Halley's Comet

• 1. Ancient Sky Watchers

  ›

  In 240 BC, Chinese astronomers recorded a comet in the Records of the Grand Historian. They described it moving from east to north. This entry marks the first confirmed sighting of Halley's Comet in human history. Babylonian tablets rediscovered in August 1984 also hold records of its appearance in 164 BC and 87 BC. These ancient observers noted the star for over a month during the 87 BC apparition. Roman historian Cassius Dio wrote about a comet hovering over Rome in 12 BC. He claimed it portended the death of Marcus Vipsanius Agrippa that same year. Jewish historian Josephus described a sword-like comet appearing over Jerusalem before the First Jewish-Roman War. The Talmud mentions a star arising once every seventy years that misleads sailors. Scholars believe this refers to the 66 AD passage. In 374 AD, the comet passed within 0.09 astronomical units of Earth. It appeared again in 451 AD, which some said heralded the defeat of Attila the Hun at the Battle of Chalons. Emperor Louis the Pious observed the 837 AD return and devoted himself to prayer. He feared the token signaled a change in the realm or the death of a prince. The tail stretched 60 degrees across the sky during that close approach.

• 2. Halley And Newton

  ›

  Edmond Halley used Newton's laws of gravity to calculate the orbits of three comets. He studied appearances from 1531, 1607, and 1682. His 1705 Synopsis of the Astronomy of Comets concluded these were the same object returning every 76 years. Halley predicted its return for 1758 after accounting for gravitational pulls from Jupiter and Saturn. Johann Georg Palitzsch, a German farmer and amateur astronomer, saw it first on the 25th of December 1758. The comet did not reach perihelion until the 13th of March 1759 due to planetary attraction. A team of French mathematicians including Alexis Clairaut and Nicole-Reine Lepaute computed this delay before the event. They made an error of only one month in their prediction. John Winthrop lectured at Harvard University about the implications for Newtonian mechanics. Francis Williams, a Jamaican polymath, independently recognized the return but his observations never reached Europe. A portrait commissioned by Williams shows his hand resting on page 521 of Newton's Principia. The white smudge in the sky likely depicts the comet relative to constellations in March 1759. Nicolas-Louis de Lacaille named the comet in Halley's honor in 1759.

• 3. Spacecraft Armada

  ›

  The Soviet Vega 1 probe began returning images of Halley on the 4th of March 1986. It captured the first-ever image of the nucleus during its flyby on the 6th of March. Vega 2 followed with a flyby on the 9th of March, coming within 8,030 kilometers of the surface. The European Space Agency launched Giotto to make the closest pass on the 14th of March 1986. Japanese probes Suisei and Sakigake also joined the effort. This group became known unofficially as the Halley Armada. Data from these missions revealed the nucleus is pitch black with an albedo of 0.04. It reflects only 4% of sunlight hitting it, similar to coal. The shape resembles a peanut or potato rather than a perfect sphere. Scientists found the surface composed largely of dusty non-volatile materials. Only about 10% of the surface was active ice. Jets of sublimating gas ejected water vapor, carbon monoxide, and carbon dioxide. The dust particles ranged down to sizes of approximately 0.001 micrometers. Ground-based observations suggested a rotation period of 7.4 days while spacecraft data indicated 52 hours.

• 4. Cyanogen Panic

  ›

  On the 19th of May 1910, Earth passed through the tail of the comet. Spectroscopic analysis discovered toxic cyanogen gas in that tail. Press speculation claimed life on Earth could be endangered by this substance. Public panic buying of gas masks occurred despite scientific reassurances. Quack anti-comet pills were sold to fearful consumers. In China, people believed the comet indicated calamity such as war or pestilence. Some doors remained unopened for half a day during the passage. No water was carried because rumors said pestilential vapors poured from the sky. A hoax reached major newspapers claiming an Oklahoma religious group tried to sacrifice a virgin. Mark Twain wrote in his autobiography that he expected to die with the comet since he was born two weeks after its 1835 perihelion. He died on the 21st of April 1910, one day after the comet's closest approach to the Sun. The 1910 apparition added unrest to China on the eve of the Xinhai Revolution. James Hutson recorded missionary accounts from Sichuan describing the fear among locals.

• 5. Retrograde Path

  ›

  Halley orbits the Sun in a retrograde direction opposite to the planets. Its orbital eccentricity measures 0.967, making it highly elliptical. The orbit extends from inside Mercury's path out to Pluto's distance. It is inclined by 18 degrees to the ecliptic plane. Much of the orbit lies south of this reference line. Astronomers classify it as a Halley-type comet due to its period between 20 and 200 years. Most short-period comets belong to the Jupiter-family category instead. Halley likely originated in the Oort cloud before gravitational perturbations sent it inward. It has probably existed in its current orbit for 16,000 to 200,000 years. Non-gravitational effects cause delays averaging four days in perihelion passage. Boris Chirikov and Vitold Vecheslavov analyzed 46 apparitions showing chaotic dynamics. David Hughes calculated the nucleus lost 80 to 90% of its mass over the last few thousand revolutions. Future projections suggest it may evaporate or split within tens of thousands of years. The comet will be ejected from the Solar System within hundreds of thousands of years.

• 6. Future Returns

  ›

  The next perihelion occurs on the 28th of July 2061 when the comet reaches its closest point to the Sun. Earth approach happens one day after perihelion on the 29th of July 2061. Apparent magnitude is expected to reach negative 0.3 making it visible to the naked eye. This return offers better viewing conditions than the 1985-1986 apparition. On the 9th of September 2060, Halley passes within 70 million kilometers of Jupiter. It then passes within 100 million kilometers of Venus on the 20th of August 2061. A subsequent return brings perihelion to the 27th of March 2134 with an apparent magnitude of negative 2.0. In 2003 astronomers observed the comet at magnitude 28.2 using Very Large Telescopes in Chile. They verified methods for finding faint trans-Neptunian objects. On the 9th of December 2023, Halley reached its farthest slowest point from the Sun traveling at 1.5 kilometers per second. The 1986 apparition was the least favorable on record due to light pollution and positioning.