Thanks to Ptolemy and his cronies, everyone used to think that the Earth was the center
of the Solar System, with the Sun, planets and even the stars orbiting around it on a
series of concentric crystal spheres.
It was a clever idea, and explained the motions of the planets… sort of.
Then Copernicus figured out in 1543, that the Earth isn’t the centre of the Solar
System.
In fact, it’s just one planet in a vast Solar System, with objects whirling and whirling
around the Sun.
With the structure of the Solar System figured out, and the crystal sphere idea in the garbage,
astronomers still had a big unknown: how big is the Solar System?
Was it a few million kilometers across, or hundreds of millions.
How big is the Sun?
How far away is Venus?
Astronomers needed some kind of cosmic yardstick to measure everything against.
Figure out one piece of the puzzle, and then you could measure everything else in relation.
In 1627, Johannes Kepler figured out that the motion of Venus was predictable, and that
Venus would pass in front of the Sun in 1631, probably in the afternoon.
This is known as a “transit” of Venus.
The first crude measurements of Venus’ motion across the Sun were made in 1639 by Jeremiah
Horrocks and William Crabtree from two different spots in England.
And with these two observations, they were able to calculate the geometry between the
Earth, Venus and the Sun.
If you recall all those memories you’re repressing from your high school geometry,
once you’ve got an angle and a side of a triangle, you can work out all the other parts
of the triangle.
Horrocks and Crabtree worked out the distance from the Earth to the Sun within about 2/3rd
accuracy.
Not bad, considering the fact that astronomers literally had no idea before this point.
Following on from this observation, astronomers returned to their telescopes with each transit
of Venus, better refining their calculations, and eventually settling on the current distance
of about 150 million kilometers.
From here on Earth, we can see a few objects pass in front of the Sun: Venus, Mercury and
the Moon.
Venus transits are the most rare, happening two times every 108 years or so.
Mercury transits happen more often, about a dozen times a century.
And a transit of the Moon, also known as a solar eclipse, happens a few times a year,
on average.
It’s all a matter of perspective.
If you’re standing on the Moon, you might see the Earth pass in front of the Sun.
We’d call that a lunar eclipse, while the lunatics would call it an Earth transit.
We can also see transits in other parts of the Solar System, like when moons pass in
front of planets.
For example, if you have a small telescope, you can see when Jupiter’s larger moons
pass in front of the planet from our perspective.
One of the questions you might have, though, is why don’t these transits happen more
often.
Why don’t we see a Mercury or Venus transit every time they line up with us and the Sun.
This is because the planets aren’t exactly lined up at the same angle towards the Sun.
All of the planets are inclined at an angle that takes them above or below the Sun at
various points of their orbit.
For example, Venus’ orbit is inclined 3 degrees off the Sun’s equator, while the
Earth is inclined 7 degrees.
This means that most of the time that Venus and Earth are lined up, Venus is either above
or below the Sun.
Are you an ageless vampire, or planning to live a long time in multiple robot bodies,
then you’re in luck.
In the year 69,163, there’ll be a double transit on the surface of Sun with both Mercury
and Venus at the same time.
Enjoy that while you contemplate the horror of your existence.
Once we become a true Solar System civilization, there will be even more opportunities for
transits.
People living on Mars will be able to see Mercury, Venus and even transits of Earth
passing in front of the Sun.
Neptunians will be bored they can see them so often.
The transit method is one of the ways that astronomers discover planets orbiting other
stars.
Using a space telescope like Kepler, they survey a portion of the night sky, watching
the brightness of thousands of stars.
When a planet perfectly passes directly in between us and a star, Kepler detects a drop
in brightness.
When you think of the geometries involved, it’s amazing this even happens at all.
But the Universe is a vast place.
Even if only a tiny percentage of star systems are perfectly lined up with us, there are
enough to help us discover thousands and thousands of planets.
Kepler has turned up Earth-sized worlds orbiting other stars, some of which are even orbiting
in their planet’s habitable zone.
Watching planetary transits is more than just a fun astronomy event, they’re how astronomers
figured out the size of the Solar System itself.
And now they help us find other planets orbiting other stars.
So, let’s agree to meet up in 2117 to catch the next transit of Venus, and celebrate this
amazing event.
Did you watch the transit of Venus or recent transit of Mercury?
How did it go for you?
Let me know in the comments below.
In our next episode, we wonder if there are antimatter galaxies out there.
Could we tell the difference?
Oh, and make sure you stick around for the blooper.
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