The length of time between night and day on Earth varies throughout the year. This time variance is explained primarily by *

Explanation:

The tilt of the earth's axis.

If you put a couple of toothpicks in an orange or an apple and make them so they are at a 45 degree angle and tilt the orange so it rotates on an axis made by the toothpicks, you will see how this works.

Turn off the lights and shine the orange with a flashlight. Now rotate it using the toothpicks as an axis. Go slowly. Notice how if you tilt the top toothpick towards the flashlight, it looks different than if make the top toothpick further away from the flashlight.

What you've done is exactly what the earth would do when its axis is closer to the sun and further away from the sun.

I hope this is clear enough.