<P> Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days . Pliny the Elder believed that comets were connected with political unrest and death . </P> <P> In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically . This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th - century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were . </P> <P> In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers . Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon . </P> <P> Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example . </P>

Who is usually acknowledged to have been the first scientific explorer