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The Shallow Sky

Mercury Transit!

Akkana Peck


On November 8th, we will see a relatively rare event:

Mercury will pass across the face of the sun. This is the first Mercury transit we’ve been able to see since November of 1999; the next one won’t happen until May of 2016. So grab your solar filter and take a look, or come join the fun at the SJAA event at Houge Park.

Any telescope will do, as long as you have a solar filter or other safe way of observing the sun. For the last transit, in 1999, people were issuing all sorts of dire warnings about how it would be subtle and difficult to detect and you’d need a big telescope.

Bosh! It was visible in binoculars (with aluminized mylar solar filters) and lovely in an 80mm short-focus refractor with an Orion glass solar filter. You might even be able to see it with the filtered but otherwise naked eye.

Be sure to use a safe solar filter: one that covers the front of the telescope, not the kind that threads into an eyepiece. The eyepiece kind can crack from the heat of all that concentrated sunlight, and you don’t want to think what the sunlight would do to your retina.

If you can’t get hold of a solar filter in time and can’t get to a public event, don’t despair. You might try eyepiece projection: use a cheap eyepiece (since concentrated sunlight could potentially damage an eyepiece) pointed *down*, at the ground, a box or a piece of paper. Be careful no one can get an eye or a hand underneath the eyepiece, and be sure to keep a cap on your finder. (So how do you point the telescope at the sun? Watch the shadow of the telescope and finder on the ground, and try to make the shadow as small as possible.)

This transit won’t be a “graze” like the 1999 event, with Mercury skimming briefly along the limb of the sun. This month’s transit is nearly an all-day affair, beginning before noon and lasting until just an hour before sunset. Here’s the breakdown:

11:12:04 First contact (Mercury first starts to become visible)11:13:57 Second contact (the full disk of Mercury is visible)1:41:04 Maximum transit4:08:16 Third contact (starting to lose Mercury)4:10:08 Fourth contact (event over, go home)

These times apply for folks using white light filters (the relatively inexpensive kind you buy from places like Orion). But if you have a hydrogen alpha filter, it gets more interesting: you may be able to see Mercury even before it begins its transit, silhouetted against the sun’s chromosphere or a conveniently located prominence. So if you do have an H-alpha filter, be sure to start watching early.

The times of second and third contact are also less than straightforward. Just as Mercury’s disk becomes tangent to the Sun’s, an illusion known as the “black drop effect” makes the point of contact appear to stretch before finally breaking. The official time of second and third contact is the time when you can just barely see sunlight all the way around the planet’s disk. You can read more about the black drop effect in my July, 2004 column.

Is it just an odd coincidence that this transit and the last one were both in November? No, as it turns out: Mercury transits currently can only happen in early May or early November, because of the inclination of Mercury’s orbit (about seven degrees). Those months are the only times when the Earth is lined up with Mercury’s “nodes”, the two points where Mercury’s orbit crosses the ecliptic. November transits are a bit more common, because Mercury is moving more slowly at that point of its orbit, and so we’re more likely to pass by when Mercury is there.

Practice now, and you’ll be all ready for the next Venus transit in June of 2012! (Just kidding — you won’t need any practice to see the Venus transit. Watch this one because it’s fun and interesting.)

It’s a good thing we have the transit, because there isn’t much else going on in the shallow sky (besides the ever-beautiful moon, of course). Uranus is high in the sky at sunset and sets a bit after midnight, so it’s still accessible if you catch it early in the evening. Go for Neptune first, even though it’s more difficult: it’s about an hour and half ahead of Uranus. Saturn rises a bit before midnight and is a nice object for night owls. Later in the month, Mercury emerges into the morning sky to create a very nice viewing opportunity for early risers. The other planets are too close to the sun to be observed this month.

And finally, in other planetary news, the dwarf planet formerly known as Xena (or 2003 UB313, for sticklers) now has a real name: she’s now called Eris, after the Greek goddess of strife and discord. Eris’ moon will not be named Gabrielle, but instead is dubbed Dysnomia, after the daughter of Eris and the goddess of lawlessness. Not a bad pair of names, really, for Xena fans!


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