The Sky This Week
U.S. Naval Observatory's Weekly Blog

A Rainbow on the Moon?

by Geoff Chester, USNO Public Affairs
19 January 2021

The Moon, Sinus Iridum and Plato region, imaged at the U.S. Naval Observatory on 2012 February 6, 23:58 UT
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor, 1.6X Antares Barlow lens, and a Canon EOS Rebel T2i DSLR.

The Moon climbs higher in the evening sky this week, passing through the winter constellations as she waxes toward her full phase.  First Quarter occurs on the 20th at 4:02 pm Eastern Standard Time.  Look for Luna near ruddy Mars on the evenings of the 20th and 21st.  On the 23rd she passes north of the bright star Aldebaran in the constellation of Taurus, the Bull.  

This is another good week to do some Moon-watching.  As Luna climbs higher along the ecliptic her light passes through less of Earth’s atmosphere, offering less turbulence to blur details on her surface.  As she waxes into her gibbous phases two dramatically different landscapes are revealed.  The two largest of her ancient impact scars contrast with the battered terrain of the “southern highlands”, where craters stand shoulder-to-shoulder as a testament to the violence of the early solar system.  The two large basins known as the Mare Imbrium (Sea of Rains) and Oceanus Procellarum (Ocean of Storms) appear relatively flat compared to the pockmarked highlands.  However, they were produced by collisions with large proto-planets billions of years ago.  Molten rock from the lunar mantle eventually flooded these vas basins, solidifying well after the age of intense bombardment that created the highlands.  The few craters that dot the surfaces of these so-called “seas” were created relatively recently on the lunar time-scale, falling within the last 3.5 billion years or so.  Owners of small telescopes can delight in these features as the terminator line slowly advances across the Moon’s face.  

One of my favorite features on the Moon will be well-placed for viewing on the evening of the 23rd.  By this time the full extent of the Mare Imbrium will be revealed.  This vast circular feature is 1250 kilometers (775 miles) in diameter and is surrounded by a series of tall mountain ranges.  On its northwestern side the ring of mountains is interrupted by a semi-circular feature known as Sinus Iridum, the Bay of Rainbows.  This was one of the first lunar features I identified with my first telescope, and I always like to come back to it when it is visible.  Another striking feature sits along the northern edge of Mare Imbrium, the dark-floored crater Plato.  This 100-kilometer (60-mile) feature is an ancient crater that was flooded with lava.  Its flat surface is pocked with small craterlets; viewing these is a challenge for your telescope’s optics.

Luna’s brightening glow gradually swallows up all but the brightest of the season’s stars, but fortunately Mother Nature has seen fit to endow this part of the sky with some of the brightest of her luminaries.  The brightest star in the sky, Sirius, is easily found trailing behind Orio as he gracefully wheels across the meridian.  To the astronomers of the 17th Century who depicted the constellations in the first star atlases Sirius represented a gleaming jewel in the collar of Canis Major, one of Orion’s two hunting dogs.  Its name is derived from the Greek word for “scorching”, and when it was observed rising just before the Sun its light was thought to amplify the Sun’s rays, causing the “dog days” of summer.  Long before the Greeks the ancient Egyptians noticed that this “heliacal rise” corresponded with the annual life-giving flood of the Nile river.  They saw it as the embodiment of the soul of Isis, one of their principal deities and watched for its first appearance each year for three millennia.  Sirius gets its dazzle from its relative proximity to us, located a scant 8.6 light-years away.  Only six star systems are closer to Earth, and only one, Alpha Centauri, can be seen with the naked eye.  

You can still glimpse the elusive planet Mercury low in the southwest as evening twilight falls.  The fleet planet reaches his greatest elongation east of the Sun on the evening of the 24th.  You should be able to spot him by around 6:00 pm local time, when he will be a bit over five degrees above the horizon.  Viewing him is a feat that, legend has it, that the great astronomer Nicholas Copernicus never achieved.  Despite this detail, he correctly placed Mercury as the innermost planet in his epic work, “De Revolutionibus Orbium Coelestium”.

Mars is steadily moving eastward among the stars of Aires, the Ram.  Early this week he floats just north of the more distant planet Uranus.  To spot this distant “ice giant” planet look at Mars with a pair of binoculars.  Uranus will appear as the brightest of the faint stars below Mars’ ruddy glow.  Point a small telescope at this “star” and you will see a tiny greenish disc that’s some 2.9 billion kilometers (1.8 billion miles) away.  The two planets will appear closest together from the 19th through the 22nd.

 
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