The Sun and the Stars • G3
The Sun’s Period of Rotation
Activity G3
Grade Level: 6–12
Source: This activity comes from the curriculum module Solarscapes: Sunspots and Rotation, ©1999 Space
Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301. 720.974.5888. It was written
by Beverly Meier (Boulder Valley School District) in coordination with scientists and educators at the
Space Science Institute and Science Discovery Program. This material is based upon work supported
by the National Science Foundation under Grant No. ESI-9453798. Copies maybe ordered from the
Space Science Institute or downloaded from the Web by following the “K–12 Curriculum” link from
www.spacescience.org
What’s This Activity About?
Four centuries ago, Galileo observed sunspots on the
surface of the Sun (by projecting our star’s image on a
screen). He then deduced that the Sun rotates because
the spots moved across its face over many days. This activity uses modern images of sunspots to show students
how we can estimate the Sun’s rotation period.
What Will Students Do?
Students examine images of the Sun showing sunspots
taken with the Yohkoh satellite. They use an overlay grid
to mark the positions of the spots and then calculate
rate at which the spots are moving to arrive at the rotational period of the Sun.
Tips and Suggestions
• Since this activity may tempt students to turn from the
images of the Sun to the real Sun in the sky, it is very
important to emphasize that looking at the Sun even
for a short period of time can cause serious eye damage. The Sun should only be viewed by projecting an
image or using a filter that is known to be safe. (If you
have access to a telescope and filter, we recommend
showing students the real Sun and helping them find
and follow sunspot groups. Note that whether or not
sunspots are visible on the Sun’s surface depends on
where the Sun is in its 11-year cycle of activity.)
•A
possible extension of the activity is to use the (admittedly fuzzy) images in the activity to get a sense of
how large the sunspot groups on the images are. The
diameter of the Sun is about 860,000 miles. Students
can measure the size of the Sun and a sunspot group
on the pictures and then set up a ratio to determine the
actual size of the sunspot group in miles.
• Th
e Sun is not a solid body, but made of gas. While
this may be beyond the level of some of the younger
students, bear in mind that the Sun does not rotate
with one period. It is actually in differential rotation,
with regions at different latitudes rotating with different periods.
•T
o learn more about many aspects of the Sun’s outer layers, an excellent web site is the Stanford Solar
Center at http://solar-center.stanford.edu
What Will Students Learn?
Concepts
Inquiry Skills
Big Ideas
• Solar rotation
• Sunspots
• Periods of rotational motion
• Observing
• Calculating
• Describing
• Recording
• Patterns of change
• Scale
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 1
G3 • The Sun’s Period of Rotation
The Sun and the Stars
ACTIVITY 4:
THE SUN’S PERIOD OF ROTATION
Saturn and its Moons
Its orbit around the Sun
Other factors affecting
density and mass
Guide to Teachers
Goal: Students
will determine the
Sun’s period of
rotation.
MATERIALS NEEDED
1
23
99
3
81
9
10
92
38
01
9
23
8
20
3
94
85
28
40
5
58
2
90
59
In this activity students apply the previous lesson to the Sun, calculate its period of
rotation, and reflect on what they have learned about the Sun compared to what
they originally knew about the Sun at the start of Solarscapes. It functions as a
REFLECT/APPLY phase in Solarscapes.
•
•
•
•
•
One copy of the student activity, “The Sun’s Period of Rotation” (included)
An overlay to determine solar latitude and longitude (Figure 4, included)
Photographs of sunspots for four consecutive days (Figure 5, included)
A photocopy of the student activity and Figures 4 and 5, preferably one
copy per student
A calculator
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 2
G3 • The Sun’s Period of Rotation
The Sun and the Stars
Procedure:
XXXXXXXXXXX
XXXXXXXXXXX
XXXXXXXXXX
DISCUSS: Open a class discussion by asking students to recall what they know about
sunspots. Make a list of the information students provide. Now ask your students to
recall how they can determine the period of rotation of an object with fixed features on
its surface and discuss this procedure. Tell them (if they have not already pointed this out
themselves) that sunspots appear to move across the face of the Sun over time. Have the
students form groups of 4. They are to list all of the possible reasons that might be
responsible for that apparent motion then share their list with the class. Four common
reasons that one might expect are:
•
•
•
•
Sunspots move across a non-rotating Sun.
The Sun rotates.
The Earth moves around the Sun, causing sunspots to appear to move.
Sunspots appear and disappear in different places, appearing to move.
Ask students to think about these explanations and to try to devise ways in which one
could distinguish among the possibilities.
EXPLORE: Hand out the worksheets to the students, who will work in groups of three
to calculate the solar rotation period. Students are to answer the questions individually as
homework.
REFLECT: Student groups report their calculated periods of rotation. The Sunspot
Motion Table is shown on page 3. It displays the measurements and calculations which
should approximate those of your students. If one (or more) groups ended up with
significantly incorrect results (the correct answer is about 26 days), ask them to go back
and review their measurementts and/or calculations. Discuss any remaining discrepancies between the answers with the students. Did they use the same sunspot groups? Did
anyone forget to correct for Earth’s orbital motion? Point out that measurement uncertainties as well as other factors (in this case the exact feature chosen) can influence the
answer. Scientists often get different results, then try to figure out what is causing those
differences.
APPLY: Ask students to discuss in their groups what they know about the Sun. Review
the brainstorming list developed at the start of Solarscapes.
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 3
G3 • The Sun’s Period of Rotation
The Sun and the Stars
Sunspot Motion Table - For Teacher Use
Longitude of
Sunspot Group
Change in
Longitude
(Ex: Day 2 - Day 1)
Corrected
Longitude
Change
Period of
Rotation
(Solve for "X")
Day 1
290
Day 2
180
110
120
30 days
Day 3
30
150
160
22.5 days
Day 4
-100
130
140
25.7 days
Average Rotational Period = 78.2 days = 26 Days
3
NOTE: Each horizontal and vertical line on the overlay (Figure 4) represents 100 latitude or longitude, respectively. Since these lines do not show tenths of a degree, students
should estimate the longitude to the nearest whole degree, then solve for “X” to the
nearest tenth of a day.
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 4
The Universe at Your Fingertips • Astronomical Society of the Pacific
Student did not
complete the task.
0
Student determines
rotation period in
group.
1
Group did not
participate.
0
Group is not able to
correctly determine
the rotation period.
1
Group is able to
correctly determine
the rotation period.
2
Student determines
rotation period in
group, and answers
questions in Student
Guide.
2
3
Student determines
rotation period in
group and correctly
answers the questions
in the Student Guide.
3
SUGGESTED USE: Make one copy per student; there is also room for you to add your own task and scoring criteria.
Determining the
Sun's period of
rotation
T ask(s)
Group Assessment (goal met if group achieves a "2")
Determining
the Sun's period
of rotation and
answering
problem
Questions
T ask(s)
Individual Assessment (goal met if student achieves a "2")
Student Name: ________________________________________
Scoring Rubric for Activity4: Rotation Period of the Sun
4
4
The Sun and the Stars
G3 • The Sun’s Period of Rotation
Page 5
G3 • The Sun’s Period of Rotation
The Sun and the Stars
Student Guide to Activity 4: The Sun’s Period of Rotation
Problem: How can we determine the Sun’s period of rotation?
Introduction
You and your buddy are having a “camp out.” It’s 4:00 AM. You’ve awakened so many
times that you’ve lost count. What was it this time? Was there some little sound, the tent
flapping in the gentle breeze, or the purr of a cat, or maybe the snapping of a twig? You
feel safe enough in your own back yard, snuggled warmly in your sleeping bag. You
aren’t really scared, but you hear every tiny sound that never before seemed so loud;
night seemed to drag on forever. Finally, the first hint of daylight appears as your eyelids
grow heavy and your brain starts wondering, “What causes day and night?”
You might answer, “Earth’s rotation,” but you’ve heard others say, “Earth’s revolution.”
Which response is correct? If you mean Earth’s spinning on its axis, then “rotation” is
correct. As Earth rotates, the Sun comes into view at daybreak. Earth continues to spin,
so that the Sun appears to move across the sky until it sets at night. The Sun is no longer
visible, but Earth continues to rotate until the Sun comes into view the next morning.
One complete rotation takes about 24 hours, or one day. In contrast, “revolution” means
the motion of a body around a point in space or another body. Earth’s tilt on its axis and
its revolution around the Sun give us seasons. After about 365 rotations, or one year, our
Earth has revolved around the Sun one time.
We know from personal experience that Earth rotates because we experience day and
night and we know that the Sun does not revolve around Earth. What about our Sun, that
sphere of glowing gas rising just beyond the eastern horizon? Does it rotate? To find
out, we can use one of the Sun’s most prominent features, sunspots.
Ancient Chinese, Japanese, and Greek literature refer to observations of spots on the Sun;
however, the first recorded telescopic observations of sunspots were not made until 1610
by Galileo, an Italian astronomer. After many years of speculation, we are still not sure
what causes sunspots, but we do know some interesting details about them.
Here, Christopher Scheiner, a contemporary of Galileo’s, and a fellow
Jesuit scientist trace sunspots in Italy, in about 1625.
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 6
The Sun and the Stars
G3 • The Sun’s Period of Rotation
A large sunspot can sometimes be 20 times larger than Earth (courtesy, Space Environment Center/NOAA)
Current theory says that sunspots are caused by horseshoe shaped magnetic fields, buried
just below the Sun’s surface, that prevent the flow of hot gases from below. Like a
horseshoe magnet, sunspots have both a magnetic north and south pole. The spot where
the magnetic field is located cools slightly compared to its surroundings. Because the
spot is cooler, it looks darker.
Sunspots occur in 11 year cycles where the sunspot number increases from a minimum of
almost zero to a maximum of over 100 spots. After one 11 year cycle, the poles of the
horseshoe shaped magnetic fields switch and another cycle begins. Consequently, we
have an overall 22 year magnetic cycle as well as an 11 year sunspot cycle.
At the beginning of an 11 year cycle, several spots appear at about 30½ north and south
latitudes. As the cycle progresses, you can track the sunspots’ motion. Generally, individual sunspots or groups of sunspots do not shift significantly in latitude over time.
However, they do seem to move across the face of the Sun. What do you think could be
the reason for this? Could we use this apparent motion to determine the Sun’s period of
rotation?
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 7
G3 • The Sun’s Period of Rotation
The Sun and the Stars
ACTIVITY 4 STUDENT WORKSHEET
1. Locate the major group of sunspots on the photographs in Figure 5.
2. Place the plastic overlay (Figure 4) on the photographs so that you can determine the
latitude and longitude for the major sunspot group. Each vertical and horizontal line
equals 100 latitude and longitude, respectively.
3. Estimate how far the sunspot group moves between day one and day two by subtracting the smaller longitude from the larger. Record this longitude change in the Table
below. Estimate this number to the nearest whole degree.
4. Repeat the same process for the next days up to day four and record your results.
5. The Earth revolves around the Sun at a rate of 360 degrees in one year (365 days) or
an average motion of about 10 per day. Since Earth revolves around the Sun in the same
direction as the Sun rotates, our motion seems to chase after the sunspots. Thus, the
apparent movement of sunspots is less than the real rotation by about 10 per day. Therefore,
you must compensate for the orbital motion of Earth by adding 10 to your computed apparent daily motion.
6. Let us assume that sunspots are features whose position on the Sun does not change
very much over the course of a solar rotation. Use the following proportion to calculate
the Sun’s period of rotation (in days):
corrected longitude change = 360 degrees
1 day
X days
7. Find the average rotational period and record it in the Table below. Calculate to the
nearest tenth of a day. Be prepared to share your result with the class.
Sunspot Motion Table - For Student Use
Longitude of
Sunspot Group
Change in
Longitude
(Ex: Day 2 - Day 1)
Corrected
Longitude
Change
Period of
Rotation
(Solve for "X")
Day 1
Day 2
Day 3
Day 4
Average Rotational Period =
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 8
The Sun and the Stars
G3 • The Sun’s Period of Rotation
Questions:
1. Since the sunspot group appears to move across the Sun’s surface, what kind of solar
motion does this suggest?
2. Based on your calculations, what is the Sun’s average period of rotation?
3. If the sunspot group had changed latitude would you have been able to calculate the
number of degrees of change per day as easily? Why or why not?
4. Answer the problem question at the beginning of this activity. Include in this discussion what you learned about the Sun and what more you would like to learn about the
Sun.
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 9
G3 • The Sun’s Period of Rotation
The Sun and the Stars
Figure 4. Solar Grid Overlay
00
300
200
100
300
200
100
00
00
-100
-100
-200
-300
-200
-300
NOTE: Each horizontal or vertical line = 100 latitude or longitude, respectively.
Instructions: Copy this onto transparencies for use with Figure 5.
The Universe at Your Fingertips • Astronomical Society of the Pacific
Page 10
The Universe at Your Fingertips • Astronomical Society of the Pacific
Day 4
Day 2
Sunspots can be used to determine how long it takes the Sun to rotate about its axis. These visible
images were taken by the Japanese Yohkoh spacecraft, during February 12th through the 15th, 1996.
Day 3
Day 1
Figure 5. Sun Rotation Series
The Sun and the Stars
G3 • The Sun’s Period of Rotation
Page 11