This doesn’t happen very often. In fact, it won’t be visible from North America again until 2049.
On the morning of November 11, from Earth’s vantage point, Mercury will “transit” — that is, pass directly in front of — the Sun.
Usually, stars and planets seem pretty two-dimensional from here. We’re accustomed to seeing Mercury by itself, just a dot, following the Sun around and reflecting its light:
But when Mercury passes in front of the Sun, we suddenly have a very three-dimensional event. It’s like seeing our friend in a different car at the other end of an amusement park ride. We get a better feeling of the scope and dimensionality of the ride, but we also get the twang of recognition, waving to our friend’s car and hoping she’s having fun.
See if you can spot tiny Mercury in this time-lapse progression, right about in the center of the frame (a little below the center of the Sun):
We really are on the same ride together. Mercury is just a little ball floating in space, and so are we.
But if Mercury has a smaller orbit than we do, why don’t we see it pass directly in front of the Sun every time it goes around? Because Mercury’s orbit is inclined by about 7 degrees compared to ours, illustrated in this nice diagram from ESO:
We only line up with Mercury’s orbit twice a year, then, in May and November. That’s when all transits of Mercury must occur.
If you’d like to observe the transit as it happens, there’s plenty of good info with tips and times at sites like THIS and THIS, so I won’t rehash all of that here.
I’m more concerned with this three-dimensional aspect and how that helped us to find out some very essential and very satisfying information about our place in the Solar System.
Before we had radar to bounce off of things and before we had spacecraft to beam signals back to us, we had no idea how far away the Sun is. None at all. It could be small and a hundred miles away, or huge and a billion miles away. How can you tell? People had been guessing at this since ancient times, and their guesses were all over the place and pretty far off.
The two main people that set the stage for getting serious about figuring this out were Nicolaus Copernicus and Johannes Kepler. Copernicus, of course, established in 1532 that the planets revolve around the Sun, and Kepler worked out by 1615 that orbiting bodies always obey a simple rule:
a3 / T2 = constant,
where a is the orbit’s semi-major axis (radius at its widest point), and T is the time it takes to complete one orbit. Kepler knew how long each planet takes to complete its orbit, so he knew the planets’ relative distances to the Sun. But no one knew the absolute scale of the whole thing.
If anyone had known the value of the gravitational constant, they could have figured it out, but no one knew how heavy the Earth is, and besides, Newton hadn’t even come along yet to explain the whole gravity thing.
Hang on … I’m stopping for a second for Fun With Keplerian Orbits! If you have a couple of circular orbits similar to those of Earth and Venus, and you draw a line between the planets every so often, you get amazing patterns like this:
No, this doesn’t help us understand Kepler’s theory, but it does make it way more aesthetcally pleasing, doesn’t it?
Anyway, this is where a transit comes in. By the mid-1600s, people certainly knew you could use parallax to estimate distances. You yourself are well aware of it too, even if you’re not concerned about the math of it. In the moving scene below, it’s pretty obvious which buildings are closest to us:
That works for celestial bodies, too. Let’s make up a planet, drop it into a sea of stars, and live on it! Below you see our planet’s view of the night sky as the years zip by. (Is my son in college already?) Again, you don’t need any math to see which ones are close to us and which are far away:
A transit is a convenient way to use parallax to measure the distance to a planet that goes in front of the Sun. You get two people very far apart on Earth to note where the planet shows up from their viewpoints against the Sun’s disk. Then you use some trigonometry, but I’ll leave that to San Francisco’s Exploratorium, which does a nice job of laying this out HERE if you’re interested.
If you do this and find the absolute distance from Earth to Mercury or Venus, you can then use Kepler’s equation to find all the distances in the Solar System, including the distance from Earth to the Sun!
People did in fact go to great lengths to do this in 1761 and 1769, when Venus made transits across the Sun, at the suggestion of Edmond Halley (of Halley’s comet fame). I was totally ready to write about those transits of Venus and their role in finding the distance to the Sun for the first time, BUT … it turns out someone had beaten them to it (whether they believed him or not): Cassini!
Yes, Giovanni Domenico Cassini, the guy they named that awesome Saturn mission after!
Cassini didn’t wait around for transits. Those are so rare. Instead, he decided he could use faraway stars as the backdrop instead of the Sun. He trusted that there were stars far enough away from Earth that they would stay essentially fixed as a background, so he and fellow astronomer Jean Richer merely waited for Mars to get its closest to Earth so that their measurement of its distance from Earth would be as accurate as it could be.
In 1672, Cassini stayed behind in Paris, while Richer went to French Guiana. Each carefully measured Mars’ position relative to the stars at particular times, noting which stars didn’t appear to move. Then Richer came back so they could compare their measurements.
It was also quite fortunate that a couple of years earlier, astronomer (and priest!) Jean Picard had made a good estimate of the Earth’s size, so Cassini and Richer had a pretty good idea of how many miles apart they had been.
Their result appears for the first time in the 1673 section of the Histoire de l’Academie Royale des Sciences, on p. 173:
33 millions de lieuës.
Depending on exactly what they meant by lieuës (because there were multiple definitions in France at the time), they appear to have calculated somewhere between 88 million and 91 million miles. The value we know and love today, of course, is about 93 million miles.
Cassini and Richer had just made the Solar System more than 10 times bigger than anyone had thought it was! At last we understood the size of the massive ride we are on.
If you want to read more about parallax, there is a fun and digestible history of its use at the Cal-Irvine website HERE.
There’ll be several groups of amateur astronomers and students all over the country measuring the position of Mercury against the Sun on the morning of November 11 to find that planet’s, and ultimately the Sun’s, distance from us. Let’s see how they stack up against Cassini!
Wishing those curious amateurs all across the country a fun and productive morning. Good luck, y’all! Good luck, youse! Good luck, yinz! Good luck, you’uns! And say hi to Mercury for me, will ya? It’s been a few years.