Medical experts are warning people in areas where the total solar eclipse is visible not to look directly at the sun, to avoid damaging their eyes.
Eclipses should only be viewed directly through proper optical filters, or through glasses that filter out the sun's ultraviolet light, which is invisible to the eye but can burn the retina -- the surface in the eye that converts visual images into nerve impulses so the human brain can "see".
Wednesday's eclipse is tracing a narrow path over nearly half the Earth's surface - from Brazil, across Africa and the eastern Mediterranean, across Turkey and Central Asia, to Mongolia.
Many areas where the eclipse will be visible are underdeveloped. This has prompted experts to issue warnings about makeshift eyewear and fake "eclipse viewers" that could result in retinal burns and permanent, partial blindness.
Reports from Ghana say sirens will blare a warning signal when the eclipse begins, so babies and unprotected children can be taken indoors. Educators in Jordan ordered schools to close for the same reason.
The eclipse, which can be seen when the moon's shadow passes precisely between Earth and the sun, can be seen in portions of 15 countries. From Brazil, where the celestial alignment will begin at 0836 UTC, the eclipse will travel across the Atlantic and curve across Africa, through parts of Ghana, Togo, Benin, Nigeria, Niger, Chad, Libya and Egypt.
The moon's shadow will slice across the eastern Mediterranean, touching a tiny Greek island, then moving across Turkey, Georgia, several parts of Russia and Kazakhstan before winding up in Mongolia at 1148 UTC.
A total solar eclipse occurs about once every 18 months, on average, but it has been 28 months since the last one on November 23, 2003, which was visible only from Antarctica. Astronomers say the next total eclipse, on August 1, 2008, can be seen from Greenland, Siberia, Mongolia and China.