Answer:
The Earth rotation
Explanation:
The Earth rotates in its own axes, that is the reason why it can be observed the Sun and some other starts with the naked area. And apparently all the sky rotates, but actually is the planet the one that is constantly moving.
If the sun is close to Earth, so the light hours during the day are longer, and the visible of sun and stars is no so common.
When the sun is apart to the Earth, so the day hours are smaller, and the probability to observe the sun and the stars at the same time increases.