Captain James Cook, navigator, explorer, and discoverer, still but a lieutenant in the British Navy, witnessed and took measurements of the June 3, 1769 transit of Venus from the south-Pacific island of Tahiti.

Why sail half-way around the world through uncharted waters to land on an as of then unmapped island to do such a thing you ask? The reason was simple, and profound for his times – to determine how far away the Sun was from Earth. In an age of sail, such a fact would be of immense value in navigating the world’s oceans.

First, a thought experiment. Erase from memory all you know of distance. Stand on a flat plain, either day or night and look towards the heavens.

How far away is the Sun, the stars? What in your human experience would offer insight into the unimaginable distances – after all, 93,000,000 miles is the closest star to Earth – you look across. The ancient Greeks, for instance, thought the planet Jupiter (Zeus in Greek mythology) was just a little distance above Mt. Olympus.

Given the limited instrumentation of Cook's day, the exact timing of the transit of Venus was believed to be the only way to precisely fix the distance to the Sun. It offered a solution to a problem of immense consequence by using a methodology known to every schoolboy of that age – geometry.

The principle Cook was to employ dated to the ancient Greek geometers: parallax. Parallax gives a means of measuring distance.

A simple way to conduct a parallax experiment is to just fix on an object across the room from where you are standing. Cover one eye as you look at it. Then, cover that eye and open the other. A "shift" or angle occurs in perception. The two different points of view of the same object, from the same spot, create an optical triangle.

The Greeks proved that if you know the distance from one eye to the other, or the two different points from which you view the same fixed object, plus the angle created by the parallax, it should be possible to figure out the distance to the object viewed.

Of course, you would need one honking large triangle to get a fix on the Sun. The base of your triangle, and the one employed in Cook's time, was the diameter of the Earth. It was impossible to get one any longer. Take a measurement of the transit from two opposite points on the globe, compute the angle and voila! Length of the sides of the triangle would equal the distance from Earth to Sun.

A skilled mathematician and surveyor, the Yorkshire farm lad so impressed his naval superiors by the accuracy of his observations of a solar eclipse in 1766 that he was given command of the ship "Endeavor." His mission: "catch" the transit of Venus for the British Royal Society and King George III.

From his south-Pacific observatory, Cook, in collaboration with others measuring the transit of Venus from other locations on the globe, would become a point at the joining of two sides of a giant triangle. (see figure 2).

Unfortunately for Cook, and scientists of the era, Venus's cloudy cover did not let observers see, or fix, the exact time at which the planet entered and exited the Sun’s orb. Variations in timing the transit were too great to allow for precise computing of the distance to our closest star.

Cook’s expedition came up with a distance of approximately 123 million kilometers to 157 million kilometers. Not the kind of accuracy that NASA could use to send a spacecraft, say, to Mars, but nevertheless, much better than previous estimates. More precise measurements of Earth’s distance from the Sun (an Astronomical Unit or AU by astronomers) would have to wait for better instrumentation and different methodologies than available to Cook.

For an excellent website showing the voyage of Cook to Tahiti, check out - South Seas: voyaging and cross-cultural encounters in the Pacific (1760-1800)

For an extended mathematical discussion of parallax check out: Approximated method for the calculation of the parallax.

Next: Actual times of Venus’s transit of the Sun on June 8.
Previous: Here comes the Sun - and Venus (by Jim Bencivenga)