Star Points for November, 1999; by Curtis Roelle Black Drop of Mercury Visible Against Sun. Two rare celestial events have astronomers looking forward to mid- November. The most anticipated is a possible Leonid meteor "storm" somewhere in the world. The only time a Leonid storm has been observed this century was in 1966. The other more subtle event is a transit of the planet Mercury across the face of the Sun. Since it occurs first we will begin with a discussion of the transit. Mercury, the closest major planet to the Sun, completes one solar orbit in 88 days compared to 365 days for Earth. If Mercury and Earth orbited the Sun in the same plane we could see Mercury "transit", or pass in front of the Sun, once during each orbit or nearly four times a year. But Mercury's orbital plane is inclined seven degrees to Earth's orbit meaning that transits of Mercury are relatively rare events occurring on average 13 times per century. The last transit of Mercury visible in the continental U.S. was on November 10, 1973. From my location back then it had started while the Sun was still below the horizon and was to end just minutes after sunrise. Together with a friend we frantically searched for any hill with a good vantage point. Anyone who has traveled through the great plains region of the country will appreciate how difficult finding a hill there can be. By the time the telescope was ready the Sun was up and the transit was over! The next transit of Mercury, on Monday afternoon November 15, has shades of the 1973 event. In 1973 the Sun rose while the tiny black dot of Mercury was projected against its face like a beauty mark on Cindy Crawford and on the 15th will set in the same way. Mercury will appear to barely prick the edge of the Sun. In fact the width of the gap that separates Mercury's tiny black disc and the outer edge (or "limb") of the Sun will be less than the apparent diameter of Mercury itself. In a way a transit of the Sun is similar to a partial solar eclipse. During the transit Mercury passes in front of the Sun and the Moon passes in front of the Sun during a solar eclipse. However, the apparent size of Mercury is much smaller than the Moon because of Mercury's greater distance. Mercury's apparent diameter will only be about 1% of the apparent diameter of the Sun. Thus the same precautions used to observe a partial solar eclipse must be employed for making transit observations. One must never look directly through a telescope at the Sun without using a safe and approved solar filter. Visual safety and selection of safe filters for observing the Sun was discussed in the August Star Points column. Eyepiece projection from a telescope or one half of a binocular (with the other half capped) onto a white card or screen is another good method for generating a magnified image of the Sun. Unfortunately, because of Mercury's small size the use of a pinhole projector (see February 1998 Star Points) will probably not render an image large enough for Mercury to be observed. Here is a nutshell timetable for the transit of Mercury on November 15 as given in Guy Ottwell's "Astronomical Calendar 1999". Mercury's black disk first nicks the edge of the Sun during "1st contact" at 6:14 p.m. EST. In Westminster the Sun will be just about six degrees above the horizon. The complete drop-like disc of Mercury will be visible against the Sun starting at "2nd contact" at 6:29 p.m. By then the Sun's altitude will have decreased to three degrees. When the edges of Mercury's dark disk and Sun meet again at "3rd contact" at 06:52 p.m. the Sun has already set. However the Sun will remain visible for an additional minute due to refraction caused by Earth's atmosphere. In the space remaining let us now turn to the 1999 Leonid meteor shower. Astronomers are expecting a good probability of a "meteor storm" with thousands of meteors visible per hour somewhere in the world. The question is, "where?" Opinions vary but several expeditions are being planned. One such expedition by a private Washington astronomy research organization is planned for the island of Cyprus in the Mediterranean. Westminster Astronomical Society Vice President Brian Eney will be travelling to the Canary Islands in hopes of seeing the once in a lifetime meteor storm. If the storm doesn't occur as predicted Brian won't be disappointed since the prospects of observing in a dark moonless sky from a remote island is a grand consolation for sky lovers such as him. In central Maryland the best time to observe the Leonid meteor shower should be between midnight and dawn on the morning of Thursday, November 18 (late Wednesday night). An observing site away from artificial outdoor lights or "skyglow" is best so that the faint as well as bright meteors can be seen. It's important to dress warmly, and find a comfortable place to relax. A lawn chair with a pillow and sleeping bag or blanket works well. Don't observe near buildings or trees which may block your view of the sky. Meteor storms are notoriously hard to predict. Last year predictions for the shower peak were off by 18 hours so that the true peak occurred over some completely different terrestrial region. Thus, even though a meteor storm is not predicted for Maryland you should observe anyway because the experts could always be wrong again. This month marks the start of Star Points' fourth year. A web archive containing the previous installments of Star Points referenced above is now available at "http://www.erols.com/roelle/starpoints/".