May 10, 2016: The Transit of Mercury and Eclipse Orbital Mechanics

We had a great outreach session at Carl Schurz Park along the East River in Manhattan yesterday for the Transit of Mercury – all 7 1/2 hours of it, starting from before 7:12am when the transit began, all the way until just after 2:40pm, after it ended.  There were half a dozen of us from the AAA (Amateur Astronomers’ Association of NY) with our scopes.  Well, maybe I shouldn’t use the term “us” because I’m not actually a member, although I do go to all the outreach sessions.  Yes, I’m cheap.  That’s why this blog is on a free website.

The Transit of Mercury is basically a partial eclipse of the sun by Mercury.  It occurs when Mercury is perfectly lined up so that looking at the sun from the earth, Mercury crosses right over the face of the sun.  It’s different from the moon shadow transits at Jupiter that I love so much, because there we’re only seeing the shadow cast by the Galilean Moon on Jupiter’s face as the moon orbits Jupiter.  There is always an offset between the location of the moon and the shadow that falls behind it as seen from earth.

Of course, since Mercury is only 3032 miles wide (smaller than Ganymede at 3273 miles and Titan at 3201 miles, and just a smidge bigger than Callisto at 2995 miles), it has no chance whatsoever at fully eclipsing the sun, which is 864,576 miles across.  The only reason our moon can do it – even though the moon is even smaller than Mercury, at 2160 miles across – is obviously because the moon is much, much closer to the earth, averaging about 238,855 miles away.  It just so happens that if you do the math, the moon’s apparent diameter in the sky, being small but very close, is about the same as the sun’s diameter in the sky, being huge but very far – about half a degree, or 30 arcminutes.

The math is easy – on average, the sun is on average 389 times further away from the earth than the moon is, but the sun is also 400 times larger than the moon.  Note the key words, “on average”.  While all orbits are elliptical, the moon’s orbit is a little bit more elliptical than others.  The moon has a perigee, or closest approach to earth, of 221,441 miles, and an apogee of 252,724 miles.  The earth’s orbit is also elliptical, although less so, with perihelion being at 91.4 million miles, and aphelion being at 94.5 million miles.  It is when the moon is both lined up properly and closer to earth that total eclipses occur; the earth can also help out by being a little further away to make the sun appear smaller as well.

The moon is slooooowly moving away from the earth, so that there will come a time in the future, regardless of whether the moon is at perigee and the earth is at aphelion, that the moon will no longer be large enough in the sky to be able to eclipse the sun.  Astronomers can detect the moon’s retreat by using the corner reflectors left on the moon by the Apollo 11, 14, and 15 astronauts.  They fire extremely high-powered lasers at these sets of mirrors, which then reflect that light back down to detectors on the earth.  By timing how long an interval it is between the pulse and the receipt of the reflection, and knowing the speed of light, they can calculate the distance to the moon.  By repeating this experiment over 37 years, they can see that the moon is moving away from us, at a rate of 1 1/2 inches per year.  (See?  I said it was slow!)

As a result of this, there will come a time in about 600 million years or so, when the moon has moved far enough away that its disc is too small in our sky to fully eclipse the sun any longer.  However, the moon will be giving America a terrific show on August 21, 2017, when there is a total eclipse of the sun that will be visible across the country, going from Oregon to South Carolina, coast-to-coast!

Mercury being 52 million miles away from earth during the transit meant that its diameter of only 12 arcseconds was far too small to fully eclipse the sun’s 30 arcminutes.  But it was enough to dim it a tad.  By the way, fun random fact – you could fit  about 17 million Mercurys inside the sun.  “Only” 1.3 million earths, though.

Interestingly, it it precisely this same “eclipse” technique that the Kepler probe uses to discover new exoplanets.  Kepler just stares, unblinkingly, at the same group of about 145,000 stars in a square about ten degrees on a side off to the side of Cygnus, for years at a time.  If an exoplanet is lined up in a perfect line between the star and the earth, then it will “eclipse” that star just like Mercury did yesterday – meaning the light output reaching Kepler dims ever-so-slightly because the smallest little bit of it is blocked out by the transiting planet.  Kepler’s incredibly sensitive photometers are able to detect that exceedingly small drop in light.

After the astronomers have ruled out other causes for such a drop, like the inherent variability in brightness of some stars, then they will announce the discovery of a new exoplanet.  They’ve discovered well over a thousand since the mission began in 2009.  Correction – they just announced today that they’ve discovered over 1200 more, bringing the total to 2326!

Mercury’s orbit is tilted at 7 degrees away from the plane that the earth orbits the sun in.  Because of the geometry necessary to line up the orbits of Mercury and the earth precisely, Mercury transits occur about 13 times a century, but at differently-spaced intervals.  The next one will occur in 2019, while the one after that will occur in 2032.  Notably, they only ever occur in May or November.

Fortunately, this is quite a bit more often then Venus transits, which occur about 105-121 years apart in pairs spaced 8 years apart.  The last pair of Venus transits were in 2004 and 2012; the last pair before that were in 1874 and 1882, and the next pair won’t occur until 2117 and 2125.  I had been looking forward to seeing a Venus transit for over 20 years, ever since I read about them when I was a kid just getting interested in astronomy in the late 1970s.

I missed the 2004 transit, as it occurred mostly at night here in New York, ending at 7:25am.  Unfortunately for me, I didn’t own a scope then, and was unaware of any public viewing opportunities that might have been occurring, as I wasn’t so plugged into astronomy on the internet back then.  Also, 7 in the morning is kinda early for me, as I am most definitely a night person.  That works out pretty well for me and astronomy, though.

I was definitely more plugged in by 2012.  I did get to see that Venus transit, the same way I was able to show people yesterday’s Mercury transit.  Some guy was out alone in Carl Schurz Park with his scope offering people a look, and I did.  It was pretty damned amazing.  Because Venus is both much larger than Mercury at 7521 miles, and much closer to earth than Mercury at only 26 million miles away, it appeared as 6 times larger going across the sun than Mercury did.  Because of this, during the transit, Venus was large enough to see with the naked eye with use of a sun screen.  Just like Mercury, through the telescope, Venus was a perfectly round, inky black disc, perfectly silhouetted against the bright face of the sun.

Any transit is described in terms of contacts.  First Contact is not just a good Star Trek movie; it’s also when you can just see the disc of the planet first touching the disc of the sun.  Second Contact is just a couple of minutes later, when the entire disc of the planet has just cleared the edge.  Third and Fourth Contacts are the reverse.  Third is when, after transiting the entire sun, the disc of planet just touches the opposite edge, and Fourth is when the last bit of the planet finally clears the Sun’s disc.

At Second and Third Contacts, a phenomenon known as the black-drop effect might be able to be be seen.  This occurs at Second and Third contacts when, even though the black disc of the planet has cleared the edge of the sun, it remains connected to the edge by a black stream, which makes the planet look like a teardrop for a few seconds.  It’s not an optical illusion caused by the eye and brain, because you can see it in this video of the 2012 Venus Transit.  Some have said that this is caused by atmospheric effects, either ours or Venus’, but that can’t be true with Mercury, which has no atmosphere, and the black-drop effect has also been observed from space by the TRACE spacecraft.

The best explanation for the occurrence of the black-drop effect is diffraction, a bending and interfering of light with itself as it goes around two objects that are very close together.  You can see this for yourself – hold your thumb and index finger in front of a lamp shade, move them very close together without touching, and observe the light coming through in between them.  As they are almost about to touch, you will see a “black-drop effect” causing there to be no light coming from in between them right at the area where they are about to touch.  This article at Sky and Telescope describes the effect very well.

For the transit yesterday, the weather was in and out.  We started out with mostly clouds just after sunrise, then it cleared up around 8, and remained that way for a few hours.  Some high clouds and haze came along after Noon and remained.  Unfortunately the sun was completely obscured exactly at Second and Fourth Contacts, when the clouds were rolling in something fierce.  Everybody else but me seemed to see the “tear drop” effect at Third Contact, although I was the first to see First Contact, so that makes up for it.  Well, a little, anyway.

Otherwise, it was a very long day, but well worth it.  Mercury is absolutely itty-bitty seen against the surface of the sun, but it was still a beautiful sight to see, especially through my own scope.  Here’s a picture one of the other guys from the AAA took through my scope with his iPhone:

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Mercury is the perfectly round little BB approaching 6 o’clock in the lower half; sunspots are at about 2 o’clock in the top half.  (Photo credit:  Jorge Colorado)

We had about 200 people show up over the course of the entire day, including two classes of kids towards the end, one 7 years old, the other 16.  Hopefully the seeds were sown for some future amateur astronomers, or at least an interest in science.  As usual, people were amazed, appreciative, and completely jazzed by what they were seeing.

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That’s me with a “customer” looking through my scope a couple of hours into the transit.  (Photo credit – Taras Hnatyshyn)

That tiny, perfectly round, perfectly black disc against the white sun is really something incredible to see.  Many had heard about it in the news and came out specifically for the event.  I always enjoy speaking with people who have some understanding of what’s going on, want to know more, and seem to “get it” once you explain things to them, and there were a couple who came by.  I can’t wait for the real eclipse next year, though!

 

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