Part II · First sights · Page 1 of 4
The stars, and the first point of Aries
Stars are so far away that they keep the same places relative to one another. Each has a ground position, just like the Sun, and all of them turn west together, once a day.
So the almanac does not list every star hour by hour. It lists one reference point, the first point of Aries (♈︎), by its GHA each hour, and gives each star's SHA (sidereal hour angle): how far west of Aries the star is. Then:
GHA of a star = GHA Aries + SHA
- GHA Aries (Greenwich to Aries, west)
- 34°33.4'
- SHA of Vega (Aries to the star, west)
- 80°32.3'
- GHA of Vega = GHA Aries + SHA
- 115°05.7'
- Declination of Vega
- N 38°48.8'
SHA hardly changes from week to week; GHA Aries does all the turning.
Declination works the same as for the Sun: the star's latitude, N or S. A star's SHA and declination change only a fraction of a minute in a week.