If you’re wondering about the mechanics behind why the Moon looks bigger than usual tonight, or what that has to do with a lunar eclipse, this quick NASA animation will explain the basics of tonight’s astronomical event.
We call this a “supermoon,” but it’s actually a pretty simple optical illusion. As you can see in the animation, the Moon will be at perigee, the part of its orbit that brings it closest to Earth, tonight. We usually just say that the Moon is about 238,000 miles away, that’s an average; the distance varies depending on where the Moon is in its orbit. At apogee (the most distant point of its orbit), the Moon is actually 251,968 miles from Earth, but at perigee, it’s just 225,804 miles away. Because the Moon is close, it will look about 14% bigger than usual.
Of course, tonight’s supermoon happens to coincide with a total lunar eclipse. Earth will pass between the Moon and the Sun, casting a reddish shadow over the Moon’s surface. That’s a combination that happens just once every couple of decades. The second half of NASA’s animation is a cute look at how life on Earth has changed from one supermoon eclipse to the next, and how we might look in 2033, when the next one happens.
If you have a good pair of binoculars, you can get a more detailed look at the lunar surface, even on an ordinary night. Tonight’s supermoon should offer an even better view than usual of the Moon’s plains and craters. Once the eclipse starts, look along the edge of the shadow, where the lighting will make the terrain features really stand out.