Smile of the Day Thursday October 7, 2021

Thursday October 7, 2021’s Smile of the Day: The Dark Side of the Moon

On this Day:

In 1959, the far side of the Moon was seen for the 1st time, courtesy of USSR’s Luna 3 space probe.

The far side of the Moon is the lunar hemisphere that always faces away from Earth, opposite to the near side. Compared to the near side, the far side’s terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria (“seas”), giving it an appearance closer to other barren places in the solar system such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. The hemisphere is sometimes called the “dark side of the Moon”, where “dark” means “unknown” instead of “lacking sunlight” – both sides of the Moon experience two weeks of sunlight while the opposite side experiences two weeks of night.

About 18 percent of the far side is occasionally visible from Earth due to libration. The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were the first humans to see the far side in person when they orbited the Moon in 1968. All manned and unmanned soft landings had taken place on the near side of the Moon, until 3 January 2019 when the Chang’e 4 spacecraft made the first landing on the far side.

Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth.

Tidal forces from Earth have slowed the Moon’s rotation to the point where the same side is always facing the Earth—a phenomenon called tidal locking. The other face, most of which is never visible from the Earth, is therefore called the “far side of the Moon”. Over time, some crescent-shaped edges of the far side can be seen due to libration. In total, 59 percent of the Moon’s surface is visible from Earth at one time or another. Useful observation of the parts of the far side of the Moon occasionally visible from Earth is difficult because of the low viewing angle from Earth (they cannot be observed “full on”).

A common misconception is that the Moon does not rotate on its axis. If that were so, the whole of the Moon would be visible to Earth over the course of its orbit. Instead, its rotation period matches its orbital period, meaning it turns around once for every orbit it makes: in Earth terms, it could be said that its day and its year have the same length. This is, as stated, a result of tidal locking.

The phrase “dark side of the Moon” does not refer to “dark” as in the absence of light, but rather “dark” as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen. While many misconstrue this to think that the “dark side” receives little to no sunlight, in reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. However, the near side also receives sunlight reflected from the Earth, known as earthshine. Earthshine does not reach the area of the far side that cannot be seen from Earth.

At night under a “full Earth” the near side of the Moon receives on the order of 10 lux of illumination (about what a city sidewalk under streetlights gets; this is 34 times more light than is received on Earth under a full Moon) whereas the dark side of the Moon during the lunar night receives only about 0.001 lux of starlight. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark. The word “dark” has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example.

The two hemispheres of the Moon have distinctly different appearances, with the near side covered in multiple, large maria (Latin for ‘seas,’ since the earliest astronomers incorrectly thought that these plains were seas of lunar water). The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, compared to 31.2% on the near side. One commonly accepted explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer. While other factors, such as surface elevation and crustal thickness, could also affect where basalts erupt, these do not explain why the far side South Pole–Aitken basin (which contains the lowest elevations of the Moon and possesses a thin crust) was not as volcanically active as Oceanus Procellarum on the near side.

The China National Space Administration’s Chang’e 4 made the first ever soft landing on the lunar far side on 3 January 2019 and deployed Yutu-2 lunar rover onto the far side lunar surface.

The craft included a lander equipped with a low-frequency radio spectrograph and geological research tools. The far side of the Moon provides a good environment for radio astronomy as interferences from the Earth are blocked by the Moon.

In February 2020, Chinese astronomers reported, for the first time, a high-resolution image of a lunar ejecta sequence, and, as well, direct analysis of its internal architecture. These were based on observations made by the Lunar Penetrating Radar (LPR) on board the Yutu-2 rover.

The far side was first seen directly by human eyes during the Apollo 8 mission in 1968. Astronaut William Anders described the view:

“The backside looks like a sand pile my kids have played in for some time. It’s all beat up, no definition, just a lot of bumps and holes.” (per Wikipedia).

First, a Story:

The restaurant on the far side of the moon is reported to have great food but alas, no atmosphere.

Second, a Song:

What’s Hiding On The Dark Side Of The Moon? Unveiled, who describes themselves as follows:

“Are you curious about the world? Unveiled answers the questions that you’ve always wondered about and tackles the mysteries of the universe.

This is Unveiled, giving you incredible answers to extraordinary questions!”

has a video on YouTube.com regarding the far side of the moon.  They state:

“The moon is our closest celestial neighbor, but the Dark Side of the Moon is still one of the most mysterious places in the Solar System. In this video, Unveiled heads to the far side of the moon to find out what’s hiding there…  A lunar wasteland? Ancient Aliens? Or something else?” (per YouTube.com)

Here is “What’s Hiding On The Dark Side of the Moon?”.  I hope you enjoy this!

(https://www.youtube.com/watch?v=JPRmbP3xWl8)

Thought for the Day:

“Some of the greatest reality television we ever had was the moon landings. When you think about it, that was human emotion and people, unscripted, working with each other – and millions and millions of people around the world, glued to their television sets to share real-time in a brand new, fascinating human experience.” – Chris Hadfield

Have a great day!

Dave & Colleen

© 2021 David J. Bilinsky and Colleen E. Bilinsky