What's up in the sky, October '09
Post date: Oct 2, 2009 1:59:23 PM
A casual look at the Moon's craters would make one think it is very hard to figure out which ones are older and which ones are newer. I got a head start on figuring out that before I was interested in the Moon. In the rural area where I grew up, people used to take pot shots at road signs. So a drive in the country had us looking for bullet riddled signs. Since the signs at that time were made of stamped steel, you could tell the older holes from the newer ones. The older holes had more rust, the newest ones were shiny and the rest fell somewhere in between. Folks used to carry bigger caliber guns so more of the older, rustier holes tended to be bigger, .38 and .45 caliber. Some where in time the .22 became the recreational gun of choice and the smaller holes begin to out number the bigger ones. But there was always a mix of sizes.
All the craters on the Moon were made by impact of something, and they vary in size from the biggest Mare Imbrium 781 miles in diameter, it would hold the state of Texas, down to the smallest diameter the size of a tooth pick. The term to describe the way the surface of the Moon was created is called gardening. Impacts have turned the Moon soil over, and over, since there is no wind or water, it is the only action taking place on the Moon, besides lava and Moon quakes.
The best place to begin to understand the craters on the Moon is in the lower southeast part of Mare Imbrium where the crater Eratosthenes and Copernicus are very prominent. Between these two craters is the ghost crater Stadius, a 42 X 42 mile crater, with just it's rim exposed. It is a very old crater, between 3.85 and 3.8 billion years old. Stadius was a prominent crater in a low lying area of the lunar highlands before Mare Imbrium was created. It is so hard to comprehend that Mare Imbrium, the Sea of Rains, was made in an instant. The big Mare's rim was thought to be a series of mountain ranges by the early observers and the portion near Stadius was named the Montes Apenninus.
Lava from deep within the Moon began to flow across the Moon, it filled Mare Imbrium and just below it, Mare Insularum. Lava from Insularum flowed over the northwest rim of Stadius and filled it and that lava was joined by flows from Mare Imbrium.
Then along came Eratostenes. This impact crater is on the tip of the Montes Apenninus, some 35 X 35 miles across and 10,800 feet deep. It has a central peak, wrinkle ridges, but no lava inside of it, and no bright rays . At first this is puzzling then it was realized that the 3.2 to 1.1 billion year old crater probably had very bright rays but over time exposure to the Sun turned them dark, because none of the craters older than Eratostenes have rays either.
Then another impact, Copernicus, a some 56 X 56 mile hole big enough to hold Yellowstone National Park, 11,400 feet deep and bright rays that can be seen easily during full Moon on Mare Imbrium. The impact threw debris that created a line of secondary craters west of Stadius.
The eras of the Moon end with Copernicus. Though the last big impact is probably Tycho whose bright rays cross the face of the Moon and one divides Mare Serenity. The Copernican Era corresponds to the Precambrian Era on Earth, which had been heavily bombarded at the same time as the Moon. It will always amaze me that almost immediately after the bombardment stopped that the first life on Earth appears in the fossil record.
Oct 02 The Moon passes 6 deg north of Uranus, 9 p.m. CDT.
04 Full Moon.
05 Mars, moving eastwrd, is 6 deg south of Pollux in Gemini.
Mercury at greatest western elongation.
08 Mercury and Saturn less than a degree aparrt in the early morning sky.
11 Last quarter Moon, and will pass 1.2 deg south of Mars 8 p.m. CDT.
18 New Moon.
21 Orionid meteor shower peaks
25 First quarter Moon.
27 The Moon is 3 deg north of Jupiter, 4 a.m. CDT.
30 The Moon ,again is 6 deg north of Uranus at 4 a.m. CDT.