Tuesday

The Moon is Earth’s only natural satellite, with a mean distance of about 384,400 km, a diameter of 3,474 km, and an apparent diameter near 0.5 degrees. It is in synchronous rotation, so the same hemisphere generally faces Earth, while its orbital period around Earth is 27.3 days relative to the stars and 29.53 days from one new Moon to the next.

At 10% illumination, this is a waxing crescent, a few days after new Moon. The terminator — the line dividing the lit and unlit halves — lies across the near side, where low-angle sunlight emphasizes relief: crater rims, mountain scarps, and basin edges cast long shadows. Early crescent phases commonly bring the eastern near-side maria into view, including dark basaltic plains such as Mare Crisium and nearby highland terrain, depending on libration.

Lunar highlands are ancient, heavily cratered anorthositic crust, while the darker maria are younger volcanic basalt flows that filled large impact basins mostly between about 3.9 and 3.1 billion years ago. The Moon has no substantial atmosphere or active weather, so its surface preserves impact features over immense timescales.