The dark shadow of Mercury is seen here as it transits the Sun's disk.   This transit was the 14th and last of the 20th century; the next will occur on May 7, 2003.  Transits give us an opportunity to directly compare the relative sizes of the Sun and the planets.  The Sun's actual diameter is 285 times that of Mercury.   For this transit, the Sun’s apparent diameter was 1,940 arcseconds, compared to 9.9 arcseconds for Mercury.  These sizes differ by a factor of "only" 196, as Mercury was closer to the Earth than the Sun was when this image was taken.

In the 18^th century, observing transits of Mercury and Venus was the key to determining the scale of the solar system.  The observations of Tycho and Galileo and the theoretical work of Copernicus, Kepler, and Newton had led to an accurate determination of the relative sizes of the orbits of the planets, in terms of astronomical units (A.U., the average distance from the Earth to the Sun).  Still unknown at the time, however, was the actual distance of the A.U. in miles or kilometers.   Transits of Mercury and Venus offered the opportunity to directly measure the distance from the Earth to the Sun.  By determining the exact time that a planet first appeared and then disappeared when silhouetted against the disk of the Sun from two or more widely separated locations on the Earth, the distances from Earth to the planet and to the Sun could be triangulated.

Instrument:  C-9.25, with 26mm Tele Vue Plossl
F-ratio:  f/78
Exposure:  1/8 second
Filter:  82mm Kendrick off-axis solar filter
Film:  Elite Chrome 200
Date:  November 15, 1999
Location:  Palm Desert, California