Part IV · The fix · Page 5 of 6
Setting the star finder for twilight
- The time of the sights, in UT: about 10 minutes after civil twilight in the evening, or before it in the morning.
- GHA ♈︎ for that time, from the almanac: the hour's value plus the increment for the minutes.
- LHA ♈︎ = GHA ♈︎ − west longitude, or + east longitude, from your DR.
- The template nearest your latitude, on the side of the base for your hemisphere.
- Turn the arrow to LHA ♈︎ on the rim, and read the stars inside the horizon.
March 10, 2027, DR N 38°, W 070°. Civil twilight ends at about 2308 UT, so plan the sights for 2318 UT.
GHA ♈︎: 23h 153°23.6', plus 4°30.7' for the minutes = 157°54.3'.
LHA ♈︎ = 157°54.3' − 70° = 87°54.3'. Template N 40°, arrow to 88.0°.
Bright stars between 15° and 65°, read off the finder, and worked exactly for N 38°:
| Star | Finder (N 40°) | Worked for N 38° |
|---|---|---|
| Procyon | 47°, 138° | 49°, 136° |
| Sirius | 32°, 165° | 34°, 164° |
| Betelgeuse | 57°, 178° | 59°, 178° |
| Rigel | 41°, 192° | 43°, 192° |
Within a degree or two: plenty to preset the sextant, face the right way and find the star as the sky darkens.
Naming a star you have shot
The other way round is just as useful. A star shows through a gap in the cloud, you shoot it and take a compass bearing, but you are not sure which star it was. Set the finder for the time of the sight and look at that altitude and bearing: the star there is the one.
The planets
Planets move against the stars, so they are not printed on the base: you pencil them in yourself. The next page shows how.
Try it
April 27, 2027, DR N 52°, W 063°36.0'. You plan morning star sights at 08:05 UT, when GHA ♈︎ is 336°20.5'.
degrees and minutes, like 51 35.6
- Choose the template for your latitude, and turn it to that LHA ♈︎ (to the nearest half degree is plenty).
- Read Dubhe off the template:
degrees and minutes, like 51 35.6
whole degrees, like 132
- Partway through the round, a bright star shows through a gap in the cloud. You shoot it: altitude about 44°, bearing about 144° true.