Tidal Locking or The Dark Side of the Moon

Have you ever wondered why there is a Pink Floyd album called "The Dark Side of the Moon"?

Maybe because our moon actually has a designated dark side, one hemisphere which is consistently facing away from the earth and thus is not directly observable to us. Similarly the opposite hemisphere is constantly facing us and -if sufficiently illuminated- always displays the characteristic image of surface patterns and moon landscapes which have become known as 'the man in the moon'.

Pixabay by Comfreak

Earth and Moon are prototypical representatives of a broader phenomenon of celestial mechanics, called tidal locking.
Tidal locking usually occurs when two celestial bodies collide with low lateral momentum and subsequently form two major collision fragments. (Which is exactly what happened to the Earth. At some point in primordial time a roughly Mars sized object slammed into the pre-Earth at low velocity, creating Earth and Moon as we know them.)
This may lead to one's rotation period to be exactly synchronized with its orbital period around the second one.
In the above animation this can be seen on the left and causes one side of the first body to constantly face the other, while the non rotatiting Moon on the right changes its alignment with respect to the earth.

Stigmatella aurantiaca, Tidal locking of the Moon with the Earth, CC BY-SA 3.0

When orbiting around a common center of mass each of the bodies succumbs to the other's gravitational fields's tidal force.
This tidal force is a result of the field's spatial dependence and basically deforms an ideally spherically symmetric object like a planet into a more eccentric (e.g. ellipsoidal or egg-shaped) one. This in turn causes a torque to be exerted on each body by the respective other gravitational field and 'locks' rotational and orbital periods in synchronization.
And by the way the term 'tidal force' is not chosen coincidentally. The very same force is what causes a much more familiar terrestrial phenomenon: The tides of the ocean (which can be seen as our Earth's deformation by a few metres)

Deformation by tidal force, Krishnavedala, Field tidal, CC BY-SA 3.0

Usually only the larger of the celestial bodies tidally locks the other (like the Earth locks the Moon) but not the other way round. Actually the terrestrial tides caused by the Moon do slow down the Earth's rotation gradually but are too weak to have an immediately noticeable effect.

But there also are examples of mutual tidal locking, even in our solar system:
Pluto and its (rather large) moon Charon are mutually tidally locked, they always face in the same direction with respect to each other

Pluto-Charon system, Wikimedia commons

But it gets even crazier:
You might ask yourself if this phenomenon is exclusively limited to planetary objects. In fact, as we discovered a few decades ago, it is not.
Upsilon Andromedae B also named Saffar is a Jupiter like exoplanet that is tidally locked to its namesake star Upsilon Andromadae in the constellation Andromeda. An unfortunate consequence of one hemisphere constantly facing a star and one facing away consists in a constant temperature difference between +1400 to +1650 degrees Celsius on the day side and -230 to -20 on its perpetual night side. A rather uninhabitable environment indeed.

Upsilon Andromedae B a.k.a. Saffar by Wikipedia

To me, this physical reality is very remiscent fictional planet 'Hagalaz' from Bioware's video game series Mass Effect, a slowly rotating planet in proximity of its star, featuring a violent perpetual storm on the perimeter of its (current) bright and dark side.
Let me know what you think in the comments.

Hagalaz by Mass Effect Wikia, BioWare, EA Inc.

Sources:
Tidal locking by Wikipedia
Upsilon Andromedae B by Wikipedia
Upsilon Andromedae B by NASA Spitzer

Are you interested in more space related stuff? Check my recent article on Cosmic expansion (https://steemit.com/science/@galotta/cosmic-expansion-suesa-s-science-challenge-2) or anything by e.g. @lemouth, @fredrikaa (those guys know what they are talking about) ;)