Opinion | The tiny dot that moved: Mercury in transit

Mercury transits the Sun only about 13 times a century, and the event occurs only in May and November when it is aligned with the Earth and the Sun

Spent Monday evening watching, on my laptop, a dot move really, really slowly across a massive orange disk. I know, I know, on the face of it, that sounds like a stultifying few hours. But before you assume this was my way of navel-gazing, or thumb-twiddling, let me point out: this was an eclipse of sorts. A solar eclipse.

Except that it wasn’t. A solar eclipse happens, of course, when the Moon passes in front of the Sun. That is, we’re briefly in the path of the Moon’s shadow as it cuts off the light from the Sun. What happened on Monday came nowhere close to doing that. And yet it was essentially the same phenomenon: a celestial object passed in front of the Sun.

Not just any celestial object, either. This was the planet Mercury, the smallest planet in our solar system and the closest to the Sun. Since it didn’t exactly eclipse the Sun, we call this instead a “transit". While Mercury is larger than the Moon, it is also much further from us than the Moon. So when we see it, it is no more than a tiny speck; and on Monday, that tiny speck transited across the face of the Sun.

For astronomers and space enthusiasts the world over, this was an exciting, thrilling event. Unfortunately, it wasn’t visible in India. This is why I was at my laptop, watching a broadcast of Mercury’s progress as seen through a telescope in the Canary Islands. (The wonders of modern technology: I’m a believer). From the moment Mercury crept stealthily onto the Sun’s orange face, precisely when it was expected, the transit was awe-inspiring and somehow charming. Not necessarily for the image itself — after all, how long can a minuscule dot against an orange background inspire awe? —but for everything the transit says about space and science and mathematics.

Let me give you a flavour of that.

Start with the dot itself. Mercury’s diameter is about 5,000km—less than half the diameter of the Earth. It is, on average, 77 million km from us. Observing it from that far away is like looking at a one-rupee coin that’s 250 metres away. That’s why it’s always just a speck of light scudding across the twilight or early dawn sky. But this transit truly put its size in perspective. The Sun is 1.4 million km in diameter—that is, you need to string nearly 300 Mercurys together to span the Sun. No wonder the planet was no more than a dot against the enormous image of the Sun on Monday.

No wonder, too, that the transit lasted so long. Mercury moves through space at 50km/second. How fast is that? Well, imagine taking 20 seconds to travel from Bengaluru to Bombay. Yet even at that breakneck speed, Mercury took over five hours to move across the face of the Sun. I promise you, it took a serious leap of imagination to believe that little black dot on my screen was actually moving that fast.

All those numbers, I hope, give you an idea of how incredibly huge the Sun is, compared to Mercury. (And for even more perspective, it’s worth remembering that as stars go, the Sun is actually a smaller-than-average star). But hold your horses, we’ve just started with the fun from the transit.

A transit of Mercury needs the Earth, the Sun, and Mercury to be in a certain alignment. That doesn’t happen often: on average, only about 13 times every century. The next one will happen on 11 November 2032. The last one was on 9 May 2016. The 10 previous ones were in November 2006, May 2003, November 1999, November 1993, November 1986, November 1973, May 1970, November 1960, May 1957 and November 1953. A pat on the back if you detected the pattern here: Mercury transits the Sun only in May and November. True, two women I hold dear have birthdays in those months, but I will admit that is probably not the reason for this curious pattern. What is it then?

Imagine the solar system as a dosa. Plonked in the middle is a dollop of masala, or cooked potatoes, representing the Sun. In this model, the planets go around the Sun essentially in the material of the dosa itself. But their orbits are not all actually in the same plane. Mercury’s orbit is tilted at an angle of seven degrees to the Earth’s, so it spends most of its orbital existence soaring above and below the plane of the Earth’s orbit. There are two points, called “nodes", at which the two orbits appear to intersect. (In truth, of course, they never actually do intersect, because the Earth is much further from the Sun than Mercury is). These nodes are in May and November. If you think about it, it’s only when both planets are at or very near a node that they and the Sun are aligned, so that we on Earth can see Mercury transit across the Sun.

Which then raises the question: why don’t we see a transit twice every year, May and November? Simple: because the two planet’s years—the time they take to orbit the Sun—are of different lengths. Mercury takes 88 days, while the Earth takes—well, a year, or 365 days. To understand this, think of plodding along the outside track at your nearest Joggers’ Park, while your Rupa mavshi zips around the inside track quite a bit faster than you. How often will your Nitin kaka, timing you at the entrance, see you both pass him at the same moment? Probably not every time you plod past. In exactly the same way, it’s not every May and November that both planets line up at those nodes together. Instead, it happens only about 13 times a century.

Venus transits are even rarer—about twice every century. I’ll leave you to answer the question of whether you will ever see Mars or Jupiter or Saturn transits; and correspondingly, whether you’d see an Earth transit if you happened to live on Saturn.

Now think of this: if Rupa mavshi was doing exactly two rounds of the track to your one, Nitin kaka would certainly notice her too, every time you passed. The same with Mercury: if its orbit was exactly 182.5 days long—half the Earth’s—we’d see a transit either every May or every November, but not both. If it was just under 122 days—a third as long as the Earth’s—we’d see a transit every May and every November. But these planets’ orbital periods are not quite so congenially related. Strictly, because 88 and 365 are relatively prime, the two planets will never actually line up anyway, affording a “perfect" transit sighting. That is, even those 13 transits every century are not optimal. Think of Rupa mavshi being slightly ahead or behind you in Nitin kaka’s line of sight as you pass. Yet you’re both close enough that he will record it as a joint sighting. That’s the analogy that best captures the nature of most Mercury transits.

There’s another intriguing feature here. Planetary orbits are not circular, but elliptical. This means they move faster through some parts of their orbits than others. In particular, Mercury is faster in November than in May. This means we’re more likely to have a transit in November than in May. Sure enough: of the 58 transits recorded since we started doing so, in 1605, 39 were in November. Of the next 13 expected—taking us till 2124, just over a century from now —nine will happen in November. And because Mercury is travelling more slowly then, May transits are generally longer than November ones. They can last almost 8 hours.

Truly, there’s a lot that we learn about our universe from events like this week’s Mercury transit. In fact, the search for planets outside our solar system—“exoplanets" — is really a search for transit events across other stars. How do we do that, especially because other stars are so far away that they appear to us, even through telescopes, as more tiny specks of light, not enormous orange disks like the Sun? We do it by identifying stars whose brightness varies. For one reason a star’s brightness would decrease is that an object passes between it and us, eclipsing some or all of its light. If that decrease happens regularly, it’s likely the object is orbiting the star, and what do we call such objects? All together now: Planets.

In 2017, a team of scientists at Ahmedabad’s Physical Research Laboratory identified a planet that orbits the star EPIC 211945201. They used this transit method.

I’m not planning a visit, though. EPIC 211945201 is nearly 600 light years away. On the other hand, I’ve got 13 years to plan how to catch the next transit of Mercury. I really don’t want to see that one on a laptop screen.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun