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Re: Almanac
From: Chuck Taylor
Date: 2002 Mar 25, 08:10 US/PACIFIC
From: Chuck Taylor
Date: 2002 Mar 25, 08:10 US/PACIFIC
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