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Sydney Transit of Venus -

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NARRATION

Yesterday, just 43 million kilometres away, Venus was sliding between us and the Sun. But here at
the Sydney Observatory, so were plenty of clouds.

Mark Horstman

What are your chances of seeing the transit today do you reckon?

Schoolboy

I don't think we'll see it.

Schoolgirl

Next to nothing.

Man

I don't expect to see very much at all to be honest.

Woman

We're waiting for that blue patch over there and we're going to run! (LAUGHS)

NARRATION

But just after the transit of Venus starts, the clouds part and people get their first look.

Crowd members

Wow! Oh Wow! Oh wow! Oh my God! Can you see the dot and the spot? Yeah! I can see it, I can see it!
Wow! That's really cool!

Crowd in unison

Hello Venus!

Mark Horstman

That is amazing. I can see Venus just like a pea on the edge of a huge golden orb. It's quite
remarkable to be able to see a planet, a whole planet, silhouetted against the nearest star.

NARRATION

In the past, timing the transit from different viewpoints provided the key to measuring the
universe.

Geoffrey Wyatt

Everybody knew the ratio of the planets from the sun but they didn't know the exact distance to any
of them. So this was kind of like unlocking the size of the solar system with the biggest ruler.

NARRATION

These days, scientists study the transit for very different reasons as Graham Phillips finds out at
Macquarie University.

Dr Graham Phillips

Back in Cook's day, the purpose of observing the transit was to try and work out how far the earth
is from the sun. Now we can do that with a radar these days. This time round, astronomers are
trying to learn about other star systems.

NARRATION

Specifically, about the planets orbiting distant suns. They're called Exoplanets. Carlos' day job
is exoplanet hunting. For the search, he's building a new, highly sensitive instrument. It will
analyze a star's light spectrum in incredible detail, allowing tiny wobbles in the star to be
detected.

Carlos Bacigalupo

We have the star, and we have the planet going around the star. Now we all know that the star is
holding the planet in place but sometimes it's hard to think that the planet is also moving the
star.

NARRATION

The planet's gravity tugs on the star, forcing the star into that tiny inner orbit.

Carlos Bacigalupo

We have to be very precise and if we are, we can actually see that movement, then if we analyze it
properly, we can see the planet.

NARRATION

But there's another way of detecting exoplanets; Watching while they transit their suns, just like
a Venus transit. Rather than measuring a wobble, blocked starlight is detected.

Carlos Bacigalupo

We can look at the luminosity of the main star and when we see that it decreases, and that happens
periodically, that could be a signature of a planet around that star. It's a very useful method
because it can give us a lot more information than just the presence of a planet.

NARRATION

The size of a planet can be worked out, from the amount of light that's blocked.

Dr Graham Phillips

The reason astronomers are so excited about the transit of Venus is they get to see a transit up
close.

Carlos Bacigalupo

We know information on the sun very well. We know information on Venus very well. So we make
calculations of what it would look like to see the sun through Venus.

NARRATION

That information can be applied to suns and exoplanets we don't know so well, leading to new
discoveries. One telescope that will be keenly watching the transit is Hubble. It's made so many
amazing discoveries over its decades in space. But its highly sensitive instruments would burn out
if pointed directly at the sun.

Carlos Bacigalupo

The Hubble Space Telescope is going to be looking at the moon and it's going to look at the
sunlight reflected on the Moon while the planet Venus is going in front of it.

NARRATION

That reflected light will have passed through Venus's atmosphere and so will carry telltale signs
of the gases encountered on the way. Astronomers can work out the make-up of Venus's skies just
from the moonshine. And that's exciting because the same technique can be used to find out about
exoplanet skies.

NARRATION

Back at Sydney Observatory, you don't need a modern telescope to view the sun.

Mark Horstman

This beautiful old instrument is the Russell telescope. It's the oldest working telescope in
Australia; it's been here since 1874 and today it becomes one of the few in the world to have
observed three Transits of Venus.

Geoffrey Wyatt

Typically you find such a magnificent instrument unusable, it's usually just an exhibit but we have
it here in use today so we're actually doing some cultural astronomy.

NARRATION

The transit inspires people of all ages to pause and wonder about our place in the solar system.

Woman

It's, it's amazing to look up at that black spot and think that's a whole planet I'm looking at.

Woman

That's the size of the earth and just how tiny, tiny, tiny, tiny we are compared to the sun.

NARRATION

Thanks very much to those who sent us your photos. Here are some of the best.

Mark Horstman

And that's it for this transit. You won't see Venus quite like this for another century or so.