Spring arrives in the northern hemisphere tonight, at the moment of the March equinox, as the Sun crosses the celestial equator. Day and night are about equal across the entire planet — hence the name “equinox,” which means “equal nights.”

The Sun is passing through Pisces, the fishes. In fact, it’s appeared in Pisces at the equinox for a couple of thousand years. Before that, it was in Aries, the ram. The shift is the result of an effect known as precession of the equinoxes.

Earth wobbles on its axis like a spinning gyroscope. As it wobbles, the Sun’s location against the background of stars moves, too. When the modern constellations were first drawn, the Sun appeared in Aries at the March equinox. So even today, the point of the equinox is sometimes called the First Point of Aries.

The Sun and equinox bring up a point about the constellations. There are actually two types of designations — one for astronomy, and the other for astrology. In astrology, the zodiac is divided into 12 roughly equal constellations. The Sun is “in” each constellation at set dates — they don’t change with precession.

In astronomy, each constellation has precisely defined borders. And the constellations are all different sizes. Pisces is fairly wide, so the Sun remains within its borders for a while. The Sun entered Pisces on March 11th, and won’t slide over into Aries — the next constellation over — for another month.

Script by Damond Benningfield