The Halley Comet

Timelines of Events

  • c.350 BCE
    • According to Aristotle, comets are a type of weather phenomenon that occur in the upper atmosphere of the planet.
  • 1577
    • Tycho Brahe determines that a comet he has seen must be located far from the atmosphere of Earth.
  • 1758
    • Comet Halley makes a comeback after 76 years.
  • 1819
    • Johann Encke, a German astronomer, finds a second periodic comet that returns once every 3.3 years.
  • 1950
    • Jan Oort, a Dutch Astronomer, he suggests that there is a vast cloud of comets surrounding the solar system and that stars could cause these comets' orbits to be disturbed.

Halley’s Comet

  • Astronomers were aware of these comets, those brilliant blotches of light and their long, stunning tails as they moved slowly across the sky over intervals of a few weeks or months, but they were unsure of their origin or their final destination.
  • But in 1577, a comet that shone exceptionally brightly for several months caused things to change.
  • Tycho Brahe, a Danish astronomer, determined the comet must be at least four times farther away than the moon by analyzing observational data from various regions of Europe.
    • They appeared to him as objects capable of unrestricted movement through the same reaches of space as planets.
    • The shape of the space-traveling comets' paths, however, was a topic on which there was disagreement both during and after Brahe's time, and for many years afterwards.
  • Johannes Kepler, a former pupil of Brahe, thought that objects moved in straight lines.
  • Johannes Hevelius, a Polish astronomer, hypothesized that a comet from 1664 had followed a curved orbit around the sun.

Newton’s View of Comets

  • The great English scientist Isaac Newton started researching cometary orbits around the year 1680 after being inspired by the appearance of a particularly bright comet that year. He was also working on developing his general theory of gravitation.
  • Newton examined and anticipated the future course of the 1680 comet using his new theory.
  • He came to the conclusion that comets, like planets, had elliptical orbits with the sun at one of their focal points.
  • However, these ellipses were so stretched out that they could be approximated to an open-ended curve known as a parabola.
  • If Newton's theory was accurate, a comet that had traveled through the inner solar system and curved around the sun would either never make another trip there or make another trip after a very long time.
  • In 1684, Edmond Halley visited Newton. He was eager to talk about the potential causes of the motions of comets and other heavenly bodies, including planets.
  • Halley was informed by Newton that he had been researching the subject on his own and had already found the solution, but that he had not yet made his findings public.
  • This encounter ultimately resulted in Halley editing and funding the 1687 publication of Philosophiae Naturalis Principia Mathematica, Newton's significant work on gravity and the laws of motion.

Historical Records

  • Halley suggested that Newton use his new theory to investigate the orbits of additional comets.
  • However, Newton's attention had shifted to other issues, so starting in the early 1690s, Halley carried out his own in-depth investigation. He studied the orbits of 24 comets in total, some of which he had personally observed and others for which he had data from historical records, over the course of more than 10 years.
  • He hypothesized that while some comets followed paths that are open-ended parabolas, as Newton had proposed, others followed elliptical orbits, suggesting that they might pass through the inner solar system and become visible from Earth more than once in a person's lifetime.
  • Halley had noticed an odd thing while studying. Each comet's orbit generally possessed a few traits that set them apart from those of other comets, such as its orientation with respect to the stars.
  • However, he noticed that the orbits of three of the comets he had studied—one he had personally seen in 1682, as well as others seen by Kepler in 1607 and Petrus Apianus in 1531—seemed to be remarkably similar.
  • He believed that these were repeated appearances of the same comet, which had an orbit that was closed and elliptical and occurred about once every 75 to 76 years.
  • Halley published Astronomiae cometicae synopsis (A Synopsis of the Astronomy of Comets) in 1705, which contained an outline of his theories.
  • He wrote:

“Many considerations incline me to believe the Comet of 1531 observed by Apianus to have been the same as that described by Kepler and Longomontanus in 1607 and which I again observed when it returned in 1682. All the elements agree. Whence I would venture confidently to predict its return, namely in the year 1758.”

  • The time gaps between the three appearances weren't exactly the same; they varied by about a year.
  • Halley believed that the gravitational pull from Jupiter and Saturn could slightly alter the comet's trajectory and cause a delay in its timing.
  • When Halley asked Newton to think about this issue, he developed gravitational calculations that allowed Halley to improve his forecast.
  • His updated forecast stated that the comet would reappear either in late 1758 or early 1759.

Halley’s Prediction

  • Three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, spent several grueling months recalculating when the comet might reappear and where it might first be seen in the night sky as the year of the comet's predicted return drew near.
  • As early as 1757, amateur and expert astronomers alike started keeping an eye out for the comet's return.
  • Johann Palitzsch, a German farmer and amateur astronomer, made the discovery on December 25, 1758.
  • Only a few months later than Halley had predicted, the comet came closest to the sun in March 1759.
  • Halley had already passed away 17 years prior, but the comet's reemergence helped him achieve fame after his passing.
  • Nicolas-Louis de Lacaille, a French astronomer, gave the comet the honorary name Halley's comet in his memory.
  • It had been established that Halley's comet orbits the sun before any other object, other than planets.
  • It also offered one of the earliest demonstrations that Newton's theory of gravity could be used to explain the gravitational pull of all celestial bodies.
  • In 1986, a spacecraft made a close approach to the comet, collecting information on the composition of its nucleus and tail.
  • It is the only short-period comet that has been observed to be visible with the naked eye and to appear twice in a person's lifetime.