How should i know that it moves down not up !?

Answer 1

If you're reffering to x and y coordinates in a computer, then here's what I've got:

If you have a screen infront of you, the upper left corner is (0, 0). The bottom of the screen is your vertical resolution, which in a high-defininition monitor is 1080 pixels. The right of your screen is the horizontal resolution, which is 1920 pixels in high-definition. So, if you want to move down the screen, you move from (0, 0) towards (0, 1080). Id est, you increase your y value.

Answer 2

it should actually be x and -y co-ordinates (in mathematics)

Answer 3

check the JavaScript file of my "multi pointed star" code