A phenomenon in the sky called a blue micromoon will appear tonight, and it's not likely to happen again until July 2053. 

It is actually a full moon, but it will seem smaller because it is farther away from Earth than usual. If it isn't too cloudy tonight, it can be seen just after sunset.

According to NASA, "tonight's micromoon will be the farthest, smallest and dimmest moon this year."

The reason it is called a blue moon is because it is the second full moon in May. When two full moons occur in the same calendar month., the second one is referred to as a blue moon — which does not refer to a color. 

The idea is captured most aptly in the saying, "once in a blue moon," which means it's an unusual event. Because a lunar cycle is about 29.5 days long, a full moon can fall at the beginning and end of a month. In this case, the first full moon this month happened on May 1. 

Predictions are that it won't be spectacular, but if the sky is clear, it will be a rare event. The next time a blue micromoon alignment will occur is expected to be in 27 years.