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 only time both can be measured together. 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 before the horizon fades or the stars are lost to daylight. Three or more bodies well spread around the compass give a tight fix; a small triangle where the lines cross confirms good sights, a large one warns of an error.
- Choose bodies well spread in azimuth, ideally around 120 degrees apart, so the position lines cross at strong angles.
- Pre-compute altitudes and azimuths so each body can be found and shot in the brief twilight window.
- Use Polaris in the northern hemisphere for a quick, direct latitude.
- Use any body's azimuth to check the compass error, day or twilight.
TIP — Spread your 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. Preparation — knowing which bodies, where to look and roughly how high — is what turns that window into a good fix.