Lunar secrets? How to see the moon’s hidden seas tonight

The combination of the slow rocking back and forth of the moon called libration brings into view three lunar maria or “seas” that are normally hidden around the backside – Mare Humboldtianum, M. Marginis and M. Smythii. To find them, you can use the easy-to-spot Mare Crisium. Credit: Virtual Lunar Altas

If everything revolved in perfect circles and all planet and moon orbits were concentric, the solar system wouldn’t be nearly as much fun. Consider the moon. Orbiting in a circle rather than ellipse,its distance from Earth would never vary. There’d be no “super moons” or full moons at the time the moon is closest to the Earth.

Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Credit: Tom Ruen

The moon’s orbital speed would also be constant and never get out of sync with its rotation rate. Because the moon moves slower when farthest from the Earth (and faster than average when closest), we can peer around the east and west limbs of the moon for a few days each month to see craters and lunar seas that are otherwise hidden. This apparent rocking back-and-forth of the moon, called libration, exposes an extra 7.9 degrees of lunar longitude for our viewing pleasure.

Similarly, if the moon’s orbit were exactly concentric with Earth’s and the moon’s axis straight up and down, we’d never be able to peek over and under its north and south polar regions. We’re grateful that the combination of the 5.1 degree tilt of the moon’s orbit and the 1.5 degree inclination of its axis exposes an extra 6.8 degrees of latitude. As you might guess, this tippy business is called libration of latitude.

Add in 1 degree of diurnal libration caused by our changing perspective at moonrise vs. moonset, and altogether we’re able to see 59% of the moon. Pretty cool, eh?

You can see the effects of libration tonight through next week if you have a pair of 10x binoculars or small telescope.

Here’s what the moon will look like on Feb. 14 when it will be full. Because of libration, two of the three featured lunar seas have now disappeared behind the moon’s eastern edge. Credit: NASA

Three lunar seas that normally are absent or appear as little more than skinny stripes along the extreme eastern edge of the moon are in good view this evening – Mare Humboldtianum (Sea of Humboldt), Mare Marginis (the Border Sea) and Mare Smythii, a sea named in honor of 19th century British astronomer Admiral Smyth.

Watch in the coming nights as the rock n’ rollin’ moon whisks them away.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

9 thoughts on “Lunar secrets? How to see the moon’s hidden seas tonight

  1. I always find it interesting to note that Mare Crisium is not circular, but is actually wider that it is high, which you never get to fully appreciate because of its location, even with libration.

  2. Hi Bob
    I remember reading last year about Don Quixote on how it was now a comet and wanted to ask you is there any missions been said this year that involves Don Quixote, I happened to see an article with the name Don Quixote in it but didn’t read it properly and it wasn’t until a few days ago that I realised that the same name had been used and I now can’t seem to find the article, so that’s why I was wondering if it was named on any recent missions, I have read a few now but none with the comet in it so was hoping maybe you knew of any missions or is there anything new with the comet and I take it that we know close approaches etc, thanks if you can help as it was maybe just an article I read and it just had the name Don Quixote in it but it wasn’t anything to do with the comet. Thanks

    • Hi Giorgio,
      No news. I don’t think they’d attempt to do anything with it in the chill lunar night when it’s in hibernation. The sun will rise for the rover on Feb. 9. Perhaps we’ll hear more then.

    • Lynn,
      Sounds like it would be a great comet to check out. I’m eager to see the photos returned when the Philae lander touches down on the surface of Comet 67P Churyumov-Gerasimenko later this year.

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