300 Years Since Halley's Big Idea

Probably since time eternal, humans have wondered about our place in space. How could you not, gazing at a dynamic night sky emblazoned with countless stars, meteors, planets, aurorae, and comets? Then, three hundred years ago, Edmond Halley proposed the math that would put us in our place.

On Monday, May 9, observers across much of the world can witness part of the 2016 transit of Mercury. While Mercury may appear as a mere dot moving across the sun over a 7-hour span, it is a reminder of how we came to understand where we fit in the grand scheme of the solar system.

When only 20 years old, an ambitious Edmond Halley sailed to the Atlantic island of St. Helena with the intention of mapping the night sky with a telescope and creating an unmatched star atlas of the southern hemisphere. He also had realized that if he packed his bags and set sail without delay, in his year on the island he'd witness two eclipses and the 1677 transit of Mercury. Halley left Oxford without completing his degree and was soon bound for an island at 16 degrees south latitude. Though the St. Helena weather was less ideal than expected, the opportunity to imagine must have been unparalleled.

The simple act of a young person watching a celestial phenomenon eventually became the inspiration for a global quest that would change the world. On the island after the transit of Mercury, Halley soon realized that one could mathematically quantify the distance to each of the planets if multiple observers timed how long it took for an inferior planet (Mercury or Venus) to cross the sun during a transit.

Nearly four decades later, in 1716, Halley published A New Method of Determining the Parallax of the Sun, or His Distance from the Earth. It was the nitty gritty math that detailed his youthful epiphany. In recalling his transit of Mercury experience, Halley notes a transit of Venus would be a better object to measure:

On observing this [transit of Mercury] I immediately concluded, that the sun's parallax might be duly determined by such observations, if Mercury, being nearer the earth, had a greater parallax, when seen from the sun; for, this difference of parallaxes is so very inconsiderable, as to be always less than the sun's parallax, which is sought; consequently, though Mercury is to be frequently seen within the sun's disk; he will scarcely be fit for the present purpose. There remains therefore Venus's transit over the sun's disk...

What followed was nothing less than a complete set of instructions on figuring out the size of the solar system, which was the leading question of the day. Imagine that--spelling out the means of answering the foremost question in science of your time.

For Halley, the only problem was that transits of Mercury are relatively frequent, whereas transits of Venus come in pairs that over a hundred years apart. The next useful transit for his purposes would be the transit of Venus in 1761, well after his lifetime. Nonetheless, in his groundbreaking publication, Halley exhorted future generatons to embrace the math. Go out and time the transit of Venus from locations around the world, he appealed.

Halley's compelling argument succeeded. In subsequent years, fleets of nations ventured around the globe to time accurately the passage of Venus across the sun. A 4-minute video Patrick McPike produced in anticipation of the 2012 transit of Venus describes the ambitous scientific endeavors inspired by Halley.

When the 2016 transit of Mercury approaches, I invite you to witness this subtle event with an appreciation for the greater endeavors to which it gave life.

In South Bend, IN, the transit of Mercury begins around 07:13:30 EDT, when the sun is fewer than 7 degrees above the horizon (sunrise at 6:31 a.m.) toward the east. The transit ends shortly after 2:41 p.m. when the sun is 63 degrees above the horizon in the southwest.

Nowadays we can measure the distance to the planets with accuracy unimagined by Halley. But it all began with a young person looking skyward, observing, and wondering. That alone is reason to encourage youth of today to stop, to take a peek at the majesty overhead, and to imagine.