Sun, stars, planets and the Moon
1The lesson in plain words
The Sun is your main daytime body. Twilight is the usual window for stars and planets. Each body moves differently, so the almanac treats each differently.
The Sun
The Sun is the yachtsman's main body: bright, easy to find and usable whenever it and a sharp horizon are visible. Its declination swings between about 23°26′N and 23°26′S over the year, and its GHA grows by about 15° an hour. The almanac gives its GHA and Dec for every hour of UT.
Stars
Stars are so distant that their SHA and Dec change only slowly. The Nautical Almanac daily pages list 57 selected stars, with SHA and Dec for each three-day opening; Polaris has its own tables, and a list at the back covers 173 stars. The stars rise about 4 minutes earlier each day, so the night sky shifts slowly through the year.
Planets
Venus, Mars, Jupiter and Saturn are the navigational planets. They move against the stars, so the almanac gives their GHA and Dec hourly, like the Sun. Venus and Jupiter are often brighter than any star. A very bright, steady 'star' near the ecliptic, not in the star list, is usually a planet.
The Moon
The Moon moves about 13° a day eastward among the stars. Its GHA grows by about 14½° an hour; its Dec can change by more than 10′ an hour. It is close, so its direction differs when viewed from the Earth's centre rather than its surface. This shift, called parallax, can approach 1°. You can often observe the Moon in daylight alongside the Sun.
When to observe
You usually observe stars and planets at twilight, the period around sunrise and sunset. You need both a visible body and a sharp sea horizon. Moonlight can sometimes reveal a usable horizon at night; bright planets can sometimes be observed in daylight. The daily pages give civil and nautical twilight times; star selection is in Celestial: stars, planets and the Moon.
2Key data
- Navigational planets
- Venus, Mars, Jupiter and Saturn
- Almanac stars
- 57 on the daily pages; Polaris tables; 173 at the back
- Sun's Dec range
- About 23°26′N to 23°26′S
- Stars
- Rise about 4 minutes earlier each day
- Moon
- GHA about 14½° an hour; parallax up to about 1°
- Spanish terms
- Astros: Sol, Luna, planetas, estrellas; crepúsculo
3Key points
- 1
Use the Sun by day; twilight usually gives both visible stars or planets and a clear horizon.
- 2
Stars: SHA and Dec nearly constant; the almanac lists 57 plus Polaris.
- 3
Venus, Mars, Jupiter, Saturn are tabulated hourly, like the Sun.
- 4
The Moon moves fast and has a large parallax: it needs its own corrections.
4Frequent mistakes
Wrong: Star sights can be taken at any time of night.
Right: You need both the body and a clear horizon. Twilight usually provides them; moonlight can sometimes reveal the sea horizon at night.
Wrong: A bright 'star' not in the list is an error in the almanac.
Right: It is usually a planet. Check the planet columns and the almanac's planet notes.
5Practice case & exam keys
On an Atlantic passage you see, at evening twilight, a very bright steady light low in the west, near the Sun's path, brighter than any star. It is not in the almanac star list. What is it most likely to be, and how do you check?
Show the answer
Venus is a likely candidate; Jupiter is another. Both can outshine every star and lie near the ecliptic. Check the almanac's planet notes for the date and the planet's visibility. Brightness alone does not confirm its identity. For detailed identification, see Celestial: stars, planets and the Moon.
Exam keys
- Use a visible body and a clear horizon; twilight is usual for stars and planets.
- Almanac: 57 selected stars plus Polaris; planets Venus, Mars, Jupiter, Saturn hourly.
- Stars rise about 4 minutes earlier each day.
- Moon: fast, large parallax, separate correction tables.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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