Or, So Passes Venus Rarely... On Tuesday, June 8, 2004, the first "transit of Venus" since Dec. 6, 1882, will occur.

What is a transit of Venus? Venus orbits the sun within Earth's orbit, so it rarely happens that, seen from Earth, the disk of Venus passes across the sun, appearing as a diminutive black spot, barely 1/30th the diameter of the sun. With the right atmospheric conditions to soften the intense sunlight, an unobstructed horizon and enough warning, a keen eye should be able to spot the transit of Venus at sunrise or sunset on June 8. (Never observe the sun directly without proper eye protection.)

Since the transit of Venus was first predicted and sighted by the English astronomer Jeremiah Horrocks in 1639, its subsequent appearances in 1761, 1769, 1874 and 1882 were met with increasing scientific and public fanfare and curiosity. In the past, scientists would measure how long it took for Venus to cross the face of the sun and could then calculate its distance from Earth as well as estimate the scale of the solar system. Transits of Venus occur in pairs, with more than one hundred years separating each pair. After the June 8th event, the next transit of Venus will occur on June 6, 2012.

In 1896 composer John Philip Sousa published his "Transit of Venus March." Sousa (1854-1932) was very interested in the 1882 event, as were millions of people of his day. However, he didn't write the "Transit of Venus March" to commemorate the transit itself, but to honor the great American physicist Joseph Henry, who had died on May 13, 1878. Henry, a rather obscure figure today, was quite famous in his own time. He had assisted Samuel Morse in the development of the telegraph, discovered several important principles of electricity, made significant progress in the development of electromagnets and created the first electric motor. As a result of his scientific achievements, Henry was named as the first secretary of the Smithsonian Institution. In 1883, five years after Henry's death, and the year following a transit of Venus, the Smithsonian Institution asked Sousa to compose a processional for the unveiling of a bronze statue of Henry in front of the building. The music was to be played while assembled dignitaries walked from the museum to a special receiving stand in front of the Smithsonian.