To start with, I am going to take you through a 60 second elevator. Our stops are not going to be different floors in a building though, but different periods in the passage of time.
First up, let’s go back over 2,250 years in time. Chinese records account for the observation of what they call a “broom star”, a comet that has appeared in this week in 240 B.C.
Fast forward over a 1000 years. The year is 1066, best known in history for the Battle of Hastings that saw King Harold defeated by William the Conqueror at Hastings. The Bayeux Tapestry, which depicts the Norman conquest of England, has an image of a comet, which was spotted ahead of the war, woven into it.
Third and final stop, 1305. Giotto di Bondone’s famous painting Adoration of the Magi includes a comet-like representation of the Star of Bethlehem. Many believe that the appearance of a comet at the turn of the century might have left a lasting impression on the Italian artist.
Have you connected the dots? Yes, these are three separate instances where the Halley’s comet has been observed and recorded in a certain way. But wait! How is it that a comet that has been spotted for over 2,000 years named after a British astronomer who was born only in the 17th century? Let’s find out.
Edmond Halley was interested in mathematics from a very young age and had a flair for astronomy. So when he came across calculus, a powerful mathematical tool that Isaac Newton had started using extensively, after having started out as an assistant to John Flamsteed, the Astronomer Royal at the Greenwich Observatory, he decided to tie the two topics together.
He set about computing the parabolic orbits of comets that had been recorded over the past few centuries and had his results ready by 1705. He then noticed that the orbits of the comets that were observed in 1531, 1607 (by Johannes Kepler) and 1682 (by himself) were almost identical. He was able to extrapolate his idea to find that comets had appeared in a similar orbit once in 75 years, including the one observed by Giotto in 1301.
Halley came to the conclusion that these weren’t different comets taking similar paths, but in fact were the same comet appearing once every 75-76 years. He went a step further and ventured to predict that the next time this comet would be visible from Earth would be in the year 1758.
Halley, however, passed away in 1742. He wasn’t around to see it, but the comet reappeared in 1758, only just. During Christmas that year, the comet was first spotted, thereby confirming Halley’s prediction. From then on, it has been called Halley’s comet.
Halley’s comet was thus the first discovered periodic comet and remains one of the most popular short-period comets. Periodic comets have an elliptical orbit around the sun, allowing it to frequent Earth’s neighbourhood repeatedly. These comets have enabled us to collect more data, helping us to confirm and build on our ideas about them. Though periodic comets were initially named after the astronomers who discovered their orbits, the convention of naming them after their discoverers has now become common.
As for Halley’s, when it last made its apparition in 1986, I wasn’t still around. 2061 remains my only chance.
ganesh.a.s@thehindu.co.in
