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Anders Reed Mohn wrote:

> I think someone once told me that the Almanac repeats every
> four years.
>
> Is that correct?

Approximately.  The major factors which change from year to
year are

1. The position of the earth in its orbit around the sun.
2. Precession of the earth about its axis (wobble).
3. Nutation (wobble on the wobble).
4. Proper motion of the stars.

Of these, the first is by far the largest influence. Another
name for this effect would be "the Leap Year effect".  If the
earth took precisely 365.00 days to orbit around the sun, it
would be in exactly the same position relative to the sun every
year at the same time. In fact the number is closer to (but not
exactly) 365.25, so every four years the earth is in approximately
the same place at the same time of year.

In other words, the next best thing to the current almanac is a
four-year-old almanac. Still, a current almanac is better.

For comparison, here are a few excerpts for March 24 from the
1998 and 2002 nautical almanacs

                     1998          2002
GHA Aries
1200                1 45.6       1 47.3
1300               16 48.0      16 49.7
1400               31 50.5      31 52.2

GHA Sun
1200              358 25.1     358 25.3
1300               13 25.2      13 25.5
1400               28 25.4      28 25.7

Dec Sun
1200             N  1 26.9    N  1 27.5
1300             N  1 27.9    N  1 28.5
1400             N  1 28.9    N  1 29.5

SHA
Alioth            166 30.4     166 27.7
Arcturus          146 06.1     146 03.5
Canopus           264 01.3     264 00.1

Dec
Alioth           N 55 58.1    N 55 56.8
Arcturus         N 19 11.4    N 19 10.1
Canopus          S 52 42.1    S 52 42.1

The orbits of the moon and planets are on different cycles,
so a four-year-old almanac is of no help whatsoever in
predicting their positions.
Chuck Taylor
Everett, WA, USA

   

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