THE SUN
Always observe the Sun with care! See here.
The Sun is our local star, a ball of gas radiating 4xl033 ergs per second. Earth receives about one two-billionths of this energy. The Sun is 865,000 miles across, about 109 times the size of Earth; it is about 93 million miles distant, and 107 similar suns could fit between it and the orbit of Earth. The Sun rotates counterclockwise (as seen from the north), the equatorial regions having a period of about 26 days, the polar regions about 37 days. We detect these relative motions by watching sunspots.
The source of the Sun’s energy is thermonuclear fusion: the conversion of hydrogen into helium at the Sun’s core, which releases energy. At the core the temperature is about 15 million degrees Kelvin, and the density of the gas is eight times that of gold. It takes millions of years for this energy to leak outward through the layers of gas surrounding the core. Theoretical models and studies of the solar system indicate the Sun has been shining for about 4½ billion years.
The photosphere is the “surface” of the Sun, where the gas becomes transparent enough to let light escape into space. A sunspot is a relatively cool region of the photosphere, produced probably by magnetic “storms.” Whereas the photosphere is at about 6,000 degrees Kelvin, a sunspot may be a thousand or more degrees cooler. It appears dark, but would be bright if isolated from the contrasting surface. The number of sunspots varies in cycles averaging about 11 years. Peaks in numbers of sunspots have come as close together as 8 years and as far apart as 15. Counting and charting sunspots is a common activity of amateur astronomers.
The chromosphere is the layer immediately above the photosphere. It is called that because during eclipses, when the photosphere is hidden, the chromosphere appears red. The Sun’s spectral lines are produced in this layer.
The corona is the Sun’s outer atmosphere. Special telescopes called coronagraphs allow the inner part of the corona to be seen at any time. The outer part, stretching far out into the solar system, can be seen only during total solar eclipses. The corona is a tenuous gas with a temperature of millions of degrees.
Spurting above the photosphere are prominences—glowing jets of gas. Many are cloudlike, forming in the corona and “raining” back down onto the photosphere. They may be hundreds of thousands of miles long.
Active regions of the Sun sometimes produce solar flares: sudden bursts of visible and invisible light lasting seconds or minutes, best seen with special filters. They eject streams of charged particles which may, days later, strike Earth’s higher atmosphere, causing aurorae or interruptions in radio communications.
SOLAR ECLIPSES
A solar eclipse occurs when the Moon blocks sunlight that normally falls on Earth. Besides being of some scientific interest, solar eclipses are perhaps Nature’s most beautiful spectacle.
The Moon’s shadow consists of two parts. The umbra is the dark inner portion, from the inside of which no part of the Sun’s surface can be seen. The lighter, outer portion of the shadow is the penumbra. For a person within this, a part of the Sun’s disk is visible.
A solar eclipse can occur only at the time of new moon, when the Moon is between Earth and Sun. Because the plane of the Moon’s orbit is tilted slightly to the ecliptic plane, in most months the Sun-Moon-Earth alignment is only approximate, the Moon’s shadow misses the planet, and no eclipse is visible from Earth. This situation is shown in the bottom illustration.
At the time of new moon a few times each year, the Moon’s shadow does touch Earth. If only the penumbra touches Earth, people within the shadow will be able to see part of the Sun blocked off: a penumbral eclipse. Even when as much as 90% of the Sun’s disk is covered, people may not notice the eclipse, because the rest of the Sun is so bright.
If the umbra passes across Earth, an umbral, or total, solar eclipse occurs. Only people within the umbra see totality; those outside see a penumbral eclipse or no eclipse at all. Both Moon and Earth are moving, so the tip of the umbra moves across the surface of Earth, tracing out the eclipse path. The shadow moves about 1,000 miles per hour eastward. The duration of totality is determined by the size of the shadow and the geometry of how it hits the surface of Earth. The maximum length is about 7½ minutes. During totality the Sun’s disk is completely blocked and the beautiful corona is visible.
Since the Moon’s orbit is elliptical, sometimes the umbra does not reach all the way to Earth. In such cases a thin ring, or annulus, of light is seen around the edge of the Moon at maximum eclipse. This is an annular eclipse.
As seen from Earth, the instant when the Moon’s disk first touches the Sun’s disk is called 1st contact. The beginning of totality (if it occurs) is 2nd contact. The end of totality is 3rd contact; the end of the eclipse, 4th contact: Just as the last part of the Sun’s disk disappears, and again as it reappears, we may see the beautiful diamond-ring effect. Small spots of sunlight filtering through valleys at the edge of the Moon are known as Bailey’s beads.
At least two solar eclipses occur each year, and a maximum of five are possible. As many as three eclipses may be total or annular, or none may be. Though solar eclipses are slightly more common than lunar eclipses, they are less familiar, because one must be inside the relatively narrow eclipse path to see the eclipse. On the average, you can expect the path of a solar eclipse to cross your location once every 360 years.
HOW TO VIEW THE SUN SAFELY
Viewing the Sun directly without proper precautions can be very dangerous. The safest way, particularly for several viewers, is to make a sunbox—a sort of pinhole camera. Take a cardboard box, cut off one side, and tape a white piece of paper on the inside of one end. In the opposite end, cut a hole about 2 inches across and tape a piece of aluminum foil over the hole. Then very carefully punch a hole in the foil about 1⁄8 inch in diameter.
To use the sunbox, simply point it at the Sun so that the Sun’s image falls on the paper. If the image is too faint, enlarge the hole. If the hole is too large, image quality will suffer. You might want to make a more durable opening by drilling the right-sized hole in a piece of aluminum or copper and then taping that over the cutout.
A more sophisticated projection technique is to use a simple lens, binoculars, or a telescope. DO NOT LOOK THROUGH IT. Use its shadow to get it pointed toward the Sun. Again use a white piece of paper as a screen. If using binoculars or a telescope with an eyepiece that has lenses cemented together, don’t keep it pointed at the Sun long. You may crack the glass or fry the cement, and ruin the lens.
Special, usually expensive filters are available for advanced amateurs for direct viewing through a telescope. Use only filters made for this purpose; use them correctly. Don’t improvise.
Neutral-density photographic filters are not suitable for visual viewing, either alone or with a telescope or through a camera with a telephoto lens. Such filters do not stop harmful infrared rays.
A filter made of film is safe for brief views of the sun. Take a roll of black-and-white (not color) film, expose it completely, and have it developed. Sandwich at least two layers of this film between glass, and tape the edges. To use it, first place the filter in front of your eye, then look at the Sun for no more than 5 seconds—sufficient to follow the progress of a solar eclipse.
For the few minutes of totality of a solar eclipse, when the Sun’s disk is completely covered by the Moon, but only then, it is safe to view the Sun without filter. The shimmering corona sheds less light than the full Moon. Once the slimmest slice of Sun reappears, use the filters or sunbox again.
Some people have the idea that the Sun’s rays are more dangerous during an eclipse. Not so; however, during an eclipse more people are looking at the Sun and so more eye injuries occur at these times. Take the proper precautions: the effects may be delayed, but you can become permanently blinded, and you may not even feel it happen. With the proper methods, though, there is no reason not to enjoy looking at the Sun.
TABLE AND MAPS OF SOLAR ECLIPSES
The table below lists all solar eclipses for the period 1990-1999 and categorizes them as total (T), annular (A), or partial (P). A few unusual eclipses may begin as annular, become total for part of the path, and then become annular again (A-T). The portion of the eclipse that is total is not indicated in the maps.
The central lines of all total and annular eclipses through 1999 are shown with year, month, and day on the facing page. A triangle marks the beginning of each path; a bar marks the end. The regions of partial eclipse are not shown. The actual duration of the annular or total phase of the eclipse depends on where you are within the path.
More detailed information can be obtained from the references in the Bibliography. To do your own eclipse timing predictions, consult the Canon of Solar Eclipses listed there. Each astronomy magazine usually has a major article, sometimes several, on each upcoming total eclipse, giving detailed timings, weather predictions, and other information. Several companies offer tours and cruises to view total eclipses; read the advertisements in astronomy magazines.
SOLAR ECLIPSES, 1998–2005
| Date | Type | Region of Visibility |
| 1998 Feb 26 | T | Pacific, Atlantic, N South America |
| 1998 Aug 22 | A | Sumatra, Borneo, Pacific |
| 1999 Feb 16 | A | Indian Ocean, Australia |
| 1999 Aug 11 | T | Atlantic, Europe, S and SE Asia |
| 2001 Jun 12 | T | Atlantic, South Africa, Madagascar |
| 2001 Dec 14 | A | Pacific, Central America |
| 2001 Jun 10 | A | Pacific |
| 2002 Dec 4 | T | S Africa, Indian Ocean, Australia |
| 2003 May 31 | A | Iceland |
| 2003 Nov 23 | T | Antarctica |
| 2005 Apr 8 | A–T | Pacific, Central America, N South America |
| 2005 Oct 3 | A | Atlantic, Spain, Africa, Indian Ocean |
