There goes the sun - the Aug. 11, 1999 eclipse

On August 11, 1999, millions of people in the Northern Hemisphere will have one last chance in this millennium to marvel at a total solar eclipse. The 50 mile wide path of totality will extend from the western Atlantic ocean, through Europe and Turkey, all the way across India into the Indian Ocean. At the instant of totality viewers situated along this narrow path can glimpse some of Nature's rarest wonders -- "Bailey's Beads," the "Diamond Ring," mysterious shadow bands, and, of course, the Sun's ephemeral corona.

Solar eclipses happen when the new Moon passes in front of the Sun. They don't take place every month because of the tilt of the Moon's orbit. The Moon's orbit around the earth is slightly elliptical, as is the earth's orbit around the Sun. Therefore, the Moon and the Sun do not always appear to be precisely the same size in the sky. The Moon's diameter ranges from 29.3 to 33.5 arcminutes while the Sun's diameter may be anywhere between 31.5 to 32.5 arcminutes. About once a year, on average, when the Moon passes directly in front of the Sun, and both are exactly the same size, we have a total eclipse somewhere on Earth.

Total eclipses are widely regarded as one of Nature's greatest spectacles. The chill and sweep of the Moon's shadow, the sparkle of Bailey's beads, and the sight of the corona blazing across the darkened daytime sky are life-altering experiences for some observers. Many become globe-hopping "eclipse addicts," die-hard lovers of totality who spend years and thousands of dollars traveling from one total eclipse to another.

This year's total eclipse on August 11, 1999 could be even better than usual, thanks to the solar maximum. Every 11 years the sun undergoes a period of heightened activity. There are frequent solar flares, lots of sunspots, and the corona expands to many times its average size. The August 11 solar eclipse will take place just 9 months before the predicted maximum in mid-2000. Even now the Sun is very active, raising expectations for an unusually beautiful eclipse.

Seeing the Sights

Most eclipse watchers consider the corona to be the most beautiful of all the phenomena of totality. The super-hot outer atmosphere of the sun glows with the intensity of the full moon when the sun's disk is completely occulted. Pearly white coronal streamers sometimes stretch several degrees across the sky during eclipses near a solar maximum.

Second only to the corona for dramatic impact, by some accounts, is the Moon's shadow. As totality approaches the shadow can be seen sweeping across the landscape at over 1000 miles per hour. According to Michael Maunder and Patrick Moore, in their book The Sun in Eclipse, "It...gives the impression of a vast dark cloak rushing toward you and then enveloping you: the general effect can only be described as eerie." Sometimes, as the Moon's shadow approaches, "shadow bands" can be seen. These are poorly understood, wavy lines of alternating shadow and light somewhat like the ripples on the bottom of a swimming pool.

The swift arrival of the Moon's shadow transforms the landscape. The darkness of totality resembles nighttime, and plants and animals react accordingly. Birds stop singing and may go to roost. Daytime flower blossoms begin to close as if for the night. The temperature drops in the coolness of the Moon's shadow. All of Nature seems to pause for this brief moment of daytime darkness.

There is a special quality to the darkness of a total eclipse. The sky near the horizon still appears bright, and this distant scattered light produces a slight reddish glow and unusual shadow effects. Because the direct light of the Sun is blocked, some of the brighter stars and planets become visible. Because this year's eclipse occurs just one day before the peak of the Perseid meteor shower, sky watchers may also see some rare daytime meteors.

The duration of totality varies depending on the eclipse and the viewing site. On August 11, 1999, the maximum duration will be 2 minutes 22 seconds for observers in south-central Romania. One of the longest eclipses on record will take place in 2009 when the "moment" of totality will endure for over 6 minutes as seen from a spot in the Pacific Ocean.

It's beautiful, but is it good for anything?

Scientists have used solar eclipses as an important research tool for hundreds of years. Eighteenth century astronomers, including Edmund Halley, routinely monitored eclipses to refine the orbits of the Moon and Earth. In the 19th century, astrophysicists realized that the chromosphere, prominences, and the corona were important physical aspects of the sun, and they followed eclipses as the only way to study these intriguing phenomena. In the early 20th century, solar eclipses provided crucial tests of Einstein's then-new successful theory of General Relativity.

Today, with space telescopes and modern coronagraphs it is sometimes thought that eclipses have little to offer in the way of science. This is not so.

To limit the scattering of sunlight, space-borne coronagraphs have had to block out the inner corona, and ground-based observations of the corona (excluding eclipses) cannot see the corona very far from the Sun. The Moon is still Nature's greatest coronagraph!

Eclipses are also unbeatable ways to precisely measure the Sun's diameter. Members of the IOTA routinely travel to solar eclipses which they observe near the edge of the path of totality. By stretching a team of observers perpendicular to the expected edge of the shadow's path they are able to measure the location of the edge with a precision to less than 100 meters. This translates into a measurement of the Sun's diameter with a precision of only 0.004 arcseconds, or 20 miles. According to such measurements, the Sun may have been 0.4 arcseconds larger in 1983 than it was in 1979. Is the Sun oscillating? It's too soon to say. Some of the eclipse measurements are controversial, and more data are needed. IOTA members will be on hand for the August 11, 1999 eclipse and they intend to continue their work during future eclipses as well.

Some of the most fascinating eclipse research has nothing to do with astronomy. Biologists and zoologists sometimes use eclipses to study the circadian rhythms of living creatures. Between 1954 and 1975, twp Polish zoologists, R. Wojtusiak and Z. Majlert, conducted a unique set of experiments in which they observed the behavior of mammals, birds, and insects during seven eclipses with varying degrees of coverage, including totality. They discovered that the daily habits of mammals were little affected by the eclipse, but that birds and especially insects were influenced. Under the Moon's shadow, many species of birds manifested anxiety and roosting behavior, and nearly all birds stopped singing (which contributes to the oft-noted quietude of totality). Insects are affected even more. During eclipses studied by the Polish team, bees returned to their hives en mass, nocturnal moths appeared, and butterflies settled in the grass as though it were night. The most sensitive species, bees, have been observed to return to their hives during partial eclipses covering only 19% of the Sun!

Other biologists have followed the ground-breaking experiments of the Polish team with observations that confirm the influence of eclipses on the behavior of birds, insects, and even nematodes and plankton! More information about these studies can be found in J.B. Zirker's book, Total Eclipses of the Sun.

There are many other research applications of solar eclipses, including exotic gravity measurements, investigations of ionization and radio propagation in Earth's atmosphere and studies of asteroid dust and cometary debris in the innermost solar system. The full range of research opportunities presented by a solar eclipse is too broad to review here. Suffice it to say that solar eclipses promise great value to scientific research for many, many years to come.

Eclipses on other planets

If the Moon were slightly smaller than the Sun, the best eclipses would be annular ones with no dramatic corona or blackening of the daytime sky. If the Moon were slightly larger, the full glory of the chromosphere and prominences surrounded by the glowing corona would be lost. Fortunately, the Moon is "just right."

Earth is the only planet in the solar system with spectacular solar eclipses. Thanks to an apparently improbable coincidence, the Sun and the Moon are almost exactly the same size as seen from Earth. The Sun is 400 times larger than the Moon, but it is also 400 times farther away. The table below, adapted from The Sun in Eclipse by Maunder and Moore, shows that there is no other planet where the angular diameter of a satellite is so close to that of the Sun. The most interesting case may be Jupiter's moon Amalthea, which subtends an angle of 7' 24" as seen from the cloud tops of Jupiter, and where the sun is nearly the same size at 6' 09". Still the sizes of Amalthea and the Sun are quite different compared to the similarity between the angular sizes of the Sun and our own satellite. Another interesting case, noted by Maunder and Moore, is that of Mars's satellite Phobos, which subtends an angle up to 12.3 arcminutes as seen from the surface of the red planet. Phobos transits the Sun about 1300 times in every Martian year and each time it does there would be a hole in the middle of the 21' Sun near "totality." Maunder & Moore argue that the novelty of such a sight would soon wear off for future Martian colonists because it happens so frequently.

It has often been asked if the similarity of the Moon's and Sun's diameters can be simple coincidence. In the absence of more data about the statistical distribution of sizes of stars, planets, and moons in solar systems other than our own, it would seem that the most likely answer is "yes." Nevertheless, it is a fortunate coincidence for the denizens of Earth.

For more information about the August 11, 1999 solar eclipse, please see Goddard Space Flight Center's Solar Eclipse home page.