float x; 
float cof = 75.0;  

void setup() {
size(500,500);
background(0);
}

void draw() {
x++;
background(0);
stroke(255,255,255);

float x1 = 250 + cos(x*PI/180)*cof;
float y1 = 250 + sin(x*PI/180)*cof;

float x2 = 250 + cos((x + 180)*PI/180)*cof;
float y2 = 250 + sin((x + 180)*PI/180)*cof;
line(x1,y1,x2,y2);
}