Nearly 130 years ago today, the premier event in astrophysics involved watching a tiny dot slowly sail across the surface of the sun. That dot was our sister planet, Venus, and observing its transit as it passed directly between the Earth and sun was a momentous scientific undertaking.
A transit of Venus is one of the rarest astronomical phenomena, occurring every 243 years. Because of the positions of Earth and Venus around the sun, the transit usually happens twice in close succession, such as the pair in 1761 and 1769 or 1874 and 1882.
The above image comes from 1882, when astronomers were very keen to make accurate measurements of the occurrence. That’s because, until this time, they did not know the exact size of the solar system. Since the 1600s, scientists had been able to calculate the relative positions of the planets but no one knew the specific distance between them.