But I’m getting ahead of myself—let me make my case for Venus properly.
The ancient Babylonians called her Ishtar, for the Greeks she was Aphrodite, and for the Romans Venus—goddesses of love, fertility, or beauty. Venus has fascinated humankind for millennia. She is the brightest star in the night sky and even visible during a clear day. Some saw her as the harbinger of morning and evening, of seasons or portentous times. She reigns as the ‘Morning Star’ or the ‘Bringer of Light’ for 260 days, and then she disappears to rise again as the ‘Evening Star’ and the ‘Bringer of Dawn’—which is the reason why many cultures thought she was not one but two planets. Not so for the Babylonians, who recorded her movements for a period of 21 years on the Venus Tablet of Ammisaduqa around the mid-seventeenth century B.C.E. The Mayans also regarded Venus as important and she was part of their religious calendar.
But enough of the ancients. Venus continued to be significant for later generations too. She played a role in the acceptance of Copernicus’s heliocentric model of the solar system. When Galileo saw Venus goes through phases like the Moon, he noticed that sunlight could only have interacted with it in this way if the planets moved around the sun.
In the 18th century, Venus, the ‘bright queen of the sky’ held the key for the size of the universe. No other planet can claim such an accolade. In the midst of the Seven Years’ War, Venus initiated a scientific race across the world—a race that united hundreds of astronomers from dozens of countries. As the war tore Europe and its colonial possessions apart, scientists ignored political, national, and religious borders in their search for knowledge. They traveled across the globe to far away places to observe the transit of Venus, one of the rarest astronomical events.
During the transit, Venus moves between Earth and sun and is visible for about six hours as a perfect black dot moving slowly across the burning face of the sun. The planet, which is almost as big as our own, is dwarfed by the immensity of the sun. The transits occur in pairs—eight years apart—but then it takes more than a century for Venus to do her show again. (The last transit was on June 6, 2012 and if you missed it, you really have missed your chance, because the next one will only be in December 2117).
For astronomers in the eighteenth century the transits of 1761 and 1769, Venus was the celestial yardstick with which to measure the size of the solar system. If they could only measure the exact time and duration of the transit, they would get the data they needed to calculate the distance between Earth and sun, and by extension the dimensions of the solar system—the holy grail of astronomy.
The only trouble was that several people at different locations had to measure the rare heavenly rendezvous. If they viewed the transit from different places as far apart as possible, they would observe Venus traversing the sun along a slightly different track. With the help of relatively simple trigonometry, these different tracks (and the differences in the duration of Venus’s march across the sun) could then be used to calculate the distance between the sun and Earth.