Barbarian
Member
- Jun 5, 2003
- 33,179
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It should be noted that some long period comets have periods of many thousands of years or even millions of years.
How do we know this? We can observe their orbits, and apply Kepler's laws to determine the orbit and the period it takes to complete one orbit. Hence the notion of 10,000 years to evaporate is silly. It takes longer than that for most of the long-period comets to complete one orbit.
How?
Kepler's first law says that orbits are ellipses, with the sun at one of the two foci.
Kepler's second law says that equal areas are swept out by the comet in equal amounts of time.
Kepler's third law says that the ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes.
So, we can take a comet that has an apohelion of one light year, and that gives us a semimajor axis of about 0.5 light year. By definition, the Earth is one AU from the Sun, and has a period of one year. There are 9,500,000,000,000 kilometers in a light year, and 150,000,000 kilometers in an AU. So then 0.5 light years are about 31667 AUs.
So.... Comet period squared/1 = Comet distance cubed/1
Which means that a comet with an apohelion of one light year would take about 5.6 million years to make one orbit.
How do we know this? We can observe their orbits, and apply Kepler's laws to determine the orbit and the period it takes to complete one orbit. Hence the notion of 10,000 years to evaporate is silly. It takes longer than that for most of the long-period comets to complete one orbit.
How?
Kepler's first law says that orbits are ellipses, with the sun at one of the two foci.
Kepler's second law says that equal areas are swept out by the comet in equal amounts of time.
Kepler's third law says that the ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semimajor axes.
So, we can take a comet that has an apohelion of one light year, and that gives us a semimajor axis of about 0.5 light year. By definition, the Earth is one AU from the Sun, and has a period of one year. There are 9,500,000,000,000 kilometers in a light year, and 150,000,000 kilometers in an AU. So then 0.5 light years are about 31667 AUs.
So.... Comet period squared/1 = Comet distance cubed/1
Which means that a comet with an apohelion of one light year would take about 5.6 million years to make one orbit.