The Nearest Stars: A Guided Tour

Activity: Observing the Sun — Safely

(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 the victim 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 fit over 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 of the 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 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 the figure. 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 the normal 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!), as shown 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 the middle. 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 not jiggle 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

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 looking straight 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 in the 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 the outer 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

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. Turn the 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 to display 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 the hole 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 of the 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.

Further Reading About The Sun

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)

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