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Moon and Spica

August 3, 2022

Spica cruises along with the Moon this evening. The leading light of the constellation Virgo is close below the Moon at nightfall, and sets before midnight.

Astronomers have studied Spica for millennia. In fact, Spica played a starring role in an important discovery about our own planet.

Around 127 B.C., Greek astronomer Hipparchus carefully studied Spica’s location. He noted its position relative to the Sun’s position at the fall equinox. He then compared his observations to those made by other skywatchers a few centuries earlier. And he found something surprising: Spica had shifted a bit. And in fact, other stars he measured showed that same change.

Hipparchus called that change the precession of the equinoxes. The entire celestial sphere — every single star — shifts by a little more than one degree per century.

That isn’t caused by the stars, though. Instead, it’s the result of a “wobble” in Earth’s rotation — like the wobble of a gyroscope that’s beginning to slow down. It takes about 26,000 years to complete a single wobble. During that time, Earth’s north celestial pole draws a big circle on the sky. Different stars along that circle take turns as the pole star.

The wobble also means that every star shifts position relative to the point of the equinox. So over the millennia, the entire celestial sphere rotates through the seasons — a discovery made possible in part by brilliant Spica.

Script by Damond Benningfield

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