Labs from YSI 95 :
Studying the Sun and Sunspots.

Rich Dreiser
CARA Yerkes Summer Institute, August 1995

Portions of this lab have been adopted with the author's kind permission from Science Projects in Astronomy, by Bill Wickett, copyright 1990, ISBN 0-9631051-0-8.

This is the staff copy of the lab.

Introduction

This lab is designed to familiarize students with some characteristics of the Sun, observing the Sun safely, and telescope operation.

During the first part of the lab we will be learning about the Sun, the closest star to Earth.

During the second part of the lab, we will learn the basics of operating the ten-inch reflecting telescope inside the South Building. This will be equally useful when using the telescope at night.

During the third part, we will use the telescope to examine the Sun. We will determine the direction the Sun appears to be moving, track the Sun with the telescope, and attempt to trace the solar disk and sunspots.

During the fourth part, we will figure out how big the sunspots are.

During the fifth part, we will determine the rotation of the Sun based on the how many degrees the sunspots traveled over time, from the data we have collected or from earlier data.

During the sixth part, we will see how to estimate the size and diameter of the Moon.

Very Important Information!

The Sun is extremely bright in both visible wavelengths and invisible infrared and ultraviolet wavelengths responsible for the heat you can feel and sunburn. Keep in mind at all times when working on this lab that we will be magnifying the SunÕs light and heat. Never look at the Sun through the telescope or binoculars. You risk permanently damaging your eyes. In fact, do not stare at the Sun with your unaided eyes.

We can safely observe the Sun by projecting an enlarged image onto a white piece of paper. The image, spread out this way, is not dangerous to our eyes. Take care though not to get too close to the telescope eyepieceÕs focal point (where the light narrows and gets concentrated) as this is hot enough to burn.

Part One : A Brief Introduction to the Sun

We will attempt to obtain images of the Sun. Among other things the Sun is: Which direction does the Sun move in the sky? What causes this motion? If the Sun is at a certain place in the sky at say ten in the morning, when will it be in the same place in the sky the next day? Is the Sun ever directly overhead at Yerkes Observatory or in Chicago? Can you think of any place on Earth where the Sun might appear to be? Can you think of any place on Earth in August where the Sun will not appear to rise for many weeks?

We can nearly always see sunspots, "temporary" cool regions that appear to float on the upper layer of the Sun, often for days or weeks at a time. Why do you suppose sunspots appear dark? We will assume that sunspots rotate at about the same rate at the Sun. Why do you think this is not precisely true?

Sunspots often appear to move in pairs. We know these and other changes that we can observe on the Sun are caused by changes in the solar magnetic field. These change may be in response to gas flow in the solar upper atmosphere. Astronomers and other scientists would certainly like to understand more about how and why magnetic fields emerge from the solar surface!

We know that sunspots change over time, sometimes considerably. Scientists cannot predict when a sunspot will appear or when a spot will disappear. We cannot tell if a spot that disappears around one edge (or limb) of the Sun will appear coming around the other side weeks later. If we are lucky, we may see an unusually large sunspot group that appears to be associated with other phenomena. About the best we can do is recognize that solar activity varies according to a 22-year cycle.

Sunspot activity is often associated with solar flares. Sometimes, solar particles from such enormous flares become trapped in the EarthÕs magnetic fields and interact with the EarthÕs upper atmosphere to create auroral displays, colorful, glowing, flickering nearly- transparent sheets or bands through which the stars may be seen.

Such displays may look beautiful, but their appearance means telecommunications will be affected, sometimes profoundly so. Essential radio transmissions may be interrupted and power grids disrupted. Excessive flare activity can cause the EarthÕs atmosphere to expand somewhat, slowing satellites, and it can be a major hazard to astronauts above the EarthÕs atmosphere.

Part Two: Operating the Ten-inch Reflecting Telescope

We will be learning the following, which will also make it possible to operate the telescope at night: unlocking and opening the dome; recognizing and learning to lock and unlock both the right ascension (or hour angle) clamps and the declination clamps; moving and aiming the telescope by hand; focusing the telescope; turning the telescope drive on and off; and rotating the dome.

Some things to keep in mind: Before opening the shutters, be certain to unlock them. Do not force them apart! To avoid hitting the wooden object alongside the dome track (it is a digital clock which doesnÕt work), the shutters must be opened before you turn the dome.

It might be a good idea to open the shutters only halfway to keep the interior as dark as possible so the projected image of the Sun will appear brighter.

Before trying to move the telescope, always check that the declination and right ascension clamps are unlocked (turned all the way counterclockwise). Never move anything by trying to force it.

The telescope is hard, heavy, and sharp: keep track of where your body and head are and you wonÕt get hurt. Before moving the telescope, watch out for where others are standing, and warn them you are about to begin moving the telescope. It is best if only one person moves the telescope at a time.

To prevent anyone from injuring their eyes, we will cover the finder telescope. To keep some of the SunÕs radiation out of the ten-inch reflecting telescope, we will cover half the aperture with a cardboard "stop."

When trying to aim at the Sun, Do Not use the finder telescope or look through the telescope eyepiece! Instead, we will unclamp the telescope in both right ascension and declination, then move the telescope until its shadow is as small as possible. An image of the Sun should appear simultaneously on our focusing screen.

When the Sun is correctly lined up, we can gently lock both the declination and right ascension axes and turn on the drive. Why do we need a telescope drive? What will happen if we donÕt turn on the drive?

If the shadow of the telescope or the projected image disappears, perhaps clouds are obscuring the Sun. Keep an eye out on the weather so we can close the shutters well before it starts raining!

Part Three: Tracing the Solar Disk and Sunspots

Before we trace the Sun and sunspots, we want to be certain each trace is oriented the same way. Turn off the telescope drive and mark the direction the Sun is drifting. To establish the east-west direction, we will use the spot where the solar edge, or limb, first drifts off.

Does the projected image move to the left or to the right? Is this what you expected? Can you think of any possible explanation?

Trace the image of the Sun and the positions of any sunspots, as accurately as you can. Note on the paper the date and time of each observation.

Make note of anything else you may notice about the projected solar image. What if a bird or cloud gets between us and the Sun? Does the image appear sharp, or does it appear to wave? (Are no sunspots visible? Does the limb of the Sun appear darker than the center? Can you think of any possible explanation for the lack of sunspots or the limb darkening?)

Part Four: Determining the Size of Sunspots

Obviously, if we know how much larger the diameter of the Sun is than our tracings, we can determine the actual sizes of the sunspots, whether from observations we have made, or from the provided solar photographs.

Since most of us may find it difficult to imagine what numbers such as 2,600 km really mean in terms of size, we will be comparing the sizes of sunspots to the Earth itself, which I hope you recognize is a reasonably large planet!

sunspot image size = actual size of the sunspot

Sun image size actual size of the Sun

Given: the SunÕs radius is 695,990 kilometers. In scientific notation, this is 6.9599 x 105 km. (Do you recall that 695,990 kilometers is the same as 695,990,000 meters? How big is our tracing of the Sun? Can you express this in some fraction of one meter?) We need to solve the equation above for the actual size of a sunspot.

Now, we want to express this in terms of the Earth. EarthÕs radius is 6378 km (6.378 x 10^2 km). The Sun is 6.9599 x 10^5 km times larger than Earth.

The Sun is _____ times bigger than the Earth. (If you do not understand scientific notation, refer to the attached sheet. Or, you may solve the problem longhand instead.) So, the sunspot in question must be _____ times the size as Earth!

Part Five: Calculating the rotation of the Sun

If the Sun were a solid sphere, we would expect it to rotate all together during a given period of time. Since it is not solid, the rotation may vary from place to place. We will assume that the Sun rotates at the same speed and does not speed up nor slow down. Can we measure such motion from day to day? Consider: if the Sun were rotating VERY slowly, we would expect to see little day-to-day change in sunspot positions.

We can only take measurements from the side of the Sun that is facing the Earth. We fill assume it takes between seven and thirty days for a sunspot to travel from one edge of the Sun to the other visible to us (remember, thatÕs only half of the rotation period). We may be able to measure this rotation in as little as four or five days.

If we can record some sunspots, we may be able to measure the direction they are moving on the SunÕs photosphere and thus determine the time it takes for the Sun to revolve. In the event that we are unable to find any sunspots, or we are hampered by cloudy weather, we will use data obtained with the 40-inch refracting telescope, in the form of 8x10-inch glossy photographs. If the weather and sunspots permit, we will compare the two sets of data.

Suppose we had been tracing images of the Sun that were one meter in diameter. Do you see how the diameter of the Sun would be approximately 1.4x10^9 times larger? Suppose our drawings were one-half of one meter in diameter. How much bigger is the Sun?

If we really could measure distances directly on the SunÕs disk, we would. Since we cannot, we will measure the same proportional distances on a much smaller disk, the tracing we have done, and enlarge as necessary. If the real Sun is one billion times larger, the distance the sunspots have moved on the Sun will be one billion times larger than on the distance they moved on our sphere.

Remember we must at all times use the same units of length. For example, if weÕre talking about the size of sunspots in kilometers, we must speak of how large the Sun is in kilometers, not meters.

More About Solar Rotation.

An astronomer made measurements of sunspots at equal intervals of time but forgot to tell me if the observations had been taken at the same time every day, every two days, every three days, or perhaps once a week!

In the attached drawing Figure One A, I show the solar disk as it appears from Earth, with the sunspot observations labeled first, second, third, fourth...and so on.

In Figure One B, I have projected the data onto a drawing of the Sun as it might appear if we were directly over one of the SunÕs poles. Note I have divided the Sun into thirty-six equal sections, each ten degrees in width, which resemble slices of pie.

The sunspot positions and solar divisions can be seen in both drawings. From our privileged position above the pole, we see that sunspots move the same distance from each observation to the next. Why from our vantage point on Earth do the sunspots appear to spend more time along the limbs?

Part Six: Estimating the Size and Distance of the Moon

The Moon is much smaller than the Sun, but because it is approximately 500 times closer to the Earth than the Sun, it just so happens to appear to take up approximately the same area in the sky, about one- half a degree. Suppose the Moon were twice as large but at its same distance. How large would it appear in the sky? Suppose it were twice its size at doouble its distance. How large would it appear in the sky? Suppose the Moon were half its size but at the same distance. How large would it appear in the sky?

Actually, both the Sun and the Moon vary somewhat in size, appearing slightly larger when they are closer to Earth. Although this is easy to see on photographs, it is not an effect that is apparent to the eye!

Given that the angular sizes of the the Sun and the Moon are the same, we can estimate the diameter of the Moon without too much effort. We are dealing once again with proportions:

    diameter of the Moon      the diameter of the Sun
   ---------------------  =   -----------------------
    distance to the Moon      the distance to the Sun 
What would you need to know to calculate the distance to the Moon? What do you need to calculate the diameter of the Moon?


Important Disclaimers and Caveats: