The spectacle of the Moon passing through the Earth’s shadow is not as simple as you might expect. Rather than simply passing into the darkness to vanish for a while, before re-emerging into the light, we see quite a complex sequence of events. This is because the Earth’s shadow is composed of two regions: The penumbra and the umbra. The umbra is the fully dark part of the shadow, and is shown in the diagram below as the dark grey cone. The penumbra is partly dark, and is shown by the light grey region. You can see the regions in your own shadow – if you hold out your hand in front of a light to cast a shadow on a wall, the fuzzy border is the penumbra, and the dark central region is the umbra. As you move your hand further from the wall, you’ll see the penumbra grows bigger while the umbra grows smaller, and the diagram below shows why this is.

As the Moon passes through these different parts of the Earth’s shadow, astronomers look for specific events.  The first of these (sometimes labelled P1) is when the Moon first touches the penumbra.  This is the very beginning of the eclipse, but because the penumbra is fuzzy it’s hard to spot the difference.  As time passes, the Moon gets deeper and deeper into the penumbra until finally it is completely engulfed.  At this point, being partly in shadow, the Moon shines less brilliantly than usual but the difference is slim enough that it’s not usually obvious to people who aren’t already looking for it.

The next stage comes shortly after, and marks the moment when the Moon first dips into the Umbra:  U1.  This is a very obvious sight, and is when casual observers consider the show to have begun.  For the next hour, we see the dark shadow moving steadily across the Moon’s surface, gradually hiding it from sight.  Eventually the last little bit slips into the shadow, and we label this U2.  But something interesting occurs, just when we would expect the Moon to be completely black:  It begins to shine with a reddish-orange light.  Sometimes this is quite dim, but can also be quite bright.  Where does it come from?  Well, the Earth’s atmosphere does a pretty decent job of refracting the Sun’s light, so that it is first split into its various colours, then bent back to shine dimly into the Earth’s shadow.  Now the blue light gets scattered by the air molecules and ends up shining in all directions (Which, incidentally, is why the sky is blue), but the red light gets through mostly untouched.  We see this red light when dust reflects it back to us at Sunset and Sunrise, or when it lights up the Moon in a lunar eclipse.  This red light, incidentally, causes the entire moon to blow eerily with a reddish-orange colour, which is why lunar eclipses are sometimes referred to as a “Blood Moon”

Eventually the moon will reach its closest point to the centre of the Umbra (occasionally it will actually reach the centre itself, but this is rare – the alignment has to be absolutely perfect for this to happen.  The closeness affects how long the eclipse lasts), and astronomers call this Totality.  The show is now exactly halfway over, and from this point on things happen exactly in reverse.  After a while, the edge of the Moon peeps out from the Shadow (U3), and moves back into the light.  After an hour or so, it leaves the Umbra completely (U4) and begins its journey out of the penumbra until it’s fully lit by the Sun again (P4).  And then everybody goes back inside for hot chocolate.

Incidentally, a lunar eclipse only ever happens when the Moon is full.  Why is this?  Well if we look at what causes the Phases of the Moon, we see that the Moon is full when it’s positioned so that we can’t see any of its unlit parts.  As anybody who’s played with stage lighting or taken photographs with a flash will tell you, this can only happen when you’re almost exactly in between the source of light (The Sun) and the Moon.  And since this is part of what causes the lunar eclipse in the first place, it’s obvious that it can’t happen at any other time in the Moon’s orbit!