Two planets and the moon will form a triangle in the southwestern sky on Sunday night. Watch as Mars and Saturn, which are already close together, are joined by the thin crescent moon. The three are in the zodiacal constellation Scorpius.

If you miss this event, you get a less interesting version of it on Tuesday night. By then, the moon will have advanced along the zodiac and be farther away. The triangle will then be far more elongated.

Mars is the reddish planet, east of the yellowish Saturn.

Though the two planets appear close together, they are far apart. Mars is 120 million miles away. Saturn is 948 million miles away.

Treasure the moment as the two planets are separating. In the nights ahead Mars will show how quickly it can move as it moves away from Saturn and heads eastward. Watch it happen over successive nights.