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Galactic Grid

August 30, 2016

The main system that astronomers use to mark the positions of stars and other objects is basically a projection of Earth’s coordinates on the sky. The celestial equator, for example, is a projection of Earth’s equator. And the celestial north pole is the projection of Earth’s pole, marked by the star Polaris.

But astronomers also use a second system. It’s based not on Earth, but on the Milky Way. The galactic equator is the plane of the Milky Way galaxy, and the poles are the points 90 degrees north and south of the galactic center.

This system was adopted in 1958. The starting point is roughly the center of the galaxy, where the galactic plane crosses the Sun’s path across the sky.

The north galactic pole is in the constellation Coma Berenices, which is in the northwest at sunset, to the lower left of the Big Dipper. There’s no bright star to mark the pole, though.

The pole is a popular region for the astronomers who study other galaxies. It’s away from the busy star fields and dark dust clouds in the Milky Way’s disk, so it’s easier to see other galaxies.

In fact, a search for dark energy that’s getting under way at McDonald Observatory is looking not far from the pole, around the Big Dipper. The project will measure a million galaxies that are billions of light-years away. That will tell astronomers how the universe was expanding long ago, and help them understand how the expansion has changed — a key to understanding dark energy.


Script by Damond Benningfield

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