What is Halley’s Comet-Structure, Size & Significance in History

What is Halley’s Comet?

Halley’s Comet is probably the most famous comet. It is a “periodic” comet that moves around the sun in an elliptical orbit & returns to Earth’s vicinity about every 75 years. The last time it was here was in 1986 & it is projected to return in 2061.

Coma & Tail
Coma & Tail that extend upto thousands of kilometers


Why is it called Halley’s Comet?

The comet is named after English astronomer Edmond Halley though he was not the one to discover Halley’s comet. He supposedly analyzed historical reports of 24 comets appearing near Earth between 1337 & 1698. Three of the comets that appeared in 1531, 1607 & 1682 seemed to be very similar in terms of orbit & other parameters, leading Halley to propose that these three comets were actually the same comet returning over & over again & predicted that it would come again in 1758. It actually appeared again in 1758 that led to the comet being named after him.



Halley’s comet has found a mention time & again in history. It has been dubbed as an omen & herald. Some theologians suggest that the star of Bethlehem was actually a comet (Halley’s comet) that marked the birth of Christ. Below are a few excerpts from historical records:

240 B.C.

a Chinese historian described a comet that appeared in the east and moved north

468 B.C.

Comet was recorded in ancient Greece between 468 and 466 BC; its timing, location, duration, and associated meteor shower all suggest it was Halley


The comet seen in England. Later King Harold II died at the Battle of Hastings. Surviving accounts from the period describe it as appearing to be four times the size of Venus and shining with a light equal to a quarter of that of the moon.


Edmond Halley predicted return of the comet


Mark Twain predicted his death with Halley’s comet. (Click)


Comet flew by close to the earth. It was captured on camera for the first time.


Space craft was launched to study the comet.


Halley’s comet is supposed to come again in Earth’s proximity.

Structure & Composition

Halley’s comet is a cold, dirty ball of rock & ice. This icy core of the comet, called the nucleus also contains volatile substances made of carbon, hydrogen, nitrogen & oxygen like water, carbon monoxide etc.

As it approaches the sun, the volatile substances in the nucleus begin to sublime & the comet soon becomes surrounded by a cloud of dust & gas called the Coma. It acts as an atmosphere extending up to thousands of kilometers. Two things happen in the Coma:

  1. Gas molecules absorb solar light & radiate at different wavelengths
  2. Dust particles scatter solar light

Because of this, it appears as an extremely bright ball of light with a tail.

Yes, it also develops a tail that is typical of comets. A fraction of the gas molecules is ionized by the ultraviolet radiation of the sun. The charged solar winds emitted from the sun draw these ionized molecules into a long tail extending up to 100 million kilometers into space.

Surface & Size

The surface of the comet shows varied topography, with hills, mountains, ridges, depressions & craters. Inspite of the vast size of the coma, Halley’s nucleus is relatively small: about 15 kilometers long, 8 kilometers wide & barely 8 kilometers thick. Its mass is relatively low (roughly 2.2 × 1014 kg) & average density is about 0.6 g/cm3, suggesting that it is made of a large number of small pieces, held together very loosely.

Is Halley’s comet always this bright?

The answer is NO. As it moves away from the Sun, there is a steady decrease of activity. When it reaches the distance of Saturn, the tail & coma disappear completely, leaving only the avocado-shaped “dirty snowball” nucleus which is pitch black with minimal visibility. It starts getting visibility only when it nears perihelion.

Believe it or Not

*It is possible that a human can see it twice in his or her lifetime

*Edmond Halley did not live to see the return of the comet.

*The Orionid meteor shower (Click) that occurs annually in October is caused by Halley’s comet.

*It’s a short period comet eventhough It appears once in 75 years!

*It has its own fleet of spacecrafts called “Halley’s Armada”


Disclaimer: Images courtesy The images may not be actual.

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