A solar eclipse offers students a unique opportunity to see a natural phenomenon that illustrates the basic principles of mathematics and science that are taught through elementary and secondary school. Indeed, many scientists (including astronomers!) have been inspired to study science as a result of seeing a total solar eclipse. Teachers can use eclipses to show how the laws of motion and the mathematics of orbital motion can predict the occurrence of eclipses. The use of pinhole cameras and telescopes or binoculars to observe an eclipse leads to an understanding of the optics of these devices. The rise and fall of environmental light levels during an eclipse illustrate the principles of radiometry and photometry, while biology classes can observe the associated behavior of plants and animals. It is also an opportunity for children of school age to contribute actively to scientific research - observations of contact timings at different locations along the eclipse path are useful in refining our knowledge of the orbital motions of the moon and earth, and sketches and photographs of the solar corona can be used to build a three-dimensional picture of the sun's extended atmosphere during the eclipse.
However, observing the Sun can be dangerous if you do not take the proper precautions. The solar radiation that reaches the surface of the earth includes ultraviolet (UV) radiation at wavelengths longer than 290 nm, to radio waves in the meter range. The tissues in the eye transmit a substantial part of the radiation between 380 and 1400 nm to the light-sensitive retina at the back of the eye. While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the concern over improper viewing of the Sun during an eclipse is for the development of "eclipse blindness" or retinal burns.
Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function which may be either temporary or permanent, depending on the severity of the damage. When a person looks repeatedly or for a long time at the Sun without proper protection for the eyes, this photochemical retinal damage may be accompanied by a thermal injury - the high level of visible light causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (the retina has no pain receptors), and the visual effects do not occur for at least several hours after the damage is done (Pitts, 1993). Viewing the sun through binoculars, a telescope or other optical devices without proper protective filters can result in thermal retinal injury because of the high irradiance level due to visible light, as well as near infrared radiation, in the magnified image.
The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the moon completely covers the Sun. It is never safe to look at an annular eclipse or the partial phases of any eclipse without the proper equipment and techniques. Even when 99.9% of the sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn, even though illumination levels are comparable to twilight (Chou, 1981, 1996; Marsh, 1982). Failure to use proper observing methods may result in permanent eye damage or severe visual loss. This can have important adverse effects on career choices and earning potential, since it has been shown that most individuals who sustain eclipse-related eye injuries are children and young adults (Penner and McNair, 1966; Chou and Krailo, 1981).
The same techniques for observing the Sun outside of eclipses are used to view and photograph annular solar eclipses and the partly eclipsed Sun (Sherrod, 1981; Pasachoff 2000; Pasachoff & Covington, 1993; Reynolds & Sweetsir, 1995). The safest and most inexpensive method is by projection. A pinhole or small opening is used to form an image of the Sun on a screen placed about a meter behind the opening. Multiple openings in perfboard, a loosely woven straw hat, or even between interlaced fingers can be used to cast a pattern of solar images on a screen. A similar effect is seen on the ground below a broad-leafed tree: the many "pinholes" formed by overlapping leaves creates hundreds of crescent-shaped images. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. All of these methods can be used to provide a safe view of the partial phases of an eclipse to a group of observers, but care must be taken to ensure that no-one looks through the device. The main advantage of the projection methods is that nobody is looking directly at the Sun. The disadvantage of the pinhole method is that the screen must be placed at least a meter behind the opening to get a solar image that is large enough to see easily.
The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most of these filters have a thin layer of chromium alloy or aluminum deposited on their surfaces that attenuates both visible and near-infrared radiation. A safe solar filter should transmit less than 0.003% (density~4.5) of visible light (380 to 780 nm) and no more than 0.5% (density~2.3) of the near-infrared radiation (780 to 1400 nm). Figure 3.1 shows transmittance curves for a selection of safe solar filters.
One of the most widely available filters for safe solar viewing is shade number 14 welder's glass, which can be obtained from welding supply outlets. A popular inexpensive alternative is aluminized polyester that has been made specially for solar observation. ("Space blankets" and aluminized polyester used in gardening are NOT suitable for this purpose!) Unlike the welding glass, aluminized polyester can be cut to fit any viewing device, and doesn't break when dropped. It has recently been pointed out that some aluminized polyester filters may have large (up to approximately 1 mm in size) defects in their aluminum coatings that may be hazardous. A microscopic analysis of examples of such defects shows that despite their appearance, the defects arise from a hole in one of the two aluminized polyester films used in the filter. There is no large opening completely devoid of the protective aluminum coating. While this is a quality control problem, the presence of a defect in the aluminum coating does not necessarily imply that the filter is hazardous. When in doubt, an aluminized polyester solar filter that has coating defects larger than 0.2 mm in size, or more than a single defect in any 5 mm circular zone of the filter, should not be used.
An alternative to aluminized polyester solar filter material that has become quite popular is “black polymer” in which carbon particles are suspended in a resin matrix. This material is somewhat stiffer than polyester and requires a special holding cell if it is to be used at the front of binoculars, telephoto lenses or telescopes. Intended mainly as a visual filter, the polymer gives a yellow image of the Sun (aluminized polyester produces a blue-white image). This type of filter may show significant variations in density of the tint across its extent; some areas may appear much lighter than others. Lighter areas of the filter transmit more infrared radiation than may be desirable. A recent development is a filter that consists of aluminum-coated black polymer. Combining the best features of polyester and black polymer, this new material may eventually replace both as the filter of choice in solar eclipse viewers. The transmittance curve of one of these hybrid filters (Polymer Plus™ by Thousand Oaks Optical) is shown in Figure 3.1. Another material, Baader AstroSolar Safety Film, can be used for both visual and photographic solar observations. It is an ultrathin resin film with excellent optical quality and less scattered light than most polyester filters.
Many experienced solar observers use one or two layers of black-and-white film that has been fully exposed to light and developed to maximum density. The metallic silver contained in the film emulsion is the protective filter; however any black-and-white negative with images in it is not suitable for this purpose. More recently, solar observers have used floppy disks and compact disks (CDs and CD-ROMs) as protective filters by covering the central openings and looking through the disk media. However, the optical quality of the solar image formed by a floppy disk or CD is relatively poor compared to aluminized polyester or welder's glass. Some CDs are made with very thin aluminum coatings that are not safe - if you can see through the CD in normal room lighting, don't use it!! No filter should be used with an optical device (e.g. binoculars, telescope, camera) unless it has been specifically designed for that purpose and is mounted at the front end. Some sources of solar filters are listed below.
Unsafe filters include color film, black-and-white film that contains no silver, film negatives with images on them, smoked glass, sunglasses (single or multiple pairs), photographic neutral density filters and polarizing filters. Most of these transmit high levels of invisible, infrared radiation which can cause a thermal retinal burn (see Figure 3.1). The fact that the Sun appears dim, or that you feel no discomfort when looking at the Sun through the filter, is no guarantee that your eyes are safe. Solar filters designed to thread into eyepieces that are often provided with inexpensive telescopes are also unsafe. These glass filters often crack unexpectedly from overheating when the telescope is pointed at the Sun, and retinal damage can occur faster than the observer can move the eye from the eyepiece. Avoid unnecessary risks. Your local planetarium, science central, or amateur astronomy club can provide additional information on how to observe the eclipse safely.
There are some concerns that UVA radiation (wavelengths between 315 and 380 nm) in sunlight may also adversely affect the retina (Del Priore, 1991). While there is some experimental evidence for this, it only applies to the special case of aphakia, where the natural lens of the eye has been removed because of cataract or injury, and no UV-blocking spectacle, contact or intraocular lens has been fitted. In an intact normal human eye, UVA radiation does not reach the retina because it is absorbed by the crystalline lens. In aphakia, normal environmental exposure to solar UV radiation may indeed cause chronic retinal damage. However, the solar filter materials discussed in this article attenuate solar UV radiation to a level well below the minimum permissible occupational exposure for UVA (ACGIH, 1994), so an aphakic observer is at no additional risk of retinal damage when looking at the Sun through a proper solar filter.
In the days and weeks before a solar eclipse occurs, there are often news stories and announcements in the media, warning about the dangers of looking at the eclipse. Unfortunately, despite the good intentions behind these messages, they frequently contain misinformation, and may be designed to scare people from seeing the eclipse at all. However, this tactic may backfire, particularly when the messages are intended for students. A student who heeds warnings from teachers and other authorities not to view the eclipse because of the danger to vision, and learns later that other students did see it safely, may feel cheated out of the experience. Having now learned that the authority figure was wrong on one occasion, how is this student going to react when other health-related advice about drugs, AIDS, or smoking is given? Misinformation may be just as bad, if not worse than no information (Pasachoff, 2001).
|American Paper Optics||3080 Bartlett Corporate Drive|
Bartlett TN 38133
|Celestron International||2835 Columbia St.|
Torrance CA 90503
|Meade Instruments Corporation||16542 Millikan Ave.|
Irvine CA 92714
|Orion Telescopes and Binoculars||P.O. Box 1815|
Santa Cruz CA 95061-1815
|Pocono Mountain Optics||104 NP 502 Plaza|
Moscow PA 18444
|Rainbow Symphony, Inc.*||6860 Canby Ave., #120|
Reseda CA 91335
|Thousand Oaks Optical*||Box 5044-289|
Thousand Oaks CA 91359
|Khan Scope Centre||3243 Dufferin Street|
Toronto, Ontario, Canada M6A 2T2
|Perceptor Telescopes TransCanada||Brownsville Junction Plaza, Box 38|
Schomberg, Ontario, Canada L0G 1T0
|Starfield Scientific||PO Box 232|
Port Kembla, NSW 2505, Australia
* sources for inexpensive hand held solar filters and eclipse glasses
For more information, contact Prof. Jay M. Pasachoff, Hopkins Observatory, Williams College, Williamstown, MA 01267, USA. Information on safe solar filters can be obtained by contacting Dr. B. Ralph Chou.
Almost any kind of camera with manual controls can be used to capture these rare events. However, a lens with a fairly long focal length is recommended to produce as large an image of the Sun as possible. A standard 50 mm lens yields a minuscule 0.5 mm image, while a 200 mm telephoto or zoom produces a 1.9 mm image. A better choice would be one of the small, compact catadioptic or mirror lenses that have become widely available in the past ten years. The focal length of 500 mm is most common among such mirror lenses and yields a solar image of 4.6 mm. Adding a 2x tele-converter will produce a 1000 mm focal length, which doubles the Sun's size to 9.2 mm. Focal lengths in excess of 1000 mm usually fall within the realm of amateur telescopes. If full disk photography of the Sun on 35 mm format is planned, the focal length of the optics must not exceed 2600 mm. However, since most cameras don't show the full extent of the image in their viewfinders, a more practical limit is about 2000 mm. Longer focal lengths permit photography of only a magnified portion of the Sun's disk. For November's total eclipse, a focal length no longer than 1500 mm is recommended in order to capture the solar corona. However, a focal length of 1000 mm requires less critical framing and can capture some of the longer coronal streamers. For any particular focal length, the diameter of the Sun's image is approximately equal to the focal length divided by 109 (Table 3.1).
A solar filter must be used on the lens throughout the partial phases (and annularity) for both photography and viewing. Such filters can be obtained through manufacturers and dealers listed in Sky & Telescope and Astronomy magazines (see: SOURCES FOR SOLAR FILTERS). These filters typically attenuate the Sun's visible and infrared energy by a factor of 100,000. However, the actual filter factor and choice of ISO film speed will play critical roles in determining the correct photographic exposure. Almost any speed film can be used since the Sun gives off abundant light. The easiest method for determining the correct exposure is accomplished by running a calibration test on the uneclipsed Sun. Shoot a roll of film of the mid-day Sun at a fixed aperture (f/8 to f/16) using every shutter speed between 1/1000 and 1/4 second. After the film is developed, note the best exposures and use them to photograph all the partial phases. The Sun's surface brightness remains constant throughout the eclipse, so no exposure compensation is needed except for the crescent phases (and annularity) which require two more stops due to solar limb darkening. Bracketing by several stops is also necessary if haze or clouds interfere on eclipse day. This is especially true for the May 31 annular eclipse which occurs low on the horizon. A camera with a built in spot meter would be of great value in determining the exposure. If the haze is thick enough, you may need to remove the solar filter in order to see and photograph the annular eclipse, but extreme caution must be used.
For November's eclipse, the most spectacular and awe inspiring phase is totality. For two brief minutes, the Sun's pearly white corona, red prominences and chromosphere are visible. The most important point to remember is that during the total phase, all solar filters must be removed! The corona is a million times fainter than the photosphere, so photographs of the corona must be made without a filter. It is completely safe to view the totally eclipsed Sun directly with the naked eye. No filters are needed and they will only hinder your view. The average brightness of the corona varies inversely with the distance from the Sun's limb. The inner corona is far brighter than the outer corona. Thus, no single exposure can capture its full dynamic range. The best strategy is to choose one aperture or f/number and bracket the exposures over a range of shutter speeds (i.e., 1/1000 down to 1 second). Rehearsing this sequence is highly recommended since great excitement accompanies totality and there is little time to think.
Exposure times for various film speeds (ISO), apertures (f/number) and features (chromosphere, prominences, inner, middle and outer corona) are summarized in Table 3.2. The table was developed from eclipse photographs made by Espenak and photographs published in Sky and Telescope. To use the table, first select the ISO film speed in the upper left column. Next, move to the right to the desired aperture or f/number for the chosen ISO. The shutter speeds in that column may be used as starting points for photographing various features and phenomena tabulated in the 'Subject' column at the far left. For example, to photograph prominences using ISO 400 at f/16, the table recommends an exposure of 1/1000. Alternatively, you can calculate the recommended shutter speed using the 'Q' factors tabulated along with the exposure formula at the bottom of Table 3.2. Keep in mind that these exposures are based on a clear sky and a corona of average brightness. You should bracket your exposures one or more stops to take into account the actual sky conditions and the variable nature of these phenomena.
Finally, an eclipse effect that is easily captured with point-and-shoot or automatic cameras should not be overlooked. Use a kitchen sieve or colander and allow its shadow to fall on a piece of white card-board placed several feet away. The holes in the utensil act like pinhole cameras and each one projects its own image of the Sun. The effect can also be duplicated by forming a small aperture with one's hands and watching the ground below. The pinhole camera effect becomes more prominent with increasing eclipse magnitude. Virtually any camera can be used to photograph the phenomenon, but automatic cameras must have their flashes turned off since this would otherwise obliterate the pinhole images.
If photography is done aboard a ship at sea, this puts certain limits on the focal length and shutter speeds that can be used. General recommendations are difficult to make since it depends on the stability of the ship as well as wave heights encountered on eclipse day. Certainly telescopes with focal lengths of 1000 mm or more can be ruled out since their small fields of view would require the ship to remain virtually motionless and this is rather unlikely even given calm seas. A 500 mm lens might be a safe upper limit in focal length. ISO 400 is a good film speed choice for photography at sea. For the total eclipse, shutter speeds as slow as 1/8 or 1/4 may be tried if the ocean is calm day. Otherwise, stick with a 1/15 or 1/30 and shoot a sequence through 1/1000 second. It might be good insurance to bring a wider 200 mm lens just in case the seas are rougher than expected. New image stabilized lenses from Canon and Nikon may also be helpful aboard ship by allowing the use of slower shutter speeds.
Cold weather and below freezing temperatures will be the greatest challenge especially for travelers to the total eclipse in Antarctica (see ECLIPSES AT COLD TEMPERATURES). The best advice is to keep observing and photography plans very simple and uncomplicated.
For more information on eclipse photography, observations and eye safety, see FURTHER READING in the BIBLIOGRAPHY.
Antarctica's interior temperatures are a far more challenging adversary. Commercial telescopes begin to fail at about -30 C, especially after several hours at those temperatures. Only manual cameras can be trusted, although shutters begin to lock up or fail at -35 C. Chemical warming packs will not work unless boosted by being tucked up against a warm human body, especially if the wind chill is significant. In such a cold weather venue, photographic plans should be modest.
Eyepieces will be quick to frost over when an errant breath of moist air condenses on their cold surfaces, so a spare should be kept within easy reach inside a pocket. Camera viewfinders are similarly affected. Metal stings immediately, and so a thin inner glove can permit fine adjustments possible at the last critical moments. Latex medical gloves can prevent hands from sticking to metal, but these provide no warmth and leave hands feeling clammy. However, they do prevent the skin from drying and cracking.
Cold and dry air brings static electrical discharges within the camera body that can leave jagged streaks across film that is advanced too rapidly. Motor driven cameras are most likely to suffer this fate. All film should be advanced slowly and carefully to avoid the discharges and breaking the brittle film. With average temperatures, cameras and telescopes should be able to tough it out in the interior. Below normal temperatures will be too much without supplementary heating but even something as simple as covering equipment in a black cloth to absorb whatever energy the Sun can offer may bring sufficient warming to make eclipse observation and photography more successful.
Cameras are best kept inside a coat against the skin and brought out for photography when needed. The eclipse is short and warm equipment will not cool so quickly that it fails in the time available. Attaching cameras to telescopes is more difficult, but even at -30 C, bare hands can be used for a half-minute to make a final adjustments. Small parts don't have a great heat capacity and won't usually burn in a short minute but large metal objects can produce a very quick frostbite.
Winds can play havoc with eclipse plans, especially along coastal Antarctica where average speeds are nearly twice those inland. For the most part prevailing winds tend to come out of the south and southeast, the same direction in which the Sun lies during the eclipse. Windbreaks will have to be selected carefully to avoid blocking the eclipse if they can be used at all.
Warmth is critical if the eclipse is to be enjoyed and in Antarctic conditions this means several layers of clothing. For average conditions on the coast with modest winds the following might suffice:
For colder interior temperatures the following should be added or substituted:
NASA eclipse bulletins can be read or downloaded via the World-Wide Web using a Web browser (e.g. Netscape, Intenret Explorer, Safari, etc.) from the GSFC SDAC (Solar Data Analysis Center) Eclipse Information home page, or from top-level URL's for the currently available eclipse bulletins themselves:
Current plans call for making all future NASA eclipse bulletins available over the Internet, at or before publication of each. The primary goal is to make the bulletins available to as large an audience as possible. Thus, some figures or maps may not be at their optimum resolution or format. Comments and suggestions are actively solicited to fix problems and improve on compatibility and formats.
The umbral path characteristics have been predicted at 2 minute intervals of time compared to the 6 minute interval used in Fifty Year Canon of Solar Eclipses: 1986-2035 [Espenak, 1987]. This should provide enough detail for making preliminary plots of the path on larger scale maps. Global maps using an orthographic projection also present the regions of partial and total (or annular) eclipse. The index page for the path tables and maps is: Central Solar Eclipses: 1991-2030.
This service is offered for the 2003 eclipses as well as for previous eclipses in which analysis is still in progress. To discuss your needs and requirements, please contact Fred Espenak.
In August 1982, the International Astronomical Union (IAU) General Assembly adopted a value of k=0.2725076 for the mean lunar radius. This value is now used by the Nautical Almanac Office for all solar eclipse predictions [Fiala and Lukac, 1983] and is currently the best mean radius, averaging mountain peaks and low valleys along the Moon's rugged limb. The adoption of one single value for k eliminates the discontinuity in the case of annular-total eclipses and ends confusion arising from the use of two different values. However, the use of even the best 'mean' value for the Moon's radius introduces a problem in predicting the true character and duration of umbral eclipses, particularly total eclipses. A total eclipse can be defined as an eclipse in which the Sun's disk is completely occulted by the Moon. This cannot occur so long as any photospheric rays are visible through deep valleys along the Moon's limb [Meeus, Grosjean and Vanderleen, 1966]. But the use of the IAU's mean k guarantees that some annular or annular-total eclipses will be misidentified as total. A case in point is the eclipse of 3 October 1986. Using the IAU value for k, the Astronomical Almanac identified this event as a total eclipse of 3 seconds duration when it was, in fact, a beaded annular eclipse. Since a smaller value of k is more representative of the deeper lunar valleys and hence the minimum solid disk radius, it helps ensure the correct identification of an eclipse's true nature.
Of primary interest to most observers are the times when umbral eclipse begins and ends (second and third contacts, respectively) and the duration of the umbral phase. When the IAU's value for k is used to calculate these times, they must be corrected to accommodate low valleys (total) or high mountains (annular) along the Moon's limb. The calculation of these corrections is not trivial but must be performed, especially if one plans to observe near the path limits [Herald, 1983]. For observers near the center line of a total eclipse, the limb corrections can be more closely approximated by using a smaller value of k which accounts for the valleys along the profile.
This publication uses the IAU's accepted value of k=0.2725076 for all penumbral (exterior) contacts. In order to avoid eclipse type misidentification and to predict central durations which are closer to the actual durations at total eclipses, we depart from standard convention by adopting the smaller value of k=0.272281 for all umbral (interior) contacts. This is consistent with predictions in Fifty Year Canon of Solar Eclipses: 1986 - 2035 [Espenak, 1987]. Consequently, the smaller k produces shorter umbral durations and narrower paths for total eclipses when compared with calculations using the IAU value for k. Similarly, predictions using a smaller k result in longer umbral durations and wider paths for annular eclipses than do predictions using the IAU's k.
The primary source for geographic coordinates used in the local circumstances tables is The New International Atlas (Rand McNally, 1991). Coordinates for research stations in Antarctica can be found here. Elevations for major cities were taken from Climates of the World (U. S. Dept. of Commerce, 1972).
All eclipse predictions presented in this publication were generated on an Apple iMac G4 computer. Word processing and page layout for the publication were done using Microsoft Word v5.1. Figures were annotated with Claris MacDraw Pro 1.5. Meteorological diagrams and tables were prepared using Microsoft Excel 98.
The names and spellings of countries, cities and other geopolitical regions are not authoritative, nor do they imply any official recognition in status. Corrections to names, geographic coordinates and elevations are actively solicited in order to update the data base for future eclipses. All calculations, diagrams and opinions presented in this publication are those of the authors and they assume full responsibility for their accuracy.