When reenacting or acting as a historical interpreter, its good to have a few historical dates and stories to share. This series will publish a few.

Edmund Halley, England’s most significant astronomer,
was born November 8, 1656

Edmond Halley was born on November 8, 1656, in Haggerston, England to a well-to-do merchant family.  His father was a London chandler whose wealth enabled him to indulge his son’s scientific curiosity and send him to Queens College, Oxford, in 1673.  At Oxford, Halley became the protégée of John Flamsteed, England’s first Astronomer Royal.  Inspired by Flamsteed’s star catalog of the northern hemisphere, Halley proposed that he would do the same for the southern hemisphere. 

The project was sponsored by King Charles II and Halley spent an entire year cataloging nearly 350 stars and their positions in the southern sky.  Halley discovered a star cluster in Centaurus and also became the first person to ever record the entire transit of Mercury.  Halley published his star catalog in 1678 and King Charles decreed that Halley be granted a Master of Arts degree from Oxford and he was invited to join The Royal Society at just 22, making him one of the youngest ever granted that honor.

Halley soon turned his attention to comets and he repeatedly failed to plot their orbits using the same logic that others (Tycho Brahe, Galileo Galilei, and Johannes Kepler, etc.) had used to describe planets.  No one really knew what comets were or how they worked, so Halley was tackling new intellectual territory.  While large celestial bodies like planets were seen to orbit the sun, and moons were seen orbiting planets, the same couldn’t be said of comets. They did not seem to have any period that anyone could measure and it was widely believed that they traveled in a straight line or followed parabolic paths that flung them back out into deep space, never to return. 

Because of his appointment to the Royal Society, Halley was able to leverage his association with Isaac Newton, Robert Hooke, and other great physicists.  In a meeting with Newton, Halley learned that Newton has essentially already worked out a solution another comet, but had set the matter aside.  When Halley asked to see the calculations, Newton conceded that he no longer had them, so he rewrote them and sent them to Halley.  This led to Newton actually publishing Philosophiae Naturalis Principia Mathematica, THE most important work in classical physics. 

Not completely satisfied with simply Newton’s work, Halley then went on to PROVE the significance if Newton’s laws by using these equations to calculate the orbit of a particular comet which was documented to have been sighted in 1305, 1380, 1456, 1531, and 1607. He went on to predict that this comet would return in 1758.  When the comet appeared as predicted on December 25, 1758, Halley’s Comet, as it came to be known, proved Newton correct and made Halley a household name, even to this day.

Halley then turned his attention to planets.  In the 18th century, astronomers had been able to calculate the relative distances between all of the major bodies in the known solar system, but they had no way of knowing what these distances were in absolute terms. The problem was that no one had any idea how big the sun actually was.

Halley proposed using the fact that we knew how large the Earth was. By observing a transit of a planet in front of the Sun from several points on the Earth’s surface then using the distance between these points, it was possible to compute distance between the Earth and the Sun through simple trigonometry.  This process leveraged a phenomenon whereupon things look different from different observation points — parallax.  Halley proposed that he COULD have computed the distance of the Earth to the Sun using the Transit of Mercury in 1677 but only one other record of the Transit of Mercury has been made, which wasn’t sufficient to identify an accurate parallax.  Fortunately, Transits of Venus, set to occur in 1761 and in 1769. Halley published a public call for international cooperation between nations to send out scientific expeditions around the world to record the Transit of Venus in order to calculate the astronomical unit and reveal the scale of the solar system for the first time. 

Scientists traveled to Tahiti; Norway; several locations in North America including Canada, New England, and San José del Cabo; Saint Petersburg; Hungary; Denmark; and the King’s new Observatory at Greenwich to observe these transits.  The results of the various observations the Royal Society was able to compute that the Earth-Sun distance was 24,000 times the Earth’s radius.  These results were soon confirmed by the American Philosophical Society in 1771.  These results were only about 3% different from the correct value (measured with microwaves by NASA in 1969). 

Edmond Halley didn’t live long enough to see his two most notable contributions to our understanding of the universe play out. He died in 1742 at the age of 85 but his instructions lived on beyond his years and enabled other to continue the work.

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