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SYFY WIRE Bad Astronomy

How to Watch the Rare Transit of Mercury Across the Sun on Monday

By Phil Plait

On Monday, a very cool astronomical event will occur: The smallest planet, Mercury, will appear to move across the face of the Sun. This relatively rare event, called a transit, happens on average only about 13 times per century.

Mercury is small, but big enough that with the right equipment it will appear as an inky black dot silhouetted against the Sun, moving slowly across its face. In the article below I’ll detail the important times of the transit, how to watch it, why this event is rare, and just why it’s so interesting.

But first:

Looking at the Sun without proper equipment is very dangerous. Like, “blinding yourself badly and perhaps permanently” level dangerous. Never look through a camera, binoculars, a telescope, or any kind of optical device at the Sun unless it has been set up to do so safely by someone who knows what they’re doing.*

OK, we good? Good. Now let’s talk transit.

Observing Mercury Slide Across the Sun

(All times are listed in Coordinated Universal Time, or UTC; subtract four hours for Eastern U.S. time, five for Central, six for Mountain, and seven for Pacific).

The transit begins on Monday at about 11:12 UTC. This is the moment the edge of Mercury first appears to touch the edge of the Sun. It takes about three minutes for the planet’s entire disk to move completely into the disk of the Sun.

Mercury reaches the midpoint of the transit more than three hours later, at 14:15 UTC. The beginning of the end of transit is at 18:39 UTC, when the leading edge of Mercury touches the inside edge of the Sun; three minutes later it’s all over as the trailing edge of Mercury leaves the Sun behind.

NASA put together a brief animation showing what it will look like:

The transit takes so long that most of the planet will see at least a part of it; the Sun rises during the transit for much of the western U.S. for example, and sets midtransit for most of Africa and eastern Europe. Sorry, Australia, Japan, and Indonesia; the transit happens at night for you and you’ll miss it.

But don’t despair! Lots of observatories are doing live webcasts of the transit:

If you want to see this for yourself, I imagine lots of astronomy clubs will be hosting events for it. See if there’s one near you!

If you already have the proper equipment, then by all means give it a shot. That includes the proper solar filter on your telescope, binoculars, or camera. Instructions for how to do this can be found in many places online; Fred Espenak has details (written for an eclipse but still appropriate), and his page on viewing eclipses is relevant as well. You might be able to project the image of the Sun onto a piece of paper and see the transit, but I’m not sure Mercury will be big enough to see this way. If you want to try it, I have links and instructions on my page describing the 2012 transit of Venus.

If it’s clear in Colorado, I plan on watching this myself. I have a specially constructed solar telescope, and while it’s difficult to get my phone connected to it correctly, I’ll give it a try. I might even stream it live on Periscope if I can get it working. Follow me on Twitter for up-to-the-minute info on that.

How This Works

So why do we see a transit? Like an eclipse, it depends on geometry. It is, after all, like a mini-eclipse!

Mercury orbits the Sun closer than Earth does. Its orbit is very slightly tilted with respect to ours, by about 7°. That’s not much, but given the geometry, it's enough that in most cases when Mercury gets between us and the Sun, it misses the Sun’s actual disk, passing above or below it.

Because Mercury’s orbit is tilted, it appears to move “up and down” compared with Earth’s orbit. It crosses the plane of our orbit twice every Mercury year (those points on its orbit are called “nodes,” which are indicated in the illustration above), physically passing through one node or the other every 44 days or so (Mercury’s orbital period, its year, is about 88 days long). From Earth’s point of view, we see one or the other of Mercury’s nodes directly in line with the Sun twice per our year as well as we orbit the Sun. As it happens, those times are in May and November.

To see a transit, Mercury has to be positioned at one of its nodes at the same time Earth happens to be aligned such that the node is in front of the Sun. We then see Mercury in front of the Sun, and we get a transit!

That’s why transits are rare; everything has to line up just so. It’s also why we only see Mercury transits in May or November. The next one will be on Nov. 11, 2019, so only about 3½ years from now. The next one after that is 13 years later, on Nov. 13, 2032. I hope you can catch this one or the next. It’ll be a long wait for the one after that.

So, What’s the Big Deal?

I know that some folks might think this whole thing is a ho-hum event. I can assure you, with no hint of bias at all, that these people are hollow shells of humans.

No, seriously, this really is pretty cool. I’ve seen a couple of Mercury transits (and two Venus transits, too), and it’s decidedly weird to see that tiny, perfect little circle slowly move across the Sun’s face. It connects you with the cycles of the Universe, shows you how wonderful and rare and grand such events can be. Just ask these folks.

It’s scientifically useful as well. Theoretically it can be used to determine the scale of the solar system, which was very important before the invention of modern techniques like radar timing to get the distances to the planets. In practice it’s very hard, because Mercury is so small. It turns out it’s hard to do with Venus for other reasons, but not for lack of trying

But the 2003 and 2006 transits of Mercury were used to determine the diameter of the Sun more accurately than ever before: 1,392,684 +/- 65 km. That’s an accuracy of 99.995 percent! Not bad.

Also, when you see this transit, you’re seeing an example of how astronomers find exoplanets, worlds orbiting other stars. If an exoplanet has an orbit aligned such that we see it edge-on, the planet passes directly in front of its host star. This transit blocks a small fraction of the star’s light, which can in many cases be detected, and tells us about the orbit and physical size of the planet. Mercury’s transit is this exact phenomenon up close!

So, in my unbiased opinion, if you get a chance to catch this event either for yourself or online via an observatory’s live stream, you should take it. The Universe rarely puts on such a well-timed and regularly scheduled event for us, so the least we can do is oblige it.

And one final note: If you want to impress your friends with your Mercury knowledge, watch my episode of Crash Course Astronomy featuring the diminutive planet. It’s actually a very cool (well, hot) place.

* Glancing at the Sun with your unaided eye probably won’t cause serious or permanent damage, but duh, it hurts, so don’t do it. It’s more dangerous for children than adults, because as we age our corneas get yellower, slightly dimming the more dangerous bluer light. Wearing sunglasses can actually make it worse, because they dim the light enough to dilate your pupils but may not block the more dangerous invisible radiation. So it’s not a good idea either way.

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