The Sun is an object in the sky that we have observed from infancy. It seemed very large to us and it gave us heat and light. As we grew older, we began to ask questions about the Sun and were given answers that may have been right or wrong, but they helped to satisfy our curiosity. What are some of the questions we may have asked at these earlier ages and how would we answer them today?
How was the Sun formed?
The presently believed theory on the origin of the Sun, is that it formed from a large rotating inter-stellar gas cloud that condensed into the spherical object we call the Sun. To help them understand why and how it became a star, and to explain how this mass of gas and dust contracted and became hot enough to allow nuclear fusion to begin, go to the following website: http://www.owlnet.rice.edu/~esci101/Lecture?ESC1101.02.SolarSystem.pdf ESCI 101: Lecture 2. Origin of the Sun and Planets, January 14, 2005.
This website shows with diagrams and explanations how the Sun was formed.
If you have access to the website: http://www.brainpop.com, you may want to look at the short movie entitled "Lifecycle of Stars" for the younger students. This movie shows the development of a star beginning with the clouds of dust to the "death" of the star. They explain that the mass of the star determines what will happen to that star at the end of the lifecycle. There is also a ten-question quiz, at the end, you may use to evaluate the students grasp of the "Lifecycle of Stars."
Another source of information on the life history of the Sun is found at: http://unitedstreaming.com, "The Death of the Sun and the Solar System."
How big is the Sun?
After they have found this information by reading assigned material, it might be fun to do the activity found at: http://www.cse.ssl.Berkeley.edu/AtHomeAstronomy/activity_03.html, "Finding the Size of the Sun and Moon." This website takes you step-by-step from the constructing the device used to measure the diameter of the Sun, how to use this device and how to calculate the diameter of the Sun. If you have them do this activity, it is fun doing a percent error comparing their results with the assigned reading figures.
Correct Value - Experimental Value
% Error = Correct Value X 100%
How far away is the Sun?
Because the Sun is so large compared to the other objects in our solar system, it appears to be very close to Earth. When we talk about this distance, we usually use the average distance of 93 million miles. Students usually want to know how we arrived at this figure.
A successful activity I have used is having them do triangulation activities. I have them do this activity at two different distances. After I give them the real measured distances, I have them compare the results of their scale drawing to the measured distance.
A good website for this activity is: http://motivate.maths.org/conferences/conf46/c46_parallax.shtml
This website also shows how parallax is used to find this distance.
Structure of the Sun
When we (middle school) talk about the structure of the Sun, we generally refer to the core, photosphere, chromosphere and corona.. A good overview for the student is to view the brainpop film "Sun." This gives some basic information about the Sun and again has questions at the end of the viewing that you may use for evaluation purposes.
Brainpop is a website that cost $134.95/yr for "Teacher Access." This allows you 35 logins per day. This website not only has information for Science but for Math, Social Studies, Technology and English.
The Sun in relation to other stars in our universe is considered an average size star. However, more stars are smaller than larger compared to the Sun. The composition of the Sun, by mass, is hydrogen 74% and helium 24%. These two elements, as a result of nuclear fusion, form the energy we receive from the Sun. Basically what happens is four hydrogen nuclei are fused to form one helium nuclei. This energy works its way to the surface of the Sun, after about one million years, and radiates energy we see as visible light as well as forms we cannot see but can detect with special instruments.
The core is located in the center of the Sun and is the location of the above mentioned fusion reaction. Both the temperature and the density of the Sun decrease as you move away from the core.
The photosphere (light) is what we see as the visible surface of the Sun. Granules cover the entire surface of the Sun, except where there are sunspots. Granules are convection cells that constantly rise to the surface, cool and after about 10-20 minutes are replaced by new granules.
The sunspots are dark areas that are cooler than the rest of the surrounding surface and may last for several days to weeks. Sunspot activity seems to peak in an eleven-year cycle allowing their magnetic fields to cause the most havoc with communications on earth. Tracking sunspot activity has helped us to learn that all areas on the sun do not rotate at the same speed, rotation being faster at the equator than at the poles.
It takes light approximately 81/2 minutes to travel the 93 million miles to Earth. There are other forms of energy the Sun sends to Earth. We cannot see these forms, but can detect their presence. These other forms of energy come as magnetic fields, ultraviolet rays, radio waves, infrared wave, x-rays, and gamma rays.
The chromosphere(color –sphere) is located above the photosphere. The temperature rises from 6,000º C to 20,000º C in this region. The activities that can be observed in the chromosphere are solar flares, prominences and filament eruptions, and post-flare loops.
A solar flare is a tremendous release of energy that occur in the neutral area between opposing magnetic fields. The energy released is as much a billion megatons of TNT. Prominences and filaments are both dense clouds material that are pushed above the surface by loops of magnetic field. Prominences are seen near at the edge of the Sun and can be seen during an eclipse. Post-flare loops are seen after a solar flare event. They are loops that condense from the hot corona and move back to the surface of the Sun.
The corona is the outer atmosphere of the Sun and can be seen during a total eclipse. During the early observations, astronomers were puzzled by the composition of the corona and believed that it was made of a new element which they named "coronium." Later investigations proved that it was composed of superheated hydrogen and helium that were stripped of their electrons.
Scientist have been puzzled by the high temperatures in the corona. The currently held view is that magnetic fields transfer energy from the photosphere to the corona. The temperature in the corona ranges from 1,000,000 to 2,000,000 degrees K. An interesting fact is that very little heat is transferred to other materials because of the very low densities.
Sunlight and Earth
We all know that life on Earth depends upon sunlight. Without this light there would be no plants to supply food and oxygen for our use. We can now appreciate that the study of the Sun-Earth relationship involves more than just curiosity. We will now investigate how we learn facts about the Earth by studying Earth-Sun relationship.
Because we wanted to get the maximum use of the Sun, communities originally had their own solar time. As railway transportation developed, much confusion was encountered reading a time schedule, as each location had a different time. Sir Sanford Fleming developed a system for Standard Time setting-up time zones that will be explained later in this unit. In some areas, we created daylight savings time to maximize daylight hours and now we are trying to use solar energy to help use with our energy problems.
Most of us, at one time, probably thought that the Sun revolved around the Earth. At such early ages, we never knew the difference between the Sun and Moon and never thought about the tides unless we lived near an ocean.
As we grew older and gained more knowledge about these things, we gained an appreciation for those who made and proved the behaviors of the Sun and Earth.
A good starting point would be to perform 3 investigations on Sun-Earth relationships. After completing these investigations, they appreciate how new information proves or disproves previous beliefs. One investigation will give the best explanations for Sun-Earth movements based on our present day knowledge.
If you have access to the book "Earth Science for Secondary Schools" by Bob Swift(see bibliography) or an earlier version entitled "Interactions of Earth and Time, these three investigations showing possible Sun-Earth movements may be used as a demonstration or a hands-on activity for the student. We begin by using the changing lengths of days as a means of determining the proper Sun-Earth relationship.
Investigation 1 has the Sun in a fixed location. The Earth is in another fixed location and rotating on its axis. To satisfy the changing hours of daylight, they have the Earth rocking back and forth toward and away from the Sun. This model satisfies the criteria for the changing length of days.
Investigation 2 has the Earth in a fixed location with a tilted axis and the Sun is revolving around the Earth. This also satisfies the changing lengths of day and night.
Investigation 3 has the Sun in a fixed position and the Earth, rotating with its axis tilted, revolving around the Sun. Again this satisfies the changing lengths of days.
These investigations make the students realize that they must start using other information to eliminate some or all of the models investigated. The first piece of evidence they use is Polaris. They know that they can go our on any night, face north, raise their arm at about a 45° angle and Polaris is always in the same spot. This knowledge eliminates investigation 1.
Investigation 2 & 3 are still accurate at this point. They use the analogy of a moving car in a rainstorm with spotting meteors. They tell the students that more raindrops hit the front windshield of the car than the rear windshield because the front of the car is moving into the rain while the back of the car is moving away. They then say we see more meteors in the morning than at night. If the Earth were stationary, you would see the same number of meteors throughout the night. This eliminates investigation 2.
Remind the student that we now have more definitive proof of Earth revolution using Cosmic Microwave background radiation.
Now that we have determined the Earth revolves around the Sun and the axis of the Earth is tilted, causing the hours of daylight to vary during the year what else might we want to investigate? We may want to investigate the direction of Earth rotation, how we maximize the hours of daylight and how we determine our location on Earth. We can use shadows to help us understand some of these questions.
From an early age we noticed that our shadow changed in size and direction. However, most of us were more interested in having fun chasing the shadow and not really thinking about what caused it. When we observe our shadow during the day, we notice it not only changes direction but also its size. In the morning we notice that it points in the direction opposite the Sun and is larger than the object that causes the shadow. At noon, its direction has changed and the shadow is shorter. In the evening, the shadow is longer again but points in the opposite direction from the morning shadow as the position of the sun has changed due to the rotation of the Earth. If we construct a gnomon, we can use the shadows from a gnomon to determine the season of the year and also our longitude. You can make your own gnomon by using a board, sheet of unlined paper and a paper clip. This is an advantage over using your shadow because you will have a written record of time of day, length and direction of the shadow. If you have access to the textbook "Earth Science for Secondary Schools" or Interaction of Earth and Time" the activities with the gnomon are set-up for you to follow. The lesson entitle "The Gnomon" shows you how to construct your gnomon, record and construct a shadow line.
The next lesson is entitled "Interpreting a Shadow Line." From this lesson you can determine the difference between solar noon and clock noon. You can also determine the difference between true north and magnetic north.
You may find your longitude using your gnomon records, knowing what the central meridian is for your time zone and the time correction in minutes:seconds.
Speed of Earth's Rotation
We know that the Earth rotates on it's axis once in about every twenty-four hours. But just how fast is this? Is it the same for all places on the Earth? If we look at a model for the Earth, we see that it is a sphere and so not all places travel at the same speed during a rotation. If we divide the circumference of the equator by 24 hours, we find that places on the equator are traveling at about 1,000 mph. To find out how fast you are traveling at your latitude, you multiply the speed at the equator by the cosine of your latitude.
Most calculators make this possible. You may look up your latitude on line or construct an instrument called an astrolabe and measure the altitude of the North Star.
One revolution around the Sun is referred as a year. Special things happen on Earth during this revolution around the Sun. Our astronomical winter, spring, summer and fall begin with these special events. The first day of winter begins on the winter solstice. This is when we have least number of daylight hours in the northern hemisphere (Dec.), after this date, we begin to have more daylight hours. Spring begins on the spring equinox (March) and all places on Earth have twelve hours of daylight and twelve hours of darkness. The days continue to get longer, more hours of daylight, until June when we have the summer solstice, the longest day of the year. After the summer solstice, the days begin to get shorter and when we reach the month of September, we have the fall equinox, equal hours of daylight and darkness. The hours of daylight continue to decrease until we reach the winter solstice and we start the whole cycle again.
During these changes of seasons, we notice a change in temperature as well as changing hours of daylight. These changes are due to the tilt of the Earth's axis. In the winter, the axis is pointed away from the Sun causing us not only to receive less hours of daylight but the angle at which sunlight reaches us. As we move from winter to summer, the hours of daylight increase and slowly we notice a temperature increase. Just the opposite happens from summer to winter. What some people fail to realize is that we are closer to the Sun in the month of March than we are in summer.
Because we want to use sunlight to our best advantage, we have time zones and in some areas, we use daylight savings time. If we divide the number of degrees in a circle (360°) by the number of hours in a day (24hrs.) we find that the Earth rotates about 15° per hour. As a result of this, our time zones are 15° in most places except where it might adversely affect the working environment of towns and cities close to one another. Daylight savings time is used in some areas in order to get the maximum use of sunlight and for safety reasons. Daylight Savings Time has communities setting their clocks ahead one hour starting in April and ending in October. We basically change the clocks so we may have an additional hour of daylight in the evening thus saving energy by maximizing sunlight.