Sextant

Like the other informational pieces, it works from your inventory, a Void Bag or a social slot. There is no reason to give it a real slot. In multiplayer, teammates standing near you read the same information off it.

The Skeleton Merchant rotates his stock by phase, and several of his better lines only show up on specific ones. Some spawn behavior tracks the phase as well. Without this you are counting nights by hand. The icon also flips to a red moon when a Blood Moon or a Solar Eclipse begins, which is a useful early warning.

It comes out of Angler quests, and he holds it back while one is already sitting in your storage, so hand your spare to a friend or a chest if you want him to move on to other rewards. Shimmer will trade it for the Fisherman's Pocket Guide or the Weather Radio if you would rather have a different readout.

A real sextant measures the angle between a star and the horizon so a navigator can work out latitude. It has nothing to say about the moon.