Finding Longitude: Take a Guess
Latitude can tell you how far north or south you are. With repeated measurements, it can confirm you're travelling in a straight line, not in circles. Latitude alone, however, can't tell you where you are on the planet. You need longitude as well.
Longitude is your distance (in degrees) east or west of the prime meridian, and finding it is a difficult proposition for celestial navigators. Calculating longitude requires knowing the Greenwich Mean Time (GMT), and you can't find that using the stars. You need a very accurate timekeeper called a chronometer. (At Greenwich, England, the prime meridian is 0 degrees. If you know, for instance, that it is 7 a.m. in Greenwich when it is noon where you are, then you know you are 75 degrees east of Greenwich -- multiplying 5 times 15 degrees. The Earth turns 15 degrees per hour or 360 degrees in 24 hours) [source: Nova].
Longitude-wise, what the celestial navigator can achieve is a very rough approximation of east/west progress. By tracking the position of an east/west-setting star at the exact same time every day (assuming you have a watch), you can get a rough idea of how far you've traveled east or west [source: Thaler].
Ideally, though, in the days of GPS, and countless mapping apps, you won't need that much luck. Unless you're stranded in a desert or your boat sinks in the open ocean after being struck by a whale, star-based navigation is more a curiosity than a way to accurately navigate the globe. For most modern navigators, mastering the technique is primarily a path to a deeper understanding of the universe, the relationships between celestial bodies, and the relationship between those celestial bodies and you.
Author's Note: How to Use the Stars to Find Your Way
While in most cases, star navigation is more a hobby than a tool in modern times, I learned in the course of my research something at once reassuring and a little alarming: Even now, your airplane pilot and cruise-ship navigator learn the art. Should their instruments fail, they can find their way the old-fashioned way. Good to know they have a back-up. Frightening to know they might have to use it.
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