Looking at the Sun is harmful to your eyes at any time, partial eclipse or no. The danger that a partial solar eclipse poses is simply that it may prompt people to gaze at the Sun, something they wouldn't normally do. The result can be "eclipse blindness," a serious eye injury that can leave temporary or permanent blurred vision or blind spots at the center of your view. Fortunately, there are many easy ways to watch the show safely.
The simplest safe way to view a partial solar eclipse is to watch the Sun's image projected onto a piece of paper. Poke a small hole in an index card with a pencil point, face it toward the Sun, and hold a second card three or four feet behind it in its shadow. The hole will project a small image of the Sun's disk onto the lower card. This image will go through all the phases of the eclipse, just as the real Sun does. Experiment with different size holes. A large hole makes the image bright but fuzzy; a small hole makes it dim but sharp.For a better view, you can reduce the amount of daylight shining on the viewing card by enclosing it in a long box (right). This lets you use a small pinhole giving a sharp image.
A much better way to do pinhole projection can be arranged at a window indoors. Find a room with a Sun-facing window, turn out any lights, and pull the shades. Arrange for sunlight to enter through a small hole punched in a card near the top of the window. Set up a white piece of paper across the room to catch the Sun's image. Again, experiment with different size holes to get the best, sharpest view. (Of course, don't look through the hole directly at the Sun! Look only at the spot of light that falls on the paper.)
If the Sun is too high in the sky for this, you can direct its image horizontally into the room by setting up a small, high-quality mirror on the sill of an open window. Hold the mirror in place with modeling clay. Tape your card with the hole right onto the mirror.
Even at its best, pinhole projection gives only a small image. The throw distance in feet, divided by 9, gives the image diameter in inches. Pretty small!
Projection with binoculars or a telescope.
You can form a much sharper and bigger Sun image by projection through a small telescope or binoculars (left). This is best done outdoors to avoid the distorting effect of a windowpane. To aim the instrument safely, look at its shadow on a white card as you swing the tube around. (Don't use your finderscope — make sure it's capped at the front end or removed completely.) When the scope's shadow nears its minimum size, a brilliant beam of sunlight will burst out of the eyepiece and fall onto the card. Turn the focus knob and experiment with the card's distance behind the eyepiece until the Sun's disk is sharp and as big as you want. Look for sunspots!
If you prefer to look directly at the Sun, you can use a square or rectangular arc-welder's glass of shade #13 or #14, available for a few dollars from local welding-supply stores. (Don't get a lower-numbered shade; the Sun will be too bright to look at safely.) Alternatively, special, cheap "eclipse glasses" (right) are widely made from safe solar filter materials.
A solar filter that's designed to be used with a telescope is also safe for viewing with the otherwise unaided eye.
Filters that are not necessarily safe, though sometimes recommended in old books, include smoked glass, stacked sunglasses, crossed polarizing shades, photographic neutral-density filters, or a filter intended to block visible light for infrared photography. While these may greatly dim the Sun's glare, thus appearing to do the job, invisible ultraviolet or infrared radiation may be getting through to damage your eyes. (See the article "Solar Filter Safety" for more details.)Telescopic viewing. The clearest and best views of the Sun are had through a properly filtered telescope. Sky & Telescope reviewed commercial solar filters designed for telescopes in the July 1999 issue, page 63 ("Solar Filters: Which is Best?") and September 2000, page 63 ("Baader AstroSolar Safety Film: A New Standard in Solar Filters").
The filter must be secured over the telescope's front to keep most of the Sun's light and heat out of the instrument (left). Never use a Sun filter at the eye end, where it could crack or melt in the concentrated heat.
Direct viewing with a telescope and proper solar filter gives the best views of sunspots and the complex details within them, as well as the progress of the Moon's jagged, mountainous edge making its way across the solar disk.
Remember, safety is paramount. Never look directly at the Sun without using a safe solar filter. If you don't have one, see our list of suppliers.