taylor
Everett, WA
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