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Article What is the Transit of Venus?

Image of the planet Venus
Image of the planet Venus, courtesy of NASA

There have been fifty-two transits of Venus across the face of the Sun since 2000 B.C., but until 1643 A.D., no human was known to have observed this astronomical rarity.

Venus orbits the Sun within Earth's orbit, so it occasionally happens that as seen from Earth, the disk of Venus passes across the Sun. It appears 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 advance warning, a keen eye can spot the transit at sunrise or sunset. To avoid blindness, never observe the Sun 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.

Astronomers quickly discovered that by carefully measuring the transit, the distance from the Sun to Earth could be calculated. In 1761, the exact value of this number was still unknown; estimates ranged from 5 million to over 150 million miles. Without its precise value, astronomers could not deduce the physical size of our solar system, or the dimensions of the universe beyond the solar system's outer reaches. The size, mass, and radiant power of our Sun were also left ill-defined. The painstaking measurements of the transit of Venus by hundreds of international expeditions and observers soon refined this astronomical unit (an "astronomical unit" is the scientific term for a unit of measure equal to the average distance from Earth to the Sun) to 95 million miles by 1769, and then to 92.79 million miles by 1891. During the twentieth century, the same radar technology that astronomers use to map the face of Mercury, or study the rings of Saturn, has yielded a precise value for the distance between the Sun and Earth of 92.9558203 million miles, with a margin of error of less than a few miles. But, regardless of this technological accomplishment, the elegant simplicity of the transit of Venus measurements made the biggest practical difference in the progression toward greater accuracy in measurement of immense distance in space. These transit observations were important to astronomers in other ways as well.

Image of a drawing of the Transit of Venus Prior to the 1761 transit, no one had supposed that planets might have atmospheres. They were thought to be merely drab balls of colored rock and craters. One of the first things that Russian astronomer Mikhail Lomonosov (1711-1765) noticed about the 1761 transit was that the black disk of Venus appeared as though surrounded by a halo of light. He recognized that only a body surrounded by a diffusing atmosphere could cause such a display. Today, astronomers search for planets orbiting distant stars by measuring the diminution of light from stars as planets pass between them and our line-of-sight. In the case of the planet orbiting the star HD 209458, instruments here on Earth can use the planet's transit to discover the chemical composition of its atmosphere.

Caption for the image: Afbeelding van den weg der planeet Venus Nicholas Ypey, 1761. Rare Book and Special Collections Division, Library of Congress. A beautiful drawing of the transit of Venus of 1761, by Nicholas Ypey. Although the coronal detail on the sun is not actually observable, the path of the transit is accurately depicted.

And, what of the public interest in the transit of Venus? There was plenty!

Benjamin Franklin observed the 1769 transit, and wrote about it in a scientific journal. Largely through his careful measurements and scientific reputation, the international community came to recognize America as a fledgling scientific partner. Later, Congress supported and funded the study of the 1874 and 1882 transits, as the political importance of a vibrant, domestic scientific capacity became increasingly valued.

Although newspaper accounts of the 1761 and 1769 transits were sketchy at best, by 1874 and 1882, increasing attention was given to all the efforts of those involved in the observation and study of the transits: their labors, their measurements, and in a few instances, their dramatic failures. The 1882 transit served also as the inspiration for a poem by Oliver Wendal Holmes (1809-1894), "The Flaneur: Boston Common, During the Transit of Venus"; cover art for the much-read Harper's Weekly (April 28, 1883); and one of the earliest marches by the young military band conductor John Philip Sousa entitled "Transit of Venus March."

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