Eclipses

It is a coincidence of nature that the apparent size of the Sun and the Moon in the sky are about the same. Thus on those rare occasions when the orbital motion of Earth and Moon cause them to align with the sun, as seen from points on Earth, the Moon will just cover the surface of the Sun and day will suddenly become night. Those who are located in the converging lunar shadow that just reaches Earth will see a total eclipse of the sun. The converging shadow cone, within which the sun is completely hidden by the moon, the umbral shadow of the moon.

One can imagine a diverging cone with the Moon at its apex in which only part of the sun is covered by the moon. This shadow is called the penumbra, or partially dark shadow. Folks on the earth located in this shadow will see the sun partially obscured or covered by the Moon.

Such an eclipse is called a partial solar eclipse. Because the base of this shadow cone is far larger than the umbral shadow, far more people see partial solar eclipses than see total solar eclipses. However, the impact on the observer of a total solar eclipse is far greater. Even a nearly total solar eclipse permits a small fraction of the solar surface to be visible, but covering the bright photosphere completely drops the light-level to a millionth of its normal value. During totality one can safely look directly at the sun and its corona, but this should not be done outside of totality during any partial or annular phases. The photospheric surface of the Sun is so bright, its focused image on the retina of the eye can do permanent damage to an individual's vision, including total blindness. Even viewing the sun through colored or smoked glass should be avoided, for the filter may pass infrared or ultraviolet light not obvious to the observer, but which can still do extensive damage. While specially designed "sun filters" may provide viewing safety, the safest approach to looking at the sun is projecting its image from a small telescope or monocular onto a screen. Direct viewing and photographs of the projected image can then be made in relative safety.

To the observer of a total solar eclipse many strange phenomena are apparent at the same time. The progressive coverage of the solar photosphere by the Moon reduces the solar heating of Earth, causing the local temperature to fall. The drop in temperature is accompanied by a rise in humidity and often a wind change. The covering of the central part of the sun's disk also brings about a subtle color shift toward the yellow. In the final seconds before totality the last bright regions of the sun's disk shine through the valleys at the limb of the moon, causing bright spots called "Baily's Beads." As the last of these disappear, the blood-red upper atmosphere of the sun called the chromosphere will briefly appear before it too is covered, revealing the winding, sheet-white corona that constitutes the outer atmosphere of the sun and is less bright than the full moon.

Birds fly to roost and animals behave as if night had truly arrived. All the senses are assaulted at once both by the changes in the local environment and the changes to the sun. A solar eclipse makes such an impression on people that it is said St. Patrick used one to convert the Celtic Irish to Christianity in the fifth century. The ancient historian Herodotus reported that a total solar eclipse that occurred during a battle between the Lydians and the Medes in 585 B.C. caused the soldiers to throw down their weapons and leave the field. Otherwise professional astronomers have been known to stand and stare at the phenomenon, forgetting to gather the data they have practiced for months and traveled thousands of miles to obtain.

Outside the narrow band traced across the earth by the tip of the moon's umbral shadow, part of the sun will be covered from those located in the expanding cone of the lunar penumbral shadow. An eclipse seen from such locations is said to be a partial solar eclipse. If the moon is near its farthest point from Earth, its dark umbral shadow does not quite reach Earth. Should this occur when the alignment for a solar eclipse is correct, the bright disk of the Sun will only be partially covered. At the middle of the eclipse a bright annulus of the solar photosphere will completely surround the dark disk of the moon. Such eclipses are called annular eclipses and may be considered a special case of a partial solar eclipse. Since part of the photosphere is always visible, one never sees the chromosphere or corona and the sky never gets as dark as during a total solar eclipse. However, there is a definite change in the color of the sunlight. Since the visible photosphere at the limb of the annularly eclipsed sun is cooler and more yellow than the center of the solar disk, the effect is for the daylight color to be shifted to the yellow. The effect is quite pronounced for eclipses occurring around local noon.

Because the area on the earth covered by the moon's umbra during a total eclipse is so small, it is quite rare for an individual to see one even though their frequency A total solar eclipse in La Paz, Baja California, Mexico, on July 11, 1991. Photograph by Francois Gohier. Photo Researchers, Inc. Reproduced by permission. of occurrence is somewhat greater than lunar eclipses. Lunar eclipses occur when the moon passes into the shadow cast by the earth. During a lunar eclipse, the bright disk of the full moon will be progressively covered by the dark disk of the umbral shadow of the earth. If the eclipse is total, the moon will be completely covered by that shadow. Should the alignment between the sun, earth, and moon be such that the moon simply grazes the earth's umbra, the eclipse is called a partial. Lunar orbital paths that pass only through the penumbral shadow of the earth are called penumbral lunar eclipses. The dimming of the moon's light in these eclipses is so slight that it is rarely detected by the human eye so little notice of these eclipses is made.

Since the moon is covered by the shadow of the earth, any point on the earth from which the moon can be seen will be treated to a lunar eclipse. Thus they are far more widely observed than are total eclipses of the sun. However, because the sun is so much brighter than the full moon, the impact of a total lunar eclipse is far less than for a total solar eclipse. Unlike a solar eclipse where the shadow cast by the moon is totally dark, some light may be refracted by the earth's atmosphere into the earth's umbra so that the disk of the moon does not totally disappear during a total lunar eclipse. Since most of the blue light from the sun is scattered in the atmosphere making the sky blue, only the red light makes it into the earth's umbral shadow. Therefore, the totally eclipsed moon will appear various shades of red depending on the cloud cover in the atmosphere of the earth.

Lunar eclipses do not occur every time the moon is full, nor do solar eclipses happen each time the moon is new. Although the line-up between the sun, earth, and moon is close at these lunar phases, it is not perfect. The orbital plane of the moon is tipped about five degrees to

the orbital plane of the earth. These two planes intersect in a line called the line of nodes. That line must be pointed at the sun in order for an eclipse to occur. Should the moon pass by the node between the earth and the sun while the line of nodes is aimed at the sun, the alignment between the sun, moon, and earth will be perfect and a solar eclipse will occur. If the moon passes through the node lying beyond Earth when the line of nodes is properly oriented, we see a lunar eclipse.

Except for slow changes to the Moon's orbit, the line of nodes maintains an approximately fixed orientation in space as it is carried about the sun by Earth's motion. Therefore, about twice a year the line of nodes is pointing straight at the Sun and eclipses can occur. If the alignment is closely maintained during the two weeks between new moon and full moon, a solar eclipse will be followed by a lunar eclipse. A quick inspection of the table of pending eclipses shows that 20 of the 47 listed eclipses occur within two weeks of one another, indicating that these are times of close alignment of the line of nodes with the sun. A further inspection shows that these pairs occur about 20 days earlier each year indicating that the line of nodes is slowly moving westward across the sky opposite to the annual motion of the Sun. At this rate it takes about 18.6 years for the nodes to complete a full circuit of the sky. Thus every 18-19 years eclipses will occur at about the same season of the year. After three of these seasonal cycles, or 56 years, the eclipses will occur on, or about, the same day. It is this long seasonal cycle that Gerald Hawkins associated with the 56 "Aubry Holes" at Stonehenge. He used this agreement to support his case that Stonehenge was used to predict eclipses and the Aubry Holes were used to keep track of the yearly passage of time between seasonal eclipses.

There are other cycles of eclipses that have been known since antiquity. It is a reasonable question to ask how long it will be before an eclipse will re-occur at the same place on the earth. The requirements for this to happen are relatively easy to establish. First, the moon must be at the same phase (i.e., either new or full depending on whether the eclipse in question is a solar or lunar eclipse). Secondly, the moon must be at the same place in its orbit with respect to the orbital node. Thirdly, the sun and moon must have the same distance from the earth for both eclipses. Finally, if the solar eclipses are to have similar paths across the earth, they must happen at the same time of the year. The first two conditions are required for an eclipse to happen at all. Meeting the third condition assures that the umbral shadow of the moon will reach the earth to the same extent for both eclipses. This means that the two eclipses will be of the same type (i.e., total or annular in the case of the sun). The last condition will be required for solar eclipses to be visible from the same location on the earth.

The interval between successive phases of the moon is called the synodic month and is 29.5306 days long. Due to the slow motion of the line of nodes across the sky, successive passages of a given node, called the nodal month, occur every 27.2122 days. Finally, successive intervals of closest approach to Earth (i.e. perigee passage) are known as the anomalistic month, which is 27.55455 days long. For the first three conditions to be met, the moon must have traversed an integral number of synodic, nodical, and anomalistic months in a nearly integral number of days. One can write these constraints as equations whose solutions are integers. However, such equations, called Diophantine equations, are notably difficult to solve in general. The ancient Babylonians found that 223 synodic months, 242 nodical months, and 239 anomalistic months all contained about 6,585 1/3 days, which turns out to be just 11 days in excess of 18 years. They referred to the cycle as the Saros cycle, for it accurately predicted repeats of lunar eclipses of the same type and duration. However, the cycle missed being an integral number of days by about eight hours. Thus, solar eclipses would occur eight hours later after each Saros, which would be more than enough to move the path of totality away from any given site. After three such cycles, sometimes referred to as the Triple Saros, lasting 54 years and a month, even the same solar eclipses would repeat with fairly close paths of totality. Since the multiples of the various months do not exactly result in an integral number of days, the repetitions of the eclipses are not exactly the same, but they are close enough to verify the predictability and establish the cycles. The Babylonians were able to establish the Saros with some certainty. Their ability to do so supports Hawkins' notion that the people who built Stonehenge were also capable of establishing the seasonal eclipse cycle.

It is tempting to look for cycles of even longer duration in search of a set of synodic, nodical, and anomalistic months that would yield a more close number of days, but such a search would be fruitless. There are other subtle forces perturbing the orbit of the moon so that longer series of eclipses fail to repeat. Indeed, any series of lunar eclipses fails to repeat after about 50 Saros or about 870 years. Similar problems exist for solar eclipses.