The Sun can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse. Partial eclipses, annular eclipses, and the partial phases of total eclipses are never safe to watch without taking special precautions. Even when 99% of the Sun's surface is obscured during the partial phases of a total eclipse, the remaining photospheric crescent is intensely bright and cannot be viewed safely without eye protection [Chou, 1981; Marsh, 1982]. Do not attempt to observe the partial or annular phases of any eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness!
Generally, the same equipment, techniques and precautions used to observe the Sun outside of eclipse are required for annular eclipses and the partial phases of total eclipses [Reynolds & Sweetsir, 1995; Pasachoff & Covington, 1993; Pasachoff & Menzel, 1992; Sherrod, 1981]. The safest and most inexpensive of these methods is by projection, in which a pinhole or small opening is used to cast the image of the Sun on a screen placed a half-meter or more beyond the opening. Projected images of the Sun may even be seen on the ground in the small openings created by interlacing fingers, or in the dappled sunlight beneath a leafy tree. Binoculars can also be used to project a magnified image of the Sun on a white card, but you must avoid the temptation of using these instruments for direct viewing.
The Sun can be viewed directly only when using filters specifically designed for this purpose. Such filters usually have a thin layer of aluminum, chromium or silver deposited on their surfaces that attenuates ultraviolet, visible, and infrared energy.
In spite of these precautions, the total phase (and only the total phase) of an eclipse can and should be viewed without filters. It is crucial that you know when to take off and put back on your glasses; see Eye safety during a total solar eclipse
Chou, B. R., "Safe Solar Filters," Sky and Telescope, August 1981, p. 119.
Marsh, J. C. D., "Observing the Sun in Safety," J. Brit. Ast. Assoc., 1982, 92, 6.
Pasachoff, J. M., and Covington, M., Cambridge Guide to Eclipse Photography, Cambridge University Press, Cambridge and New York, 1993.
Pasachoff, J. M. Field Guide to the Stars and Planets, 4th edition, Houghton Mifflin, Boston, 2000.
Golub, L. and Pasachoff, J. M. Nearest Star: The Exciting Science of Our Sun, Harvard University Press, 2001.
Reynolds, M. D. and Sweetsir, R. A., Observe Eclipses, Astronomical League, Washington, DC, 1995.
Sherrod, P. C., A Complete Manual of Amateur Astronomy, Prentice-Hall, 1981.
