EARLy Observations

In ancient times, without interference from streetlights or urban pollution, comets could be seen by everyone. Their sudden appearance was interpreted as an omen of nature that awed people and was used by astrologers to predict flood, famine, pestilence, or the death of kings. The Greek philosopher Aristotle (4th century BCE) thought that the heavens were perfect and incorruptible. The very transient nature of comets seemed to imply that they were not part of the heavens but were merely earthly exhalations ignited and transported by heat to the upper atmosphere. Although the Roman philosopher Seneca (1st century CE) had proposed that comets could be heavenly bodies like the planets, Aristotle's ideas prevailed until the 14th century CE

Finally, during the 16th century, the Danish nobleman Tycho Brahe established critical proof that comets are heavenly bodies. He compared the lack of diurnal parallax of the comet of 1577 with the well-known parallax of the Moon (the diurnal parallax is the apparent change of position in the sky relative to the distant stars due to the rotation of Earth). Tycho deduced that the comet was at least four times farther away than the Moon, establishing for the first time that comets were heavenly bodies.

In 1619, the German astronomer Johannes Kepler still believed in 1619 that comets travel across the sky in a straight line. It was the English physicist and mathematician Isaac Newton who demonstrated in his Principia (1687) that, if heavenly bodies are attracted by a central body (the Sun) in proportion to the inverse square of its distance, they must move along a conic section (circle, ellipse, parabola, or hyperbola). Using the observed positions of the Great Comet of 1680, he identified its orbit as being nearly parabolic.

Telescopes Mastery

Telescopes Mastery

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