Sunspots Lesson - University of Wyoming

Sunspots Lesson Web resources:

• Based on the following lessons (http://solar-center.stanford.edu/teachers/lessons.html)

• Solar filters (http://www.thousandoaksoptical.com/ecplise.html) • Latitude/Longitude grid (http://solar-center.stanford.edu/solar-

images/latlong.html) • SOHO website (http://sohowww.nascom.nasa.gov/data/realtime-images.html) • Archived images (e.g.,

http://www.solen.info/solar/images/AR_CH_20141001.jpg) Activity 1: Observing the Sun

Warning: Do not look directly at the Sun without a filter. Even if you can do it once, over time you will damage your eyes.

Warning: NEVER look through an unfiltered telescope, period. As one of my professors told me you can actually do this twice: if you’re stupid enough to do it again with the remaining eye. Galileo was the first European to discover sunspots, and he did so by looking through his unfiltered telescope. He was blind at the end of his life.

Draw the Sun. For each sunspot group, name the group and record its latitude/longitude. Activity 2: Collect More Raw Data Repeat the Sun observations every day for a month. If you have limited time as we do today, use the archived images linked above. Activity 3: Analysis How long does it take for the Sun to do a full rotation (spin on its axis)? Extension: Make a graph of Latitude as a function of date (one graph for all sunspots).

10/20/2014 Projecting the Sun

http://resources.yesican-science.ca/trek/eclipse0602/pinhole2.html 1/2

adapted from Society for Popular Astronomy SPA

Background

You can easily and safely observe the Sun by projecting it through a tiny hole onto a white

sheet of paper.

Purpose

To construct a simple sun-projecting device from a telescope or binoculars

Materials

1 sheet of stiff white papertelescope or binoculars

(optional) tripod

Procedure

1. Point a telescope or binoculars at the Sun. Do not attempt to view the Sun directlythrough the telescope or the binoculars! In the case of a telescope, make sure that anysmall finder telescope is capped, and keep the cover on one half of the binoculars. The

easiest way to find the Sun is tilt your instrument to get the smallest shadow.

2. Hold a piece of white card about 15 cm behind the eyepiece to act as a screen on whichyou can catch the image. You should see a bright circle of light, probably blurred, on the

screen. Focus the instrument until the circle is sharp. This is the disc of the Sun itself. If

the eclipse is in progress you should see the Moon as a dark bite out of one edge.

3. Experiment with moving the card farther closer and further away. What effect does the

distance from the card have on the image?

4. Try mounting the binoculars or telescope firmly on a tripod. The advantage of this type ofprojection is that several people can see the image at once.

10/20/2014 Projecting the Sun

http://resources.yesican-science.ca/trek/eclipse0602/pinhole2.html 2/2

10/20/2014 ASP: The Nearest Stars: A Guided Tour

http://www.astrosociety.org/edu/publications/tnl/05/stars2.html 1/4

The Nearest Stars: A Guided Tour

Activity: Observing the Sun — Safely

by John R. Percy, University of Toronto

(c) 1986 Astronomical Society of the Pacific

[Editor's note: Since other stars are so far away, much of the progress we are making in

understanding stars in general comes from studying our own "hometown'' star, the Sun. Although

most of us associate the study of astronomy with the night, in this month's activity our newsletter's

newest contributing editor shows that the Sun can lend itself to useful daytime astronomy activities.

(Dr. Percy is professor of astronomy at the University of Toronto in Canada and one of the world's

leaders in the field of astronomy education.)]

We must begin with an important warning: Never look directly at the Sun, especially when using

binoculars or a telescope. Direct sunlight can cause permanent eye damage in seconds, without thevictim being aware of it until it is too late.

For safe direct viewing of the Sun, #14 welder's glass can be used, or a proprietary material known as

Solar Skreen (Roger W. Tuthill, Inc., 11 Tanglewood Lane, Mountainside, NJ 07092). Although some

telescopes are equipped with Sun filters, many of these are not reliable, and should not be used unless

you are absolutely sure of what you are doing. The only reliable filters are some (but not all) which fitover the front of the telescope, and reflect away most of the light.

The best way to view the Sun with binoculars or a telescope is by projection — looking at an image ofthe Sun rather than at the Sun itself. Instructions for doing this are given below.

We should note that some school officials feel that all viewing of the Sun should be forbidden. Even

though there are safe ways to view the Sun, there is always a chance that some student will not take

the necessary precautions, or will disobey instructions, and an accident will occur. The projection

methods described below are quite safe, however — and the number of astronomy-related school

accidents is far less than the number encountered in other science subjects!

Viewing the Sun by Projection

This method is relatively safe and, with it, many people can view the Sun at once. You will need a pair

of binoculars or a small telescope, a piece of plain cardboard about 30 centimeters square for the

"collar,'' and a second piece of white cardboard (or paper) at least 10 centimeters square for the

screen. If you use a telescope, you should mount it on a tripod. If you use binoculars, you can hold

them in your hand, but it is much more convenient (and you will have a steadier image) if you

improvise some sort of stand or tripod to hold them.

This demonstration can be done at any time of the day when it is clear and when your class has access

to direct sunlight.



Note: Do not use binoculars whose front lenses are 50 millimeters across or wider. (Binoculars usually

are described by a pair of numbers separated by an 'x', such as "7 x 3'' or "7 x 50'';; the number to the

right of the 'x' is the diameter of the front lenses in millimeters.) Big lenses gather a lot of light, and

the heat generated by direct sunlight in side large binoculars can damage their complex optics.

Method

1. Make a cardboard collar to fit around the front end of the binocular or telescope, as shown in thefigure. This shades the area where the image will be from sunlight. and (in the case of

binoculars) will cover the lens which you are not using.

2. Focus the binocular or telescope on infinity by looking at a distant object (not the Sun!) in thenormal way. (If you are using a telescope. use a low-magnification eyepiece.)

3. Point the binoculars or telescope at the Sun (do not look through the instrument to do this!), asshown in the figure, and adjust the direction of pointing until the image of the Sun appears on

the screen. (This may take a minute or two. One useful trick is to watch the shadow of the

binoculars or telescope tube: if pointed directly toward the Sun, then the sides of the tube will

cast no shadows, and the instrument's shadow will be as small as it can be.)

4. Move the screen toward or away from the eyepiece until the image of the Sun fits neatly in themiddle. and adjust its tilt until the Sun's image is circular.

5. Jiggle the binoculars or telescope very slightly. Any specks on the image of the Sun which do notjiggle along with the image when you do this are specks in the binoculars or telescope (or

smudges on the screen), and not spots on the Sun itself.

Observations

When you and your students examine an image of the Sun, you will notice the following properties:

1. The image is brighter in the middle of the disc than at the edges. This effect is called limb-darkening. It occurs because, when we look at the middle of the Sun's disc, we are lookingstraight down into the hotter part of the Sun. At the edges of the disc, we look more obliquely,

and see only the cooler, less bright gases, higher in the Sun's atmosphere.



2. The image moves slowly across the screen. This is due to the east-to-west motion of the Sun inthe sky, caused by the rotation of the Earth. The direction of motion of the image therefore tells

you which direction on the screen (and on the Sun's image) is west.

3. There may be small darks spots on the image. These are called sunspots and are regions in theouter layers of the Sun which are cooler and therefore not as bright as their surroundings. In

sunspots, the Sun's magnetic field is exceptionally strong, and astronomers suspect that this is

connected to their being darker than the material around them. Sunspots, when examined

closely with a telescope, are seen to be very complex. They can form within a few days, and may

endure and evolve for weeks or months.

An Alternate Way to Project An Image of the Sun

This method produces an image which is a bit fuzzy, but good enough to show large sunspots, and it is

particularly suitable for observing a partial eclipse of the Sun. It is very safe, and can be used to show

an image of the Sun to an entire class. You will need a small pocket mirror or hand mirror. a piece of

plain cardboard to fit over the mirror (or some tape to cover it), and a piece of white cardboard or

paper to use as a screen.

Method

1. Cut the plain cardboard or paper so it fits over the mirror.2. Cut or punch a very small hole, about 5 millimeters in size. in the middle of the plain cardboard.You could also use tape to cover all but a small portion of the surface of the mirror.

3. Put the mirror on a window sill in the sunlight such that it catches the rays from the Sun. Turnthe room lights off and draw the window blinds so that as little as possible of the room other

than the mirror is in sunlight.

4. Reflect the sunlight onto a wall of the darkened room.5. Put the white cardboard or paper on the wall at this point, so you can use it as a screen todisplay the image of the Sun.

Observations

1. You will notice that the image of the Sun is round (unless an eclipse is in progress), even if thehole which you cut or punched in the plain cardboard or paper was square!

2. You can also demonstrate that the size of the image of the Sun is proportional to the distance ofthe screen from the mirror. The larger the distance. the larger (and fainter) the image. In a more

advanced class, you might want to develop an explanation for these two observations.

If you do not have a classroom in which there is a sunlit window, you can do the activity outdoors. Find

a place where you can catch the sunlight with your mirror, and can reflect it onto a shaded wall.

(Better still, reflect it into a darkened classroom.) Again, you can use a sheet of white paper or

cardboard as a screen. It takes a few minutes to discover the best arrangement for the mirror and the

screen, but once you have done so, it is easy to set up the demonstration again on any following day.

Further Reading About The Sun

Robert Burnham: "Observing the Sun,'' Astronomy, August 1984, p. 51.

B. Ralph Chou: "Safe Solar Filters,'' Sky & Telescope, August 1981, p. 119.

Alan MacRobert: "Close-Up of a Star'', Sky & Telescope, May 1985, p.3 97.



Simon Mitton: Daytime Star (1981, Scribner's)

<< previous page | 1 | 2 |

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Living With a Star EG-2001-1-005-GSFC12

*These lessons can be adapted for higher grade levelsby including telescope mirrors and observing eclipses.Teachers can also project the Sun’s image through a tele-scope resulting in a larger image for tracking sunspotsand other solar activity.

There are several ways you canobserve the Sun, and hopefullysunspots, for yourself. The easiestand safest is to project the Sun by buildingyour own pinhole camera. If you have a telescope, you will have to equip it with asolar filter or use a solar telescope that youcan access via the Web.

12 Projecting the Sun

13 Using Remote Solar Telescopes

13 Using Your Own Telescope

14 Observing Solar Eclipses

15 Sunspot Drawings

PAGE ACTIVITY

Hands-on activities for use in the classroom.

Classroom Activities

Observing the Sun for

Yourselfhttp://solar-center.stanford.edu/

observe/observe.html

Classroom ActivitiesGrade Level 3-5*

Courtesy of the Stanford Solar Center

Partial solar eclipse image from Fred Espenak’s Eclipse Home Page at NASA’s Goddard Space Flight Center.

http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

CAUTION!Don’t EVER look directly

at the Sun, with orwithout a telescope

(unless you have the proper filters).

star 1/31/01 9:51 AM Page 14

Projecting the Sun

You can easily and safelyobserve the Sun by projecting

it through a tiny hole onto awhite sheet of paper.

This simple device is called a “pinhole camera.”

1. With the pin, punch a hole in thecenter of one of your pieces of paper.

2. Go outside, hold the paper up andaim the hole at the Sun. (Don’t look atthe Sun either through the hole or inany other way! )

3. Now, find the image of the Sunthat comes through the hole.

4. Move your other piece of paper back and forthuntil the image rests on the paper and is in focus(i.e., has a nice, crisp edge). What you are see-ing is not just a dot of light coming through thehole, but an actual image of the Sun.

Experiment by making your hole larger or smaller.What happens to the image? What happens when

you punch two holes in the piece of paper? Trybending your paper so the images from the twoholes lie on top of each other. What do you thinkwould happen if you punched a thousand holes inyour paper, and you could bend your paper so allthe images lined up on top of each other?

In fact, optical telescopes can be thought of as acollection of millions of “pinhole” images allfocused together in one place!

You can make your pinhole camera fancier byadding devices to hold up your piece of paper, ora screen to project your Sun image onto, or youcan even make your pinhole camera a “real” cam-era by adding film.

If you want to learn more about how light works, youcan join artist Bob Miller’s Web-based “Light Walk”

at the Exploratorium. It’s always aneye-opening experience for studentsand teachers alike. His unique discov-eries will change the way you look atlight, shadow, and images!

You’ll need:

• 2 sheets of stiff white paper

• 1 pin• A sunny day• Perhaps a

friend to help

Bob Miller’s Light Walkhttp://www.exploratorium.edu/light_walk/lw_main.html

Several sites give instructions for building more exotic pinhole cameras forobserving the Sun:

Cyberspace Middle School http://www.scri.fsu.edu/~dennisl/CMS/sf/pinhole.html

Jack Troeger’s Sun Site http://www.cnde.iastate.edu/staff/jtroeger/sun.html

Related Resources

Activities courtesy of the Stanford Solar Centerhttp://solar-center.stanford.edu/observe/observe.html





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Using Remote Solar Telescopes

UsingYour OwnTelescope

Using Mike Rushford’s robotic solar observatory inLivermore, California, you can get a real-time view of the Sunby controlling a telescope from your Web browser. At cloudy times,there are other things to do as well!

The safest way to look at the Sun through your own telescope is NOT to!

Looking at the Sun can cause serious damage, evenblindness, to your eyes, unless you have proper filters.

The safest practical way to see the Sunis by eyepiece projection. Line up yourtelescope with the Sun, but instead oflooking through the eyepiece, hold asheet of white paper behind the eye-piece. You’ll see a solar image project-ed onto the paper. What happenswhen you move the paper farther back?

Experiment with the paper to get asharp viewing contrast. You should beable to see the largest sunspots withthis method.



Viewing the Sun With a Telescope http://www.sunspot.noao.edu/PR/answerbook/telescope.html#q15

Dr. Sunspot gives more detailed information about safely viewing the Sun witha telescope and filters.

Observing the Sun in H-Alpha http://www.4w.com/pac/halpha.htm

This site gives technical information on how to observe the Sun with your owntelescope using an H-alpha filter. Includes detailed information on what fea-tures of the Sun are best seen in H-alpha. By Harold Zirin, Peter V. Foukal,and David Knisely.

Related Resources

Eyes on the Skieshttp://sunmil1.uml.edu/eyes/index.html

Related Resources




Solar Eclipse © 1999 Paul Mortfieldhttp://www.backyardastronomer.com

Galileo Galilei used telescopes to observe and tracksunspots c.1600. Picture from The Galileo Project.

http://es.rice.edu/ES/humsoc/Galileo/

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A solar eclipse occurs when the Moon, during its monthly revolution around

Earth, happens to line up exactlybetween Earth and the Sun. Why isn’t there

an eclipse every month? Because solar eclipsesoccur during a new moon, but not at every

new moon. Most often the Moon passes a littlehigher or a little lower than the Sun. There is a

solar eclipse about twice a year, when theMoon’s and the Sun’s positions line up exactly.

You can safely observe a TOTALLYeclipsed Sun with the naked eye, butyou will need a pinhole camera, anappropriate type of welder’s glass,or special Mylar glasses to safelyobserve the beginning and ending ofa full or partial eclipse.



Observing Solar Eclipses

Fred Espenak’s Eclipse Home Pagehttp://sunearth.gsfc.nasa.gov/eclipse

Eclipse: Stories From the Path of Totality http://www.exploratorium.edu/eclipse

Solar Data Anaylsis Center Eclipse Information http://umbra.nascom.nasa.gov/eclipse

Eclipse Paths http://umbra.nascom.nasa.gov/eclipse/predictions/eclipse-paths.html

Related Resources

The glory of a solar eclipse comes from thedramatic view of the Sun’s corona, or outeratmosphere, which we can see only when thebrilliant solar disk is blocked by the Moon. Thecorona is not just light shining from around thedisk: It is actually the outermost layer of the solaratmosphere. Although the gas is very sparse, it isextraordinarily hot (800,000 to 3,000,000Kelvin), even hotter than the surface of the Sun!(The heating of the corona is still a mystery.) Thecorona shows up as pearly white streamers, their

shape dependent on the Sun’s current magneticfields. Thus every eclipse will be unique and glo-rious in its own way.

A solar eclipse is only visible from a small area ofEarth. It’s unlikely that, during your lifetime, you willever see a total solar eclipse directly over the placeyou live. Many people travel long ways to experi-ence a total solar eclipse. If you’re lucky, you mightsomeday see a partial solar eclipse (one where theMoon doesn’t quite cover all the Sun’s disk) nearby.

Solar eclipse image from Fred Espenak’s Eclipse Home Page at NASA’sGoddard Space Flight Center.http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html




15Living With a Star EG-2001-1-005-GSFC

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Until recently, astronomers havehad to rely on drawings or sketches

to document what they’ve seen.

Charge-coupled device (CCD) camerasand other technological wonders have

changed all that. Historic drawings, however, are still very important.

And even today, drawings are still more accurate at recording exactly what the eye sees, unaltered by the

processing of fancy electronics.

Galileo’s drawings of sunspots (c. 1600) still survive today. And thesolar telescope at Mt. Wilson, abovePasadena, California, has been collecting sunspot drawings since1917. The tradition continues. Youcan check current sunspot drawingseach day at the Websites listed here,and compare them with your own.



SunspotDrawings

Daily Sunspot Drawing Observations at Mt. Wilson http://www.astro.ucla.edu/~obs/150_draw.html

Daily Sunspot Images from SOHOhttp://sohowww.nascom.nasa.gov/latestimages

Galileo’s Sunspot Drawings http://es.rice.edu/ES/humsoc/Galileo/Things/g_sunspots.html

Sunspots at the Exploratoriumhttp://www.exploratorium.edu/sunspots

These classroom activities can be found at: http://solar-center.stanford.edu/observe/observe.htmlCreated by Deborah Scherrer, April 1997. Last revised by DKS on 2 December 1997.

Related Resources

Galileo Galilei (left) and sunspot drawings (above) from The Galileo Project.http://es.rice.edu/ES/humsoc/Galileo/




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10/20/2014 sungrid-0.gif (550×550)

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10/20/2014 solar-center.stanford.edu/solar-images/worksheet.html

http://solar-center.stanford.edu/solar-images/worksheet.html 1/1

Sunspot Data Recording Worksheet

Name: _______________ Date: _______________

Complete one of these each day of your observations

For Each Sunspot Group

Spot Group/ID

Give each group an identifying nameLatitude Longitude Comments/Changes Observed

10/20/2014 The Spinning Sun

http://solar-center.stanford.edu/spin-sun/spin-sun.html 1/2

The Spinning Sun

Does the Sun spin?Galileo Galilei, back in 1612, noticed something interesting about the Sun when he observed itssunspots. Let's see what he discovered.

Data | Compute Rotation | Observe Rotation | Your & Galileo's Discovery | Other Pages

Collect Your Data

If you haven't already done so, collect solar data for a couple weeks by looking at imagesfrom the SOHO spacecraft.

Arrange your copies or drawings of the solar disk and sunspots in order from longest-ago tothe present, and flip through them slowly as if they were an animated "flip-book". Are thesunspots permanent, or do they come and go? Do they stay in the same place, or seem tomove? If they move, do they move in relatively straight lines or do they wander?

Galileo noticed that the spots seemed to move directly across the disk of the Sun. He thoughtthat, if they were on the surface of the Sun, their movement might indicate that the Sun wasrotating. What do you think?

Compute the Sun's Rotation Rate

Let's try to calculate the rotation rate of the Sun:

10/20/2014 The Spinning Sun

http://solar-center.stanford.edu/spin-sun/spin-sun.html 2/2

By Estimating

By Calculating the Spots' Angular Velocity

Observe the Sun Rotate and check your answers.

What Did you Discover?

Would you like to compare your sunspot sketches with those made by Galileo?

Galileo's drawings and animations.

Did Galileo's sunspots move horizontally across the solar disk? Do your sunspots movehorizontally across the solar disk? If your pictures differ from Galileo's, why do you think that is?

This page is http://solar-center.stanford.edu/spin-sun/spin-sun.html Created by Deborah Scherrer

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10/20/2014 Getting Solar Images

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Retrieving Solar Images

To discover various characteristics of the Sun, you'll need to observe it. Your "eyes" will be theSOHO spacecraft, currently circling the Sun about 1,000,000 miles from Earth. With SOHO's 12specialized scientific instruments, we can explore everything from the Sun's glorious halo or"corona", to the violent magnetic storms on its surface, to the sound waves which help usunderstand the mysteries of the Sun's deep interior.

Using daily pictures collected by the SOHO spacecraft, you are going to observe and recordinformation about the currently visible sunspot groups. What do you think we can learn fromwatching sunspots?

What You'll Need | What to Do | Getting the Images | Examples | For the Teacher |Exploring Other Images

What You'll Need to Get Started:Sunspot Recording Worksheet. Print out and make enough copies for each day of your observations.

Latitude/longitude grids. Print these out. If you can, copy the grids onto transparency paper.

An image of the Sun every day, for about 2 weeks. You will pick these up from the web(see below). Your images will look something like this, only bigger. (The sunspot groupsshow up as black and white blotches):



What you are going to do:You are going to observe and track the movement of sunspots (actually, magnetically "activeregions") across the Sun's visible disk.

Every day, using the web, print out a copy of the internet solar image (we tell you howbelow). If you don't have a printer, sketch the image and sunspot groups you see. (If youhave to sketch, try placing the latitude/longitude grid directly over the image on yourscreen to find exactly where to sketch your spots. Be careful to always have the imagestraight up and down.)

For each of the major sunspots groups, record on your Sunspot Recording Worksheet:

The name of each spot group. Make up any name you want, but make sure tokeep track of which group has which name.Where (i.e. at what latitude and longitude) the spot groups lieNote whether there were any observable changes in your sunspot groups (has thegroup changed size, shape, disappeared altogether?)

Collect images every day for 10-14 days. After you've collected your data, go on to theother activities.

Getting the Images:The images you will be getting are called either intensitygrams or magnetograms. They are



retrieved every 96 minutes by the MDI instrument on the SOHO spacecraft.

Before going any further, read About the Images. Don't cheat and skip this part!

When you look at the image lists, use the images labeled:SOHO MDI, Magnetogram, longi. comp., Full Disk

or SOHO MDI, Intensitygram, Full Disk

If there is more than one magnetogram or intensitygram available, pick the one doneearliest in the day (there will be a time given with each).

SOHO Daily Images

Help-- I Need Some ExamplesAn example set of magnetograms (images).

An example of a completed Sunspot Recording Worksheet.

For the Teacher

Exploring Other ImagesThere are many interesting sites which provide solar images. The following are particularly goodcollections of images from various places around the Earth and in space:

SOHO Synoptic Database -- Repository of recent images from other sites

Space Environment Center -- A collection of images from other sites.

Solar Data Analysis Center at NASA Goddard Space Flight Center in Greenbelt,



Maryland. A collection of images from other sites.

For more image sites, see our About the Sun page.

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10/20/2014 Estimating the Sun's Rotation Period

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Estimating the Sun's Rotation Rate

How can you use your sunspot data to figure out how long it takes for the Sun to spin around once? Toestimate the Sun's rotation rate, let's assume that the Sun is a flat disk, just like it appears on your copies orsketches.

You can use a calculator for this exercise.

On your data sketches, pick a sunspot group which travels a long distance across the Sun's disk. A goodchoice would be a sunspot which starts out closest to the left limb (edge) of the Sun. Let's call this spot"George". You are going to figure out how long it took George to move across the Sun.



Find your picture with George closest to the left limb of the Sun. With a (metric) ruler, measure how faraway from the left edge of the Sun's disk George is.

Now, find the picture with George closest to the right limb of the Sun and measure it's distance. Makesure you again measure the distance starting from the left limb.



Now, measure the distance across the entire disk of the Sun (ignoring George and any of his friends).You will need to multiply this by 2 to include the back side of the Sun.

Look again at your data sheets and find out what time your first sketch of George was taken. Find thetime for your last sketch of George. How long did it take for George to travel from the first place to the last?(Subtract the last time from the first. In our example, it was 7 days.

Now, how far around the Sun did George go? In our example, George went 6 cm (7 cm - 1 cm) and theSun was 24 cm around. So, in this example, it took George 7 days to get 1/4 of the way around the Sun, whichmeans that George would need 4*7 = 28 days to go all the way around (assuming he could last that long).

If your numbers are more complicated than George's, then you can use your calulator to figure it out:

Sun's rotation time = George's-time * (Sun's-distance / George's-distance) 28 days = 7 days * (24 cm/6 cm)

If you picked a different spot or group, do you think your answer for the Sun's rotation rate would be the same?Try to find out by doing the calculation for groups at higher or lower latitudes (that is, groups that are closer orfarther from the Sun's poles).

You have just estimated the solar rotation rate. Your data will be more accurate if you use the AngularVelocity exercize.

To see how close your estimate is and also watch an animation of the Sun

spinning.

Return To Sunspot Rotation Activity

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10/20/2014 Calculating the Sun's Angular Velocity

http://solar-center.stanford.edu/spin-sun/angvel.html 1/2

Calculating the Sun's Angular Velocity

What is Angular Velocity?

Angular velocity is how fast something travels in a circle (or on a sphere): it is the angle by which

an object spins in a certain time -- its rotation rate.

Imagine a clock (the old-fashioned kind with hands). The minute hand goes around the clock, a

circle -- 360 degrees, in 60 minutes. To find its angular velocity, or rotation rate, you need to

divide the number of degrees by the number of minutes:

360 degrees / 60 minutes = 6 degrees per minute

So, the minute hand's angular velocity, or rate of rotation, is 6 degrees per minute.

10/20/2014 Calculating the Sun's Angular Velocity

http://solar-center.stanford.edu/spin-sun/angvel.html 2/2

How Can I Determine the Sun's Angular Velocity?

We can use the movement of a sunspot to determine the Sun's rate of rotation, just like we used

the minute hand of a clock. To do this accurately, you will have to use a transparent template for

your sketches of the Sun's disk. The template has marked on it the degrees of longitude on a

sphere.

From your sketches, pick the sunspot group which is the most long-lived (that is, which

appears in most of your sketches). Using your template, figure out how many degrees of Sun the

spot group traveled across, and how long it took to move that far.

Sun's Angular velocity = degrees the spot has covered / time it took for the spot to travel

Sun's rotation rate = 360 degrees / angular velocity

Going Further

If the Sun has been particularly active and you have sunspots sketched at different latitutes on

the Sun's surface, compute the angular velocity for different sunspot groups which appear at

varying latitudes. Do you get the same solar rotation rate for each set? If not, why not?

Want more information on the Sun's rotation?

Return To Sunspot Rotation Activity

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Sunspots Lesson - University of Wyoming

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