A pair of cat’s eyes glows just above the north-northwestern horizon as darkness falls. The glowing eyes drop from sight in a hurry. And they’ll drop even lower during the coming nights, before disappearing entirely in the evening twilight.

The “eyes” are the stars Pollux and Castor. They mark the heads of the constellation Gemini. The stars are described as “twins,” but that’s mainly because they’re so close together. Pollux is actually twice as bright as Castor, which is close to its right.

Like all the stars in the night sky, Pollux and Castor rise and set about four minutes earlier each day. They and the other distant stars return to the same point in the sky every 23 hours and 56 minutes. But during that time, Earth moves a little farther in its orbit around the Sun. So Earth has to turn a little bit longer for the Sun to return to the same spot. As a result, the entire background panorama shifts position from night to night.

Gemini is at its best during winter, when it’s in view for all or most of the night. In early spring, it’s in view for about half the night. And now, as spring gives way to summer, only the twins remain in view — but not for much longer. They’ll soon vanish from the evening sky once again. But they’ll return to view a couple of months from now — this time in the dawn twilight — beginning another year-long circle across the night sky.

Tomorrow: “gentlemanly” astronomy.

Script by Damond Benningfield