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The Moon stands high in the south not long after sunset this evening, with three planets trailing to its lower right. Mars is the closest to the Moon, with Venus, the “evening star,” farther along the same line. Mercury is farther still, just above the horizon, and visible mainly from the southern half of the country.
For the past two and a half centuries, such an alignment has been more than a pretty picture for navigators at sea. It’s helped them determine their longitude — their position east or west on the globe.
Such a determination was made possible by two inventions. First was clocks that could keep accurate time at sea. And the second was the Nautical Almanac, a book first published by the British Astronomer Royal 250 years ago.
The almanac contained a lot of basic skywatching information. For navigators, though, its most important feature was a daily table of the Moon’s precise distance from bright stars and planets. The distance was listed at three-hour intervals for Greenwich, England. By measuring the angles between the Moon and the other objects, and calculating the difference from Greenwich, a navigator could determine his longitude — a critical detail for safe travels.
The United States began publishing its own almanac in the 1850s. The two almanacs linked up in 1960, and in 1981 became the Astronomical Almanac. By then, they’d been joined by one more publication — a key tool for pilots in World War II. More about that tomorrow.
Script by Damond Benningfield