Look south to southwest at dusk and nightfall to view tonight’s grand and glorious moon. The moon’s disk is about 50 percent illuminated by sunshine and 50 percent engulfed in its own shadow. This is what’s known as a first quarter moon, which comes this evening at precisely 8:21 p.m. Eastern Time, 7:21 p.m. Central Time, 6:21 p.m. Mountain Time or 5:21 Pacific Time. By the way, the star next to tonight’s moon is Aldebaran, the brightest in the constellation Taurus the Bull.
Aldebaran: Fiery eye of the Bull
Half the moon is always illuminated in space. In other words, the moon has a day side and a night side, just as Earth does. Due to the angle between the sun, Earth and moon tonight, we’re seeing equal portions of the moon’s day side and night side. The part of the moon that isn’t in sunlight is often called the moon’s dark side. Just realize that – because of the moon’s motion around Earth – the portion of the dark side that we see from Earth constantly changes.
There is a permanent far side of the moon. But there is no permanent dark side of the moon, because any given lunar location experiences night for about two weeks, followed by daylight for about two weeks.
Does the dark side of the moon really exist?
The moon does rotate on its axis. But billions of years of Earth’s strong gravitational pull have slowed it down such that today the moon takes as long to rotate as it does to orbit once around Earth. Astronomers would say that the moon is tidally locked with Earth. For that reason, one side of the moon always faces Earth, but it is not always dark – as you can see just by looking at the sky tonight.
Incidentally, the moon’s gravitational effects on Earth are much smaller, but – given billions of years of time – the Earth will slow down and keep one face always toward the moon.
Look for the first-quarter moon tonight – right next to Aldebaran in the constellation Taurus the Bull.
