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The term “supermoon” is used to imply that a full moon is at its closest point to earth while orbiting around the planet. Monday’s supermoon was described as the biggest and brightest in 69 years. And the next time the moon will be as close to earth won’t be till 2034.

The significance of the supermoon on November 14 is examined by Noah Petro, the deputy scientist for NASA’s Lunar Reconnaissance Orbiter mission, in the video above. He explains that the reason the distance between the Earth and the moon keeps changing is that the moon doesn’t orbit our planet in a perfect circle.

“The main reason why the orbit of the moon is not a perfect circle is that there are a lot of tidal, or gravitational, forces that are pulling on the moon,” Petro says. “You have all of these different gravitational forces pulling and pushing on the moon, which gives us opportunities to have these close passes.”

When the full moon is at perigee – the point nearest to earth – it appears up to 14 percent bigger and 30 percent brighter in the sky. And so, supermoon.