An image of Halley's Comet taken in 1986. (Image credit: NASA)

On this Day:

March 30, 240 BC was the 1st recorded perihelion passage of Halley’s Comet.

In 240 B.C. Chinese astronomers observe a new broom-shaped “star” in the sky. It’s the first confirmed sighting of Halley’s Comet. Some have made the case that a sighting in the third millennium B.C. is responsible for the alignment of the Sphinx and the pyramids of Giza.

Halley’s Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–76 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the only naked-eye comet that can appear twice in a human lifetime. Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.

Halley’s periodic returns to the inner Solar System have been observed and recorded by astronomers around the world since at least 240 BC. But it was not until 1705 that the English astronomer Edmond Halley understood that these appearances were reappearances of the same comet. As a result of this discovery, the comet is now named after Halley.

During its 1986 apparition, Halley’s Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple’s “dirty snowball” model, which correctly predicted that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide, ammonia, and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.

Computation of orbit

The orbital path of Halley, against the orbits of the planets (animation);
Halley was the first comet to be recognized as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth’s atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. Many were still unconvinced that comets orbited the Sun, and assumed instead that they must follow straight paths through the Solar System.

In 1687, Sir Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, in which he outlined his laws of gravity and motion. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton’s Comet), he was unable to completely reconcile comets into his model.

Ultimately, it was Newton’s friend, editor and publisher, Edmond Halley, who, in his 1705 Synopsis of the Astronomy of Comets, used Newton’s new laws to calculate the gravitational effects of Jupiter and Saturn on cometary orbits. Having compiled a list of 24 comet observations, he calculated that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler). Halley thus concluded that all three comets were, in fact, the same object returning about every 76 years, a period that has since been found to vary between 74 and 79 years. After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758. While he had personally observed the comet around perihelion in September 1682, Halley died in 1742 before he could observe its predicted return.

Halley’s prediction of the comet’s return proved to be correct, although it was not seen until 25 December 1758, by Johann Georg Palitzsch, a German farmer and amateur astronomer. It did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a retardation of 618 days. This effect was computed before its return (with a one-month error to 13 April) by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute. The confirmation of the comet’s return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of Newtonian physics, and a clear demonstration of its explanatory power. The comet was first named in Halley’s honour by French astronomer Nicolas-Louis de Lacaille in 1759.

Some scholars have proposed that first-century Mesopotamian astronomers already had recognized Halley’s Comet as periodic. This theory notes a passage in the Bavli Talmud, tractate Aggada that refers to “a star which appears once in seventy years that makes the captains of the ships err.”

Researchers in 1981 attempting to calculate the past orbits of Halley by numerical integration starting from accurate observations in the seventeenth and eighteenth centuries could not produce accurate results further back than 837 owing to a close approach to Earth in that year. It was necessary to use ancient Chinese comet observations to constrain their calculations.

Orbit and origin

Halley’s orbital period has varied between 74 and 79 years since 240 BC. Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet’s orbit when it is nearest the Sun, is just 0.6 AU. This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 AU (roughly the distance of Pluto). Unusual for an object in the Solar System, Halley’s orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun’s north pole. The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic. (Because it is retrograde, the true inclination is 162°.) Owing to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System. The 1910 passage was at a relative velocity of 70.56 km/s (157,838 mph or 254,016 km/h). Because its orbit comes close to Earth’s in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October. Halley is the parent body to the Orionids. Observations conducted around the time of Halley’s appearance in 1986 suggested that the comet could additionally perturb the Eta Aquariids meteor shower, although it might not be the parent of that shower.

Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less. This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley’s orbit is atypical. Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less) are called Jupiter-family comets. Those resembling Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets. As of 2015, only 75 Halley-type comets have been observed, compared with 511 identified Jupiter-family comets.

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System. If Halley was once a long-period comet, it is likely to have originated in the Oort cloud, a sphere of cometary bodies that has an inner edge of 20,000–50,000 AU. Conversely the Jupiter-family comets are generally believed to originate in the Kuiper belt, a flat disc of icy debris between 30 AU (Neptune’s orbit) and 50 AU from the Sun (in the scattered disc). Another point of origin for the Halley-type comets was proposed in 2008, when a trans-Neptunian object with a retrograde orbit similar to Halley’s was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.

Halley has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations. The non-gravitational effects can be crucial; as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes cause delays in its perihelion of four days, average.

In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley’s Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales. Halley’s projected lifetime could be as long as 10 million years. These studies also showed that many physical properties of Halley’s Comet dynamics can be approximately described by a simple symplectic map, known as the Kepler map. More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years. Observations by D. W. Hughes suggest that Halley’s nucleus has been reduced in mass by 80 to 90% over the last 2,000 to 3,000 revolutions (per Wikipedia).

First, a Story:

NASA put out a call for assistance. The Muppets volunteered and teamed up with NASA to help name a newly discovered celestial object.

Upon its first sighting, the Jim Henson Company issued a press release, “Comet Defrog here.”

Second, a Song:

Bill Haley & His Comets were an American rock and roll band, founded in 1952 and continued until Haley’s death in 1981. The band was also known as Bill Haley and the Comets and Bill Haley’s Comets (and variations thereof). From late 1954 to late 1956, the group placed nine singles in the Top 20, one of those a number one and three more in the Top Ten. The single “Rock Around the Clock” became the biggest selling rock and roll single in the history of the genre and retained that position for some years.

Bandleader Bill Haley had previously been a country music performer; after recording a country and western-styled version of Jackie Brenston and his Delta Cats “Rocket 88”, a rhythm and blues song, he changed musical direction to a new sound which came to be called rock and roll.

Though the group was considered to be at the forefront of rock and roll during the genre’s formative years, the arrival of more risqué acts such as Elvis Presley and Little Richard by 1956 led the more clean-cut Haley and his Comets to decline in popularity. Haley would remain popular in Europe and go on to have a comeback as a nostalgia act in the 1970s, along with many of his contemporaries. Following Haley’s death, no fewer than seven different groups have existed under the Comets name, all claiming (with varying degrees of authority) to be the continuation of Haley’s group. As of the end of 2014, four such groups were still performing in the United States and internationally (per Wikipedia).

OldTapes on YouTube.com states:

“Welcome to the official Old Tapes Retro Music YouTube channel.

Interested in global 60s,70s and 80s unforgettable music videos? Our YouTube channel has all this and more, bringing you specially selected clips from the world’s Number 1 Music Archives.

The official Old Tapes YouTube channel is operated by Number1 Digital Media.”

Here is Bill Haley His Comets “Rock Around The Clock” OST 1956 Remastered And Colorized by Artificial intelligence Technology for Old Tapes Followers. I hope you enjoy this!

(https://www.youtube.com/watch?v=VsAlSuEG26A)

Thought for the Day:

” I came in with Halley’s Comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don’t go out with Halley’s Comet. The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.’ ” – Mark Twain

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Have a great day!

Dave & Colleen

© 2022 David J. Bilinsky and Colleen E. Bilinsky

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