What is a supermoon?
The Moon orbits our planet in an elliptical path, with Earth closer to one side of the ellipse. Every month, the Moon reaches the point closest to Earth (perigee) and the point farthest from Earth (apogee). When the Moon is at or near its closest point to Earth and is full, it is known as a “supermoon.” During this event, the full moon appears larger and brighter in the sky because it is a bit closer to us than usual.
The Moon’s orbit wobbles and changes depending on the positions of the Sun and Earth in their orbits. As a result, the exact distance between the Moon and Earth varies at its closest and furthest points. Due to this, the Moon can appear up to 14 percent larger at its closest point (perigee) than at its farthest point (apogee).
What is a blue moon?
A monthly blue moon occurs when we see the full moon twice in a single month. The Moon’s cycle is 29.5 days, so just a bit shorter than the average length of a calendar month. Eventually that gap results in a full moon happening at the beginning of a month with enough days still remaining for another full cycle ― so a second full moon in the same month. In other words, a full moon that happens on the 1st or 2nd of a month will probably be followed by a second full moon on the 30th or 31st. This happens every two to three years.
A seasonal blue moon occurs when there are four full moons in a single season (spring, summer, fall and winter) instead of the usual three. The third moon in this lineup is a blue moon.
When does the Moon appear to be bigger and more “super?”
You probably won’t notice a big difference in size when the Moon is closest to Earth (a “supermoon”). It can look up to 14 percent bigger than when it’s farthest from Earth. This is similar to the size difference between a quarter and a nickel. Because the Moon will be close to us in its orbit, it will appear a bit brighter than usual.
Moon vs. Supermoon
Use the slider to compare and contrast the Moon’s apparent size at its farthest and nearest points to Earth.
Do blue moons and supermoons always occur together?
No. The term “supermoon” is used to describe a full Moon that occurs within a day or so of perigee, so they happen three to four times a year. About 25 percent of all full moons are supermoons, but only 6 percent of full moons are blue moons (seasonal and monthly). The time between super blue moons is quite irregular ― it can be as much as 20 years ― but in general, 10 years is the average. However, if you like to celebrate both seasonal and monthly blue moons, the gap is closer to five years.
Monthly blue moons always occur in the last two or three days of the month. A monthly blue moon in January is usually followed by another one in March of the same year. And in fact, the next monthly super blue moons will occur as a pair, in January and March 2037. Seasonal blue moons always occur almost exactly one month before an equinox or a solstice. The next seasonal blue moon will be on Aug. 21, 2032.
