Halleys Comet

Halley’s Comet, officially designated 1P/Halley, is the only known periodic comet regularly visible to the naked eye. Appearing approximately every 72–80 years—with most recorded apparitions occurring after intervals of 75–77 years—it last passed through the inner Solar System in 1986 and is expected to return in 2061. Its recurring visibility has made it one of the most famous celestial objects in human history, with documented observations dating back to at least 240 BC. The English astronomer Edmond Halley first recognised in 1705 that the comet seen in multiple historical reports was the same object, establishing it as periodic and giving the comet its name.
During its 1986 apparition, Halley’s Comet became the first comet studied closely by spacecraft. Missions such as Giotto provided the earliest detailed observations of a comet nucleus, validating key predictions of the “dirty snowball” model, while also revealing that Halley’s surface is dominated by dark, non-volatile dust with only limited exposed ice.

Pronunciation

The pronunciation of “Halley” varies:

• Commonly /ˈhæli/, rhyming with valley
• Sometimes /ˈheɪli/, rhyming with daily
• Colin Ronan, a biographer of Edmond Halley, preferred /ˈhɔːli/, rhyming with crawly

Historical spellings of Halley’s surname were numerous—including Hailey, Haley, Hayley, Halley, Haly, and Hawley—making the original pronunciation uncertain. Modern bearers of the surname generally favour the version rhyming with valley.

Computation of orbit

Before the seventeenth century, comets were widely believed to be atmospheric disturbances, a view rooted in Aristotelian philosophy. This was challenged in 1577 when Tycho Brahe used parallax measurements to show that comets must lie beyond the Moon. Despite this, many astronomers continued to assume comets travelled in straight paths.
A major shift occurred after Isaac Newton published the Philosophiæ Naturalis Principia Mathematica (1687), outlining laws of gravitation and motion. Newton suspected that comets seen in 1680 and 1681 were the same object pre- and post-perihelion, but he struggled to integrate comets fully into his mathematical model.
In 1705, Edmond Halley applied Newtonian mechanics to comet observations. Comparing the orbital elements of comets seen in 1531, 1607, and 1682, he concluded they were the same comet returning on a roughly 76-year cycle. Anticipating perturbations by Jupiter and Saturn, he predicted the comet’s return in 1758—a forecast confirmed when the comet was detected on 25 December 1758 by German observer Johann Georg Palitzsch. Halley died in 1742 and did not see his prediction proven.
A team of French mathematicians—Clairaut, Lalande, and Lepaute—later refined gravitational calculations and predicted perihelion in April 1759, close to the actual date of 13 March 1759. The return of Halley’s Comet became one of the earliest triumphs of Newtonian physics.
The comet was formally named in Halley’s honour by French astronomer Nicolas-Louis de Lacaille in 1759.
Historical records, particularly from China, have been essential in reconstructing Halley’s earlier orbits. Computational attempts in the late twentieth century showed that orbital integrations become unreliable before the comet’s close approach to Earth in 837 AD, necessitating ancient data to constrain modelling.

Orbit and origin

Halley’s Comet follows a highly elongated elliptical orbit with an eccentricity of about 0.967:

• Perihelion: between the orbits of Mercury and Venus
• Aphelion: roughly equal to Pluto’s orbital distance

Unlike most Solar System bodies, it travels on a retrograde orbit, moving clockwise as viewed from above the Sun’s north pole. Its orbital inclination is often given as 162°, equivalent to 18° relative to the ecliptic but reversed in direction.
During its 1910 passage, Halley’s Comet encountered Earth at a particularly high relative velocity. Because its orbit crosses Earth’s path twice, the comet is the parent body of two major meteor showers:

• Eta Aquariids (early May)
• Orionids (late October)

Despite being classified as a short-period comet—defined as one completing an orbit in under 200 years—Halley’s orbit is unusual for this category. Most short-period comets belong to the Jupiter-family group, with orbits under 20 years and low inclinations. Halley’s longer period and retrograde orbit align it instead with Halley-type comets, which range from 20 to 200 years and exhibit higher inclinations.
Over millennia, Halley’s orbital period has fluctuated between 74 and 80 years, influenced by gravitational interactions with the planets.

Scientific exploration

The 1986 apparition was a turning point in comet science. The European Space Agency’s Giotto probe approached within about 600 kilometres of the nucleus, capturing unprecedented images and data. Other spacecraft—including Vega 1, Vega 2, Suisei, and Sakigake—also contributed, forming the first multispacecraft comet mission.
Key findings included:

• Confirmation of a dark, low-albedo nucleus
• Identification of sporadic jets releasing gas and dust
• Evidence supporting a mixture of volatile ices and non-volatile dust
• Insights into coma formation and solar wind–comet interactions

These data fundamentally shaped modern cometary science.

Cultural and historical significance

Halley’s Comet has been observed across diverse civilisations, often regarded as an omen or portent. Scholars have proposed that Mesopotamian astronomers may have recognised its periodicity. A reference in the Babylonian Talmud describes a star appearing every seventy years that confuses sailors, though interpretations differ.
Its 1758 return prompted widespread interest in Europe and the Americas. At Harvard, John Winthrop lectured on its importance for Newtonian physics, while Jamaican astronomer Francis Williams independently made observations commemorated in a unique commissioned portrait. Radiographic analysis in 2024 revealed that the painting’s star field accurately corresponds to the comet’s predicted 1759 position.