Walking on Sunshine

or most of human history, the idea of “walking on the Moon” was more or less the same as the idea of “walking on the Sun”. We always knew the Moon was different to the Sun, but it was still the less-bright light that comes out at nighttime. We thought it was a source of light itself, rather than merely a reflector. We took it for granted.

More than that actually, because to our ancestors, the way the Moon changes shape over the course of a month, and the way these phases affect everything from the size of the tides to when various plants bloom and animals migrate, wasn’t so much a source of wonder as a source of an extremely reliable and predictable celestial clock. The Moon. Not much of a name is it? Most cultures name it either after some kind of minor deity (often female), while others are more practical or prosaic. For instance, you may have heard that the word “month” comes from “moon”, since a month is more-or-less the time it takes for a new Moon to fully wax and then wane. But actually, it’s the reverse - “moon” comes from words meaning month and, interestingly, “to measure”.

That’s because for thousands of years, the Moon’s main job was to let farmers know when to sow and when to harvest. Animal reproduction is also linked to a lunar cycle - indeed, so is our own - so having a big bright disc in the sky every night that gets bigger and smaller over time to let you know when to get things ready for little lambs or little piggies (or even little people) has, in many ways, been key to our success.

The irony? Our Moon is one of the darkest in the Solar System. It’s albedo - the amount of sunlight it reflects - is just 0.12. That

Fmeans only 12% of the sunlight that hits it gets bounced back at us. Or about the same amount as a road covered in black asphalt.

Compare that to Saturn’s moon Enceladus. Its icy surface has an albedo of 0.99 - it returns almost all the light that hits it. Were Encedalus in orbit around Earth, at the same distance as the Moon, it would be a tiny but blinding and blazing coin, about a tenth the apparent size of the real Sun (and 0.14 the actual size of our current Moon).

Of course, albedo is an ideal number. The Moon actually does look a bit brighter than its albedo suggests, especially due to something called the “opposition effect”. When the Moon is full, that means the Sun is behind us as we view the Moon. The light comes straight back at us. When the Moon appears as a crescent, that means the Sun is off to the side (well, the Moon is off to the side really, because it’s orbiting Earth, and the Earth itself spins and orbits the Sun and... look, it gets complicated). Point is, the Moon looks plenty bright to us!

Eventually of course, humans got around to inventing the telescope. If you’ve never looked at the Moon through a properly powerful telescope, it’s worth trying to arrange it. To the naked eye, even on a very clear night, the Moon looks like a disc. In a telescope, it bulges out toward you. It’s really, almost confrontingly obvious that the Moon is a huge, three-dimensional thing.

And all along the dark edge you can see what are unmistakably mountains. There is a country up there, a place of desolate beauty. A landscape that, because of the lack of atmosphere and eroding water cycle, is - at human scale - grander and more epic than anything here on Earth.

It’s definitely time to go back.