The planet Mercury is now finishing up a fine apparition in which it is has been readily visible in the western evening sky for the past few weeks. It will soon disappear into the bright evening twilight, but the rocky innermost planet is scheduled to make a most unusual, albeit brief encore of sorts in early May.

Astronomers call the event a transit.

Mercury and Venus are the only major planets we can ever see crossing the face of the Sun, because the two planets both orbit inside Earth's yearly path. Transits of Venus happen less than twice a century (the next one will be June 8, 2004). Mercury transists are some 10 times more frequent. Yet only 14 transits of Mercury occur during the 21^st Century -- or about seven years apart on average. The May 7 event will be the first of these.

The entire transit will be an east-to-west passage taking just under 5 hours and 19 minutes. The Sun will be above the horizon for the entire transit in Europe, Africa, and Asia. The Sun sets with Mercury still on its disk for Japan, Australia and New Zealand. The event is already in progress when the Sun rises across much of eastern Canada and the northeastern United States.

Anyone who lives to the east of a line running roughly from Marquette, Michigan to Charleston, South Carolina should have good seats. Those to the west of this line are left out.

The end of the transit may also be observed from the Lesser Antilles Islands of the Caribbean, as well as the eastern half of South America.

The transit will begin within a couple of minutes of 5:13 GMT (Greenwich Mean Time) at every site from which it is visible. It will end within a couple of minutes of 10:32 GMT.

For the Middle-Atlantic and Northeastern United States, as well as for much of eastern Canada, the Sun rises with Mercury already on its disk. As soon as they have come above the east-northeast horizon, the planet should be recognizable near to the upper right limb as a tiny, black, sharp-edged dot, having only 1/158 the Suns diameter.

The gradual travel of this dot will bring it to the Suns upper right edge. This will be an especially interesting time. The moment when the two disks become internally tangent is known as Contact III. For most U.S. and Canadian locations where the transit will be visible, Contact III should occur within several seconds of 6:29:40 a.m. EDT (add one hour for Atlantic and 1 hours for Newfoundland Time).

Egress then lasts about 4 minutes until the last notch in the Suns limb disappears, marking Contact IV, which should occur within several seconds of 6:34:10 a.m. EDT.

With a large, filtered telescope at high power (see the "Warning" below regarding safe viewing), a viewer can time when Mercurys edges cross the limb of the Sun. Alert folks might briefly catch sight of the so-called "black drop" illusion around the moment of Contact III, where Mercurys disk appears stretched like taffy (an effect of Earth's atmosphere called "seeing") toward the Suns limb. Another way to describe it is that the planet will seem attached to the Suns limb by a thin black column or thread.

Do not look directly at the Sun. It will damage your eyes. Transits of Mercury are not visible with the unaided eye anyway. A telescope must be used, magnifying at least 50 to 100-power to bring out the "dark dot" of Mercury in silhouette against the Suns disk. This, too, must be done in specific ways to avoid eye injury.

To see Mercury as an actual disk will be a challenge, because its angular diameter is small -- in astronomers' terms only 12 seconds of arc at this inferior conjunction. As Mercury moves across the face of the Sun, it will appear absolutely jet black in contrast to the lighter gray of any sunspots that may also be present on the solar disk.

Observing a transit is somewhere between studying sunspots and watching a solar eclipse. You have to be ready at a particular time, and you may have to travel far from home. Your exact location, however, is much less critical than it is at a total solar eclipse. For those in North America where this will be an early morning event, your observing site should have a low horizon to the east-northeast. It is a good precaution to check the Suns rising point a day or two beforehand, to verify that trees or buildings do not block your view. Many newspapers carry local sunrise times.

Projecting the Suns magnified image through a telescope and on to a white card, wall or screen is relatively safe and can be used for group viewing, but remember that Mercurys disk will be quite tiny. To do this, set up a white background a few inches or perhaps a foot or two behind the eyepiece. Point the telescope at the Sun and focus the image until the Sun's edge is sharp. [See diagram near top right of this page.]

For serious transit observing, a telescope with a full-aperture solar filter is much better. Such filters are attached on the side of the telescope that is facing the Sun, not the side facing your eye. The filter blocks most of the sunlight from entering your telescope.

The transit should be watched only with an appropriate solar filter a solar filter that is sold by a reputable outlet of astronomical equipment. If your telescope comes with a filter that screws into the eyepiece, discard it immediately! Such filters have been known to crack under the intense heat of the Suns magnified image.

Lastly, never look at the Sun directly through your telescope, or even through a smaller finder scope. In fact, you should cover the finder before setting up to watch the event, so as to avoid looking through it accidentally.

Should poor weather hinder or completely obscure your view of Mercurys transit, you can console yourself with the fact that the next one is only about 3 years from now, scheduled to occur on Nov. 8, 2006. That one will be visible across much of the Western Hemisphere, west to Australia, Japan and easternmost sections of Asia.s.