Observers in Africa, Europe and Asia have the best seats and will see the whole event, from the first contact at 5:13 UT, to Mercury’s closest approach to the Sun’s center (7:52 UT), to egress at 10:27 UT (just touching the solar limb) and five minutes later “contact IV,” when Mercury leaves the limb. These times are “geocentric,” for an imaginary observer at the center of the Earth, but are accurate to several minutes anywhere on this planet. Most of the US, including much of the East Coast, misses out entirely because the transit ends before the Sun rises. Circumstances are hardly better on Delmarva. Sunrise at my house in Milton, Delaware is 5:57 local daylight time and the egress begins a half an hour later when the Sun is less than 6 º up.

Despite the low sun-angle, solar-safe observing practices are a must for this event. A low, unobstructed horizon is required, and haze and turbulence will surely make viewing at 50 - 100x unsteady at best. Egress timings can be used to pinpoint Mercury's celestial position. But since transits no longer hold much scientific value, this is essentially a pure test of observing skill. Sighting Mercury is always a challenge and knowing exactly where to look—the Sun makes a great target—is imperative.

This May’s transit is the first of 14 such events this century, occurring only in May or November when Mercury is near an orbital node where the triple alignment is sufficiently precise. May transits usually last longer than November’s because this (ascending) node is near aphelion (the farthest point from the Sun), and thus Mercury is moving most slowly along its orbit. There, Mercury comes a bit closer to us and appears somewhat larger. Unfortunately Mercury is only about 1/200 of the Sun’s apparent diameter. On May 7th, Mercury will be just 12” (arc seconds) across. 

The next transit of Mercury is in 2006 on November 8th and favors Pacific Rim observers. In the meantime, we can anticipate a far rarer event, a transit of Venus, about a year from now. Since the last occurred in 1882, no one alive today has witnessed such a transit. By coincidence, the circumstances (locations of visibility and duration) are almost identical to this May’s Mercury event. So mark you calendars for June 8th, 2004 and think of this month’s transit of Mercury as just a warm-up event.

Here are some valuable resources for May’s transit. The most useful is: “2003 Transit of Mercury” web page by Fred Espenak (NASA/GSFC) at http://sunearth.gsfc.nasa.gov/eclipse/OH/transit03.html, and there is much more is at his “Eclipse Home Page." In hardcopy: “Mercury transits the Sun” by Roger Sinnott, Sky & Telescope, May 2003; “Transit: When planets cross the Sun” by Michael Maunder and Patrick Moore (2000). Springer; and “Transits” by Jean Meeus (1989). Willmann-Bell.

Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 April 2003. Text and images copyright © 2003 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.