Finding the Moon's position in orbit

[bass_maniac]

I'm trying to make a program in which the Earth (a circle) is at the center of the screen. It has a radius of 25. I want to have the moon orbit the Earth at a distance of 100 pixels from the center of the Earth in a counter-clockwise direction.

If the initial values are:
earthX := maxx div 2
earthY := maxy div 2
moonX := earthX + 100
moonY := earthY

How do I calculate the next position of the moon?

[Tony]

use an equation of the circle

code:

x**2 + y**2 = r**2

btw, Earth is not at the center of the world, make it orbit the sun

[Mazer]

tony wrote:

btw, Earth is not at the center of the world, make it orbit the sun

The Earth isn't at the center of the world? Damn. But why would you want the moon to orbit the Sun?

From my understanding, you just want to use trig, right? So that for whatever angle you have, the moon's x is equal to 100 * the cosine of the angle. Same goes for the y coord, just replacing cosine with sine. (Don't forget to add half the screen width and height to the coordinates)

[bass_maniac]

Thank you Coutsos. Just for interest's sake, here's what I've come up with so far,

code:

View.Set ("graphics:max;max,offscreenonly") var earthX := maxx div 2 var earthY := maxy div 2 var moonX : real := earthX + 100 var moonY : real := earthY var angle := 0.0 loop     drawfilloval (earthX, earthY, 25, 25, 7)     drawfilloval (round (moonX), round (moonY), 3, 3, 7)     moonX := 100 * (cos (angle)) + (maxx div 2)     moonY := 100 * (sin (angle)) + (maxy div 2)     angle += .05     delay (30)     View.Update     cls end loop