When the Earth's axis is perpendicular to the incoming from the Sun, in spring the Sun stands directly over the equator at noon. As a result, daylight hours equal nighttime hours everywhere on Earth.
The earth rotates on its axis around the sun and half of the earth is always facing away from the sun (night time) and the other half if facing it (daytime) as the earth rotates the side that was in darkness comes to face the sun and the other side turns away!
