Celestial (Stellar) Navigation

The practice of navigating by the stars is as old as antiquity. In the modern age, light pollution has reduced the night sky to only a few constellations. Fortunately, when sailing, we often find ourselves miles from any electric light source. Here we can appreciate the innumerable stars, watch the movements of the planets, and navigate by the constellations. A rudimentary knowledge of the stars can help you find your way.

Learning the Constellations

Step 1There are a few constellations that every navigator should be aware of. In the Northern Hemisphere, these are the northern constellations of Cassiopeia and the Big Dipper. The Big Dipper and Cassiopeia are visible all year round, revolving around the North Star. Take some time to become familiar with these two constellations so you can spot them any month of the year. (Remember, the stars revolve throughout the year, so at some times of the year expect to find Cassiopeia upside down.)

Step 2Enjoy learning about some of the other constellations as well. The zodiacal constellations mark the ecliptic, and their rising and setting can be useful for marking east and west, and determining the time and season of the year.

Step 3Watch the phases of the moon, which is a very useful guide. The phases of the moon depend on its relationship with the sun. A full moon, for example, is in opposition to the sun, and will rise exactly as the sun is setting.
A new moon will always appear in the western sky. A new moon is a crescent with the points pointing to the left (or east). It is like the shape made with the right hand's thumb and forefinger extended in a 'C' shape.
A first-quarter moon will appear overhead, and a line drawn down from it perpendicular to the horizon, will mark south at sunset.

Step 4The zenith of the ecliptic, or highest point the stars travel as they appear to rotate around the Earth, will mark south.

Locating the North Star

Step 1Once you've learned to recognize the Big Dipper, you can always find Polaris, the North Star. The Big Dipper somewhat resembles a frying pan, with a long handle and a rectangular head. Looking at the "head," imagine a straight line from the two furthermost stars (see diagram). The line should extend perpendicular to the tail of the Dipper, and will point directly toward the North Star. You now have north.

Step 2Cassiopeia also points toward Polaris, and is on the other side of Polaris, opposite the Big Dipper. Polaris is almost equidistant between them. Cassiopeia resembles a W on its side. Polaris makes the form of an arrow with one of Cassiopeia's points. (See picture.)

Step 3The North Star does not move, but appears to stay fixed as all the other stars rotate around it. The picture below shows the Big Dipper rotating around the North Star.


Determining Latitude

Step 1Measuring the angle between the horizon and Polaris will provide your latitude. The North Star is on the horizon at the equator (0 degrees), and directly overhead at the North Pole (90 degrees). The angle drawn between the horizon and the North Star for any location will yield the latitude of the observer.

Step 2If a protractor or sextant isn't available, you can approximate degrees using your fist. Extend your arm toward the horizon, and make a fist. Your fist will take up approximately 10 angular degrees. This is constant for everyone, because people with longer arms typically have larger hands. This can give you a fairly good idea of your latitude.

Step 3Also worth noting: The full moon, when directly overhead, has an angular size of approximately 30 minutes of arc.

Works Cited