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The Moon

The one thing here you can see from a lit street, and the best first night anybody has.

Photographed by TPN Explorer at the Scottish Dark Sky Observatory on 16 July 2019, down the eyepiece of the observatory telescope with a phone. The Earth’s shadow is the soft dark curve across the bottom. Tap the picture for a larger version.

Seeing it from the UK

Everything else on this page wants a dark sky, no moon and a drive out of town. This wants none of it. You can see the Moon from a lit street in the middle of a city, and any pair of binoculars braced against a wall will show craters. The one thing worth knowing is that a full Moon is the worst time to look, not the best: the sunlight comes straight down onto it and nothing casts a shadow, so it looks flat. Wait for a half or a crescent and look along the line between the lit and the dark part, where the sun is low and the crater walls throw long shadows. That line moves a little every night, so the same crater looks different from one evening to the next. The picture here was taken by holding a phone to a telescope eyepiece, which is a perfectly good way to start.

The Moon is a quarter the width of the Earth and about 384,000 kilometres away, which is far enough that its light takes a little over a second to reach us. It is the only other world anybody has stood on.

It keeps the same face turned towards us. That is not a coincidence and it is not the Moon being still: it turns exactly once for every orbit it makes, because our gravity has slowly dragged it into step over billions of years. The far side is no darker than this one, it is simply never pointed our way.

The dark patches are not seas, although that is what they were named. They are plains of solidified lava, flooded out across the surface after enormous impacts more than three billion years ago, and they are smoother and younger than the pale cratered ground around them. That contrast is what makes the face people see in it.

It is also leaving. Tides drag on it and hand it energy, and it is drifting away from us by about four centimetres a year, which is roughly the rate your fingernails grow.

Lunar eclipses, and what the photograph is showing

The dark curve across the bottom of that picture is not the usual line between the lit and unlit halves of the Moon. It is the shadow of the Earth. The photograph was taken during the partial lunar eclipse of 16 July 2019, with the Moon most of the way through it.

A lunar eclipse happens when the Earth passes exactly between the Sun and the Moon and our shadow falls across it. That can only happen at a full Moon, and it does not happen every month because the Moon’s orbit is tilted, so most full Moons pass above or below the shadow rather than through it.

You can tell the difference between a shadow and a phase by the edge. The line between day and night on the Moon is sharp, and craters along it stand out because the sunlight is coming in sideways. The edge of the Earth’s shadow is soft and slightly warm, because our atmosphere bends a little red light into it, and the ground it falls on shows no relief at all.

They are also the easiest thing in astronomy to watch. Nothing is needed, nothing is dangerous, it lasts for hours rather than minutes, and unlike a solar eclipse everyone on the night side of the Earth sees the same one at the same time.

The numbers

Distance
About 384,000 kilometres
Width
3,475 kilometres, about a quarter of the Earth
Light takes
A little over one second
One orbit
27.3 days, and it turns once in the same time
Gravity
A sixth of ours
Drifting away by
About 4 centimetres a year
Best time to look
A half or a crescent, never a full Moon
Equipment needed
None. Binoculars make it better

To find out whether it is up tonight, and when it gets properly dark where you are, see Sky Tonight.