<P> 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 that refers to "a star which appears once in seventy years that makes the captains of the ships err ." </P> <P> 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 due to a close approach to Earth in that year . It was necessary to use ancient Chinese comet observations to constrain their calculations . </P> <P> Halley's orbital period has varied between 74--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 ° .) Due 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 Aquarid meteor shower, although it might not be the parent of that shower . </P> <P> 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 . </P>

What is the path of halley's comet
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