问在pygame中想运行一个行星环绕，最后出现了这个问题，求大神帮帮忙？？？

import sys,math,pygame
from pygame.locals import *

class Point(object):
    def __init__(self,x,y):
        self._x=x
        self._y=y
def getx(self):
    return self._x
def setx(self):
    self._x=x
x=property(getx,setx)


def gety(self):
    return  self._y
def sety(self):
    self._y=y
y=property(gety,sety)

    
def __str__(self):
    return  "{X:"+"{:.0f}".format(self._x)+\
            ",Y:"+"{:.0f}".format(self._y)+ "}"
def print_text(font,x,y,text,color=(255,255,255)):
    imgText=font.render(text,True,color)
    screen.blit(imgText,(x,y))

def wrap_angle(angle):
    return angle % 360

pygame.init()
screen=pygame.display.set_mode((800,600))
pygame.display.set_caption("Orbit Demo")
font=pygame.font.Font(None,18)

space=pygame.image.load(r"C:\Users\Zengfeng\Desktop\银河.PNG").convert_alpha()
planet = pygame.image.load(r"C:\Users\Zengfeng\Desktop\行星.PNG").convert_alpha()
ship=pygame.image.load(r"C:\Users\Zengfeng\Desktop\超人.PNG").convert_alpha()

width, heigth = ship.get_size()
ship = pygame.transform.smoothscale(ship,(width//2,heigth//2))

redius=250
angle=0.0
pos=Point(0,0)
old_pos=Point(0,0)


while True:
    for event in pygame.event.get():
        if event.type== QUIT:
            sys.exit()
    keys=pygame.key.get_pressed()
    if keys[K_ESCAPE]:
        sys.exit()
    screen.blit(space, (0, 0))

    width, heigth = planet.get_size()
    screen.blit(planet, (400 - width / 2, 300 - heigth / 2))

    angle=wrap_angle(angle-0.1)
    pos.x=math.sin(math.radians(angle)) * redius
    pos.y = math.cos(math.radians(angle)) * redius

    delta_x=(pos.x-old_pos.x)      #这个地方报错
    delta_y=(pos.y-old_pos.y)      #这个地方报错
    rangle=math.atan2(delta_y,delta_x)
    rangled=wrap_angle(-math.degrees(rangle))
    scratch_ship=pygame.transform.rotate(ship,rangled)

    width,heigth= scratch_ship.get_size()
    x=400+pos.x-width//2
    y=300+pos.y-heigth//2
    screen.blit(scratch_ship,(x,y))

    print_text(font,0,0,"Orbit:" + "{:.0f}".format(angle))
    print_text(font, 0, 20, "Rotation:" + "{:.2f}".format(angle))
    print_text(font, 0, 40, "Position:" + str(pos))
    print_text(font, 0, 60, "Old Pos:" + str(old_pos) )
    # screen.blit(ship, (50, 50))

    pygame.display.update()
    old_pos.x = pos.x
    old_pos.y = pos.y