Astronomers were scattered across the globe 150 years ago today to watch a black dot move across the face of the Sun – a transit of the planet Venus. They’d use their observations to measure the distance to the Sun, revealing the scale of the entire solar system.

Astronomers already knew that distance to within a few million miles. But they wanted a more accurate reading. They’d get that by watching the transit from many locations. Venus would cross the Sun at a slightly different angle from each location. Measuring the difference would reveal the distance to both Venus and the Sun.

So the nations of the world mounted dozens of expeditions. They spread out from Cairo to Honolulu, from Cape Town to Tasmania. Each trip required years of planning, plus months of travel. The scientists carried tons of equipment, including prefab observatories and housing. They spent weeks setting up. And they had to calculate the precise longitude of each site, which was a key factor in measuring the angles to Venus and the Sun.

Clouds blocked the view from many of the sites. Even so, the astronomers still managed to snap thousands of pictures of the event, and take many more measurements visually. Then came years of tedious calculations. And the results were disappointing. The transit didn’t improve the known distance to the Sun at all. So astronomers fanned out once again for the next transit, eight years later.

Script by Damond Benningfield