How can you tell an object is moving through space - through doppler shift.
By getting its doppler shift, you can say if the object is moving away from you (if you see the object with a more red) or coming closer to you (if you see the object with a more blue).
And?
You can get the the speed of that object by calculating the wavelength of light. When an objects moves away from you, light waves are stretched, giving them a red color. And if an object moves closer to you, light waves are compressed giving the observer a blue color. Through that process you can know the direction and the speed of the object.
