The moon has been much in the news this August. For most of the month, uncrewed spacecraft from Russia and India were in a hot cosmic race to see which country would earn the bragging rights of being the first to safely land a ship in the moon’s south polar region. On Aug. 20, Russia’s plans came to ruin, after an engine burn intended to fine-tune the ship’s descent went awry. Three days later, India—very much a newbie in the moon game—successfully stuck its landing, earning this year’s top honors for lunar exploration.
But the moon hardly needs human intervention to make cosmic headlines. Tonight (Aug. 30) at shortly after 8:30 p.m. eastern time, through the morning of Sept. 1, a rare “super blue moon” will rise in the skies, creating a spectacle that thrills photographers, amateur sky watchers and even seasoned astronomers.
The ”super blue moon” label requires a little unpacking. For starters there is nothing remotely blue about the way the moon will look tonight. Instead, the label–whose provenance is unsettled—refers to the second full moon in any single month. The last one to fill the skies was Aug. 1, so tonight’s full moon comes in just under the wire to qualify for the blue distinction. Blue moons are not common. Only 3% of full moons are blue moons, according to NASA, so tonight’s moon would be noteworthy no matter what. But then there’s the “supermoon” label to consider too.
Read More: How India Became the First Country to Reach the Moon's South Pole
The moon was born in violence. About 4.5 billion years ago, a small, Mars-sized planet collided with the infant Earth, sending up a cloud of debris from both worlds that eventually coalesced into the moon. That kind of explosive release of energy was not likely to lead to a tidy, circular orbit—and it didn't. Instead the moon’s orbit is elliptical, with its closest approach to Earth—or perigee—measuring 357,000 km (222,000 miles) compared to an apogee—or farthest approach—of 406,000 km. That difference doesn’t show itself much when a moon is a mere crescent. But when it is a full moon, things are different. A full moon at perigee appears 14% larger and 30% brighter than a full moon at a more distant remove. The phenomenon is especially striking when the moon is close to the horizon, where it benefits from an optical illusion. Comparing its size to smaller surface features like houses and trees makes the lunar disk look far larger still.
For all of the hoopla created by supermoons, they’re actually quite common. About 25% of all full moons are supermoons, NASA says. This week’s is the third one of the year, with a fourth one on tap for Sept. 29. Another four will appear from August through November of next year. But a supermoon that’s also a blue moon? That’s a much rarer beast. Sometimes 20 years will elapse between the appearances of super blue moons. But if you miss this one, you don’t have to wait quite so long to see another one: According to NASA, there will be two super blue moons in January and March of 2037.
But why miss this one at all? The best viewing begins at 9:36 p.m, in the east- southeastern sky, when the moon rises fully above the horizon. The spectacle will continue through Friday morning, Sept. 1, when the moon will continue to appear full before finally entering its waning phase later that day. The sky show is gorgeous, the sky show is free, but the sky show doesn’t happen every day. It’s best to catch this one while you can.
Write to Jeffrey Kluger at firstname.lastname@example.org.