Looking Up column: Moon keeps facing us and nods
Full moon occurs this Monday, Nov. 30. As we do every 27.322 days, our lovely satellite bathes in full sunlight, the “Man in the Moon” showing its fullest as we gaze back in wonder.
The Chinese launched Chang’e 5, another robotic mission, on Nov. 23, with the ambitious goal of bringing some moon rocks back to Earth Dec. 16-17. Touchdown on the lunar surface was slated Nov. 27 near the edge of the near side, on the upper left as we in the Northern Hemisphere face Old Luna.
Or in less scientific terms, in the right brow of the Man of the Moon, to the right of his right eye (on our left). Neither the Chinese, NASA or anyone’s space agency refers to their landing zones that way, as far as I ever heard.
Designed like the manned Apollo lunar missions, a lander will depart from the orbiting spacecraft and, once the rocks are aboard, will launch an ascent vehicle to meet back with the orbiter for the return to Earth.
China plans to bring back 4.4 pounds of the moon; the first lunar sample returned since the Russians had a similar unmanned mission in 1976.
NASA still plans to return astronauts to the moon by 2024 aboard their Artemis missions.
We do live in amazing times.
Even with the lure of reaching out to other worlds with our robots and someday again with our astronaut boots, there can be no diminishing of the enjoyment we share, the satisfaction we can have, in just “looking up.” From the casual admirer of the night sky to the professional astronomer, we have the universe all around us in common, including the moon’s shining face.
This Saturday night, the almost-full moon is situated between the bright, red-orange star Aldebaran (below the moon) and the glittering star cluster the Pleiades, to the upper right.
Why do we always see the same side of the moon? The reason is the moon actually rotates once around on its axis in the same time it takes to go around the Earth. The effect is the same side keeps facing the Earth. This is called synchronous rotation.
The orbit is not quite circular; the moon varies in distance from its farthest point (apogee) and closest point (perigee) by 12%.
Not only does the Man in the Moon, in its regular dance, approach and withdraw from us, the lunar face nods on a regular rhythm.
The slight eccentricity of the moon’s orbit results in a changing of its angular velocity as it goes around. When the moon is at perigee (closest), its moving faster than its rotation. This lets us see up to eight degrees of longitude on its eastern (right) far side.
Conversely, when the moon is at apogee (farthest), it is moving slower than its rotation, causing it to peek 8 degrees of longitude on its western (left) far side.
That makes the moon rock (no pun intended) to the left and right.
As if that wasn’t enough to make our head spin, the lunar orbit is also inclined to the Earth’s orbit around the sun by 5.1 degrees. This makes the rotational axis of the moon seem to rotate towards and away from us during one full orbit. The effect is we get to see the Man in the Moon bow its head toward us, or put its chin up (the moon is “looking up”) - allowing us to view almost seven degrees in latitude beyond the pole on the far side.
Someone standing on the Earth’s North or South Poles can actually see a little more of this nod - a full degree more in lunar latitude.
All this nodding about is called “libration” and amounts to us seeing a grand total of 59% of the moon through the course of a lunar revolution.
Imagine this set to music. It would be a long, drawn-out song to be sure.
Being able to see a tad more on one lunar limb or the other means at certain times, a crater, mountain chain or lava plane (maria) comes into view and appears less elongated than at other times. Examining the rugged, lunar horizon in our telescopes, we can plainly appreciate the spherical shape of our Moon as libration gives us tantalizing peeks of the 41% perpetually hidden from view.
Astronomers are still theorizing why the moon’s far side is so different than the near. The vast maria that mark the familiar “Man in the Moon’s” features are almost entirely on the side facing Earth. The back of the “Man’s head” is mostly mountains and craters.
The Soviet Union’s Luna 3 probe first photographed the lunar far side in October of 1959. NASA’s Ranger 4 was the first spacecraft to impact the far side in April 1962, but failed to send back any scientific data. In January 2019, the Chinese landed the first space probe (Chang’e 4) successfully on the far side.
There were nine Apollo missions that orbited the moon between 1968 and 1972, involving six landings. On board were 27 astronauts, the only persons to have ever seen the far side with their own eyes.
In a few years, there could be more.
It must be an amazing experience to be on the “back” of the moon, the Earth hidden from sight with no radio communication, thankful their craft would soon bring them around to see the Earthrise once again.
Keep looking up at the sky!
Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at firstname.lastname@example.org. Please mention in what newspaper or website you read this column.