Giant red moons are so last year. Look at the sky this evening and check out a pink mini-Moon. So what's going on up there?
The Moon's orbit around the Earth is elliptical rather than perfectly circular. This means that sometimes it's farther away from the planet than others. When the Moon is at its closest to the Earth, it is said to be at perigee. When it is at its farthest point, it is at apogee. From our vantage here on Earth, close means big and far means small. When the Moon is full, the difference in size is especially evident, as it will be tonight.
WHY DO WE GET FULL MOONS, ANYWAY?
The Moon's phases have nothing to do with Earth's shadow. (When the Earth's shadow obscures the Moon, you get a partial or total eclipse.) Rather, lunar phases are the result of the Moon's position relative to the Sun and Earth. As you might have noticed, the Moon does not spin. Over the course of a full revolution around the Earth (about 27 days) different parts of the Moon are illuminated by the Sun. No matter where the Moon is in its orbit, half of it is lit up—but not always on the side we can see.
When the Moon is opposite the Sun (i.e., in the order of: Sun, Earth, Moon), it is "full," which is to say the half of the Moon that we can see is in full illumination. As days pass and positions change, the part of the Moon we see receives less and less light until the Moon is nearer to the Sun (i.e., Sun, Moon, Earth). At that point, the side of the Moon that we can't see is lit in totality, and the nearer half of the Moon is thus in darkness. The darkened Moon is said to be "new," and the process repeats itself, lighting until full, and darkening until new, and so on.
Here's a video from NASA that demonstrates the lighting of the Moon relative to its position to Earth.
WHAT COLOR WILL IT BE?
When orbital dynamics place the Moon at its fullest and its orbit at perigee, you get a super Moon. (Last year's super moon coincided with a lunar eclipse.) When the Moon is full and at apogee, on the other hand, it's said to be a mini-Moon.
Contrary to its name, tonight's Moon will not actually be pink. The full moon in April was first called the "pink moon" by Native Americans, as its appearance corresponds with a pink wild flower that blooms around this time of year. In this, the name comparable to a "blue moon," which is sometimes described as a rare second full Moon in a single calendar month, and is not actually blue.
Tonight's pink mini-Moon is the smallest that the Moon will appear this year. If it's too cloudy and you don't catch it, you'll have to wait until June 9, 2017 to see it again.