Planet rotation speed?

BlitzMax Forums/BlitzMax Beginners Area/Planet rotation speed?

Shortwind

(Posted 2011) [#1]

Hi, how do I calculate the speed of different sized planets? This is basic geometry, but I'm having a heck of a time getting it...

Ok, let's say we have a body that is about 22,000 miles in diameter.
We know that the surface is moving about 1,000 miles per hour.
We also know that the rotation is 1 per 24 hours.

Now, how do they calculate that?

I want to calculate basically the same thing, but with a planet that is 50,000 miles in diameter. At the same rotation rate (1 per 24 hours), how do I find the speed at the surface?

Thanks,
Shortwind

Last edited 2011

Warpy

(Posted 2011) [#2]

circumference divided by time per full rotation

Think about the path followed by a point on the surface. As the planet rotates, the point traces out a circle, with the same diameter as the planet. So in a full rotation, the distance travelled by the point is equal to the circumference of the circle.

The formula for the circumference of a circle is Pi*Diameter

Finally, speed is distance travelled divided by time taken.

So, for your example:

speed at surface = Pi*50,000 / 24.

(this only applies to points on the equator. For a point at any latitude, multiply by Cos(latitude))

Last edited 2011

Last edited 2011

Last edited 2011

Brucey

(Posted 2011) [#3]

Something like this?

SuperStrict

' earth
Print "Earth:"
SurfaceSpeed(12756.2)

Print ""
Print "Big Planet:"
SurfaceSpeed(80467.2) ' 50000 miles


Function SurfaceSpeed(diameter:Double)
	
	' circumference = 2&#960;r
	Local circumference:Double = diameter * Pi
	
	' speed = distance / time
	Local speed:Double = circumference / 24
	
	Print "Diameter = " + diameter
	Print "Speed    = " + speed + " kph"
	Print "Speed    = " + (speed / 1.609344) + " mph"
End Function

Brucey

(Posted 2011) [#4]

Warpy, I thought circumference was 2 Pi r ?

I guess math has changed since I was at school.... :-/

Shortwind

(Posted 2011) [#5]

Thanks Brucey, that helps a lot. :)

Warpy

(Posted 2011) [#6]

Brucey: I originally wrote that in terms of the radius, then changed to diameter, and it looks like I forgot to change one of the copies of the formula! Oops.