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Does The Sun Rotate?

A mosaic of 4 images taken of the Sun on Nov. 13, 2011. Credit: Leonard Mercer.

A mosaic of 4 images taken of the Sun on Nov. 13, 2011. Credit: Leonard Mercer.

The rotation of the Sun is kind of hard to pin down. That’s because a day on the Sun depends on which part of the Sun you’re talking about. Confused yet? It kept astronomers puzzled for years too. Let’s look at how the rotation of the Sun changes.

A spot on the equator of the Sun takes 24.47 days to rotate around the Sun and return to the same position. Astronomers call this sidereal rotation period, which is different from the synodic period – the amount of time it takes for a spot on the Sun to rotate back to face the Earth. But the Sun’s rotation rate decreases as you approach the poles, so it can actually take 38 days for regions around the poles to rotate once.

The Sun’s rotation is seen by observing sunspots. All sunspots move across the face of the Sun. This motion is part of the general rotation of the Sun on its axis. Observations also indicate that the Sun does not rotate as a solid body, but it spins differentially. That means that it rotates faster at the equator of the Sun and slower at its poles. The gas giants Jupiter and Saturn also have differential rotation.

And so, astronomers have decided to measure the rotation rate of the Sun from an arbitrary position of 26° from the equator; approximately the point where we see most of the sunspots. At this point, it takes 25.38 days to rotate and return to the same spot in space.

Astronomers also know that the interior of the Sun rotated differently than the surface. The inner regions, the core and the radiative zone, rotate together like a solid body. And then the outer layers, the convective zone and photosphere, rotate at a different speed.

The Sun and the entire solar system orbits around the center of the Milky Way galaxy. The average velocity of the solar system is 828,000 km/hr. At that rate it will take about 230 million years to make one complete orbit around the galaxy. The Milky Way is a spiral galaxy. It is believed that it consists of a central bulge, 4 major arms, and several shorter arm segments. The Sun and the rest of our solar system is located near the Orion arm, between two major arms, Perseus and Sagittarius. The diameter of the Milky Way is about 100,000 light years and the Sun is located about 28,000 light-years from the Galactic Center. It has been suggested fairly recently that ours is actually a barred spiral galaxy. That means that instead of a bulge of gas and stars at the center, there is probably a bar of stars crossing the central bulge.

So when someone asks you what the rotation of the Sun is, ask them which part.

Here’s an article from Universe Today about the Sun’s magnetic field flip, and here’s an article about how there were no sunspots on the surface of the Sun.

Here’s more information on the topic from Windows on the Universe, and here’s an article from NASA.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

About 

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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