The moon · photographed, then lit here

The near side,
and tonight’s light on it.

The disc is the moon’s near side as NASA’s Lunar Reconnaissance Orbiter mapped it — a real photograph, projected as a sphere for the face the Earth is actually shown. The light on it is not in the photograph. The phase, the terminator, which limb is lit and the shading across the globe are worked out here, from the date and nothing else.

The phase is the TRUE one, taken from the angle between the moon and the sun rather than from an average month — which matters, because the two part company by up to fourteen hours, and are more than six hours apart on three nights in four. Walk a month with the scrubber, or press play and watch a whole lunation cross in half a minute.

Tonight’s moon What comes next The observatory

Tonight

The moon, right now.

The disc is the near side as the Lunar Reconnaissance Orbiter mapped it — Tranquillitatis, Crisium up on the eastern limb, Procellarum spread across the west, Tycho throwing its rays across the southern highlands — projected as a sphere for the face the Earth is actually shown. The light on it is not in the photograph: the phase, the terminator and the shading are worked out here from the true angle between the moon and the sun, rather than from an average month, so it is right on the nights the average is wrong. Walk it forward or back a month, or press play and watch a whole lunation cross in about half a minute.

The moon

What comes next

Walk the month

Method

What is photographed, and what is computed.

The ground

The near side comes from NASA’s Scientific Visualization Studio — a global albedo map assembled from Lunar Reconnaissance Orbiter Wide Angle Camera imagery, with topography from the LRO Laser Altimeter. NASA imagery is not copyrighted. It is an equirectangular map, so it is projected rather than drawn: each point on the disc is turned back into a latitude and longitude and the photograph is read at that point.

It was drawn from primitives three times before this — flat shapes, then a stipple engraving of the real maria — and it never stopped looking drawn. A photograph of the moon is a hundred thousand craters at every scale, and no procedural rule reproduces that. The engraving survives: it is one press away, and on paper it prints where a photograph turns to grey.

The light

Phase, distance and the dates are Meeus’s lunar and solar theories — the moon’s position to about ten arcseconds, the phase instants to about a minute. The shading is Lommel-Seeliger, the photometric law for a dusty regolith: brightness goes as µ₀/(µ₀+µ), which is exactly one half everywhere at full moon. That is why a full moon is flat and bright right out to its edge, and why a plain cosine law — which falls to a quarter at the limb — would ring it in a shadow that is not there.

What is not modelled