If our planet is rotating and shooting through space, why do we see stars as dots of light instead of blurred lines across the sky?

Question

I’m sitting here trying to sleep and this is keeping me awake.

In: Physics

Answer 1

It’s not about linear velocity. It’s about angular velocity.

You are a point on the circumference of a circle. The star is a point at the center of the circle. There is an imaginary line (the radius) connecting you and the star.

As you move around the circumference of the circle, the angle of the radius changes at a rate related to two variables: (1) your speed and (2) the length of the radius of the circle.

Think about it. If you walk 1 meter along the circumference of a circle with a circumference of 1 meter, that’s a whole lap. But if the circumference of the circle is 10 meters, a 1 meter walk is 1/10 of a lap. Your speed is the same, but the speed the angle changes at gets slower as the radius gets bigger.

It’s this angular velocity that will determine how quickly the object will move across your visual field, and thus how blurry it will look. In the case of you and the stars, the length of the radius is vast.

edit – obviously, in real life you are not orbiting the stars in the night sky. My example is about how angular velocity determines the blur, and without being able to draw a diagram, I thought the circle example was the easier to envision and communicate.