Friday, February 11 - With the Moon looming high overhead as darkness falls, this looks like a good weekend to explore our nearest astronomical neighbor! Tonight we will begin our lunar explorations as we look to the far north and identify the “Sea of Cold”—Mare Frigoris. This long, vast lava plain extends 1126 kilometers across the surface from east to west, yet never ranges more than 72 kilometers from north to south. Look for the unmistakable dark ellipse of landmark crater Plato caught on Frigoris' southern central shore.
Plato Crater - Credit: Damien Peach
Named after the famous philosopher, this mountain-walled plain with a dark floor is a Class V crater. Its slightly oval shape spans approximately 101 kilometers in diameter but is a shallow 1 kilometer deep - .appearing far more elliptical due to its northern latitude. Plato's floor is its most curious feature. Consisting of 2,700 square miles of unique lava, and only broken by a couple of very minor and supremely challenging craters, Plato is one of the very few areas on the lunar surface that seems to have changed in recent history. The bright rim of Plato's enclosure is very ragged and can rise as high as 2 kilometers above the surface, casting unusual shadows on the lava covered floor. At around 3 million years old, Plato is more ancient than Mare Imbrium to its south. For 300 years astronomers have been keeping a watchful eye on this crater. Hevelius called it the “Greater Black Lake,” due its low albedo (surface reflectivity). Despite its dark appearance, Plato is well known as a home for lunar transient phenomena such as flashes of light, unusual color patterns and areas that could be outgassing. Enjoy this lunar feature which will point the way to others in the future!
Saturday, February 12 - Let's begin our lunar studies tonight with a deeper look at the "Sea of Rains." Our mission is to explore the disclosure of Mare Imbrium, home to Apollo 15. Stretching out 1123 kilometers over the Moon's northwest quadrant, Imbrium was formed around 38 million years ago when a huge object impacted the lunar surface creating a gigantic basin.
The basin itself is surrounded by three concentric rings of mountains. The most distant ring reaches a diameter of 1300 kilometers and involves the Montes Carpatus to the south, the Montes Apenninus southwest, and the Caucasus to the east. The central ring is formed by the Montes Alpes, and the innermost has long been lost except for a few low hills which still show their 600 kilometer diameter pattern through the eons of lava flow. Originally the impact basin was believed to be as much as 100 kilometers deep. So devastating was the event that a Moon-wide series of fault lines appeared as the massive strike shattered the lunar lithosphere. Imbrium is also home to a huge mascon, and images of the far side show areas opposite the basin where seismic waves traveled through the interior and shaped its landscape. The floor of the basin rebounded from the cataclysm and filled in to a depth of around 12 kilometers. Over time, lava flow and regolith added another five kilometers of material, yet evidence remains of the ejecta which was flung more than 800 kilometers away, carving long runnels through the landscape.
Crater Eratosthenes – Credit: Damien Peach
Look along south shore of Mare Imbrium right where the Apennine mountain range meets the terminator. At 58 kilometers in diameter and 12,300 feet deep, Eratosthenes is an unmistakable landmark crater. Named after the ancient Greek mathematician, geographer and astronomer Eratosthenes, this splendid crater will display a bright west wall and a black interior hiding its massive crater capped central mountain 3570 meters high! Extending like a tail, an 80 kilometer mountain ridge angles away to its southwest. As beautiful as Eratosthenes appears tonight, it will fade away to almost total obscurity as the Moon approaches full. See if you can spot it again in five days.
Sunday, February 13 - We start tonight's lunar tour with a northern landmark that can even be spotted with unaided vision - Plato. Located in the northern hemisphere of the Moon, its dark ellipse is unmistakable. Plato's floor consists of 2700 square miles of lava fill and is considered by some observers as the darkest single low-albedo feature on the Moon. Because of its low reflectivity, this crater has the distinction of being one of the only mountain-walled plains that doesn't "disappear" as the Moon grows full. With Plato in the center of the field note the pyramid-like peak of Pico due south in northeastern Mare Imbrium. East of Pico is an unnamed dorsum - or lava wave - terminating just above crater Piazzi Smyth to the south. Power up in a telescope and check out the triangular peak near its end.
Sinus Iridum – Credit: Peter Lloyd
Now let's go to the lunar surface to have a look through binoculars or telescopes at tremendous impact region located to the lunar west of Plato. Sinus Iridum is one of the most fascinating and calming areas on the Moon. At around 241 kilometers in diameter and ringed by the Juras Mountains, it's known by the quiet name of the Bay of Rainbows, but was formed by a cataclysm. Astronomers speculate that a minor planet around 200 kilometers in diameter impacted our forming Moon at a glancing angle, and the result of the impact caused "waves" of material to wash up to a "shoreline," forming this delightful C-shaped lunar feature. The impression of looking at an earthly bay is stunning as the smooth inner sands show soft waves called "rilles," broken only by a few small impact craters. The picture is completed by Promontoriums Heraclides and LaPlace, which tower above the surface, at 1800 meters and 3000 meters respectively, and appear as distant "lighthouses" set on either tip of Sinus Iridum's opening. For a great telescopic challenge, imagine that Sinus Iridum is a mirror focusing light - this will lead your eye to crater Helicon. The slightly smaller crater southeast of Helicon is Leverrier. Be sure to power up to capture the splendid north-south oriented “wave -like” ridge which flows lunar east. Enjoy this serene feature ...
And enjoy your weekend!