The world should revolve around me

by Alicia
(Sydney, Australia)

I'm doing a maths investigation at the moment and I'm looking at how the speed of one complete rotation around the sun differs for each planet. I was wondering if you would be able to give me the formula in which you need figure this out?

Barry's Response - Good question, Alicia:

Depends on what you know about them. If you have the planet's year length, in days, and its average distance from the sun, in miles. Here's how it would go.

Speed = (approximately) pi (3.1415926) x distance from the sun (miles) x 2 divided by the number of days.

This will give a pretty good estimate in miles per day. Divide by 24 for miles per hour OR 86400 to get miles per second. If you prefer kilometres, you can then multiply the result by 1.609

It becomes more complicated if you consider the fact that these orbits are not perfect circles. For this exercise, you probably don't need to worry about that. It's a good way to estimate average speed.

The orbits of planets with low eccentricities, like Earth, are nearly circular, so they have relatively constant orbital speeds. High eccentricity planets have elliptical orbits and experience varying distances from the Sun, resulting in different speeds, and longer times to complete orbits because of extended periods in the slow-moving, distant part of the orbit.

Here's a list of relevant data you can use to calculate average speeds:

- - - - - - How Long - - How far (average)
Mercury: . . 88 days . 36,800,000 miles
Venus: .. . 225 days . 67,200,000 miles
Earth: .. . 365 days . 93,000,000 miles
Mars: . . . 687 days . 141,600,000 miles
Jupiter:. 4,333 days . 483,600,000 miles
Saturn:. 10,759 days . 886,500,000 miles
Uranus:. 30,687 days . 1,783,700,000 miles
Neptune: 60,190 days . 2,795,200,000 miles
Pluto: . . 2977 months 3,670,100,000 miles

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