Watch Mercury Lap The Sun

7:50 minutes

On Monday, November 11th, Mercury will slice a path across the sun—an occurrence that happens only about 13 times a century. These days, it’s fairly easy to observe a transit of Mercury—many local observatories or science centers hold viewing parties. But several centuries ago, transit chasers sailed the globe to observe these relatively rare events, in an effort to use them to calculate the size of the solar system. 

How To View The Transit

a time lapse image showing the transit of mercury across the sun
The 2016 Mercury transit. Credit: NASA

When: Monday, November 11, 7:35 a.m. to 1:04 p.m. ET.

Where: Only in the daytime, where the sun is visible. Check with your local observatory, museum, or astronomy club to see if they’re hosting a transit-viewing event. Viewers on the west coast won’t be able to see it when it starts, but since it lasts for five hours, they’ll be able to witness a good portion of it. If you aren’t able to observe it outdoors, check out this list of webcasts.

What You’ll Need: Proper safety equipment. If you’re viewing the transit through binoculars or a telescope, make sure it has a sun filter so you don’t damage your eyes.

Further Reading

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Segment Guests

Dean Regas

Dean Regas is outreach astronomer at the Cincinnati Observatory and co-host of the PBS program Star Gazers in Cincinnati, Ohio.

Segment Transcript

IRA FLATOW: Next up, it’s time to talk about another object eclipsing the sun, only this one’s much smaller. We’re talking about the planet, Mercury. Because this coming Monday, November 11, astronomy enthusiasts will be able to catch a so-called transit of Mercury. It only happens about 13 times a century, making it what, it’s really rare.

And it too, like total solar eclipses, are crucial for early astronomers trying to make sense of the mechanics of the heavens. Joining me to talk about the history, and how to see– you want to see the transit of Mercury for yourself? Dean Regas is here to tell us– Astronomer at the Cincinnati Observatory. Welcome back, Dean.

DEAN REGAS: Oh, great to be with you.

IRA FLATOW: All right. Just tell us exactly what the transit means.

DEAN REGAS: Well, this is when the little planet Mercury goes between us and the sun. It’s exactly lined up just perfectly where you will look up and see a little black dot going across the sun. I’ll spare you my Sting impression, but it is super tiny. And I consider this the nerdy eclipse. This is very small and very subtle, and it also takes a lot of safety to be able to see.

IRA FLATOW: How long does it last for?

DEAN REGAS: Well, so it will begin precisely at 7:35 AM Eastern time. That’s when Mercury will be in silhouette in front of the sun. And so Mercury will still be about 47 million kilometers from the sun, and so just be right in between us. So they look like they’re touching. But 7:35 is when it starts and then it will go across the disc of the sun for about 5 and 1/2 hours and exit at 1:04 PM Eastern time.

IRA FLATOW: Now how easy would it be for me to see it? I can’t see it with my backyard telescope, can I? Is it dangerous?

DEAN REGAS: Well, that is definitely dangerous to do this without the right equipment because if you were to just be outside, you would not notice anything different. The sun wouldn’t dim noticeably. You couldn’t just use the eclipse glasses and look at it with the naked eye safely.

You wouldn’t even see the dot, it’s that small. So you do need a telescope. You need to magnify, maybe about 30 to 40 times, to be on the safe side to be able to make it out. And then, you definitely have to have a filter on your telescope to be able to see it safely.

IRA FLATOW: So the best place to go is like, let’s say, oh, to Dean Regas’ place at the Cincinnati Observatory or other observatories?

DEAN REGAS: Absolutely. This is the kind of stuff that we get excited about because astronomers really love to share the universe with folks. And so, yeah, check with your local science club and Astronomy Observatory and see what they’re doing. Most likely, they’re doing something. The weather forecast, in the Midwest, is not looking great. So, Ira, I’m looking for places to go. Any of your listeners want to host me? I’m looking for places to go on Monday.

IRA FLATOW: Well, we’ll have to see what happens. What can be learned about this? Why is this such a– why are astronomers so excited about this?

DEAN REGAS: Well, so for me, I like the history of it. This was first viewed in 1631 and this was something that– it really lets us know how the solar system works. Johannes Kepler predicted that– he figured out the motions of the planets so well that he could figure out when Mercury was going to cross in front of the sun. This is some detailed math to be able to figure out.

And only one person saw it. In 1631 a French astronomer named Pierre Gassendi. And he couldn’t even fathom what he saw because Mercury looks so small, he didn’t believe it when he saw it. He thought it was just a sunspot.

And so then, future expeditions thought this is something to do to be able to triangulate the distance to the sun. If you can measure Mercury going in front of the sun from different places on the earth, you can actually compute the distance to the sun. And this was a big thing that nations around the world tried to do.

IRA FLATOW: I was surprised to see how rare this event is because Mercury zips around the sun, so much faster than we do, I thought there might be more opportunities, but there aren’t.

DEAN REGAS: No, getting these three bodies lined up, the Earth, Mercury, and the sun is a rare thing because– so, yeah, Mercury goes around the sun every 88 days, and we take a year to go around the sun. So we all have to be in the same place at the same time. And Mercury’s orbit is the most tilted of any the planets. So it’s sometimes too high or too low. And on the 11th, it’s going to be just right.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios, talking with Dean Regas, astronomer at the Cincinnati Observatory about the event, the transit of Mercury coming up next week. We were just talking, in the segment before, about Einstein and the 100th anniversary of him talking about the experiment that proved warp space. But that was very much involved. Einstein was very much involved in explaining, right, Mercury going around the sun?

DEAN REGAS: Well, that was one of the problems was that Newtonian physics wasn’t explaining where Mercury should be. It wasn’t– it was kind of not in its place where we expected it to be. And it was, yeah– things like observing the solar eclipse in 1919 and other things where Einstein’s theory really took hold and it turned out to explain exactly where Mercury goes. And so now, we can watch where Mercury is. We know where Mercury will be every day from now until centuries.

IRA FLATOW: The transit of Venus, there’s also that, right? It’s much more elusive. It happens very rarely. And there’s a good story about a scientist in the 1700s who pretty much risked everything to see it, right?

DEAN REGAS: Yes. I mean, we’ve talked about a lunar eclipse– solar eclipses where the moon blocks the sun. We’ve talked about Mercury going in front of the sun. And Venus is the only other thing that can do that. And we had one, a transit of Venus in 2004 and 2012. And I hope, Ira, you saw one of those because the next one won’t be till 2119.

IRA FLATOW: Better eat my vitamins.

DEAN REGAS: Yes, that’s right. Start to sleep and pretty well. I mean, this is– it’s going to be tough to make that one. And there was a fella in the 1700s, a French astronomer named Guillame Legentil who made a journey to the Indian Ocean to see the transit of Venus in 1761. He missed it because of war, politics, and they wouldn’t let him land.

And he decided, I’m going to stay around here for eight more years till the next transit instead of going all the way back home to France. He stays another eight years. And guess what happens? He got clouded out.

So 11 years after leaving France, he comes back home and finds himself declared legally dead. His family divided up his estate. His wife remarried. And so Legentil is the tragic victim of the transit of Venus. I’m hoping that won’t happen to me with the transit of Mercury here coming up.

IRA FLATOW: Well, so that– he stayed the whole time. He misses it. Stays the whole time for 11 years. Comes back. Finds out he’s declared dead and loses there, it’s his fortune and everything.

DEAN REGAS: He lost everything because of the stupid transit of Venus. You’ve got to think. It makes me think of travel in the 1700s–


DEAN REGAS: Going from Europe to Asia and Australia, that was like– that was more difficult than us going to the moon today. It was quite a journey.

IRA FLATOW: That was great. Well, we’ll look forward to it, Dean. And we’ll see how many people can get out to an observatory and hope you have good luck and the weather breaks for you.

DEAN REGAS: I’m hoping so too because this– the next one won’t happen in the United States till 2049. So I’m a patient man. But that’s tough.

IRA FLATOW: Dean Regas, astronomer at the Cincinnati Observatory. And if you want more information on the transit of Mercury, how to see it for yourself, including some live web streams, yeah, visit ScienceFriday.com/Mercury. Thank you, Dean.

DEAN REGAS: My pleasure.

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