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“Darkside of the moon” was a great album of Pink Floyd, and when it was released in the 70s of the last century we were all fascinated and amazed by the electronic tunes and the philosophic lyrics. We never asked where in the real world this eponymic dark side of the moon should be. At least nowhere on the earth moon. And I have doubts (from a scientific point of view) that a moon anywhere in the universe can have a dark side.

On the time lapse video below you can see clearly how the illumination of the moon changes within 27.4 days, i.e. during a full circle rotation of it around the earth. And despite the fact that the moon keeps indeed one side hidden from the earth’ view, it is nicely illuminated from all sides throughout the period of a full month. Something like a rotating gyros on a barbecue.

The three crew member of Apollo 8, Borman, Lovel and Anders on the first ever manned spaceflight around our earth trabant (1968) could indeed show that the hidden side of the moon is not dark at all.

Moons hidden side
Oblique view of the lunar surface taken from Apollo 8 spacecraft

Depending on the moons position on its orbit around earth, it changes from a sunny to a dark state (like day and nigth on our earth). The fact, however that the moon faces the earth always with the same side (except slight periodic rolling movements) has to be attributed to the strong gravitational interaction between earth and moon. Originally, when the moon was born 4.51 billion years ago after the collission of the young earth with a big asteroid, the moon had a strong rotational impuls independent on its orbit around the earth. At those time, one would have recognized a regular change of the moons orientation on the night sky. But during the last 4.51 billion years the rotational movement of the moon around its own axis became entangeled and fully synchronized with its rotation around the earth (“tidal locking”). This means that during its 27.xxx days orbit around the earth, the moon rotates exactly ones around its own axis.

anim_lunation

Time lapse video of the moon during its 29 days orbit (view from the earth)

My initial question was if one could imagine a moon (somewhere else in our galaxy or anywhere in the universe) that has indeed a dark side (opposite to a sunny side). The astronomic condition for such a case would be that any of a planets satellites (for instance one of the 4 moons surrounding Jupiter or one of the 9 Saturn moons) permanently faces the sun with one and the same side. This would be equivalent to a situation when this particular moon has either a self rotational impuls with exactly the same period and direction as the orbital movement of its parental planet around the sun (case A), or if the moon rotates around its parental planet on a orbital plane that is very much tilted against the eclipse of the planet (case B).

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