Multiple Earths Stability

import os, sys
import traceback
from math import *

with open(os.devnull, "w") as f:
    oldstdout=sys.stdout; sys.stdout=f; import pygame; sys.stdout=oldstdout
from pygame.locals import *


if sys.platform in ["win32","win64"]: os.environ["SDL_VIDEO_CENTERED"]="1"

pygame.display.init()
pygame.font.init()


screen_size = [1024,768]

N = 30
dt = 86400.0 #one day

au = 149597870700.0
G = 6.67408e-11

icon=pygame.Surface((1,1)); icon.set_alpha(0); pygame.display.set_icon(icon)
pygame.display.set_caption("N-Body Orbit - Ian Mallett - 2019")

surface = pygame.display.set_mode(screen_size)

aspect = float(screen_size[0]) / float(screen_size[1])
bounds_y = [ -1.3*au, 1.3*au ]
bounds_x = [ aspect*bounds_y[0], aspect*bounds_y[1] ]
scn_recip_bounds_extents = [ screen_size[0]/(bounds_x[1]-bounds_x[0]), screen_size[1]/(bounds_y[1]-bounds_y[0]) ]

def rndint(x): return int(round(x))
def rotate(vec,angle_rad):
    c=cos(angle_rad); s=sin(angle_rad)
    return [ c*vec[0]-s*vec[1], s*vec[0]+c*vec[1] ]
def pos_to_scn(pos):
    x = rndint( (pos[0]-bounds_x[0]) * scn_recip_bounds_extents[0] )
    y = rndint( (pos[1]-bounds_y[0]) * scn_recip_bounds_extents[1] )
    return (x,screen_size[1]-y)

class Object(object):
    def __init__(self, mass, pos,vel, radius):
        self.mass = mass

        self.pos = pos
        self.vel = vel
        self.force = [0.0,0.0]

        self.radius = radius

    def draw(self):
        scn = pos_to_scn(self.pos)
        pygame.draw.circle(surface,(255,)*3,scn,self.radius)

objs = []

#Sun
objs.append(Object( 1.98847e30, [0,0],[0,0], 10 )) #6.957e8

#Earths
for i in range(N):
    pos=[1.0*au,0.0]; vel=[0.0,(2.0*pi*au)/(86400.0*365.2425)]; angle_rad=radians(360.0*i/float(N))
    pos=rotate(pos,angle_rad); vel=rotate(vel,angle_rad)
    objs.append(Object( 5.9722e24, pos,vel, 3 ))

def get_input():
    keys_pressed = pygame.key.get_pressed()
    mouse_buttons = pygame.mouse.get_pressed()
    mouse_position = pygame.mouse.get_pos()
    mouse_rel = pygame.mouse.get_rel()
    for event in pygame.event.get():
        if   event.type == QUIT: return False
        elif event.type == KEYDOWN:
            if   event.key == K_ESCAPE: return False
    return True

def update():
    for obj in objs:
        obj.force = [0.0,0.0]
    for i in range(0,len(objs),1):
        obj1 = objs[i]
        for j in range(i+1,len(objs),1):
            obj2 = objs[j]
            vec12 = ( obj2.pos[0]-obj1.pos[0], obj2.pos[1]-obj1.pos[1] )
            r2 = vec12[0]*vec12[0] + vec12[1]*vec12[1]
            force_magn = G*obj1.mass*obj2.mass / r2
            sc = force_magn / (r2**0.5)
            force2on1 = ( sc*vec12[0], sc*vec12[1] )
            obj1.force[0]+=force2on1[0]; obj1.force[1]+=force2on1[1]
            obj2.force[0]-=force2on1[0]; obj2.force[1]-=force2on1[1]
    for obj in objs:
        accel = ( obj.force[0]/obj.mass, obj.force[1]/obj.mass )
        obj.vel[0] += accel[0]*dt
        obj.vel[1] += accel[1]*dt
        obj.pos[0] += obj.vel[0]*dt
        obj.pos[1] += obj.vel[1]*dt

def draw():
    surface.fill((0,0,0))
    for obj in objs: obj.draw()
    pygame.display.flip()

def main():
    clock = pygame.time.Clock()
    while True:
        if not get_input(): break
        update()
        draw()
        clock.tick(60)
    pygame.quit()

if __name__ == "__main__":
    try:
        main()
    except:
        traceback.print_exc()
        pygame.quit()
        input()