Kind: captions
Language: en
[Applause]
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?
Uh because of the the earth blocks the
the light that comes from the sun.
A full moon is basically where we're
seeing the full uh circle of the moon
reflected back to Earth,
but the light is actually coming from
the sun. And the cresant where the
cresants 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.
The earth's between the moon and the
sun. We won't see the moon and then it
as it goes around the earth gets out of
the way. So more of the sun shines on
the moon and we can see the moon I
think.
So let's try to resolve it. What really
causes the phases of the moon? phases of
the moon. Um, 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. So, 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
times still when the moon has moved
further around 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's 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 5°. So virtually every full
moon, the moon is moving just above or
just below the Earth'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 would be a full moon, but the full
moon happens when the moon is in Earth's
shadow. So, the moon has passed through
Earth's shadow. Um, it's gone through
the outer fainter part of the shadow,
gone into the inner darker part and
turned red.
So, why does it look red when it's in
the middle of Earth's shadow?
Ah, there's three three effects here
going on. One is that the Earth's
atmosphere is refracting the light into
towards the moon. A second bit like a
prism, is it?
Bit like a prison. Yes, exactly like
like that. Um it the blue light however
is being scattered out by the Earth's
atmosphere just like a sunset. You see a
red sunset because the blue light is
scattered outwards. Um 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 it um 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, I would have said it
was had no importance at all. Not much
importance anyway. Beautiful thing to
look at but not much scientific
importance. But nowadays um we know that
there are planets orbiting around other
stars, alien stars, ex exoplanets we
call them. And if we can measure um 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. We can detect things like ozone or
carbon dioxide in Earth's 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 um ozone or carbon dioxide in
that atmosphere.
Could you detect water vapor as well?
You could detect water vapor as well.
Yes.
And that would obviously be a pretty
exciting sign if we saw that.
That would be a very exciting sign. Yes.
Yes. All those things together and
perhaps some methane as well might
suggest that there's life out there in
the universe.