Eclipse of the Moon Monday 28 September

On (very!) early Monday morning we  have a good chance to see a total eclipse of the moon. These take place when the earth gets exactly in between the sun and moon in the sky. Once a month, at full moon, the moon, earth and sun are approximately lined up in this way, but because the moon’s orbit of the earth is not quite in the same plane as the earth’s orbit around the sun, on most months the moon misses the earth’s shadow. But this month the moon will go straight through it, and, if our skies are clear, we will see it looking very different.

When seen from the earth, the moon and sun have (by coincidence) about the same apparent diameter, so total eclipses of the sun by the moon are rare, confined to a narrow strip of the surface of the earth,  and very short, lasting only a few minutes in any given place. Partial eclipses of the sun last for longer and can be seen more widely, and similarly total eclipses will be seen more extensively as partial eclipses, as was the case for us in March this year.

But the earth is about 4 times as wide as the moon, and consequently from the moon will appear about 4 times the width of the sun. Moon people will therefore see eclipses of the sun by the earth much more frequently, and they will last for some hours. We see the same thing as an eclipse of the moon. The eclipsed moon can be seen from anywhere on the earth where the moon is above the horizon if the sky is clear, and for the same reason will last for some hours.

The moon in eclipse is still visible with the naked eye, but it is much darker than usual, and its colour might be anything between a dull dark grey, to brown, to a coppery red. Imagine you are on the moon during the eclipse. You will see the black ball of the earth in front of the sun, but you will also see a very bright ring around it as the sunlight bends through the earth’s atmosphere. The ring might be relatively faint, or bright, or red – for the same reason as our sunsets and sunrises can be red. It depends on the weather at those parts of the earth where the sunlight is passing through.

Lunar eclipses also start with a partial phase, but this we only see as a gradual dimming of the full moon. The main eclipse starts at about 2am, when you will see the earth’s shadow begin to encroach over the moon. Totality is reached at 3 , and will last until about 4.20. That period is the best time to see the colour of the eclipsed moon. And from 4.20 to 5.20 the earth’s shadow moves off the moon, though it will be another hour or so before it is back to full brightness.

All this can be seen with the naked eye if the sky is clear; binoculars probably give the best view; and there is no harm in looking with a telescope if you want to.