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Saturday, November 4, 2006

Astronomy: mercredi le 8 novembre

In Latin, the seven days of the week were named after the seven principal celestial bodies, or “luminaries”: the five planets known in antiquity (Mercury, Venus, Mars, Jupiter, and Saturn), plus the Sun and Moon. Many other languages around the world name days of the week in the same manner, while in other languages the names of weekdays are derived from their numerical order in the week, usually starting on Sunday or Monday.

Modern Romance languages such as French, Italian, and Spanish retain the Latin-derived names for all of the days except Sunday, renamed the “Lord's day”. In French, for example, Wednesday is “mercredi”—Mercury's day.

Next Wednesday, mercredi le 8 novembre, will indeed be Mercury's day, for observers in the Western Hemisphere, Pacific, East Asia, Australia, and New Zealand will be treated to the spectacle of a transit of Mercury across the disc of the Sun, the second in the pair of transits which began with the May 2003 transit, and the last such event until May 9th, 2016.

I posted a heads up here on October 17th, encouraging folks in areas from which the transit will be visible to outfit their telescopes with a safe solar filter, as that is by far the best way to observe the event. (Unlike a transit of Venus, which is visible to the unaided eye with “eclipse specs” or a similar filter, Mercury in a November transit subtends only 10 arc-seconds and requires optical assistance: a magnification between 50 and 100 diameters is best.)

If you have suitable telescope but lack a solar filter for it, it's probably too late by now to obtain one before the transit, but you can still glimpse the event by using eyepiece projection onto a piece of white cardboard held some distance from the eyepiece; hold the cardboard perpendicular to the light emerging from the eyepiece and move it back and forth until the Sun's disc is in focus. The focal length of the eyepiece will determine the size of the image produced by this method. If your telescope has a finder scope, be sure to leave its objective lens covered (or cover it with aluminium foil if you don't have a lens cap for it), to avoid accidentally burning yourself with the intense light emerging from it. You can aim the telescope at the Sun without need of a finder simply by moving it until the shadow of the telescope tube on the ground is as small as possible. I've never had a problem using eyepiece projection to view sunspots before I had a full aperture solar filter, but some people warn that the intense sunlight passing through the eyepiece may heat it sufficiently to cause damage; it's wise to use an inexpensive eyepiece and not keep the telescope aimed at the Sun for too long. Also, never leave the telescope unattended, even for a moment, as some kid might look directly through it and be instantly blinded.

If you don't have a telescope, or aren't in a location from which the transit will be visible, you can still observe the event as it occurs via the Webcasts planned from Kitt Peak, Arizona and several sites in Hawaii.

Clear skies!

Posted at November 4, 2006 15:37