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Saturday, May 12, 2012

Sextants for Venus

Next month (June 5 or 6 depending on where you live) the transit of Venus across the face of the sun will be visible for much of the planet. A lot of different sites have been discussing various ways to watch this once-in-a-lifetime event (actually twice in our lifetime, because Venus transits occur in couplets spaced eight years apart), such as pinhole viewers and special filters for binoculars and telescopes. All of which work fine.

However, earlier this week I was practicing some sunlines with my Astra 3b sextant, and I noticed that the large sunspot group AR 1476 was quite visible. Given that any time Venus is visible (even in broad daylight) it is very easy to see its planetary disk with a 4x40 sextant telescope, and given that during the transit Venus will be about 67,000,000 miles closer to earth than it is during apparent aphelion, it should be very easy to observe the transit using a standard marine sextant with its sun filters in place. If you happen to have a 7x35 scope for your sextant, the viewing will be that much better, but this is not at all necessary.

To use your sextant as a solar telescope, zero the index arm, and ensure that the filters are on both the index mirror and the horizon glass. Then, just look directly at the sun through the sextant telescope. Start with too many filters and then start taking them away until you can see the sun as a crisp disk. Do NOT look at sun unfiltered!! Note that the sextant in the graphic above does NOT have adequate filtering to safely look at the sun; it is a costume piece from a steampunk website, not an actual working sextant.

If you happen to live here in Seattle, you may have the added advantage of having the sun further filtered by dense layers of nimbostratus. Because in Seattle, that's how we roll.

Will be posting more about the transit soon, meanwhile there is much information here (click on image then right click and select "view image" to enlarge):

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