The Moon tonight

How to read these numbers

The Moon as it will be tonight above your place: its phase, when it rises and sets, and above all the craters, mountains and rilles that will look their best, where the Sun is rising or setting on it.

The Sun’s colongitude tells where the terminator is. It is about 0° at first quarter, 90° at Full Moon, 180° at last quarter and 270° at New Moon, and grows by about 12° a day. The morning terminator (where the Sun is rising) lies at longitude 360° minus the colongitude.

Libration is a slight apparent rocking of the Moon: depending on the day, we see a little further round one edge or another, up to about 8°. A positive libration in longitude uncovers the eastern limb (the Mare Crisium side), a positive libration in latitude the north pole. Over the months we see 59% of the lunar surface.

Directions are those of the Moon itself (International Astronomical Union convention): lunar east is on the Mare Crisium side, on the right when north is up. A telescope shows the image upside down, and a refractor with a star diagonal shows a mirror image: choose the orientation that matches your instrument.

The picture is computed for the chosen time: the shadows in the craters come from a relief map, and earthshine faintly lights the night side.

Surface maps: NASA Scientific Visualization Studio, CGI Moon Kit (Ernie Wright), from data of the LROC camera and the LOLA altimeter on Lunar Reconnaissance Orbiter. Feature names and positions: IAU Gazetteer of Planetary Nomenclature (USGS); selection inspired by Charles Wood’s “Lunar 100”. Computations: astronomy-engine, and the formulas of J. Meeus, Astronomical Algorithms, chapter 53.