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The Dark Side Is Weak With This One
NASA’s Deep Space Climate Observatory (DSCOVR) has captured a series of unusual images showing the astronomical phenomenon known as the transit of the Moon across the Earth. They are unusual because out of the handful spacecraft are beyond the orbit of the Moon, very few are close enough to perceive the Earth and Moon as larger than a speck. The video below is not a computer simulation; it is a series of actual photographs taken on July 16, 2015.
As the Moon passes in front of the Earth, the visible portion of the Moon is what we Earthlings refer to as the far side, not the “dark side” as reported in many news outlets. Since the Moon is tidally locked with the Earth — meaning it rotates at exactly the same angular rate that it orbits — it always presents us the same face. In other words, the Moon has a permanent far side that cannot be seen from Earth. The far side has an entire set of craters and other surface features that were completely unfamiliar to us until the Space Age.
But, as any child can observe, the portion of the Moon that is illuminated by the sun goes through phases, meaning that different parts of the Moon are illuminated at different times. So the “dark side” is not a fixed feature but rather a cyclical phenomenon exactly like night and day on Earth. The far side of the Moon sees just as much sunlight as the near side, only on the opposite schedule. 
DSCOVR is positioned at a unique point in space called the Sun-Earth L1 Lagrange Point, or L1 for short. In general, any system of orbiting bodies has a set of Lagrange points, and a two-body system has five such points. Think of our Lagrange points as the balancing points between the Sun’s and Earth’s gravitational pulls and the orbital centrifugal force. An object placed there will not fall toward the Earth or Sun, nor will it pass ahead or behind. Its orbital angular speed will match the Earth’s, and it will seem to be suspended in place relative to the sun and Earth.
The L1 point is the equilibrium point that lies about one million miles on a direct line from the Earth toward the Sun. From the Earth’s perspective, an object at L1 always remains directly in front of the sun. If the Moon passes between the Earth and L1, the far side of the Moon is visible from the L1-orbiting spacecraft, fully and brightly illuminated by the sun. It’s disheartening the way so many news outlets are reporting this exactly wrong. Viewed from L1 the Moon is full, so the only part of the Moon not visible in the images is the dark side, which, incidentally, is coterminous the Moon’s near side at that time.
Now, since you are a Ricochet member — and I know you love to do so — you can go around correcting people for the rest of the day.
Published in General
Wow! Thank you.
love this!
One useful technique to explain why “dark side” is a misnomer: at any given point, the Earth also has a “dark side” and it’s no more a place than is the Moon’s.
Also, Lagrange Points are so darn cool.
Why is the moon tidally locked with the earth?
I understand the point, but I’ve never had a problem with “dark side.” It’s more poetic for one thing, but also for most of human history that was the dark side in the sense of being dark to us.
Can’t believe this wasn’t APOD.
The part of the moon closest to earth experienced a stronger gravitational pull than the part of the moon further from the earth. This causes the moon to bulge toward the earth creating a heavier and lighter side. The heavier side is always “falling” toward the earth so we see the same face of the moon. The moon exerts a similar force on the earth which causes our tides and slows the earths rotation a little each year. After a long enough time the earths rotation will match the lunar rotation and only one side of the earth will know there is a moon.
I doubt that anyone knows for certain, as I think that there is no currently viable theory for the formation or existence of the Moon. Well, other than “God put it there,” with which I certainly agree in general, but which doesn’t rule out more materialistic explanations for this type of specific detail.
There is a force similar to “drag” that causes slowing in the rotation of a body in orbit with another body. This “drag” has a greater effect when the two bodies are closer and a greater effect on the smaller body. Thus, a number of moons (including Earth’s) are believed to have had their rotational speed gradually decreased until their rotational period corresponds to their orbital period, thus presenting the same face to their primary. The effect is called “tidal locking” or “gravitational locking” and is explained in additional detail in Wikipedia here.
Is it just me, or does there seem to be way fewer craters on the far side than on the near side?
That seems counter-intuitive. You’d think that the Earth would protect the near side from asteroid collisions, no?
Maybe asteroids actually get a gravity-assist when passing close to the Earth and are then flung at the moon, thereby creating more (and bigger, more spectacular) craters on the near side?
I’m just spit-balling here.
Re the Moon’s origin, I think that the two leading theories are “capture,” positing that the moon formed elsewhere in the Solar System and was later gravitationally captured by the Earth, and “impact,” positing an ancient impact between the early Earth and a roughly Mars-size object which resulted in an ejection of material from the Earth’s mantle which formed the Moon.
These are both inconsistent with other physical facts, though — for example, the low density of the Moon (compared to Earth) and certain isotope ratios that are the same on Earth and the Moon are inconsistent with the formation of the Moon elsewhere in the early Solar System, and the isotope ratio is also inconsistent with the impact hypothesis.
Your intitution is correct; it’s just that they don’t show great in the video. The far side has far more impact craters, but lacks the maria that the near side has, which are (at least in part) volcanic in origin. Here’s a clearer comparison:
Just a linguistic quibble, here, but I think that “dark” in the phrase “dark side of the Moon” means “unknown,” rather than literally dark. The phrase dates back at least hundreds of years in English (I haven’t tried to trace it back in Latin or other languages), including by astronomers, and we’ve had a correct physical understanding of the illumination of the Moon since the ancient Greeks. So I think it’s not a misnomer, but rather a different meaning of “dark,” which is easily misunderstood.
For example, the phrase is used in this 18th century astronomy book, along with a correct description of the illumination: https://books.google.com/books?id=TS0PAAAAQAAJ&pg=PA96&dq=%22dark+side+of+the+moon%22&hl=en&sa=X&ved=0CCMQ6wEwAGoVChMI6JPzvICVxwIVS9keCh1CRQzo#v=onepage&q=%22dark%20side%20of%20the%20moon%22&f=false
Aa similar use of “dark” occurs today in “dark energy,” which is not literally dark like dark matter. Rather, dark energy is an “unknown” form of energy causing the universe’s accelerated expansion.
Anyway, speaking as an astronomer, I agree that it’s easy to misunderstand “dark” in this context, so I explain it to my students and then try to consistently use “far side” as I teach them. They have enough conceptual issues with the Moon as it is.
Tim, I think what you said is plausible but I don’t think the passage you cited is an example of the usage you’re talking about.
I think in this passage the “dark Side” of the moon is in fact the near side, which for a new moon faces away from the sun, toward the Earth, and is illuminated by the reflected Earthlight.
Oops, you’re right, Mark. I had misread that as the Sun illuminating the “dark” (far) side (which would be the same time as a full Earth), but I see he’s talking about Earthshine.
I thought everybody knew that the dark side of the moon is where the aliens have their moon base mining operation. When Apollo went to the moon they found the aliens and the base and were warned to stay away which is why we have never been back to the moon. It is also where all the spacecraft come from for the UFO sightings, crop circles, capture people for experiments, etc.
The dark side has nothing to do with light and dark but speaks to the evil of the aliens.
That’s a relief. I thought we were going to discover it’s a racial bias thing.
So where will I meet Pink? He’ll see me on the dark side of the moon…
It is aliens are a different race. So naturally the Dems are for allowing them to come in and take over the world. The Dems love nothing better than an illegal alien. It should be interesting to see how they spin the whole white humans holding them down thing.
And everything under the sun will be in tune.