As a preposition, under means below something, either resting beneath it or covered by it. Picture a cat sitting on the floor with a table above it: The cat is under the table.
We sat under the big oak tree and ate our lunch. The branches were above us, and the tree gave some shade.
The floorboards creaked under my feet. Here my feet and the boards are actually touching, so English uses under and not below: I found a coin under the floorboards, not below the floorboards.
One nearby word is below, and the two do not always swap places. For something in a lower position than something else, English often prefers below, not under, especially with things in the sky or on a chart.
A quick check: Venus is just below the moon right now. Saying Venus is just under the moon sounds odd to most English speakers, so reach for below in that kind of sentence.
You do not need a rule for tonight; just remember the picture. Under fits the cat under the table, where one thing rests beneath or is covered by another. For things in the sky or points on a chart, below is the safer word, so Venus is just below the moon sounds more natural to most English speakers. When you are not sure, picture the cat: if something rests beneath or is covered by something else, under works well.