As the Christmas holiday approaches, millions of people will retell the story of “the first Christmas” in homes and churches around the world. In the version of the story relayed by Saint Matthew, three magi — or astrologers — followed a bright star to Bethlehem, where they found the infant Jesus.
No one knows for sure what that bright star could have been, though many astronomers have taken the question seriously enough to offer their own theories. Opinions differ on whether the Star of Bethlehem was comet, a nova exploding in a neighboring galaxy, a planet, a conjunction of several planets, or some other phenomenon entirely. Some believers argue that the star wasn’t a natural occurrence at all, but a miracle that falls outside the normal laws of the universe.
While we may never know what those magi saw in the sky all those years ago, the idea of using the stars as a guide isn’t anything particularly mystical. Travelers — explorers, naval commanders, merchants, and others — have been using the sky to find their course for thousands of years.
Celestial navigation, also known as astronavigation, was especially important to ocean voyagers, who had no roads or landmarks to guide their way. A seafarer is entirely dependent on his ability to calculate his location in relationship to his destination.
The most common star to navigate by is our own Sun. Because it rises each day in the east and sets in the west, it is easy the determine whether you’re traveling in the right direction just by noting the Sun’s position throughout the day. Other popular navigational objects include the Moon, the planets, and several prominent stars, including Polaris, also known as the North Star.
Celestial navigation was invented by the ancient Phoenicians more than 4,000 years ago. By using basic charts and observations of the Sun and stars, they were able to determine their direction. Several centuries later, the Greeks improved upon this system by inventing the astrolabe, a tool that could accurately determine the position of the Sun, Moon, and various stars for any given day or time. The Greeks were also the first to theorize that the Earth was round, and created a system of longitude and latitude lines on their navigation maps. This gave a clearer understanding of their position in relationship to their destination.
Later navigation tools, including the quadrant, nocturnal, and the sextant made it easier for travelers to calculate their position. These tools would be coupled with an Almanac that listed the positions of various sky objects throughout the year.
Although we now have access to sophisticated Global Positioning Systems, it’s still possible to determine your location by the stars. Here’s a quick look at how:
While there are 57 official navigational stars, you only need to find Polaris, the North Star. In the Northern Hemisphere, you can easily find Polaris by first finding Cassiopeia and the Big Dipper, which are visible all year round. The upper outside star of the Dipper’s “pan” points to Polaris from one side, while the central peak of Cassiopeia, which forms a “W” shape, points to it from the other.
You can find out your latitude by using a protractor to measure the angle between the horizon and the North Star. At the Equator, the North Star sits on the horizon (0°). At the North Pole, it sits directly overhead (90°). For all other Northern hemisphere locations, the star sits somewhere between these two extremes. The angle of the star to the horizon is the same number as your latitude.
Finding your longitude is much more complicated, and requires knowing the exact local time, the difference between that time and Greenwich Mean Time, and the position of the stars and Moon throughout the night. Mariners only began to be able to measure longitude during the early 18th Century, using a precision timepiece called a chronometer.