A total solar eclipse is probably the most spectacular astronomical event that most people will experience in their lives. There is a great deal of interest in watching eclipses, and thousands of astronomers (both amateur and professional) and other eclipse enthusiasts travel around the world to observe and photograph them.
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 orbits 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.
Observing the Sun, however, can be dangerous if the proper precautions are not taken. The solar radiation that reaches the surface of the Earth ranges from ultraviolet (UV) radiation at wavelengths longer than 290nm, to radio waves in the meter range. The tissues in the eye transmit a substantial part of the radiation between 380 - 400nm 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 primary 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 eye protection, this photochemical retinal damage may be accompanied by a thermal injury - the high level of visible and near-infrared radiation 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 become apparent 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 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 disk of the Sun. It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques. Even when 99% 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 and 1996, and Marsh 1982). Failure to use proper observing methods may result in permanent eye damage and severe visual loss. This can have important adverse effects on career choice and earning potential, because 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, and Michaelides et al. 2001).
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 and Covington 1993, and Reynolds and 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 and no more than 0.5% (density ~2.3) of the near-infrared radiation from 780 - 1400nm. (In addition to the term transmittance [in percent], the energy transmission of a filter can also be described by the term density [unitless] where density, d, is the common logarithm of the reciprocal of transmittance, t, or d=log10[1/t]. A density of '0' corresponds to a transmittance of 100%; a density of '1' corresponds to a transmittance of 10%; a density of '2' corresponds to a transmittance of 1%, etc.). Figure 23 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 aluminum polyester that has been made specially for solar observation. (Note that this material is commonly known as "mylar," although the registered trademark "Mylar¨" belongs to Dupont who does not manufacture this material for use as a solar filter. Note that "Space blankets" and aluminum polyester film used in gardening are NOT suitable for this purpose!) Unlike the welding glass, aluminum polyester can be cut to fit any viewing device, and does not break when dropped. It has recently been pointed out that some aluminum polyester filters may have large (up to approximately 1mm 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 aluminum 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 aluminum polyester solar filter that has coating defects larger than 0.2mm in size or more than a single defect in any 5mm circular zone of the filter, should not be used.
An alternative to aluminum polyester 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 film 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 (aluminum 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. The advent of high resolution digital imaging in astronomy, especially for photographing the Sun, has increased the demand for solar filters of higher optical quality. Baader AstroSolar Safety Film, a metal-coated resin, can be used for both visual and photographic solar observations. A much thinner material, it has excellent optical quality and much less scattered light than polyester filters. Filters using optically flat glass substrates are available from several manufacturers, but are quite expensive in large sizes.
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 aluminum polyester or welder's glass. Some CDs are made with very thin aluminum coatings which are not safe - if the CD can be see through in normal room lighting, it should not be used! 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 (i.e., chromogenic film), film negative 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 23). The fact that the Sun appears dim, or that no discomfort is felt when looking at the Sun through the filter, is no guarantee that the 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. Local planetariums, science centers, or amateur astronomy clubs can provide additional information on how to observe the eclipse safely.
There are some concerns that UVA radiation (wavelengths from 315 - 380nm) in sunlight may also adversely affect the retina (Del Priore 1999). 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. The solar filter materials discussed in this article, however, attenuate solar UV radiation to a level well below the minimum permissible occupational exposure for UVA (ACGIH 2004), 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 viewing the eclipse at all. This tactic may backfire however, 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 later learns 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 (Pasachoff 2001)? Misinformation may be just as bad, if not worse, than no information.
Remember that the total phase of an eclipse can, and should, be seen without any filters, and certainly never by projection! It is completely safe to do so. Even after observing 14 solar eclipses, the author finds the naked-eye view of the totally eclipsed Sun awe-inspiring. The experience should be enjoyed by all.
Section 3.1 was contributed by:
B. Ralph Chou, MSc, OD
The following is a brief list of sources for filters that are specifically designed for safe solar viewing with or without a telescope. The list is not meant to be exhaustive but is a representative sample of sources for solar filters currently available in North America and Europe. For additional sources, see advertisements in Astronomy and or Sky & Telescope magazine. (The inclusion of any source on the following list does not imply an endorsement of that source by either the authors or NASA.)
|American Paper Optics||3080 Bartlett Corporate Drive|
Bartlett TN 38133
|(800) 767-8427 or (901) 381-1515|
|Astro-Physics, Inc.||11250 Forest Hills Rd.|
Rockford, IL 61115
|Celestron International||2835 Columbia St.|
Torrance CA 90503
|Coronado Technology Group,||1674 S. Research Loop, Suite 436|
Tucson, AZ 85710-6739
|(520)760-1561 or (866) SUN-WATCH|
|Meade Instruments Corporation||16542 Millikan Ave.|
Irvine CA 92714
|Rainbow Symphony, Inc.*||6860 Canby Ave., #120|
Reseda CA 91335
|Telescope and Boncular Center*||P.O. Box 1815|
Santa Cruz CA 95061-1815
|Thousand Oaks Optical*||Box 5044-289|
Thousand Oaks CA 91359
|Kendrick Astro Instruments||2920 Dundas Street W.|
Toronto, Ontario, Canada M6P 1Y8
|Khan Scope Centre||3243 Dufferin Street|
Toronto, Ontario, Canada M6A 2T2
|Perceptor Telescopes TransCanada||Brownsville Junction Plaza, Box 38|
Schomberg, Ontario, Canada L0G 1T0
|Baader Planetarium GmbH||Zur Sternwarte|
82291 Mammendorf, Germany
|0049 (8145) 8802|
The eclipse may be safely photographed provided that the above precautions are followed. Almost any kind of 35mm camera with manual controls can be used to capture this rare event; however, a lens with a fairly long focal length is recommended to produce as large an image of the Sun as possible. A standard 50mm lens yields a minuscule 0.5mm image, while a 200mm telephoto or zoom produces a 1.9mm image (Figure 24). A better choice would be one of the small, compact catadioptic or mirror lenses that have become widely available in the past 20 years. The focal length of 500mm is most common among such mirror lenses and yields a solar image of 4.6mm.
With one solar radius of corona on either side an eclipse view during totality will cover 9.2mm. Adding a 2X teleconverter will produce a 1000mm focal length, which doubles the Sun's size to 9.2mm. Focal lengths in excess of 1000mm usually fall within the realm of amateur telescopes.
If full disk photography of partial phases on 35mm format is planned, the focal length of the optics must not exceed 2600mm. Because most cameras do not show the full extent of the image in their viewfinders, a more practical limit is about 2000mm. Longer focal lengths permit photography of only a magnified portion of the Sun's disk. In order to photograph the Sun's corona during totality, the focal length should be no longer than 1500 - 1800mm (for 35mm equipment); however, a focal length of 1000mm requires less critical framing and can capture some of the longer coronal streamers. Figure24 shows the apparent size of the Sun (or Moon) and the outer corona on a 35mm film frame for a range of lens focal lengths. For any particular focal length, the diameter of the Sun's image is approximately equal to the focal length divided by 109 (Table 22).
A solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. Such filters are most easily obtained through manufacturers and dealers listed in Sky & Telescope and Astronomy magazine (see Section 3.2, "Sources for Solar Filters"). These filters typically attenuate the Sun's visible and infrared energy by a factor of 100,000. The actual filter factor and choice of ISO film speed, however, will play critical roles in determining the correct photographic exposure. Almost any speed film can be used because 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 from 1/1000s to 1/4s. 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 narrow crescent phases, 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.
Certainly the most spectacular and awe-inspiring phase of the eclipse is totality. For a few brief minutes or seconds, the Sun's pearly white corona, red prominences, and chromosphere are visible. The great challenge is to obtain a set of photographs that captures some aspect of these fleeting phenomena. The most important point to remember is that during the total phase, all solar filters must be removed! The corona has a surface brightness a million times fainter than the photosphere, so photographs of the corona are made without a filter. Furthermore, it is completely safe to view the totally eclipsed Sun directly with the naked eye. No filters are needed, and in fact, they would only hinder the 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., 1/1000s down to 1s). Rehearsing this sequence is highly recommended because great excitement accompanies totality and there is little time to think.
Exposure times for various combinations of film speeds (ISO), apertures (f/number) and solar features (chromosphere, prominences, inner, middle, and outer corona) are summarized in Table 23. The table was developed from eclipse photographs made by F. Espenak, as well as from 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, the recommended shutter speed can be calculated using the 'Q' factors tabulated along with the exposure formula at the bottom of Table 23. Keep in mind that these exposures are based on a clear sky and a corona of average brightness. The exposures should be bracketed one or more stops to take into account the actual sky conditions and the variable nature of these phenomena.
An interesting, but challenging, way to photograph the eclipse is to record its phases all on one frame. This is accomplished by using a stationary camera capable of making multiple exposures (check the camera instruction manual). Because the Sun moves through the sky at the rate of 15° per hour, it slowly drifts through the field of view of any camera equipped with a normal focal length lens (i.e., 35 - 50 mm). If the camera is oriented so that the Sun drifts along the frame's diagonal, it will take over 3h for the Sun to cross the field of a 50 mm lens. The proper camera orientation can be determined through trial and error several days before the eclipse. This will also ensure that no trees or buildings obscure the view during the eclipse. The Sun should be positioned along the eastern (left in the Northern Hemisphere) edge or corner of the viewfinder shortly before the eclipse begins. Exposures are then made throughout the eclipse at ~5min intervals. The camera must remain perfectly rigid during this period and may be clamped to a wall or post because tripods are easily bumped. If in the path of totality, remove the solar filter during the total phase and take a long exposure (~1s) in order to record the corona in the sequence. The resulting photograph will consist of a string of Suns, each showing a different phase of the eclipse.
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 cardboard 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 because this would otherwise obliterate the pinhole images.
Several comments apply to those who choose to photograph the eclipse aboard a cruise ship at sea. Shipboard photography puts certain limits on the focal length and shutter speeds that can be used. It is difficult to make specific recommendations because 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 because their small fields of view would require the ship to remain virtually motionless during totality, 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. If it is a calm day, shutter speeds as slow as 1/8 or 1/4 may be tried. Otherwise, use a 1/15 or 1/30 shutter speed and shoot a sequence through 1/1000s. It might be good insurance to bring a wider 200 mm lens just in case the seas are rougher than expected. A worst case scenario is when Espenak photographed the 1984 total eclipse aboard a 95ft yacht in seas with wave heights of 3ft. He had to hold on with one hand and point his 350 mm lens with the other! Even at that short focal length, it was difficult to keep the Sun in the field, however, any large cruise ship will offer a far more stable platform than this. New image stabilizer lenses from Canon and Nikon may also be helpful aboard ship by allowing the use of slower shutter speeds.
Consumer digital cameras have become affordable in recent years and many of these may be used to photograph the eclipse. Most recommendations for 35 mm single lens reflex (SLR) cameras apply to digital SLR (D-SLR) cameras as well. The primary difference is that the imaging chip in many D-SLR cameras is only about 2/3 the area of a 35 mm film frame (see the camera's technical specifications). This means that the Sun's relative size will be about 1.5 times larger in a D-SLR camera so a shorter focal length lens can be used to achieve the same angular coverage compared to a 35 mm SLR camera. Another issue to consider is the lag time between digital frames required to write images to the camera's memory card. It is also advisable to turn off autofocus because it is not reliable under these conditions; focus the camera manually instead. Preparations must be made for adequate battery power and space on the memory card.
For more on eclipse photography, observations, and eye safety, see the "Further Reading" sections in the Bibliography.
The total phase of an eclipse is accompanied by the onset of a rapidly darkening sky whose appearance resembles evening twilight about half an hour after sunset. The effect presents an excellent opportunity to view planets and bright stars in the daytime sky. Aside from the sheer novelty of it, such observations are useful in gauging the apparent sky brightness and transparency during totality.
During the total solar eclipse of 2006, the Sun will be in southern Pisces. Three naked-eye planets and a number of bright stars will be above the horizon within the total eclipse path. Figure 25 depicts the appearance of the sky during totality as seen from the central line at 10:30 UT. This corresponds to central Libya south of Jalu.
The most conspicuous planet visible during totality will be Venus (mv= - 4.2) located 47° west of the Sun in Capricornus. Mercury (mv = +1.0) is also west of the Sun at an elongation of 25°, however, it will prove more challenging to detect because it is five magnitudes (~100 x) fainter than Venus. Mars (mv = +1.3) lies 73° east of the Sun and is slightly fainter than Mercury.
Although no bright stars will be close to the Sun during the eclipse, a number of them will be above the horizon and may become visible during the eerie twilight of totality. Deneb (mv = +1.25), Altair (mv = +0.76), and Vega (mv = +0.03) are 65°, 71°, and 87° northwest of the Sun, respectively. Betelgeuse (mv = +0.45), Rigel (mv = +0.18), Aldebaran (mv = +0.87), and Capella (mv = +0.08) are to the northeast at distances of 80°, 71°, 61°, and 75°, respectively. Finally, Fomalhaut (mv = +1.17) is 40° southwest of the Sun. Star visibility requires a very dark and cloud free sky during totality.
At the bottom of Figure 25, a geocentric ephemeris (using Bretagnon and Simon 1986) gives the apparent positions of the naked eye planets during the eclipse. Delta is the distance of the planet from Earth (in Astronomical Units), App. Mag. is the apparent visual magnitude of the planet, and Solar Elong gives the elongation or angle between the Sun and planet.
For a map of the sky during totality from Asia, see NASA's Web site for the 2006 total solar eclipse.
Precise timings of beading phenomena made near the northern and southern limits of the umbral path (i.e., the graze zones), may be useful in determining the diameter of the Sun relative to the Moon at the time of the eclipse. Such measurements are essential to an ongoing project to detect changes in the solar diameter.
Because of the conspicuous nature of the eclipse phenomena and their strong dependence on geographical location, scientifically useful observations can be made with relatively modest equipment. A small telescope, shortwave radio, and portable camcorder are usually used to make such measurements. Time signals are broadcast via shortwave stations WWV and CHU, and are recorded simultaneously as the eclipse is videotaped. If a video camera is not available, a tape recorder can be used to record time signals with verbal timings of each event. inexperienced observers are cautioned to use great care in making such observations.
The safest timing technique consists of observing a projection of the Sun rather than directly imaging the solar disk itself. The observer's geodetic coordinates are required and can be measured from United States Geological Survey (USGS) maps or other large scale maps. If a map is unavailable, then a detailed description of the observing site should be included, which provides information such as distance and directions of the nearest towns or settlements, nearby landmarks, identifiable buildings, and road intersections.
The method of contact timing should be described in detail, along with an estimate of the error. The precisional requirements of these observations are ±0.5 s in time, 1arcsec (~30 m) in latitude and longitude, and ±20 m (~60 ft) in elevation. Commercially available Global Positioning System (GPS) receivers are now the easiest and best way to determine one's position to the necessary accuracy. GPS receivers are also a useful source for accurate UT as long as they use the one-pulse-per-second signal for timing; many recievers do not use that, so the receiver's specifications must be checked. The National Marine Electronics Association (NMEA) sequence normally used can have errors in the time display of several tenths of a second. The International Occultation Timing Association (IOTA) coordinates observers worldwide during each eclipse. For more information, contact:
Dr. David W. Dunham, IOTA
Reports containing graze observations, eclipse contact, and Baily's bead timings, including those made anywhere near or in the path of totality or annularity can be sent to Dr. Dunham at the address listed above.
For high resolution maps of the umbral path, the coordinates listed in Tables 7 and 8 are conveniently provided in longitude increments of 1° to assist plotting by hand. The coordinates in Table 3 define a line of maximum eclipse at 5 min increments. If observations are to be made near the limits, then the grazing eclipse zones tabulated in Table 8 should be used. A higher resolution table of graze zone coordinates at longitude increments of 7.5' is available via the NASA 2006 total solar eclipse Web site.
Global Navigation Charts (1:5,000,000), Operational Navigation Charts (scale 1:1,000,000), and Tactical Pilotage Charts (1:500,000) of the world are published by the National Imagery and Mapping Agency. Sales and distribution of these maps are through the National Ocean Service. For specific information about map availability, purchase prices, and ordering instructions, the National Ocean Service can be contacted by mail, telephone, or fax at the following:
NOAA Distribution Division, N/ACC3
It is also advisable to check the telephone directory for any map specialty stores in a given city or area. They often have large inventories of many maps available for immediate delivery.