The Moon — grazing light over a landscape of rubble

Everything else in this gallery is light without a place: nebulae, clusters, galaxies — surfaces of glow, hundreds to millions of light-years away. Only one object has landscape. On the Moon you see mountains, valleys and craters as shapes, not even one second of light travel away. The gallery's two lunar entries are also the oldest images here — October 2012, a Newtonian, a planetary camera — and together they are a complete lesson in what a photograph of the Moon actually shows.

Light is everything

The Moon has two layers of information, and only ever one of them can be seen well. The first is topography: along the day-night boundary, the terminator, sunlight strikes so obliquely that every rim and every mountain range casts a long shadow — the landscape stands out in relief as if under a desk lamp. The second is the albedo, the brightness of the material itself: dark basalt, bright highlands, dazzlingly bright fresh ejecta. It only emerges once the shadows disappear — at full moon.

The same crater, two lightings. On the left, grazing light turns heights into shadows — and ever since Galileo, shadow lengths have been used to compute the first mountain heights on another world. On the right the relief is switched off; what remains is the material map.

This is exactly the contrasting pair formed by the two gallery entries: a waning three-quarter moon whose terminator runs through Clavius, Plato and the Montes Apenninus — and three weeks later a moon 99.9 percent full, on which the same regions appear flat and white, while instead the ray systems of Tycho and Copernicus dominate the image. Anyone who lays the two side by side understands why lunar observers regard full moon as the dullest and the half phases as the best time.

A biography in four acts — all of them at the beginning

The formation story favored today is drastic: about 4.5 billion years ago the young Earth collided with a body roughly the size of Mars — Theia. The Moon gathered from the disk of debris knocked out of it. It began as a glowing sphere with a global magma ocean; as it solidified, the lightest rock floated to the top and became the bright crust that we see today as the highlands. Then came the bombardment: enormous impacts tore open the basins of Imbrium, Serenitatis and Crisium — and only millions of years later did thin basaltic lava fill these basins to form the dark maria, the “seas”.

All the drama sits in the earliest youth. Since the end of the mare flooding almost nothing has happened — Tycho, the youngest major event on the near side, lies closer to the dinosaurs than to the formation of the Moon, and is nevertheless ancient compared with anything human.

That is what makes the Moon something special: it is an archive. Earth has erased its own early history completely through weather, water and plate tectonics — the Moon has none of that and preserves every scar. Reading its photograph means doing stratigraphy: whatever lies on top of something else came later. Tycho's rays lie on everything, so Tycho is young; the lava of the maria drowns old crater rings, so it came after them.

A few things worth knowing about it:

We always see the same side — but not exactly. Earth's tidal forces locked the Moon's rotation to its orbital period long ago. But the slight wobble of libration tilts the limb zones toward us by varying amounts: over many months a total of about 59 percent of the surface becomes visible.
It is leaving — measurably. The Apollo missions left laser reflectors behind that are still being shot at from Earth today. The result: the Moon is receding by about 3.8 centimeters per year, and Earth's rotation is slowing down in return. Total solar eclipses are therefore on their way out — in the distant future the Moon will be too small for them.
The eclipse coincidence is pure chance. The Sun is about 400 times larger than the Moon and happens to be about 400 times farther away — which is why the two disks cover each other almost exactly in the sky. No physics enforces this; we simply live in the epoch in which it fits.
The “silvery” Moon is as dark as asphalt. Its mean albedo is around 0.12 — it reflects only a good tenth of the sunlight. It looks bright solely through the contrast with the black sky. And even its night side is never entirely dark: the ashen earthshine is sunlight thrown back by Earth.
The far side is another world. It has hardly any maria — its crust is thicker, and the lava came through less often. The “face” of the Moon made of dark seas is purely a near-side phenomenon, recognized as such only in 1959 with the first probe images of the far side.
Lunar photography is deep-sky the other way around. With nebulae light is missing and you expose for hours against the noise — as the article on the night describes. On the Moon light is there in abundance; the adversary is atmospheric turbulence. So you shoot video and, using lucky imaging, keep only the moments in which the atmosphere briefly holds still — which is how the two panoramas in this gallery came about too.