The twilight fix
Morning and evening twilight are the short windows when the brighter stars and planets are visible at the same time as a distinct horizon. The navigator pre-computes the approximate altitude and azimuth of the chosen bodies with a star finder or the almanac, so they can be found and shot quickly, and chooses bodies well spread in azimuth so the position lines cross at strong angles. Three or more good sights give a tight fix; a small triangle confirms it, a large one warns of an error in a sight, the time or the working.
- Choose bodies well spread in azimuth (about 120 degrees apart) for a strong fix.
- Pre-compute altitudes and azimuths so each body is found and shot in the short window.
- Use Polaris in the northern hemisphere for a quick, direct latitude.
- Cross three or more sights; a small triangle confirms the fix, a large one warns of error.
TIP — Spread the bodies around the sky Bodies bunched in one direction give lines that cross at a shallow angle and a weak fix. Choosing stars spread around the horizon makes the lines cross squarely and the fix reliable.
NOTE — Twilight is a short window There are only a few minutes when both the stars and the horizon are visible together. Preparation — which bodies, where and roughly how high — is what turns that window into a good fix.