The Motions of the Earth

The Earth goes through three types of motions: rotation, revolution, and precession. Let’s review them one by one.

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How the Earth rotates. Credit: Dennis Nilsson/Creative Commons

Earth Rotation

The rotation occurs when a body, such as the Earth, spins on its axis. The axis is an imaginary line that passes through the center of the Earth, going through the North Pole and exiting through the South Pole. It is what gives us night and day.

One complete Earth rotation equals 23 hours, 56 minutes, and 4 seconds. The Earth’s axis is not perpendicular to its orbital plane. Instead, it is tilted 23.5 degrees. However, the value changes as the axial tilt vary between 22.1 to 24.5 every 41,000 years (one Milankovitch Cycle). The change in the axial tilt is called obliquity.

Because of the Earth’s axial tilt, different regions in the Earth experience varying intensities of seasons and different lengths of daytime. The winter solstice is the shortest day of the year, while a summer solstice is the longest day of the year. An equinox occurs when day and night are equal in length. 

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How the Earth revolves around the Sun. Credit: Tauʻolunga/Creative Commons

Earth Revolution

As the Earth rotates, it orbits around the Sun in a process called revolution. One complete Earth revolution requires 365 days, 6 hours, and 9 minutes at an average speed of 30 km/s.

When the Earth is at its aphelion, it means that the Earth’s position in its orbit is farthest from the Sun. When the Earth is at its perihelion, it is closest to the Sun.

The Earth experiences a third type of motion that is slower and less prominent– precession. Axial precession represents the “wobble” of the Earth as it rotates on its axis, much like a spinning top. Throughout a period of 26,000 years, the direction in which the axis points changes until it completes 360 degrees.

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How the Earth wobbles every 26,000 years. Credit: Department of Geography/UCSB

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Ruth Raganit

Ruth Raganit obtained her Bachelor of Science degree in Geology from the University of the Philippines – Diliman. Her love affair with Earth sciences began when she saw a pretty rock and wondered how it came to be. She also likes playing video games, doing digital art, and reading manga.

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