Part II · First sights · Page 3 of 4
Polaris sits almost over the pole
Polaris, the Pole Star, lies almost exactly above the North Pole. Its light reaches the Earth along lines parallel to the Earth's axis, so the angle you measure up to it from your horizon equals your latitude.
Predict You sail south across the equator to 10° S. What happens to Polaris?
- Your latitude
- 40° N
- Altitude of the pole above your horizon
- 40°
Polaris is so far away that its light reaches every point on Earth along lines parallel to the axis. The angle between your horizon and that light is your latitude.
Almost. Polaris is about 38′ from the true pole, and circles it once a day. Depending on where it is on that small circle, it can look up to 38′ higher or lower than the pole would. Here is a close-up over the pole:
- LHA Aries
- 200°00.0'
- Polaris from the pole
- 0°37.5'
- Along your meridian
- 33.4′ beyond the pole
- So Ho needs about
- +33.4′
Polaris is beyond the pole, so it looks lower than the pole would: add to Ho.
The a0 table does exactly this for you, with a constant added so it is never negative.
Which way Polaris lies from the pole depends on the turning sky, measured by LHA Aries: GHA Aries plus your east longitude, or minus your west longitude. The Polaris tables take it from there.
Polaris only works in the Northern Hemisphere: south of the equator it is below your horizon, and in the tropics it sits so low that it is hard to see against the twilight horizon.