Thursday, May 17, 2012

Transit Trivia: Jeremiah Horrocks

The above painting is an artist's conception of the first observed transit of the planet Venus, as predicted and observed by Jeremiah Horrocks in 1639. The portrait is held in the collection of Astley Hall Museum and Art Gallery Chorley and the property of Chorley Council. Image Credit: Because of its age, the image is in the Public Domain.

Kepler's Rudophine Tables proved invaluable to many astronomers, not the least of which was Jeremiah Horrocks (1618 – 1641), an English astronomer, the son of a Liverpool farmer, and a clerk at the local St. Michael's Church in Much Hoole, Lancashire. In the tables, Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. But Horrocks found and corrected an error in Kepler's calculation for the orbit of Venus. He then realized that transits of Venus would occur in pairs that were eight years apart, and so predicted the transit in 1639.

On December 4th, 1639 (November 24th under the Julian calendar, which was then in use in England), Horrocks prepared to observe from Carr House in Much Hoole, near Preston in England. Although he was uncertain of the exact time, Horrocks calculated that the transit would begin at approximately 3:00 PM. The day being Sunday, Horrocks spent much of the time attending to his clerk duties at St. Michael's. It was just as well, for clouds obscured the sun for most of the day, but cleared by about 3:15, when Horrocks was finished with his other obligations. He was able to observe a portion of the event for about 30 minutes until the local sunset. Aside from Horrocks, only one other person observed the transit: English astronomer, mathematician, merchant, and friend to Jeremiah Horrocks, William Crabtree (1610 - 1644). Crabtree  observed from his home in Broughton, near Manchester.

Horrocks understood that looking directly at the sun without proper filtering would cause blindness, a tragedy for anyone, but especially bad for an astronomer. Horrocks therefore focused the image of the sun through a simple refractor telescope and onto a piece of card, where the image could be safely observed.

Horrocks' observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the distance between the Earth and the sun. He estimated that distance to be 59.4 million miles (0.639 AU) – about two thirds of the actual distance of 93 million miles (149.6 million km) but a more accurate figure than any suggested up to that time.

The life of Jeremiah Horrocks was indeed impressive. This son of a farmer had managed to study at Cambridge University. He had reviewed, found fault, and corrected the works of earlier greats, including those of Kepler. He correctly predicted a transit of Venus, was one of the first two people to observe the event, and he used his observations to calculate a new distance between Earth and the sun which was the most accurate to that time. And for all of this, Horrocks was only about 21 years old.

In addition, Horrocks determined the orbit of Earth's moon and correctly hypothesized that its shape was elliptical rather than circular. He even suggested that Earth and the sun influenced the moon's orbit, anticipating Isaac Newton's theories on gravity. And Horrocks had begun a detailed study of tides in an attempt to explain the lunar causation of tidal movements. Sadly, Horrocks would not complete this or any other work. He died at his home, from unknown causes, at the age of 22. As his friend William Crabtree expressed, "What an incalculable loss!"

About twenty years later, in 1662, the Polish-Lithuanian astronomer Johannes Hevelius (1611 - 1687) published, at his own expense, the transit work of Jeremiah Horrocks. The treatise was entitled Venus in sole visa (Venus in transit across the Sun). The paper caused great excitement when it was presented to members of the Royal Society. It contained much evidence of Horrocks' enthusiastic and romantic nature, including humorous comments and passages of original poetry. When writing on the century which separated his and the next transit of Venus, Horrocks rhapsodized,

"Oh! then farewell, thou beauteous queen!
Thy sway may soften natures yet untamed,
Whose breasts, bereft of the native fury,
Then shall learn the milder virtues.
We, with anxious mind, follow thy latest footsteps here,
And far as thought can carry us;
My labours now bedeck the monument for future times
Which thou at parting left us. Thy return
Posterity shall witness; years must roll away,
But then at length the splendid sight
Again shall greet our distant children's eyes."

At St. Michael's Church in Much Hoole, a stained glass window on the east wall commemorates Horrocks and his achievement in observing the transit. Nearby, at Carr House, a plaque recognizes the location where Horrocks made his observations.

The next transit of Venus will occur over June 5-6. To learn more, visit these links.

2012 Transit of Venus, NASA/Goddard Space Flight Center:

Transit of Venus, Sun-Earth Day 2012, NASA:

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