« Why is the Sky Blue? | Main | Jupiter: You've Got a Friend in the Solar System »
The Moon: Our Nearest Companion
The Solar System >
As the sun goes down this evening, the Moon will be following. It is near First Quarter, that is, it's about a quarter of a way through its orbit around the Earth, an orbit which "begins" at New Moon. Many people call this phase Half Moon because our celestial friend appears to be half lit up in the Sun's light. Go out tonight and take a look at it --- no, really, take a long look. And if you have some binoculars, or better, a telescope, take those with you.
The First Quarter is a prime time to see the Moon in all its craggy glory. Not next week when a bright Full Moon will appear flat and dull due to the direct hit it takes from the light of the Sun. Full Moon is a big bright blah.
But at First Quarter, when the sun lights it from the side, the surface of the Moon takes on a three-dimensional appearance that can make this dead rock come alive.
One thing you'll notice is the jagged terminator, not to be confused with Arnold Schwartzenegger, the aged Terminator. The terminator is the imaginary line that divides night and day, the moon's shadowlands.
This is where one can see the "mountainous" areas next to and highly contrasted with the Moon's flatter geographies.
Those "mountains" are not what they seem. On earth we know mountains form by the squeezing together of the tectonic plates that lay across our surface like the broken shell on a hard-boiled egg. Crashing together the plates rise into the atmosphere forming mountains like the Himalayas.
On the Moon, however, mountains were formed from colossal impacts that have plagued the Moon since its creation. As the impacting bodies --- some of them tens of miles across --- slammed into the Moon they lifted up material in great rings around the place of impact, and threw a lot of other lunar crud hither and yon. All this violence left the high mountains you can see through your binoculars.
So what are the flat places and why are they so devoid of craters? The Moon was once alive; it had guts of molten rock. Magma would ooze up through cracks in its crust and bleed liquid rock all over the place filling great basins with new smoothness. The flat areas you see are the results of those floods.
Galileo noticed these through his primitive telescope and thought they might be oceans of some sort. He called them maria, the Latin for "seas." Even today we refer to them thusly, as in "Apollo 11 landed on the Sea of Tranquility."
A planetary geologist will tell you that the maria are lacking many impact craters because they were formed after the barrage of impacts that hit us and all the other planets billions of years ago, when the solar system was a dangerous and polluted place to be. It's been relatively quiet since then and the relatively young and undamaged maria are evidence of that silence.
But the fact that there are any impacts on the maria at all also tells us that the Moon has been asleep, probably dead, for a long, long time. Earth just keeps on pumping out the magma --- every year there are massive volcanic eruptions. But the Moon remains quiet.
Even in death, though, the Moon serves its purpose well. Adjusting and fine-tuning our own spin and wobble and tilt, the dead and haggard Moon keeps us all alive and well.
Any questions? Good! Write me here.
Posted by Administrator at 2002.02.16 03:12 PM | Comments (0)
