Astronomy Picture of the Day 4-12-03

Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while before sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. By the middle of this month, Mercury will again be well placed for viewing above the western horizon at sunset, but by the end of April it will have faded and dropped into the twilight. On May 7th, Mercury will cross the Sun's disk.

Notice the last sentence in the article above. When Mercury (or Venus) crosses the Sun's disk it is called a transit.

Transits of Mercury occur in either early to mid-May at intervals of 13 or 33 years, or early to mid-November at intervals of 7, 13, or 33 years. The November transits are more common because Mercury is then near perihelion (a planet's closest approach to the Sun). This means it is moving more quickly and is more likely to cross the plane of the Ecliptic (Solar System orbital plane) at a critical time. This explains the May and November transits, too: Mercury's orbit is inclined 7 degrees with respect to the Ecliptic.

Here is a table of upcoming transits of Mercury. "Separation" is the distance in seconds between the Sun's nearest limb and Mercury's limb at maximum transit.

Transits of Mercury: 2001-2100

Date UTC Separation

2003 May 07 07:52         708"
2006 Nov 08 21:41         423"
2016 May 09 14:57       319"
2019 Nov 11 15:20           76"
2032 Nov 13 08:54         572"
2039 Nov 07 08:46        822"
2049 May 07 14:24       512"
2052 Nov 09 02:30         319"
2062 May 10 21:37     521"
2065 Nov 11 20:07          181"
2078 Nov 14 13:42          674"
2085 Nov 07 13:36         718"
2095 May 08 21:08         310"
2098 Nov 10 07:18           215"

Transits of Venus are much more rare; none occurred in the twentieth century! The wait is almost over, though; here are Venus' transits for the next 200 years:

2004 Jun 08----08:19------627"
2012 Jun 06----01:28------553"
2117 Dec 11----02:48------724"
2125 Dec 08----16:01------733"

The 2004 transit of Venus will be the first one since 1882.

CAVEAT! WARNING! DANGER! NEVER look at the Sun without proper precautions. I'm not talking squinting or wearing dark sunglasses here. That won't do. Even if the Sun is attenuated through layers of clouds, fog, or smog, its radiation can damage your vision very quickly. Your retina has no pain receptors; you won't know the damage is done until it's too late.

Even a GLANCE at the Sun through unfiltered binoculars or telescopes can INSTANTLY blind you. Remember using a pair of binoculars to focus sunlight to burn paper or hapless ants? (Well, I do.) Same thing will happen to you if you don't take the proper precautions.

The mylar eclipse glasses will work for the Venus transit (Mercury is too small to pick out against the Sun's disk with the naked eye). Same for welder's glasses (16 or darker). Make sure there are no pinpricks in the eclipse glasses (test at a bright lightbulb) or cracks in the welder's glasses.

Special solar filters are available for your telescope or binoculars. Make SURE you don't get the cheapie brands. Also DON'T use the type that fit over the eyepiece. Those won't protect the instrument from overheating - and possibly damaging the filter and your vision.

Perhaps the best way to observe the transits is to view them at a near-real-time site like the SOHO LASCO camera's site. Latest LASCO/EIT images

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