Many objects in space rotate. While Earth completes its rotation in about 24 hours, Venus takes a whopping 243 Earth days. The moon’s rotation period is approximately 27 days. It turns out that the sun also rotates. So, how long does it take for the sun to go around once?
The answer depends on your cosmic perspective and the part of the sun you’re measuring.
In 1612, Galileo Galilei observed the Sun through a telescope, drew what he saw, and observed that sunspots (dark areas near the Sun’s surface) moved across the Sun’s surface over time. “Galileo tracked a lot of information [sunspots] J. Todd Hoeksema, a solar physicist at Stanford University, told Live Science. By measuring the speed at which sunspots moved around the sun, Galileo discovered that the sun rotates once every 28 days. However, this number does not tell the full story of the sun’s rotation speed.
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Centuries later, in the mid-1800s, British astronomer Richard Carrington also measured the sun’s rotation rate using essentially the same method as Galileo, but with a better telescope, Hoksema said. Carrington determined how fast sunspots rotate in the specific region where they are most frequently observed (about 30 degrees of solar latitude). According to Carrington’s measurements, the sunspot traveled at a speed that took about 27.3 days to complete one revolution around the sun.
Hoksema said most sunspots appear and disappear within a week or two, so they don’t rotate completely. Still, astronomers like Galileo and Carrington were able to map the sunspot’s movement over several days to determine the speed of the sun’s rotation and, from there, how long it would take for the sun to complete a rotation, Hoksema explained.
However, the Earth’s rotation makes these calculations difficult. Because the Earth moves around the Sun and in the same direction as the Sun’s rotation, measurements of the Sun’s rotation taken from Earth capture the speed of the Sun’s rotation relative to the Earth’s movement. This type of measurement, called synodal rotation rate, is nearly two days longer than measurements related to the star’s motion, called stellar rotation rate, said Nicolene Viall, a research astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Carrington’s 27.3 days included two additional days, Viall said.
That means the sun actually rotates more than once during Carrington’s rotation period of 27.3 days, Viall said. Nevertheless, Carrington’s rotation rate “ended up being adopted by everyone as the norm,” Hoksema said.
But compared to the star’s slow motion, which is negligible in this context, Viall said scientists now know that at the latitude of the sun where Carrington observed the sunspot, it takes about 25.4 days for the sun to rotate.
“From a pure physics perspective, stellar velocity is the correct rotational speed,” Viall says. Therefore, from now on in this article, we will use the stellar rotation rate.
latitude and depth
Researchers like Carrington had to rely on visible features of the Sun, such as sunspots, to determine the Sun’s rotation rate. The problem, Hoksema says, is that not all regions of the sun have sunspots. He noted that researchers who rely on sunspots to measure the speed of the sun’s rotation are limited by where on the sun they can make measurements, since there are “essentially no sunspots” at the poles and “relatively few” at the equator.
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Hoeksema said the speed of the sun’s rotation depends on its latitude and depth, so measurements at different points on the sun are needed to get the full picture.
“What’s interesting is that there is no single rotation rate that can explain the sun,” he says. There seems to be a rate for each part. This phenomenon, called differential rotation, occurs because the Sun is made of gas. On the other hand, the Earth is solid and does not rotate differentially. All parts of it must work together.
Starting in the 1970s, scientists began observing the sun’s rotation using methods other than visual observation. One of them is heli-seismology, which “determines the properties of the sun using sound waves traveling inside the sun,” Hoeksema said.
Scientists can also measure the sun’s rotation by observing the Doppler shift. Doppler shift is when the light waves emitted by a rotating sun become shorter or longer depending on whether they move toward or away from you.
By combining these data sources, scientists found that the sun rotates fastest at the equator, completing one revolution every 24.5 days, and slowest at the poles, where each revolution takes more than 34 days. This latitude-based variation occurs from the surface of the Sun to the bottom of the convective zone, a layer of the Sun that extends about a third of the way to the center.
In the same region, the speed of the sun’s rotation also changes with depth, Hoksema said. Further inside the Sun, the radiation zone between the convective zone and the center of the Sun rotates like a solid body at a rate of about 26.6 days, regardless of latitude.
Scientists don’t fully know how fast the sun’s core rotates because they don’t have enough measurements, Hoeksema said.
That’s “something people will understand in the future,” Hoksema said.
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