How fast is the Earth moving on a cosmic scale?

The earth, this blue planet, not only dances on its axis and moves leisurely around the sun, but also moves into the depths of the universe following the pace of the Milky Way.

No matter from which dimension you look at it, the Earth is in constant motion. The blue planet rotates on its axis, completing one revolution approximately every 24 hours. While rotating, the Earth also orbits the Sun, covering a distance of nearly 1 billion kilometers per year. The entire solar system, including the Sun, the planets and their satellites, is located in the Milky Way, orbiting around the center of the Milky Way on a time scale that transcends human history.

In addition, the Milky Way moves within the Local Group of galaxies, which itself travels through interstellar space.

Although we can hardly feel the movement of the Earth on Earth, through observations of the universe, we can accurately understand the movement of the Earth on different cosmic scales.

Image caption: Photos of the Earth taken by NASA's Messenger probe

Earth's rotation speed

The speed at which the Earth rotates may seem like a simple question, but it actually has a complex and varied answer, which depends entirely on your specific location on the Earth.

When we consider the Earth to be a "rigid body," we mean that the motion of each point on its surface relative to other points remains relatively constant. The Earth rotates around its axis once, and almost every point on its surface undergoes a complete rotation, which takes about 23 hours, 56 minutes and 4.09 seconds, not the 24 hours we usually think of.

The difference between these two times comes from the influence of the Earth's revolution around the sun , which means that from one sunrise to the next sunrise, or from one sunset to the next sunset, the Earth needs to rotate more than 360 degrees.

The average radius of the Earth is about 6,371 kilometers, but the rotation speed at a specific point will be different due to the undulating terrain and the overall shape of the Earth (the equator is bulging and the poles are compressed).

For example, a person at the equator rotates with the Earth at 1,676 kilometers per hour, while a person at 45 degrees north or south latitude moves slower, at about 1,183 kilometers per hour. At the North or South Pole, people rotate on their own axis but do not actually move.

The Earth's rotation also brings about other interesting phenomena. For example, at high latitudes, because they are closer to the center of the Earth, the acceleration of gravity is greater. The faster rotation speed near the equator means that less energy is required to launch a rocket into space, which is why tropical regions are the preferred place for rocket launches.

In addition, the tidal effects of the moon and the sun are gradually slowing down the Earth's rotation, causing the Earth's rotation period to increase by about 14 nanoseconds per year. About 4 billion years ago, in the early stages of the solar system, the Earth's rotation speed was much faster than it is now, and a day at that time was only 6 to 8 hours instead of 24 hours.

The speed of the Earth's revolution around the Sun

Compared to its rotation, the speed of the Earth's revolution around the Sun is more significant. The average distance between the Earth and the Sun is about 150 million kilometers. In order to maintain a stable near-circular orbit, the Earth must move at a speed of about 30 kilometers per second.

This speed varies as the Earth moves from perihelion to aphelion, reaching a maximum of 30.29 km/s and a minimum of 29.29 km/s, a difference of about 3%.

Image caption: The location of the solar system in the Milky Way

How long does it take for the solar system to orbit the center of the Milky Way?

The Milky Way has hundreds of billions of stars that orbit around the supermassive black hole at the center of the Milky Way under the gravitational pull of the star. Our solar system is also located in the Milky Way, and although it is about 27,000 light-years away from the center of the Milky Way, it is also "bound" by the huge gravity to revolve in an elliptical orbit around the center of the Milky Way.

The time it takes for the sun to orbit the Milky Way is extremely long, roughly between 220 million and 250 million years. That is to say, when the sun was last in its current position in the Milky Way, dinosaurs had just appeared on Earth.

It is estimated that the Sun's orbital speed in the Milky Way reaches about 220 km/s, which is 7 times the speed of the Earth's orbit around the Sun. It should be noted that there is an uncertainty of about 20 km/s in this value, which means that the accuracy of scientists' calculation of the Sun's orbital speed in the Milky Way is only about 10%.

Image caption: The orbital plane of the solar system planets is about 60 degrees from the sun's orbit in the Milky Way.

How fast is the Milky Way in the Local Group of galaxies?

However, the Milky Way is not only an independent galaxy in the universe, it is also a member of the Local Group of about 60 galaxies, interacting with other galaxies including the Andromeda Galaxy.

The largest and most massive galaxy in our local group is the Andromeda Galaxy , which is about 2.5 million light-years away and may be twice as massive as the Milky Way.

Because the Andromeda Galaxy is so far away that we cannot accurately track its position over time on a human time scale, but we can measure its speed. Astronomers' observations show that we and the Andromeda Galaxy are approaching each other at a speed of about 301 kilometers per second.

Combining the direction and speed of the Sun's movement within the Milky Way, we can conclude that the Milky Way and the Andromeda Galaxy are approaching each other at a speed of 109 km/s; this means that we are in the process of a collision between these two galaxies, and it is expected that in about 4 billion years, a spectacular galaxy merger will slowly begin.

Image caption: This set of photos depicts the gradual merging of the Milky Way and the Andromeda Galaxy as it appears in the sky above Earth.

How fast is the Local Group of galaxies traveling through interstellar space?

The situation is more complicated and interesting when it comes to the speed at which the Local Group travels through intergalactic space. In the wider universe, the Virgo Supercluster is a powerful gravitational source, pulling the Local Group toward it at a speed of about 300 km/s.

However, at the other end of the universe, a region with little matter is repelling us at a similar speed. The combined force of this attraction and repulsion makes the net movement speed of the Local Group of galaxies in the universe reach about 600 kilometers per second.

On a cosmic scale, where are we rushing?

When we combine the Earth's rotation and revolution, the movement of the Sun in the Milky Way, the relative motion of the Milky Way and the Andromeda Galaxy, and the movement of the Local Group of galaxies affected by the surrounding cosmic structure, we can draw a conclusion: at any moment, the total speed we move in the universe is about 368 km/s, pointing in the direction of Leo.

This speed fluctuates over the course of a year due to the Earth's rotation around the sun, by as much as 30 kilometers per second. Measurements of the cosmic microwave background radiation support this conclusion: the cosmic microwave background radiation is hotter in the direction we are moving (Leo) and cooler in the opposite direction (Aquarius).

To accurately answer the question "how fast is the Earth moving through the Universe?" we must be clear about our reference. Is it the Earth's own rotation speed, or its motion relative to the Sun? Is it relative to the Milky Way or the Local Group of galaxies, or to other large structures in the Universe? Or is it relative to the cosmic microwave background left over from the Big Bang? There is no absolute stillness in the Universe; everything is constantly moving and interacting.

With the deepening of scientific research and the improvement of the accuracy of the Milky Way map, we hope to more accurately predict the movement of the Earth and the Local Group of Galaxies in the universe in the future, with an accuracy far beyond today's imagination.

References

https://bigthink.com/starts-with-a-bang/fast-earth-move/

Compiled by: Wen Xing

Reviewed by: Li Xin, Research Librarian of Beijing Planetarium