Venus Transit

June 5 brings the last Venus transit until 2117, when Venus will pass across the face of the sun – the second of the only two Venus transits any of us are likely to see in our lives. (The first was in 2004, visible on the east coast of the US but not from here.)

Of course, this being a solar event, you need a safe solar filter to see it. No need for a fancy H-alpha filter – a white light filter is fine, preferably a glass filter over the aperture of the telescope.

(Don’t use the kind that screw into the eyepiece! They can overheat and crack while you’re looking through them.)

You don’t need a big telescope, either: I used an Orion filter on my little 80mm f/7 refractor for the last Mercury transit and it worked great. And Venus is much larger than Mercury, so any telescope, or even binoculars, will show the transit. Venus’s size is about 50 arcseconds (just under one arcminute), while the sun is about half a degree (30 arcminutes). Mercury only gets to 11 or 12 arcseconds, so if you’ve seen a Mercury transit, you can imagine how much easier and more spectacular a Venus transit can be.

If you use binoculars, either make sure that you have solar filters for both sides, or keep one side covered at all times.

If you absolutely can’t find a solar filter in time for the transit, you can set up your telescope to project the sun’s image onto a white board or sheet of paper. Use a cheap, low powered eyepiece for this: the eyepiece will get quite hot, and you don’t want to risk damaging a fancy eyepiece. Be careful with solar projection – make sure nobody nearby can walk between the telescope and the surface you’re using as a projection screen, or place their hands or eyes in the light path.

A web search for “solar projection” will uncover other tips.

When does this all happen?

Venus begins its ingress onto the disc of the sun on 3:06 on the afternoon of June 5. The transit continues until after the sun sets at 8:26. So we won’t get to see egress. Venus’s exit from the face of the sun, but it’s the mirror image of what we’ll see at ingress.

Ingress has two parts: first contact, when the edge of Venus’s disk first touches the outside of the sun’s disk, and “internal ingress” or second contact, when Venus’s disk is fully inside that of the sun. Second contact is the most interesting period of the transit: it’s when the famous “black drop effect” occurs, where the black circle of Venus can seem to elongate into a teardrop shape as it “tears away” from the edge of the sun. The black drop caused a lot of problems for early Venus transit observers, since it makes it difficult to record the exact second contact time. The effect seems to be caused by blurring from our own atmosphere (poor seeing) combined with telescope diffraction. So the steadier your seeing is, and the bigger and better your optics, the less likely you are to see the black drop.

If you have especially good seeing before second contact, try looking for the aureole, an arc of light just outside of the solar disk made by the refraction of sunlight through Venus’s atmosphere.

Amazingly, the aureole has the same surface brightness as the sun’s surface, and is said to be possible to see even through a solar filter. That’s something you’ll never see in a Mercury transit!

Here’s the time table for San Jose (from the table on NASA’s eclipse website):

First contact: 3:06:20 p.m.

Internal ingress: 3:23:56 p.m.

Maximum transit: 6:25:30 p.m.

Sunset: 8:26 p.m.

At first contact, the sun will still be high, 60 degrees up. By maximum transit the pair will have sunk to 21 degrees, still plenty high enough to see the spectacle. Photographers will want to wait around for sunset for a chance at some spectacular photos, such as this sunrise photo (see above) taken by SJAA member Bill Arnett from Chicago during the 2004 Venus transit.

Want more details or times from other locations?

Go to transitofvenus.org for the full scoop.

What about other planets to watch this month? Who cares? Okay, maybe you do care. So here’s the list:

Saturn rules the evening sky, transiting just before midnight.

Mars, too, hangs in the evening sky and is visible until midnight, though it’s small, distant and not very high.

Neptune doesn’t rise until around midnight.

Mercury moves into the early evening after the first week of June.

Jupiter moves into the dawn sky, where it joins Uranus.

Pluto, at opposition on June 29, is visible all night ... but it’s in the heart of the Sagittarius Milky Way next to M25, so you’ll have a tough job finding it. But you’ll see some great scenery along the way!