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 November 12, 1999: Solar maximum
is just around the corner, which means that the Sun is peppered
with sunspots. Just today there were 5 groups and at least 50
individual spots visible on the surface of our star. Sunspots
-- cool areas created by twisted magnetic field lines poking
through the sun's surface -- move rather slowly. They're usually
visible for about two weeks as they move from east to west with
the solar rotation. On Monday, November 15, observers in the Pacific hemisphere can catch a glimpse of a different kind of sunspot -- a black dot that zips across the sun in little more than an hour. It's not really a sunspot; it's Mercury, the nearest planet to the sun! Right: This white light image of the sun shows several sunspot groups on November 12, 1999. On Monday, November 15, another tiny dark spot will appear briefly near the Sun's northeastern limb when the planet Mercury crosses in front of the Sun.
On 1999 November 15, Mercury will cross the visible disk of the Sun for the first time since 1993. At approximately 2115 UT (4:15 p.m. EST) the black disk of the planet will appear at the Sun's northern limb, about a third of the way around from North to East. These cardinal directions are easy to figure by simply nudging an equatorial mounted telescope back and forth on both axes. The black disk of the planet will be small -- 9.9 arcseconds across -- and blaker than any normal sunspot. Monday's transit is an especially unique event called a grazing transit. Mercury's motion across the Sun will follow a short path near the Sun's northeastern limb. From some parts of the Earth observers will see only part of Mercury's black disk superimposed on the Sun. From other observing sites Mercury's entire disk will be visible just inside the sun's northern limb. The next grazing transit like this one won't happen until the year 2314! The partial phase of the transit will be visible from most of Australia, New Zealand, and Antarctica. The total transit will be visible from Papua-New Guinea, northeastern Australia, Hawaii, western South America and most of North America, where the event occurs shortly before sunset. Although we say that the transit will be "visible," you should never look directly at the sun. Instead, use eyepiece projection or a telescope with suitable solar filters.
The first hint of Mercury's ingress onto the solar disk is called first contact (or Contact I). Second contact (Contact II) occurs at the instant that the entire disk of Mercury is visible against the Sun. Third contact (Contact III) takes place just as Mercury's disk touches the solar limb on its way to exit the solar disk. Finally, fourth contact (Contact IV) is when the last trace of the planet disappears. Accurate timings of these contacts can have scientific value. Edmund Halley first realized that transits could be used to measure the Sun's distance, thereby establishing the absolute scale of the solar system from Kepler's third law. Unfortunately, his method is somewhat impractical since contact timings of the required accuracy are difficult to make. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance. 
Nowadays the distances to the Sun and to Mercury are known very
precisely. Planetary transit timings have very little to offer
for improving these values. However, there is another mystery
that such timing measurements can address. There is growing evidence
that the sun's radius varies by a fraction of an arcsecond during
the solar cycle. Accurate timings, especially from a grazing
transit like the one on November 15 can be used to calculate
the radius of the Sun. Since Mercury is only 1/194 the Sun's apparent diameter, a telescope with a magnification of 50x to 100x is recommended to watch this event. Naturally, the telescope must be equipped with proper filters to ensure safe solar viewing. NEVER look directly at the Sun! For information on solar filters and safe observing methods click here. The visual and photographic requirements for observing a transit are identical to those for sunspots. The most valuable scientific contribution the amateur can make is to time the four contacts at ingress and egress. Observing techniques and equipment are similar to those used for lunar occultations. Observing tips may be found at the Association of Lunar and Planetary Observers website (click here). A telescope equipped with a solar filter and a CCD video camera would be ideal for timing observations. To make your observations useful to scientists, you'll need to record an accurate time signal on the audio track of the video recorder. The easiest way to do this is to tune a shortwave receiver to WWV, which transmits time signals at 2.5, 5, 10, 15, and 20 MHz, and position the receiver within listening distance of your camcorder's microphone. To hear what a WWV signal sounds like, you can phone (303) 499-7111. Another good source of time information is the Canadian radio station CHU, which broadcasts at 3.330 MHz and 7.335 MHz.
If you feel that you have made accurate and/or interesting observations, you may contact Dr. Tony Phillips, who can help by forwarding them to the appropriate scientists. Pictures or video of the transit are also welcomed for use in a future Science@NASA headline. The author acknowledges Fred Espenak's excellent web site on the Mercury transit for much of the information that appears in this article. |
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1999 Transit of Mercury -- by Fred Espenak of the NASA/Goddard Space Flight Center This week's sky at a glance -- from Sky & Telescope Jack Star Gazer -- Jack Horkheimer's naked eye astronomy web site Mercury - from the SEDS Nine Planets web site The Sun - from the SEDS Nine Planets web site |
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