Page 89 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

THE

DANJON

SCALE
Watch as
the full Moon
starts to darken, The Danjon Scale, named after its
before becoming creator, the French astronomer André-
orange-red as Louis Danjon, describes how dark the
it is completely Moon gets during a total lunar eclipse.
covered by It measures the darkness of an
Earth’s shadow eclipse from L0 through to L4. All lunar
eclipses are very different in colour,
going from light orange to dark brown.
This is because during an eclipse, the
Moon is only lit by light that has passed
through Earth’s atmosphere. So the
colour will depend on how much dust,
volcanic ash and water vapour is in the
atmosphere to affect the sunlight’s path.
The eclipse in 1884, after the huge
volcanic eruption of Krakatoa, was so
dark that the Moon could only just be
This is what you’d see if made out, such was the amount of dust
you were on the surface in the atmosphere.
of the Moon during a total
lunar eclipse. As the Moon
moves into Earth’s shadow to L0
totality at stage 4, our planet’s
atmosphere glows red

1

L1

The beauty of a lunar eclipse is that it is
visible wherever you are on the night-side
of Earth. The Moon can be in shadow for 2
over an hour and a half, so the whole thing
is quite a gentle process and you won’t need
any special equipment to see it. L2

RED MOON AT NIGHT
As for the other eclipse types: a partial
eclipse is when only part of the Moon 3
travels through the dark umbral shadow.
It can be quite noticeable though, as you
L3
can see for yourself by watching the next
partial event on 25 April 2013. You may
even have seen a penumbral eclipse
without noticing. This is when the Moon
only passes through the lighter part of 4
Earth’s shadow; sunlight still reaches the
Moon, so you only see a slight dimming. L4
You can see one on 28 November 2012.
But if it’s a total lunar eclipse you want
to see, there’s a bit of a wait sadly: the
next one visible in Europe isn’t until
28 September 2015!
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