Marianne Dyson, November 2017
Did you know that the sun rotates on its axis about once a month? Since all sides of the sun look essentially the same, how can scientists tell how fast it goes around? The sun has spots! These spots act as markers for what part of the sun is facing Earth. By tracking their motion, scientists can clock the rate of motion of the surface.
Back in 1610, Galileo was the first to notice spots moving across the Sun. But some people didn’t believe him. They said the spots were planets crossing between the Earth and Sun, casting shadows like Mercury and Venus. Galileo explained that the spots changed shape and sometimes appeared and disappeared, unlike the known planets.
Galileo also noted that the speed at which the spots crossed from the “left” side to the “right” side of the Sun was not constant. It usually takes about 11 days for a spot to make a crossing. (Because the sun is not a solid, the high latitudes take 36 days and the equatorial region takes 25 days for a full rotation.) Spots near the edge appear to move faster than when they are moving across the middle third of the disk. This effect is called foreshortening. A spot coming around the limb of a sphere is moving towards the viewer even though the disk appears flat from a distance. So it appears to be moving faster than when it is crossing the middle of the disk.
You too can prove the sun rotates by tracking sunspots. But please be careful! Never ever look directly at the sun, and especially not with binoculars or a telescope that isn’t covered by a special filter. As Galileo sadly discovered, looking at the Sun for just a few minutes can cause permanent blindness.
If you don’t see any spots, there might not be any. The number of spots varies over a 11-12-year solar cycle. The minimum number of spots is expected in 2019 or 2020. During solar minimum, the spots are closer to the equator and generally smaller than during maximum years.
NASA’s “The Sun Now” shows the state of the Sun every day. The images can be printed out and used to plot the progress of sunspots across the disk of the Sun. (I recommend dividing the diameter into at least six equal sections and drawing vertical lines. Then write the time it took the center of the spot to move from one line to the next to see foreshortening for yourself.)
Galileo proved through observations that the Sun has spots and rotates. Scientists now know that sunspots are areas where the magnetic fields are about a thousand times stronger than other areas. The magnetic fields cause the plasma to “bunch up,” and, to keep the pressure constant (T is lower since PV=nRT), cool off compared to surrounding areas. (Spots are 4000 versus 6000 degrees K.) These cooler areas appear as dark spots to human eyes, though I hope thinking about them has “brightened” your day!
Writing about Space
As my Twitter followers and Facebook friends know, my house was flooded by Hurricane Harvey. We are slowly rebuilding the downstairs while “camping” in my upstairs office. We hope to have floors and bedrooms by Christmas, and a new improved kitchen by early in the New Year. We are very grateful to friends and neighbors who have helped us deal with this disaster. I also appreciate all of you who subscribe to my blog and buy copies of my books and eBooks for yourselves or as gifts for others. You have really lifted my spirits! Thank you!
And I’m happy to announce that the new book I’m coauthoring with Buzz Aldrin for National Geographic Kids now has a title: To the Moon and Back! This book is Buzz’s personal story of the historic first trip to the Moon, brought to you in 3-D pop-up format by the extraordinary paper engineering wizard, Bruce Foster. Expected release is the fall of 2018 in time for the 50th anniversary of the first manned Apollo flight.
Speaking about Space
If you’d like me to visit your school or speak at your event in the spring or summer, please visit my website and send an email with program and schedule preferences for 2018.
December 5-7, 2017, I’m attending SpaceCom Expo at GRB in Houston. Will Buzz make a surprise appearance? I don’t know, but he did last year!