The moon will be out till fairly late tonight. But the second-brightest celestial object to light up the evening sky – the planet Venus – looms low in the southwest at dusk and nightfall. It sets fairly soon after nightfall, so be sure to get an eyeful of this blazing beauty before she departs at early evening.
The moon reaches perigee – its nearest point to Earth for the month – this Thursday, October 10, at 23:00 Universal Time. That’s 7:00 p.m. EDT, 6:00 p.m. CDT, 5:00 p.m. MDT or 4:00 p.m. PDT at our U.S. times zones. One day later – on Friday, October 11 – the moon’s half-lit first quarter phase will come at precisely 7:02 p.m. EDT, 6:02 p.m. CDT, 5:02 MST or 4:02 p.m. PDT.
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 a little more of its night side than day 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.
Understanding moon phases
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 about two weeks of daylight.
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.
- EarthSky
