Here's why:

* Day and night are caused by Earth's rotation: The Earth spins on its axis, completing a full rotation approximately every 24 hours. This rotation is what causes day and night, with one side of the Earth facing the sun (day) and the other side facing away (night).

* Orbital shape affects Earth's distance from the sun: The elliptical orbit means Earth is closer to the sun at perihelion (around January) and farther at aphelion (around July). This difference in distance does affect the amount of solar radiation Earth receives, which is why the Northern Hemisphere experiences winter when Earth is farthest from the sun and summer when it is closest.

* But the orbital shape doesn't impact Earth's rotation: The rotation speed remains relatively constant throughout the year, so the length of day and night doesn't change significantly due to the elliptical orbit.

Key takeaway: While the Earth's orbit has a significant impact on seasons, it doesn't directly affect the length of day and night. The length of day and night is primarily determined by Earth's rotation.