Eclipse
From Encyclopædia
{ee-klips'} An eclipse is observed when a celestial body is seen to move in front of another. The SUN is eclipsed when the
Moon passes in front of it. The
Moon is eclipsed when the Earth casts its shadow on the
Moon. The Galilean satellites of Jupiter eclipse one another. Some close
binary stars are eclipsing binaries. The word eclipse is a general one; when the
Moon moves in front of a
star, however, the event is called an OCCULTATION, and when Mercury or Venus moves between the Earth and the Sun, the event is called a TRANSIT.Eclipses of the Sun and
Moon have long made a deep impression on humankind. The blotting out of
light from a familiar and apparently predictable source was generally considered a bad omen. Predicting eclipses was one of the duties of ancient Chinese astronomers, who labored under the threat of death if they failed. Today eclipses can be predicted to within seconds, and
interest in them is both scientific and aesthetic.Partial, Total, and Annular EclipsesThe shadow cast by a body has two components. In the
case of the
Moon, these are (1) the umbra--a cone with its apex an
average of 373,000 km (231,700 mi) from the
Moon, and in which the Sun is entirely obscured; and (2) the penumbra--a region becoming wider the farther it is from the
Moon, and in which the Sun is partially obscured. Since the
average distance between the Earth and
Moon is about 4,800 km (3,000 mi) greater than the length of the umbra, the tip of the umbra does not, on the
average, quite reach the Earth's surface. But because of the eccentricity of the
Moon's orbit, the
Moon can come closer than
average, and the umbra can intersect the Earth's surface in a circle with a diameter as great as 269 km (167 mi). As the umbra sweeps over the surface of the Earth, the longest time for a fixed observer to remain inside it is about 7.5 minutes. An observer within the umbra sees a total eclipse; one in the penumbra sees a partial eclipse. A solar eclipse is called annular when the tip of the umbra fails to reach the Earth; then only the center of the Sun is obscured, leaving a bright ring, or annulus.An eclipse of the
Moon can be total or partial, but not annular. It can be observed from any point on the Earth where the
Moon would be visible. The
Moon is never wholly dark during a lunar eclipse because of sunlight refracted by the Earth's
atmosphere; the amount of indirect illumination depends on the atmospheric conditions at the time.Conditions for OccurrenceAn eclipse of the Sun can take place only at new
Moon, and that of the
Moon only at full
Moon; and the Sun, Earth, and
Moon must be very nearly, if not precisely, aligned. Because the orbit of the
Moon is inclined at about 5 deg to the ECLIPTIC, during most months new and full
Moon will occur when the
Moon is either too far north or south of the Sun for an eclipse to take place. But when the Sun is close to the ascending or descending node of the
Moon's orbit, an eclipse is possible.The Sun circles the ecliptic once a year; the
Moon's ascending node moves in the opposite sense, taking 18.6 years for a circuit. The interval between successive passages of the Sun through the ascending node is called the eclipse year and is 346.6 days. Nineteen eclipse years is nearly the same as 223 synodic months (the interval between successive full
Moons, or 29.53 days). This period is called the
saros and is equal to 18 years and 11.33 days (if the interval contains 4 leap years). After this interval a pattern of eclipses is repeated, but because of the partial 0.33 day, the repetition is displaced nearly 120 deg in longitude. The
saros was known to ancient Babylonian astronomers.Scientific ImportancePartial eclipses of the
Moon and the Sun are of little scientific
interest. Total solar eclipses are of great importance, and are also stunningly beautiful. While a small part of the Sun's visible glowing surface (the PHOTOSPHERE) is still visible, the sky appears bright and milky because of scattered sunlight. At this
stage it is still dangerous to look toward the Sun, and many people have suffered irreparable eye damage when doing so. Just before totality, the last sunlight gleams through irregularities on the
Moon's surface, giving the phenomenon called Bailey's beads. Then, suddenly, the sky darkens and
stars are visible.Most remarkable of all, the solar CORONA becomes visible during this period of darkness; this is sometimes accompanied by solar
prominences extending like red
tongues from the CHROMOSPHERE. For a long time these could be studied only at times of solar eclipse, but the CORONAGRAPH and
spectroheliograph now make it possible to study the inner corona and
prominences at other times. The solar chromosphere can be observed as a thin red arc in the moments just after the photosphere disappears and just before it reappears. Observed through a
prism, this
light breaks up into bright arcs called the flash
spectrum. This emission
spectrum is nearly a reversal of the dark-line absorption
spectrum of the photosphere; differences occur in gases at higher levels in the chromosphere.One of the most famous tests of Einstein's theory of general relativity was made in 1919 during a solar eclipse. Einstein had predicted that a ray of
light just grazing the surface of the Sun would change direction by 1.75 seconds of arc. The field of
stars near the Sun was photographed during total eclipse; the same field had been photographed months earlier when it was in the night sky. The photographs were compared, and on this occasion, as well as on many similar occasions since, the observations were consistent with Einstein's prediction.J. M. A. DanbyBibliography: Abell, G.,
exploration of the Universe, 3d ed. (1982); Link, F., Eclipse Phenomena in Astronomy (1969); Lowenthal, James, The Hidden Sun: Solar Eclipses and Astrophotography (1984); Menzel, D. H., Our Sun (1949); Meeus, J., et al., Canon of Solar Eclipses (1966).
