Let Me Be Blunt For A Moment

The not-quite-full waxing gibbous moon's in Virgo this evening and forms a largish triangle with Saturn and Spica, Virgo's brightest star. Created with Stellarium

Tonight we’ll see a blunted moon. Look closely at its left or eastern edge and you’ll notice that it appears slightly flattened. That’s because we’re still a day away from full phase, so the moon hasn’t quite filled out its circular outline.

Tomorrow the moon will be directly opposite the sun in the sky with the Earth square in the middle. This monthly lineup of the three most cherished celestial bodies in the solar system guarantees us a full moon.  This particular one is the Full Pink Moon, named after the flower moss pink or wild ground phlox. Other names for the April full moon include the Egg, Sprouting Grass and Fish.

Saturn appears the same size in the sky as the swimming pool-like crater Plato in the moon's northern hemisphere. The planet's ring plane is some 175,000 miles across while the crater measures 68 miles. Saturn's enormous distance of 803 million miles is responsible for the shrinkage factor. Credit: Tom Ruen (left), Bob King

Earlier this week Tom Ruen of Minnesota used his trusty 6-inch Edmund reflector to take closeup pictures of Saturn and moon. He shot them at the same scale and discovered that Saturn neatly fit inside of Plato, one of the moon’s best known craters. How cool is that! You can see Plato for yourself tonight in just about any pair of binoculars. Look for a dark, neat oval near the top (north) of the moon.

The limb of Saturn's moon Mimas appears flattened in this photo taken from a distance of 161,000 miles on Jan. 31 this year. The planet's rings are in the background. Credit: NASA/JPL/SSI

Let’s continue on our blunted path. Recently, the Cassini probe in orbit about the planet Saturn, sent back a photo of the moon Mimas (Me-mas or MY-mas) that showed something odd. If you look closely, you’ll notice the right side of the satellite is flattened. Hmmm … what could that be? Unlike the moon, phase has nothing to do with this dent despite the resemblance. Instead, we’re seeing the moon’s largest crater Herschel edge-on.

The crater is 80 miles across, which for a moon measuring only 242 miles in diameter is gigantic. The impact that excavated Herschel removed a significant portion of the moon’s circumference leaving a 6-mile deep crater with 3-mile high walls. Seen from the side or edge-on, the profile of a mountain-ringed crater appears linear, hence the odd flattening shown in Cassini’s camera angle. Tip a bowl and you’ll see the same thing.

At left is Saturn's moon Mimas with a great view of the crater Herschel. At right is the Death Star from the movie Star Wars. Mimas is composed mostly of water ice. Click the photo for a spectacular hi-res image of Mimas. Credit: NASA

Mimas was discovered by astronomer William Herschel in September 1789 and named after one of the Titans in Greek mythology. Nowadays Mimas has acquired the nickname of the ‘Death Star’ moon because of its striking similarity to the Death Star weapon featured in Star Wars. Since the first closeups of the moon weren’t available until after the movies were made, the resemblance is coincidental.

We trust Mimas has no ill intentions.

4 Responses

  1. la vie, c'est dur

    mimas is only 242 miles in diameter. is that correct? if so, it’s a rather small object, more like an asteroid. was herschel’s telescope powerful enough to detect such a small object at such a distance? just wondering.

    1. astrobob

      Yes indeed he did see it – the first person to do so. It’s visible nowadays in a good 10-inch scope when the brightness of Saturn’s rings don’t interfere. No doubt it’s brighter (13th magnitude) than some objects its size, because the moon is mostly water ice and therefore a good reflector of light.

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