There Goes the Sun
"There is nothing in the whole of Nature to rival the glory of a total eclipse of the Sun."
Michael Maunder & Patrick Moore, 1998, The Sun in Eclipse
Above: One frame from a 159 KB Quicktime Animation of the Moon's shadow sweeping across Earth during a total solar eclipse. Credit: Digital Radiance. Note that this animation is intended to illustrate the basic features of Solar eclipses and is not to scale.
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 the Moon is the same size or larger than the Sun, we have a total eclipse somewhere on Earth.
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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.
Right: Just before the Moon completely covers the Sun tiny specks of light called "Bailey's Beads" appear. Caused by sunlight shining through valleys on the edge of the Moon, these points of light are spaced irregularly around the disappearing edge of the Sun, forming the appearance of a string of beads around the dark disk of the Moon. Bailey's beads make their brief appearance up to 15 seconds before totality. When a single point of sunlight remains, a beautiful "diamond ring" effect is created against the outline of the Moon. This final sparkling instant signals the arrival of the moon's shadow. Bailey's Beads and the Diamond ring are seen again in reverse order at the end of totality when the Moon moves away from the Sun. During totality colorful prominences and the reddish chromosphere are also frequently visible. The animation pictured here is based on art by Duane Hilton.
It's beautiful, but is it good for anything?
for lesson plans and activities related to the August 11, 1999 total solar eclipse.
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.
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 global 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.
|As seen from||Object||Apparent diameter|
|Based on data from Chapter 14 (pp 183-184) of The Sun in Eclipse by Michael Maunder and Patrick Moore.|
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.
Audio eclipse may fill the sky - August 4, 1999 story on investigations of ionization and radio propagation in Earth's atmosphere during the eclipse
Peering through a Hole in the Sky - June 17, 1999 story on exotic gravity measurements to be carried out during the eclipse
The Millennium's Last Solar Eclipse -- from Sky &Telescope
Solar Eclipse Home Page -- at the NASA/Goddard Space Flight Center
Total Eclipses of the Sun, J.B. Zirker, 1995, Princeton University Press
The Sun in Eclipse, Michael Maunder and Patrick Moore, 1998, Springer-Verlag
Totality: Eclipses of the Sun, Mark Littmann, Ken Willcox and Fred Espanek, 1999, Oxford University Press
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