Orbit and recorded sightings
The period of Halley's orbit has been fairly consistent over the last 300 or so years as it has been 75–76 years, although it has varied in the past. Since 240BC the orbital period has varied between 74–79 years.[4]
Halley's orbit of the Sun is highly elliptical, typical of most comets, it has an orbital eccentricity of 0.967 (0 being a perfect circle and 1 being a parabolic trajectory).
The farthest point on its orbit is 35 AU from the sun, more or less the same distance as Pluto, which can be equated to 3.25345325 x10^9 miles, this point is referred to as the aphelion, in comparison to the closest point in the comets orbit to the sun, which in this case is 0.6 AU. S.[5]
The comet orbits the sun in a retrograde fashion, which is highly unusual in the solar system, in Lehman's terms, it orbits the sun in the opposite direction to the planets. The inclination of the orbit is at 18° South of the plane of the Solar System (the ecliptic).
Relative to the Earth the comet has one of the largest velocities of any object in the Solar System, this is mainly down to the retrograde orbit. The 1910 passage was at a relative velocity 254,016 km/h (or 70.56 km/s).[6]
Halley's orbit of the Sun is highly elliptical, typical of most comets, it has an orbital eccentricity of 0.967 (0 being a perfect circle and 1 being a parabolic trajectory).
The farthest point on its orbit is 35 AU from the sun, more or less the same distance as Pluto, which can be equated to 3.25345325 x10^9 miles, this point is referred to as the aphelion, in comparison to the closest point in the comets orbit to the sun, which in this case is 0.6 AU. S.[5]
The comet orbits the sun in a retrograde fashion, which is highly unusual in the solar system, in Lehman's terms, it orbits the sun in the opposite direction to the planets. The inclination of the orbit is at 18° South of the plane of the Solar System (the ecliptic).
Relative to the Earth the comet has one of the largest velocities of any object in the Solar System, this is mainly down to the retrograde orbit. The 1910 passage was at a relative velocity 254,016 km/h (or 70.56 km/s).[6]
A short period comet is one which has an orbit that take 200 years or less to complete, rather than a long period comet, which typically have orbits lasting for thousands of years, Halley a short period comet.[7] Halley's orbit is not normal for the majority of short period comets, as short period comets tend to have an orbital period of 6-7 years and an average inclination relative to the solar plane of only ten degrees.[4] Short period comets that are similar to Halley in the way that orbital period are within 20 to 200 years, with a tilt of up to 90 degrees are described as Halley-type comets.[8]
It is likely that Halley-type comets, including Halley itself were once long period comets, that have had their orbits unsettled and changed by the gravity of large masses such as the giant planets, which has caused the comets to be directed towards the inner parts of the solar system. This can be determined from the orbits of various Halley-type comets, and this means that it is highly likely that these former long period comets, Halley in particular, may have originated as part of the Oort cloud. [7]
It is likely that Halley-type comets, including Halley itself were once long period comets, that have had their orbits unsettled and changed by the gravity of large masses such as the giant planets, which has caused the comets to be directed towards the inner parts of the solar system. This can be determined from the orbits of various Halley-type comets, and this means that it is highly likely that these former long period comets, Halley in particular, may have originated as part of the Oort cloud. [7]