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First Leap

March 25, 2018

It might not sound wise, but a gazelle leaps past the feet of the Great Bear. In ancient skylore, in fact, it makes three leaps — each marked by a pair of stars.

The stars that mark the first jump are known as Alula Borealis and Alula Australis — the northern and southern first leaps. As night falls this evening, they’re almost due east, far to the right of the stars that mark the inner edge of the bowl of the Big Dipper — the most prominent stars of the great bear. The two “alulas” are quite close together, so they look like a pair of eyes.

Alula Australis holds an important distinction in the history of astronomy.

A telescope reveals that it’s a “double” star — two stars that appear quite close together. And by measuring the relative motions of the stars, in the late 18th century, astronomer William Herschel proved that they’re bound to each other. That made the system the first confirmed binary — two stars that move through space together, tied by their mutual gravitational pull.

A few decades later, it became the first binary to have its orbit accurately measured. The two stars orbit each other once every 60 years, at an average distance of more than 20 times the distance between Earth and the Sun.

In more modern times, astronomers found that both stars are binaries on their own — each has a small, faint companion in a tight orbit. So the first leap of the gazelle consists of at least four stars, moving through the galaxy as a family.


Script by Damond Benningfield

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