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Earth’s Rotation

Earth's Rotation

Earth, as imaged by the MESSENGER space probe. Credit: NASA

What if someone were to tell you that at any given moment, you were traveling at speeds well in excess of the speed of sound? You might think they were crazy, given that – as best as you could tell – you were standing on solid ground, and not in the cockpit of a supersonic jet. Nevertheless, the statement is correct. At any given moment, we are all moving at a speed of up to 1,674 kilometers an hour, thanks to the Earth’s rotation,

By definition, the Earth’s rotation is the amount of time that it takes to turn around once on its axis. This is, apparently, accomplished once a day – i.e. every 24 hours.. However, there are actually two different kinds of rotation that need to be considered here. For one, there’s the amount of time it take for the Earth to turn once on its axis so that it returns to the same orientation compared to the Universe. Then there’s how long it takes for the Earth to turn so that the Sun returns to the same spot in the sky.

To answer the second question first, it takes exactly 24 hours for the Sun to return to the same spot in the sky, which would seem obvious. 24 hours is what we think of as being a complete day, and the time it takes to transition from day to night, and back again. But the answer to the second question is more interesting. It actually takes the Earth 23 hours, 56 minutes, and 4.09 seconds to turn once on its axis compared to the background stars.

6 hours of rotation. Credit: Chris Schur

The night sky, showing 6 hours of rotation captured by long-exposure. Credit: Chris Schur

But why is there a difference? Well, that would be because the Earth is orbiting around the Sun, completing one orbit in just over 365 days. If you divide 24 hours by 365 days, you’ll see that you’re left with about 4 minutes per day. In other words, the Earth rotates on its axis, but it’s also orbiting around the Sun, so the Sun’s position in the sky catches up by 4 minutes each day.

The amount of time it takes for the Earth to turn once on its axis is known as a sidereal day – which is 23.9344696 hours. Because this type of day-measurement is based on the Earth’s position relative to the stars, astronomers use it as a time-keeping system to keep track of where stars will appear in the night sky, mainly so they will know which direction to point their telescopes in.

The amount of time it takes for the Sun to return to the same spot in the sky is called a solar day, which is 24 hours. However, this varies through the year,  and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. This is caused by two factors, the first being the Earth’s elliptical orbit around the Sun, which causes it to move faster when it is closest (perihelion) and slower when it is farthest (aphelion).

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit as viewed from the Sun during the Northward equinox. Credit: NASA

Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit as viewed from the Sun during the Northward equinox. Credit: NASA

The second factor is Earth’s axial tilt, which means that when the Sun crosses the equator at both equinoxes, it’s daily shift relative to the background stars is at an angle to the equator. In June and December when the sun is farthest from the celestial equator a given shift along the ecliptic corresponds to a large shift at the equator. So apparent solar days are shorter in March and September than in June or December.

And as stated earlier, the Earth’s is spinning rather rapidly. In fact, scientists have determined that Earth’s rotational velocity at the equator is 1,674.4 km/h. This means that just by standing on the equator, a person would already be traveling at a speed of 1,674.4 km/h in a circle. However, the planet is slowing slightly with the passage of time, due to the tidal effects the Moon has on Earth’s rotation.

Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. The Earth’s rotation also goes from the west towards east, which is why the Sun rises in the east and sets in the west.

Portrait of Our Dusty Past
Artist’s impression what the Solar System looked like in the early stages of formation, as a dust cloud circling a star. Credit: JPL/NASA

Another interesting thing about the Earth’s rotation is how it all got started. Basically, the planet’s rotation is dude to the angular momentum of all the particles that came together to create our planet 4.6 billion years ago. Before that, the Earth, the Sun and the rest of the Solar System were once part of a giant molecular cloud of hydrogen, helium, and other heavier elements.

As the cloud collapsed down, the momentum of all the particles set the cloud spinning. The current rotation period of the Earth is the result of this initial rotation and other factors, including tidal friction and the hypothetical impact of Theia – a collision with a Mars-sized world that is thought to have taken place approx. 4.5 billion years ago and formed the Moon.

This rapid rotation is also what gives the Earth it’s shape, flattening it out into an oblate spheroid (or what looks like a squished ball). This special shape of our planet means that points along the equator are actually further from the center of the Earth than at the poles.

In short, the world has been spinning since its inception. And, contrary to what some might say, it actually is slowing down, albeit at an incredibly slow rate.

We’ve written several articles all about Earth for Universe Today. Here’s an article about how fast the Earth rotates, and here’s an article about why the Earth rotates.

If you’d like more information on the Earth’s rotation, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.



Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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