Monday, June 04, 2012

Transits of Venus

I'm looking forward to watching the Transit of Venus tomorrow. We're heading over to Bart and Xy's house, and we've got our cocktail picked out: The Blond Venus. This is my last shot to see the Transit of Venus, as it won't occur again until 2117.

All of this has me thinking quite a bit about the planet, partly because I've been reading Richard Holmes' The Age of Wonder. It's a book about "science" during the Romantic Age at the end of the 18th century. The first chapter is about Captain Cook's voyage to Tahiti in 1769 to measure the Transit of Venus. England also sent out expeditions to North Cape Norway and Hudson Bay Canada, with the idea that through triangulation they could then measure the distance to the sun. This concept of solar parallax is credited to James Gregory who published Optica Promota more than a century prior to Cook's Tahitian voyage. Cook's team measured 4 "phases" of this transit: when Venus first appeared to touch the outer solar sphere, when Venus was entirely within the solar sphere but still touching the outer rim, when Venus had crossed the solar sphere and first touched the outer sphere, and finally, when Venus had exited the solar sphere but still touched the solar rim. Using the data collected from this 1769 transit, 2 years later Thomas Hornsby published that "the mean distance from the Earth to the Sun (is) 93,726,900 English miles." That's pretty accurate, given that today it is believed that the distance is 1/8 of one percent shorter: 92,955,000.

The Venus Tablet of Ammisaduqa, British Museum

Observing the planet Venus is also key to understanding chronology in the ancient Near East. This is mostly because of a cuneiform tablet known as the Venus Tablet of Ammisaduqa which was part of the library unearthed at Nineveh. While copies of this text tend to date to the 8th and 7th centuries BCE, the text is believed to have been composed much earlier during the reign of Ammisaduqa, who ruled Babylon about a century after Hammurabi. The tablet records the exact date of the heliacal rising and setting of the planet Venus for 21 years. Modern astronomers use these dates to reconstruct when the referenced 21 year period occurred. The problem is that there are questions about the accuracy of the recordings, and that the events recorded accurately fit into three different time periods. Thus scholars must choose between the so-called High Chronology, Middle Chronology, and Low Chronology. Thus Hammurabi's reign begins in 1848 BCE, 1792 BCE, or 1736 BCE. I'm personally a Middle Chronology kind of guy. So Hammurabi was reigning in 1769 BCE, and James Cook recorded the Transit of Venus in 1769 CE, and in another 1769 years, I'll be dead.


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