Here's why:
* Stars appear fixed: Stars are incredibly far away, so from our perspective on Earth, they seem stationary.
* Earth's rotation: As the Earth spins on its axis, it appears as though the stars are moving in a circle around the celestial pole.
* Long exposure: To capture star trails, photographers use long exposure times (often several hours). This allows the apparent movement of the stars to be recorded as streaks of light.
The circular pattern of the trails indicates that the rotation axis of the Earth is tilted relative to the plane of the Earth's orbit around the Sun. This tilt causes the stars to appear to move in a circle around a point near the North Star.
