Coincidences are, almost by definition, unusual. They're also fun! In astronomy there are several major types of coincidence that are striking enough to bear unique names. A conjunction occurs when a couple of moving objects (e.g., planets) come together in the sky, to a greater or lesser degree of closeness. Since there are quite a few moving objects this happens almost all the time. Rarer is an occultation, the passage of a nearby and apparently-large body (like the Moon) in front of a distant bright object (such as a star or planet). Lunar occultations take place many times per month, though the more dramatic ones that are easily seen by naked eye or through binoculars only happen a few times annually. An eclipse is an occultation involving both the Sun and the Moon.

More exotic is a transit, as a little body crosses the face of a more distant but bigger one. The satellites of Jupiter, for instance, can often be seen with a telescope as they creep across the disc of that planet. From Earth the only possible planetary transits with the Sun involve Mercury and Venus, since they're the only planets with smaller orbits. Mercurial transits happen every several years, but Venereal (sorry, that's the proper adjective) events are much rarer. A pair of them takes place less than once per century. We're lucky — one is due on 8 June 2004.

The existence of artificial satellites makes a new kind of transit feasible: the passage of a tiny man-made artifact in front of the Moon. Most of those won't be visible except with large telescopes.

But there's an exception. Certain communications satellites have large flat antennas which, at the right moments, can reflect sunlight down with brilliant intensity. These so-called IridiumFlares cause a glint brighter than any star or planet. But the spectacle only lasts for a few seconds.

Now what if an Iridium flare occurred just as the satellite in question were passing in front of the Moon? That would be true drama! I haven't been able to learn whether any such Iridium transits have been observed, and they should be extremely rare — but there's no reason for them not to exist. My rough guess is that, within 50 miles or so of any given location, there might be an Iridium Lunar transit visible once every few years. I want to see one!

(see also CoincidentalTaxonomy (19 Oct 2001), IridiumFlares (30 Dec 2001), CoincidentalTaxonomy2 (14 May 2002), SarosCycle (6 Nov 2003), ... )

TopicScience - 2004-03-01

About the Venereal transit; the NASA page describing the event, and when to look to see it, is at http://sunearth.gsfc.nasa.gov/eclipse/OH/transit04.html - RadRob

(correlates: IncomprehensiblePhenomena, LookingBack, SandBox, ...)