In Berlin a waxing crescent is a thin bright sliver on the right-hand side. In São Paulo the same crescent, the same night, is on the left. Neither view is the correct one, and nothing about the moon has changed between them.
You are standing the other way up
An observer in the southern hemisphere is, relative to one in the north, tilted towards being upside down — by roughly the sum of the two latitudes. Someone in Berlin at 52° north and someone in São Paulo at 24° south are about 76° apart in orientation. They are looking at the same object with their heads at different angles, so they see it rotated.
The moon always keeps the same face towards Earth, so the pattern of dark plains is the same for everyone. From the south it simply appears turned around: the markings a northern observer calls the top are at the bottom.
Why it matters for a website
Almost every site draws the northern view and leaves it at that. For a reader in Brazil, Australia or Argentina the picture is then plainly wrong — the lit side is on the wrong edge from the one they can see out of the window.
Our moon is drawn rather than photographed, from the lit fraction and the direction of the cycle, so flipping it is free. Pages for southern cities mirror it, and so do the Portuguese pages, which are rendered for São Paulo.
Near the equator it stands on its side
Strictly, the rotation is gradual rather than a flip. Near the equator a crescent can appear as a bowl, lit along the bottom — the "wet moon" that shows up in tropical folklore. A simple mirror image is an approximation of the full geometry, and it is the one that matters for telling the moon’s shape apart at a glance.
See it on the moon phase today page, or on any city page south of the equator.
