now I've been around Sydney and I've
asked people what causes the phases of
the Moon and you know what they say how
do we get the phases of the Moon because
of the the earth blocks the delight that
comes from the Sun a full moon is
basically where we're seeing the full
circle of the moon reflected back to
earth but the light is actually coming
from the Sun and the present whatever
the presents are caused by the shadows
of the Earth from the Sun onto the moon
the phases of the Moon is that about the
Sun and the moon and the and the
something goes in front of something
that's between this moon and the Sun we
won't see the moon and then it goes
around the earth gets out of the way so
more of the Sun shines on the moon and
we can see them it I think so
let's try to resolve it what really
causes the phases of the Moon phases of
the Moon and we need to know a couple of
things first one is there's only one
source of light in the solar system and
that's the Sun at the center of the
solar system it produces all the light
so both the earth and the moon are half
illuminated by that one source of light
as the Moon moves around the earth our
perspective on it changes because
sometimes we see just the unlit face of
the moon when the moon is between us and
the Sun that's a new moon at other times
we'll see the moon has moved around the
earth and we see the moon half
illuminated and half dark we'd call that
first quarter at other time still when
the moon is moved further round the
earth we see the full illuminated face
of the moon and we call that full moon
but it's just because the moon is half
illuminated by the Sun and it's our
perspective on that half illuminated
moon which gives the phases why doesn't
the earth block the light when we're
seeing a full moon
you know if it's Sun Earth Moon why
doesn't the earth block out the light so
we can't see the moon you would think
that would happen every full moon you
would think the earth the moon would go
through the earth shadow at every full
moon but in fact the moon's orbit is
tilted slightly to the orbit of the
earth around the Sun how much is it
tilted by about five degrees so
virtually every full moon the moon is
moving just above or just below the
it's shadow I'm here with Andrew Jacobs
at the sydney observatory and we've just
witnessed a total lunar eclipse so can
you tell me what that is
so total lunar eclipse is when normally
there will be a full moon
but the full moon happens when the moon
is in Earth's shadow so the moon is
passed through a shadow it's gone
through the outer faint a part of the
shadow going to the inner darker part
and turned red so why does it look red
when it's in the middle of a shadow ah
three three effects here going on one is
that the Earth's atmosphere is
refracting the light into towards the
moon it's accurate like a prism is it be
like a prism yes exactly
like that the blue light however is
being scattered out by the Earth's
atmosphere just like a sunset see a red
sunset because the blue light is
scattered outwards
so that leaves the red light
preferentially to get through to the
moon and then if there's dust perhaps
volcanic dust in the Earth's atmosphere
that dims the light so leads to either a
brighter or darker red color on the face
of the Moon what can we use the lunar
eclipse for why is it important to us a
few years ago would have said it with
had no importance at all not much
importance anywhere beautifully to look
at but not much scientific importance
but nowadays and we know that there are
planets orbiting around other stars
alien stars now expec so planets we call
them and if we can measure and the light
that's going through Earth's atmosphere
and reflecting off the moon it can give
us an idea of what we might expect to
see if the light is coming through the
atmosphere of an exoplanet orbiting
around another star so we can use the
observations of a lunar eclipse around
the Earth to infer what's what the
atmosphere of a exoplanet may be like so
we can detect things like ozone or
carbon dioxide in atmosphere by looking
at the spectrum that reflects off the
moon and if we see a similar spectrum
when we look at a exoplanet around
another star we could infer there is an
ozone or carbon dioxide in that
atmosphere do you detect water vapor as
well you could just take water vapor as
well yes and that would obviously be a
pretty exciting sign if we saw that that
would be very exciting sign yes yes all
those things together have some methane
as well
might suggest that there's life out
there in the universe