FOR some 300 million people across North America, next Tuesday (May 10) is a special day. It's the only opportunity until Aug. 21, 2017 when most of the continent will have a grandstand view of a nearly complete solar eclipse.
Moreover, it's not the ordinary type of eclipse where the moon's disc covers the sun, and the solar corona (atmosphere) shines forth. It's an annular eclipse where the moon covers 88 percent of the sun, leaving a bright ring, or annulus, surrounding the eclipsed center. You won't see the corona with 12 percent of the sun still shining.
That makes it uninteresting for solar astronomers. But as geophysicist Forest M. Mims III of Sun Photometry in Seguin, Texas, notes, next week's annular eclipse ``is a very important event for the atmospheric people.'' They can use that 12 percent sunshine to help monitor what happens in the atmosphere as solar heating is diminished and then brought back to full strength. Observers can monitor changes in the absorption of selected wavelengths of sunlight to track changes in the ozone, oxygen, or water-vapor content of the atmosphere above their observing sites.
Mr. Mims has organized Project Halo for this purpose. He says that 21 observing teams now are preparing to work at a network of sites located along the path of eclipse totality, extending from Baja California, Mexico, to the northeastern United States. Seven universities are involved.
Weather permitting, many people can share in this experience at least to the extent of seeing the eclipse themselves. The sun will rise partly eclipsed as seen from Hawaii and will be high in the sky over North America during the full eclipse phases. It will set partly eclipsed as seen from western parts of Europe. For parts of Morocco, it will set during annularity to display a spectacular ring of fire on the western horizon, according to Sky & Telescope magazine.
THE map above shows the times the eclipse can be seen and the extent of the eclipse - partial or full - for North American sites. Times are given in universal time (UT). Subtract four hours from UT to get Eastern Daylight Time, five for Central Daylight Time, six for Mountain Daylight Time, and seven for Pacific Daylight Time.
Although it is rare to see a total eclipse from any given site, eclipses are a routine phenomenon of the Earth-moon system. By coincidence, the sun is some 400 times larger than the moon and 400 times more distant than our natural satellite. Thus, the lunar and solar discs are about equal size as seen from Earth.
Since the Earth-moon distance varies as the moon goes around its orbit, the lunar disc - as seen from Earth - does not always completely cover the sun. Next Tuesday, the moon will be near its apogee - the farthest point from Earth. Hence the annular eclipse.
With 12 percent of the sun still showing, it would be dangerous to view the eclipse directly even during totality. Experts suggest using filters designed for eclipse viewing or a piece of welders' glass of shade number 14. Smoked glass, photographic filters, sunglasses, exposed photographic film, and other such materials do not provide adequate protection.
Punching a pinhole in cardboard and using that to focus the sun's image on a piece of white paper is also a safe way to view the event. In fact, some people may see ``pinhole'' images of the eclipsed sun sparkling on the ground as light filters through leaves. Shadow edges should also be sharper than usual during totality, since the eclipsed sun is a smaller light source.
Meanwhile, Project Halo observers will be measuring atmospheric pressure and humidity and natural background radiation, as well as tracking changes in the high atmosphere. Among other things, they will be looking for pressure waves and waves in the stratospheric-ozone layer that some previous observers have reported.