The Moon and two companions form a wide, skinny triangle in the western twilight this evening. They all drop from view within a couple of hours of sunset, though, so there’s not a lot of time to look for them.

The companions are the star Aldebaran and the planet Mercury. Aldebaran stands to the left of the Moon, with Mercury about the same distance to the lower right of the Moon. Aldebaran and Mercury are equally bright.

Aldebaran and Mercury will remain roughly in step with each other until they disappear in the twilight in a week or two.

Aldebaran is sinking as a result of the annual migration of the stars. It takes Earth about 23 hours and 56 minutes to make one full turn on its axis relative to the background of stars. But our planet moves along its orbit from day to day. That means it takes about four minutes longer each day for the Sun to reach its highest point in the sky. As a result of that difference, the stars rise and set about four minutes earlier each day. So over the seasons, stars arc across the night sky.

Mercury’s motion across the sky is the result of the combined orbital motions of Mercury and Earth. As the planets circle the Sun, our viewing angle to Mercury changes. Right now, that combination means Mercury stays fairly still compared to the background of stars. So like the stars, it’s dropping lower in the sky — soon to disappear in the evening twilight.

Tomorrow: final glimmers.
 

Script by Damond Benningfield

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