Objectives 1. To determine the size of sunspots 2. To determine the rotation period of the Sun. 3. To study some images of Coronal Mass Ejections from the sun. Equipment 1. Metric ruler with centimeter markings 2. Calculator 3. Only print pages 1 through 7 of this lab Introduction Ever since Galileo saw sunspots on the surface of the Sun with his telescope, we have been intrigued by these “freckles” and their study has helped us to determine many properties of the Sun. In the last few decades, astronomers have developed some fantastic tools, one of which is SOHO, the Solar and Heliospheric Observatory. This is a 2-ton spacecraft located between Earth and Sun, about 1.5 million kilometers from earth. It continuously “looks” at the Sun and its many instruments have beamed back a tremendous amount of information since its launch in 1995. The cameras on SOHO capture many different types of images, which you will use in this experiment. The photosphere is the yellow surface of the Sun we see. While Galileo and others like Thomas Herriot of England recorded sunspots after 1611, historical records from Chinese astronomers indicate they were also aware of this phenomenon. However, sunspots are very small, and because it is dangerous to look at the Sun for long periods of time, we have to use special precautions for viewing it. We can use specially filtered solar telescopes or make a special device with a pinhole, lens and mirrors to “project” the sun’s image onto a piece of paper. The latter is a relatively simple way to view the Sun safely, and we have a special device to do this conveniently on campus. You must NEVER focus a telescope or binoculars on the Sun! These devices work by concentrating light to produce a sharp image. In the case of the Sun, the concentrated light can melt the glue holding the lenses in place and completely destroy the telescope or binoculars. More dangerously, the concentrated light can permanently damage the retina in your eyes. For these reasons, it is very important to view the Sun safely. Simply using dark glasses especially during eclipses is not enough. A total solar eclipse is a particularly fantastic sight and a great opportunity to study some features of the Sun. During a total solar eclipse, the Sun, Moon and Earth all lie in exactly the same plane, with the Moon between the Sun and Earth. By coincidence, the Sun is 400 times bigger than the Moon, but it is also 400 times further away. This means they have the same angular size in the sky, about 0.5 degrees, and the size of the Moon’s disk is the same as the Sun. When they are lined up exactly, and the Moon is at the proper distance from Earth, the Moon’s disk can completely cover up the Sun. However this condition lasts for less than 7.5 minutes and is visible from only a few places on Earth. This makes total solar eclipses rare events that can be frightening if you are not aware of what’s happening! Daylight gradually gives way to darkness and you can actually see the Sun with a “bite” taken out of it. As the bite increases, the sky gets darker, stars become visible, and at “totality” you see a black circle surrounded by a ghostly white light. The black circle is the Sun covered up by the Moon, and the white light is the Sun’s corona, which is always there, but since its light is too dim, it is 2 obscured by the brighter light from the photosphere. Since the Moon is moving, within a few minutes it will uncover the Sun, the corona will once again become invisible, the Sun’s bite will appear in the opposite direction, the sky will gradually brighten and in less than an hour, everything will go back to ‘normal.’