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Weekend Star Party: Explore Lunar Seas, Mountains, Rilles
Weekend Star Party: Explore Lunar Seas, Mountains, Rilles

Use your binoculars or telescope to find Mare Imbrium, the ""Sea of Rains,"" the pyramid-like peak Mons Pico, and Sinus Iridum, the ""Bay of Rainbows"" on the weekend of February 7-9. 20-14.

Friday, February 7 - On this day in 1889, the first national astronomy organization in the USA was born: the Astronomical Society of the Pacific. Congratulations to these fine people who continue to contribute to public astronomy awareness! We appreciate you, and the Night Sky Network!

Let's begin our lunar studies tonight with a deeper look at the ""Sea of Rains."" Our mission is to explore 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.

Mare Imbrium Courtesy of NASA

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 deep as 100 kilometers. 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.

Saturday, February 8 - Today celebrates the discovery of the Sayh al Uhaymir 094 ""Mars Meteorite."" Found this day in 2001, scientists had long known Mars' surface was home to many impact craters which may have caused space-born debris. It was only a matter of time before a bit of this debris would be captured by Earth's gravity and be brought down as a meteorite. Upon study, tiny gas deposits were discovered in its composition that nearly matched the atmosphere of Mars as measured by the Viking landers, and its mineral composition also leads science to believe the meteor originated from Mars.

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 Mons 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.

Mons Pico Courtesy of David Campbell

Sunday, February 9 - Tonight let's go to the lunar surface to have a look through binoculars or telescopes at a 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.

Sinus Iridum - Credit: Clementine/NASA

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 lunar feature.


Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She?s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Date Taken: 02/27/2017
Author: Tammy Plotner
Category: Observing Guides

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