dipper@utastro.UUCP (Debbie Byrd) (11/12/84)
Sometimes an inner planet appears to move across the face of the sun. More on transits -- and the safest way to see them -- after this. November 12 A Transit of Mercury On today's date two years from now the planet Mercury will transit the sun. A transit occurs when a planet inward from Earth appears to move across the face of the sun. We see transits only of Mercury and Venus since those are the planets orbiting inside the orbit of Earth. Transits don't happen every time a planet is between the Earth and sun. Usually the planet passes above or below the sun from our point of view. The last transit of the planet Mercury was eleven years ago -- and Venus isn't due for one until the year 2004. The long times between transits are because the planets orbit the sun only roughly in the same plane. As a planet moves around the sun, its orbit will sometimes intersect the plane of Earth's orbit. If that intersection occurs when a planet is between the sun and Earth -- then from our vantagepoint that planet appears as a small black dot moving across the sun's face. Mercury is the planet nearest the sun. It also has a high degree of inclination -- seven degrees to the plane of Earth's orbit. So transits of Mercury are rare. On the other hand, transits of Venus are even more rare. They occur in pairs about every 120 years. Viewing a transit requires special precautions. For Mercury, you need a telescope -- and some means of protecting your eyes from the blinding light of the sun. But you'll have plenty of time to prepare -- for a transit of Mercury on this date in 1986. Script by Diana Hadley. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin