The Moon and the stars are not painted on the sky.
By: Fiona Annand
Tags: Astronomy, Space, Science, Universe, Nature

We don’t usually give the Moon a lot of deep thought, and when we do describe it, we tend to do this as a backdrop to our human activities — romantic, beautiful, or picturesque. But if the Moon is another world, how can we get a sense of perspective of this?

Next time you look at the Moon — it’s easily seen in the daytime, if in the right part of the sky — you may gain some sense of its size by knowing it’s a bit smaller than Australia. This is because the Moon’s width is a bit over one quarter of Earth’s. While you look at it, you could try imagining Earth hanging there in your view as well — four times as wide as the Moon, with green and brown continents, swirls of cloud, and sunlight glinting off blue oceans. This is how Earth would appear to you in the sky if you were standing on the Moon, an extraordinary view seen only by a select few, but captured for us in the famous Earthrise photograph by the Apollo 8 crew:

Image: Earthrise, 1968, William Anders.

Now look at the Moon again, and think of its size and spherical shape there in space, as it travels around Earth.

The Moon’s reflection of the Sun’s glare washes out the night sky, and astronomers love to hate it. However, through a telescope the three dimensions of the stark lunar landscape come into view, and the Moon is transformed into a fascinating object of study. The best view isn’t when it is full, with the Sun on the opposite side of our sky and its rays striking the Moon’s surface full on from our viewpoint. Its surface at this time is glaringly bright and the definition of features is very low. Instead, it’s best to look when the Moon is in its orbit around us where we can only see part of its sunlit half. Then we can see the terminator — the moving line between light and dark, or the point on the surface where the Sun would be seen to be setting or rising.

Near that line, the shadows cast by the terrain lie long across the landscape — just as they do on Earth at the time near sunset or sunrise — bringing the features near there into dramatic relief for us. The Moon no longer appears as a smooth disk, but instead shows itself to be a world of desolate mountain ranges, serrated craters, smooth lava plains, and trenches lying sharply across the landscape.

Mountains can be seen in the centres of some craters, which sprang into creation when violent impacts by solar system debris splashed the molten surface upwards. Since the Moon has no atmosphere, or water or ice that moves across the surface, features have remained in place for us to view, millions and billions of years after their formation. This mountain group inside Tycho crater reaches 2 km above the crater floor and is about 15 km wide, the size of a medium city. The crater itself is nearly 5 km deep. You can see how the shadows define the contours:

Image: NASA Goddard/Arizona State University

In 1610, Galileo Galilei turned the recently invented telescope towards the heavens, and was the first to properly examine the surface of the Moon. He realised the broken landscape that he saw there showed that the Moon was not a pure, untouched sphere of the heavens, as had been thought throughout the ages. He saw that in many ways its landscape features resembled those of Earth’s. Along with his other famous discoveries of that year, his observations began a profound shift in human thought about the nature of the heavens and of Earth’s place within them.

But if the Moon is one quarter as wide as Earth, why does it look small in the sky? That is because it is almost 400 000 km away. If that’s a bit meaningless, it may help that this distance is about thirty times the diameter of the Earth. Earth and the Moon can be seen here together in space, captured by the telescopic camera which is on-board NASA’s Mars Reconnaissance Orbiter. The Moon is further off in the distance behind Earth in this image:

Image: NASA/JPL-Caltech/University of Arizona.

Although it looks stationary, the Moon is soaring around Earth at over 3 500 km/hr. You can see this motion yourself though, just by noting its position next to a pattern of stars and then looking again a couple of hours later. Occultations of the planets by the Moon also give us glimpses of this movement, along with a little bit of perspective into the scale of the solar system.

An occultation of Saturn by the Moon was visible from northern Australia in 2019. If you were lucky enough to watch this through a telescope, it was spectacular — the little pale-yellow Saturn, complete with rings, being gradually swallowed up by the huge grey edge of the Moon, later to peek out from behind the other side before sailing out again into the blackness. In reality we saw a small, relatively nearby world passing across our view of a giant planet, seemingly tiny due to its distance away out in the outer solar system. During that hour or so, the Moon travelled its own width across our sky.

But if the size of the Moon compared to Earth seems large to you, this is actually the case. In fact, in our solar system, it is the largest by far compared to its planet. This point could possibly be very important for us here on Earth.

The large ocean tides we have here on Earth are due to the gravity of our Moon, and it may have been inside the tidal pools at the edges of the oceans where primordial life began, as smaller molecules combined to form self-replicating macromolecules. Later on in Earth’s history, the aquatic creatures left behind in pools as the tides receded across the rocks may have been the ancestors of those animals which learnt to live on land.

The Moon’s gravity also appears to have helped stabilise Earth’s axis of spin — by not wobbling too much in space our climate has remained stable over the ages. This could have been vitally important for life’s slow evolutionary journey over billions of years. And in addition to these questions we would like to know how likely the creation of such a large moon was, around a planet in a star’s habitable zone.