One of the stalwarts of the evening skies of spring and summer is disappearing from view. In fact, tonight is your last best chance to see it, because it’s close to the crescent Moon.

Spica is the leading light of the constellation Virgo. The bright star stands well to the left or lower left of the Moon as evening twilight fades away.

Spica climbs into view in the east in the evening skies of April. It rises about four minutes earlier each night, so it’s a little farther across the sky from one night to the next.

That change in position is caused by Earth’s motion around the Sun.

As Earth spins on its axis, the stars rise and set every 23 hours and 56 minutes. During that period, though, Earth moves a little farther in its orbit. That changes the viewing angle to the Sun. So our planet has to turn a little farther for the Sun to reach the same point in the sky — an interval of 24 hours. If you do the math, you’ll see that on our 24-hour clock, the distant stars must rise four minutes earlier each day.

Now, that progression is beginning to carry Spica out of sight. It’s so low in the western twilight that it’s hard to pick out, even though it’s pretty bright. By two or three weeks from now, depending on your latitude, it’ll be too low in the sky to see at all.

As always, though, Spica won’t stay hidden for long. It’ll return to view in the dawn sky in early November — then return to the evening sky in the spring.

Tomorrow: disappearing planets.

Script by Damond Benningfield