Astronomers don't explain stars moving in a straight line across the night sky because that's not what happens. Stars appear to move in an arc across the sky due to the rotation of the Earth.

Here's why:

* Earth's Rotation: Our planet spins on its axis, completing one rotation roughly every 24 hours. This rotation causes the apparent movement of stars in the sky.

* Perspective: As the Earth rotates, our perspective on the stars changes. Imagine looking at a distant object like a streetlight while riding in a car. The streetlight appears to move even though it is stationary. The same principle applies to stars.

* Celestial Sphere: Astronomers use the concept of a celestial sphere to visualize the movement of stars. This imaginary sphere surrounds Earth and is used to map the positions of celestial objects. As the Earth rotates, it appears as if the celestial sphere, with all its stars, is rotating around us.

Exceptions:

* Polaris (North Star): The North Star appears relatively stationary because it's located almost directly above Earth's North Pole.

* Planets: Planets do exhibit slight changes in their apparent motion compared to stars, due to their own orbits around the Sun.

In Summary:

The apparent motion of stars across the night sky is a result of the Earth's rotation and our changing perspective. It's an illusion, not actual straight-line motion of the stars themselves.