Vero Beach Public Library
No LL

Obviously, cruisers are interested in navigation and navigation methods. Slightly less obvious is the fact that it is much easier to measure latitude than longitude. (If you don't understand why, then you must read the fascinating book Longitude.) Given that preface, you can understand why I was delighted this morning to find on APOD a way of determining latitude that was completely new and novel to me.

In the Norhern Hemisphere, we know that all stars at night seem to travel in circular motions, except Polaris which is at the exact center of the circle.



Look at the above picture. At the upper left, the light trail of the star's apparent motion is in a circle centered at the upper left. Starts at the lower right seem to move in a circle centered at lower right. Right in the middle, the starts seem to move in a straight line. How can that be? It can be so only if one is looking up from the equator. Stars in the plane of Earth's equator mark the boundary between circular paths centered on the north and south poles.

Of course, low tech time exposures can not be taken from the deck of a ship at sea. But it will work when ashore. Using an inexpensive web cam plus suitable algorithms, it should be possible to made a continuous digital latitude estimator based on the movement of heavenly objects that would not depend on GPS nor require measurements relative to the horizon.