Q:

What causes the alternation between day and night?

A:

Quick Answer

The Earth rotates around its axis, allowing half of the globe to face the sun at a given time, with a full day taking approximately 24 hours. The planet is always separated into a day side and a night side, with the day side facing the sun. The lengths of day and night are influenced by the tilt of Earth's axis; it is not a vertical line relative to the ecliptic, or the orbital plane.

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Full Answer

From the point of view of an observer on Earth, the sun appears to move from east to west. However, the planet is actually rotating from west to east. Therefore, an observer standing outside watching the sunrise would see the sun crest over the eastern horizon. This is the point at which the observer's location enters daytime. Standing there the whole day would show the sun tracing an east-west path across the sky, with the relative height of the sun depending on the person's latitude and the time of year. At sunset, the spot on Earth would be rotating into the night side.

To account for the rotation of Earth, the planet is divided into 24 different time zones, with Greenwich, England serving as meridian time. Each successive time zone to the east is an hour ahead of the last, while those to the west are an hour behind their neighbors.

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