The closest that Halley’s Comet has come to the Earth was on 10th April 837 with the distance from the Earth is 0.04 astronomical units.
The sky we look upon hold many stories that have mesmerized generations. Each has a legend of its own and is beautifully woven together by “storytellers” of the society. Now we are going to uncover one among them and delve into the magical world.
Many years ago, an unwelcome guest made its presence felt in the sky for several days. The scientifically illiterate and superstitious crowd considered it as a bad omen. Naturally, people started thinking that the Gods must have been angry with them. Superstitions of such nature were prevalent during those times.
People started thinking that the unusual sighting would bring diseases, destroy crops, dry up rivers, and bring a multitude of other destructions with it. Evidently, people began to describe it as an awful thing they witnessed in their lifespan and began to think that the Doomsday was near. Poor people! They hardly knew that reality.
The appearance of Halley’s Comet can be traced back to around the third millennium before Christ. Without scientific knowledge, ancient cultures often turned to myth for the explanation of such a thing. Many records and literature are describing Halley’s Comet.
Its impact on literature and culture has also been well documented. The earliest of the records that have been discovered are from China. The Chinese records dating back to 240 BC. There are reasons to believe that they are older documents which are yet to be discovered. This leaves us with a tiny chance to at least lay our hands on one such document.The Chinese, by nature very circumspect, had a dedicated team of astronomers who kept a close watch on the sky and documented anything unusual. It further went on to interpret its portents for the safety and security of the Emperor. They have a ginormous volume of information related to the sightings in the sky since ancient times.
There are other speculations related to the sightings of Halley’s Comet. The most popular among them are about its observations in Greece between 467 BC and 466 BC.
Not until the 17th century, a wise man Edmond Halley, an English astronomer, scientifically investigated this Comet. He took up the documented reports of a comet coming at a sighting distance from the Earth in 1531, 1607, 1682 and analyzed them. In 1705 he published his observations in the catalogue of orbits of 24 comets.
His calculations showed a stark similarity in the orbits of the Comets. This convinced him that these comets were not different but rather the same one coming periodically in Earth’s vicinity. These observations allowed him to study and predict the orbit and also the return of the Comet. He prophesied that the Comet will revisit in 1758.
Halley did not live long enough to see his prediction come true. He passed away in 1742, and the Comet was visible in late 1758. Hence Halley’s Comet as you might have guessed it was named after its discoverer Edmond Halley to honor him.
Halley’s Comet makes a periodic visit to the Earth’s vicinity after approximately 75 years. If lucky, one can see the Halley’s Comet twice in a lifetime. This periodic visit made it possible to decipher that the Comet revolves around the Sun but in the opposite direction of the planets. This observation also made it possible to give scientific backing to the fact that some comets are members of the solar system.
Halley’s Comet on 8 March 1986 Ref: NSSDC’s Photo Gallery (NASA)Let’s gauge how easy or difficult tracing a comet can be. A comet, like other planets, is a celestial body. Like all celestial body, a comet also has a gravitational force and is acted upon by the gravitational force of other planets. So, there is a constant tug-of-war going on in different directions. This modifies the path and subsequently, the orbit of the Comet. In simple terms, the perpetual tugging either elongates or shrinks the orbit of the Comet.
As you guessed it, if the orbit is elongated, the Comet will take longer to return; else if the orbit shrinks, the Comet will return faster. This result in a varied orbital period ranging from 74.5 years to 79 years or at times slightly more. Halley’s Comet inspired several artworks too. Famous among them are Bayeux Tapestry.
It might have also inspired the form of the Star of Bethlehem that was used by the Italian painter Giotto in his painting The Adoration of the Magi which was painted in around 1305.
In 1986 appearance of Halley’s Comet provided an opportunity for scientists to study a comet closely. Many probes and satellites were sent in space to gather multi-wavelength data and information about it. Scientists also observed it from space stations and space shuttles.
The Appearance Of Halley’s Comet
Transcending lousy omens; Halley’s Comet became a sensation, making Halley the only known short-period Comet that is regularly visible to the naked eye from Earth. A comet is a small part of the solar system composed of ice, dust, and small rocky particles. It gets warm and begins to release gases when it is closer to the Sun, this is known as ‘Outgassing.’
This gives the tail to the Comet which is also known as the coma, which is mainly the effects of solar radiation and the solar wind acting upon the nucleus, which is solid core structure of the Comet. Comet nuclei range from a few hundred meters to tens of kilometers. The dust particles acting as scatters and gas molecules which absorbs and re-radiate solar light makes tail visible, which may extend up to more than 100 million kilometers into space.
The closest that Halley’s Comet has come to the Earth was on 10th April 837 with the distance from the Earth is 0.04 astronomical units. (A.U.: 6 million km)
Though the tail of Halley’s Comet is quite large in size, compared to that its nucleus is barely 15 kilometers long, 8 kilometers wide and perhaps 8 kilometers thick. Its mass is roughly 2.2 × 1014 kg. Observations of close pictures taken by spacecraft showed that the gases ejected from the nucleus were 80% water vapour, ~17% carbon monoxide and 3–4% carbon dioxide, with traces of hydrocarbons, methane, and ammonia.
This proved the hypothesis of Fred Whipple‘s “dirty snowball” model, which says Halley would be composed of a mixture of volatile ices such as water, carbon dioxide, ammonia, and dust. This disqualified the alternate theory of the sandbank model purported by an English astronomer R.A. Lyttleton between period 1930s to ’80s. He theorized that the nucleus was not a solid body, but it was rather composed of absorbed gassed with a cloud of dust.
Halley’s orbital period varies between 74–79 years, and has a highly elliptical orbit. It orbits the Sun in the opposite direction to the planets. Halley falls under the class of a periodic comet; one which has an orbit lasting two hundred years approximately.
An interesting insight: Dust particles and other debris as a result of the slow fragmentation over the millions of years are distributed along the orbit which results in meteor showers during October and May.
Next appearance of Halley’s Comet is predicted in 2061. Which will provide opportunities for further studies.
Moving from superstitions to science allowed us to admire the beauty of Halley’s Comet. Even life and water on Earth are now predicted to be from a comet itself.
It is difficult to study comets closely for many decades. Yet scientists continue to look for other comets in the solar system similar to Halley. An example of this was the Rosetta probe which looked at the Comet 67P/Churyumov-Gerasimenko between 2014 and 2016.
Its notable finding was that the Comet has a different kind of water than that of the Earth. Many potential research projects are taking place to check this possibility like famous Balloon experiment. Accepting scientific approach allowed us to unlock many possibilities in this breath-taking universe.