Mars Is About to Disappear Behind a Full Cold Moon in Rare Occultation

Mars is about to pass behind a full cold moon in an occultation Wednesday night that can be seen across a large portion of North America and Europe.

Mars' occultation is essentially an eclipse of the red planet, with the moon passing directly between the Earth and Mars. The planet will be seen starting to dip behind the moon Wednesday at around 10:30 p.m. ET, reappearing about an hour later. Even outside of the occultation viewing area, stargazers will be able to see Mars passing close by the full moon in what's known as an "appulse," near the eastern horizon, as seen from North America.

This event marks the coincidence of three rare astronomical events: a full cold moon (December's full moon), the occultation of Mars and Mars' "opposition."

Mars is our second-closest planetary neighbor, orbiting the sun once every 1.88 Earth years, or 687 days. On December 1, it hit the point in its orbit around the sun where it is as close to the Earth as possible, only 38.6 million miles. This occurs just once every 26 months. At its farthest point from Earth, Mars is about 250 million miles away.

The night of December 7 marks the point in Mars' orbit where it is in "in opposition" to Earth, which means it is on the direct opposite side of Earth relative to the sun, with the three bodies aligning in a straight line.

At the point of the occultation, the sun, Earth, moon and Mars will all line up at once.

Wednesday's cold moon is also special because it is the last full moon of the year. This time of year, only three weeks before the December 21 winter solstice, means that the sky will be very dark, creating the perfect conditions for observing the moon, stars and planets. The name "cold moon" comes from Mohawk tradition because of the freezing temperatures typical of December. The Mohicans named this final full moon the long night moon after its proximity to the longest night of the year, on the solstice.

The combination of these factors means that the occultation will be visible with the naked eye, with the bright glow of Mars visible even next to the full moon as it passes behind it.

But using binoculars or a telescope will allow stargazers to watch the event in much greater detail.

"I'm really curious if we'll continue to see Mars with the unaided eye right up to the moon's edge. I suspect we will," veteran skywatcher Bob King wrote on SkyandTelescope.com. "Also, at what point will the planet disappear from view as it dips behind the moon? Keep those binoculars handy! Telescopic observers will get fabulous views, especially those living within the southern graze zone, where the moon will partially occult the planet."

The occultation will be visible throughout most of North America, except for much of Alaska and the Southeastern coastal states. In Los Angeles, Mars will disappear at 6:30 p.m. local time and reappear at 7:30 p.m. Those outside of the viewing zone can watch the occultation on a livestream.

The next lunar occultation of Mars will occur in January 2025.

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