An Astronomical Ruler

Astronomers used the transits of the 18th and 19th centuries to develop a basic ruler for measuring distances throughout the solar system: the astronomical unit.

The first recorded Venus transit took place in 1639. British astronomer Jeremiah Horrocks predicted it, but he didn’t complete his calculations in time to alert other astronomers, so Horrocks and a friend were the only ones to see it.

Jeremiah Horrocks, who made the first confirmed transit observation in 1639, at his telescope.Jeremiah Horrocks, who made the first confirmed transit observation in 1639, at his telescope.A few decades later, Edmond Halley, for whom Halley’s Comet is named, realized that astronomers could use future transits to calculate solar system distances.

The idea was to place teams of astronomers at widely spaced locations, where they would note the starting and ending times of the transit and measure Venus’ track across the Sun. By comparing the angle and the timing from different locations and applying some trigonometry, they expected to measure the distance to Venus. Astronomers already knew the relative distances to the Sun and planets. They knew, for example, that Venus was only about seven-tenths as far from the Sun as Earth is. But they didn’t have absolute distances; estimates of the Sun-Earth distance varied by tens of millions of miles. A precise distance to Venus would allow astronomers to calculate the distances to the Sun and the other planets.

As the next transits — in 1761 and 1769 — approached, scientists from the two great superpowers, Britain and France, worked together to arrange several expeditions. The two empires were at war, however, and combined with foul weather and other conditions, they were dangerous transits.

One of the French expeditions, for example, sailed for the Indian Ocean. It was chased by the British fleet around the Cape of Good Hope, and was blocked from its intended destination by the war. The expedition did see the transit, but from a ship, where it was impossible to record the time, making the observations useless.

The British vessel HMS Seahorse faced an equally dangerous journey. It left port in December 1760, bound for Sumatra, where it would observe the transit six months later. Seahorse was attacked by a French warship just hours after leaving home, however, so it limped back to port with 11 dead, more than 30 wounded, and two passengers who were ready to pack it in: Charles Mason, an assistant to Britain’s astronomer royal, and Jeremiah Dixon, a surveyor and amateur astronomer.

After the attack Mason and Dixon tried to abandon their expedition. But the Royal Society, which chartered and funded the trip, threatened both men with lawsuits and financial ruin if they didn’t complete their mission. So they quickly reconsidered, and headed back out to sea in early 1761.

Mason and Dixon never made it to Sumatra, though, because France had captured their intended destination. Instead, they watched the transit from Cape Town, South Africa. And despite the tribulations, their observations were superb. In fact, just a couple of years later, their work earned them a new assignment: surveying a dividing line between American colonies — the Mason-Dixon Line.

Transits in 1761 and 1769 provided some data, but observers encountered several problems. Venus appeared to cling to the edge of the Sun’s disk for a few moments, for example, making it impossible to obtain highly precise timings. Teams of astronomers spent years reducing the observations and making calculations, but came up with conflicting numbers.

The next pair of transits, in the 19th century, provided better results, allowing astronomers to come close to the correct value of about 93 million miles (150 million km) from Earth to the Sun. That distance forms one of the most important measuring sticks in modern astronomy, known as the Astronomical Unit. 

Transits by the Dozens

Artist’s concept of a planetary system discovered by Kepler, with three planets ‘transiting’ the star NASA/Tim PyleArtist’s concept of a planetary system discovered by Kepler, with three planets ‘transiting’ the star [NASA/Tim Pyle]No risky transit expeditions are on tap for this year. Instead, astronomers have found a new use for transits: as a way to discover planets orbiting other stars. A transit blocks a tiny bit of the star’s light, producing a slight dip in its brightness. By measuring several transits, astronomers can not only discover an exoplanet, they can determine its orbital period (the length of its “year”) and measure its mass and its distance from the star. They can even detect whether a planet has an atmosphere and get a rough reading of the atmosphere’s composition.

The transit technique has yielded dozens of new planet discoveries, and the Kepler space observatory, which is looking for planets around 150,000 stars, has a list of about 2,000 additional candidates that astronomers are trying to confirm.

So while transit observations are no longer life threatening, they still play a role in the effort to gain a better understanding of how the universe is laid out.

(Little) Ring of Fire

In addition to measuring the distances to the planets, Russian astronomer Mikhail Lomonosov planned to use the transit of June 6, 1761, to measure Venus’ diameter. As he watched the transit begin, though, Lomonosov made an even more important discovery: Venus has an atmosphere. As Venus neared the solar disk, Lomonosov noticed that the planet’s “edge” looked a bit fuzzy, and for a moment, it was partially encircled by a ring of bright light. The conclusion was obvious: Venus must have an atmosphere, which was “bending” sunlight around it.

More about historic transit expeditions

Mason-Dixon, 1761

Jean-Baptiste Le Genti, 1761 and 1769

Captain James Cook in Tahiti, 1769

Don Jose de Galvez, 1769

British Expedition to Hawaii, 1874

British, American Expeditions to South Africa, 1882


©2015 The University of Texas McDonald Observatory