Illustration of Transit of Venus

1639 Transit of Venus

The book of science

Tom Sharp

Jeremiah Horrocks astronomy Illustration of Transit of Venus

Transit of Venus

Jeremiah Horrocks proposed timing the transit of Venus across the face of the sun from different places on Earth to calculate our distance to the sun geometrically. Horrocks measured the transit of Venus in 1639 and roughly estimated the distance to the sun, coming closer to the actual distance than anyone previously had. The next opportunities for making these measurements were long after Horrocks had died. Scientists coordinated their efforts and organized expeditions across the globe over the next two hundred years.

Black drop effect

Optical effects bedevilled measurements. Turbulence of Earth’s atmosphere. Imperfections in the instruments. Darkening of the sun at its rim. The disk of Venus, sticking to the limb of the sun like a teardrop, made timing its entrance difficult.


Venus and Earth orbit the sun with different periods and at different inclinations. All these terms seem metaphorical; however, people are not planets orbiting alone in space. Transits of the sun are separated by hundreds of years. Alignments of people are separated by fears and stupidity, by habits and ignorance, and their irregular orbits rarely conjoin.

In 1769, the young Joseph Banks accompanied Captain James Cook on his voyage to Tahiti to measure the transit of Venus. Measurements in 1761 and 1769 improved Horrock’s number but, because of optical problems, were not definitive.

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