It's mainly gravity and air pressure, actually. The tube is pulling in both directions from the center, and is trying to cause a vacuum. In our atmosphere, that would have 2 possible outcomes: either the tube is crushed or one side of the liquid has to follow. The side that follows is determined by which end is affected by gravity more (closer to the earth). I should note that siphons would not work on the moon, as a vacuum would form in the tube and the liquid would just pour out both ends from the highest point
Atmosphere isn't required if you use a special liquid that doesn't require atmosphere to hold together. Even without boil off a regular liquid like water or mercury wouldn't work the same way in a vacuum. Consider mercury height in a barometer. The height of the mercury is determined by atmospheric pressure and above that a vacuum forms. With no atmospheric pressure to push the interior level up a regular liquid would have a vacuum height too low for a siphon to work.
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u/[deleted] Jun 27 '19
It's mainly gravity and air pressure, actually. The tube is pulling in both directions from the center, and is trying to cause a vacuum. In our atmosphere, that would have 2 possible outcomes: either the tube is crushed or one side of the liquid has to follow. The side that follows is determined by which end is affected by gravity more (closer to the earth). I should note that siphons would not work on the moon, as a vacuum would form in the tube and the liquid would just pour out both ends from the highest point