Eclipses

Eclipses, occultations, and transits occur when three celestial bodies line up, causing the middle body to block the path of light between those on the two ends. In particular, solar and lunar eclipses witnessed from Earth are spectacular phenomena that have been objects of awe, study, and speculation since ancient times. Once understood, they became powerful tools of science used to investigate topics as diverse as geodesy and general relativity.

Overview

Eclipses of the Sun and Moon are impressive events that captivate global audiences. Once considered great omens or portents, they have become among the most powerful means by which science tests theories.

A lunar eclipse (or eclipse of the Moon) occurs when the Moon passes through the shadow of the Earth. For this to happen, the Moon must be on the side of the Earth opposite the Sun when the Moon’s phase is full. A solar eclipse (or eclipse of the Sun) occurs when the Moon passes between the Earth and the Sun, blocking all or part of the Sun as seen from Earth. For this to happen, the Moon’s phase must be new.

Not every full or new Moon results in an eclipse since the orbit of the Moon lies in a plane, tilted about 5° to the ecliptic plane, the plane containing the Earth’s orbit around the Sun and in which the Earth and the Sun always lie. Unless a full or new Moon occurs when the Moon is very close to crossing the ecliptic plane, the Earth’s shadow will miss the Moon (at full Moon), or the Moon’s shadow will miss the Earth (at new Moon), and no eclipse will take place. This condition has been known and used to predict eclipses since ancient times and is the source of the name of the ecliptic plane.

A lunar eclipse is visible from all points on Earth where the Moon is above the horizon. It may be either umbral or penumbral. During an umbral lunar eclipse, at least part of the Moon passes through the Earth’s umbra, the dark inner shadow in which the Earth blocks light from all parts of the Sun. If the entire Moon passes through the umbra, it is called a total lunar eclipse, but if only part of the Moon passes through the umbra, it is called a partial lunar eclipse. During a penumbral lunar eclipse, the entire Moon misses the umbra and passes only through the Earth’s penumbra, a partial shadow surrounding the umbra in which the Earth cuts off light from some parts of the Sun. An observer on the Moon during a penumbral lunar eclipse would see part of the Sun covered by the Earth and part of the Sun extending beyond the edge of the Earth.

The Moon is dimmed slightly while in the penumbra but does not darken unless it enters the umbra. When the Moon enters the umbra, the previously bright surface of the full Moon darkens to a much dimmer reddish glow, illuminated only by sunlight that has been refracted and scattered around the Earth by Earth’s atmosphere. The brightness and color of this illumination can vary markedly from one umbral lunar eclipse to another (from orangish-red to a dull reddish brown to a ghostly brownish gray), depending on the atmospheric conditions on Earth. Occasionally, some areas of the Moon will seem less illuminated than others.

A total lunar eclipse can last several hours. When the Moon begins to enter the umbra, it takes about an hour for the eclipse to become total. Totality can last nearly two hours, with another hour required for the Moon to leave the umbra entirely. The limb (or edge) of the Moon nearest the observer’s eastern horizon enters the Earth’s shadow first, and at the end of the eclipse, this limb brightens first.

Solar eclipses are more complex. By coincidence, the Sun and the Moon have nearly the same apparent size or angular diameter—about one-half degree of arc—as viewed from Earth. The Sun’s actual diameter is about 400 times larger than the Moon’s, but the Sun is also about 400 times farther away from the Earth than the Moon. Because the orbit of the Moon around the Earth and the Earth-Moon system orbit around the Sun are both slightly elliptical, the apparent (angular) size of the Moon and Sun varies as seen from the Earth. On average, the angular diameter of the Moon, 0.518°, is slightly less than the angular diameter of the Sun, 0.533°, as seen from Earth. However, when the Moon is at perigee (its closest approach to Earth), its angular diameter increases to 0.548°. When the Earth is at perihelion (its closest approach to the Sun), the Sun’s angular diameter increases to 0.542°. Thus, when the Moon is near perigee, its angular size consistently exceeds that of the Sun, and if a solar eclipse occurs, the Moon can completely cover the Sun.

The Moon’s umbra is conical, with its base at the Moon narrowing to its apex (or tip) as it nears Earth. If the Moon is near the perigee, the apex of its umbra will fall inside the Earth, and observers in the region of the Earth’s surface within the umbra will see the silhouette of the Moon completely covering the Sun’s visible surface (its photosphere)—a total solar eclipse. The region of the Earth’s surface within the umbra at any moment is relatively small, never more than a few hundred kilometers across. Due to the orbital motion of the Moon around the Earth and the Earth’s rotation on its axis, its umbra sweeps a path (the eclipse track) thousands of miles long across the Earth’s surface from west to east at speeds consistently exceeding 1,700 kilometers per hour relative to the Earth’s surface. The maximum duration possible for totality is about seven and a half minutes. However, the complete eclipse may take more than four hours, including the partial phases at the beginning and end, as the Moon slowly covers and uncovers the Sun.

Because the Moon’s average angular size is slightly smaller than the Sun’s average angular size, the tip of the Moon’s umbra will not always reach the Earth’s surface during a solar eclipse. If so, the Moon will not completely cover the Sun’s photosphere. When the Moon is centered on the Sun, a narrow ring, or annulus, of the Sun’s bright photosphere remains visible around the Moon’s silhouette. This is called an annular solar eclipse. Annular solar eclipses are about 20 percent more frequent than total solar eclipses.

Beyond the Earth’s surface region in which a total or annular solar eclipse is seen, there is an area thousands of kilometers wide inside the Moon’s penumbra. Within this area, the silhouette of the Moon covers part but not all of the Sun’s photosphere. This is called a partial solar eclipse.

Occultations and transits are phenomena similar to eclipses in which the apparent angular size of the body in front is substantially larger or smaller than the apparent angular size of the body in back. A large body moving in front of a smaller one is called an occultation. The Moon frequently occults bright stars, which are seen to wink out instantly when they pass behind the limb of the Moon. The Moon also occasionally occults planets, and planets are seen to occult their satellites and stars. (Other planets can occult and eclipse their satellites as seen from Earth. The distinction between the two phenomena is that the satellite moving behind the planet is an occultation, while the satellite passing through the planet’s shadow is an eclipse. The two events are not necessarily coincident in time because the planet’s shadow cone will not be directly behind the planet as seen from Earth unless the planet is on the side of the Earth almost exactly opposite the Sun.) The Sun also occults objects, but because of the Sun’s brightness, such solar occultations cannot be seen at visible wavelengths; however, they have been observed at radio wavelengths when the object being occulted is a source of radio emission. A small body moving in front of a larger one is called a transit. On rare occasions, it is possible to witness the transit of the planets Venus or Mercury across the Sun from Earth. When this happens, the planet appears like a small black dot moving across the face of the Sun. Transits of satellites and their shadows across their parent planets also can be observed.

Another related phenomenon is that of eclipsing binary stars. Some stars seen in the sky are, in fact, pairs of stars orbiting one another. If the Earth lies near the plane of their mutual orbit, the combined light of the system is seen to vary in brightness as the stars alternately block each other from Earth’s view. By observing these variations in brightness, astronomers can determine some of the characteristics of the individual stars (such as relative sizes and surface temperatures) and study their interactions.

Methods of Study

Ancient peoples, who used astronomical observations to keep track of planting seasons and the like, usually imputed magical or spiritual significance to eclipses and consequently tried to predict them. They did this by watching the changing position of the Moon against the background of the stars and by recording patterns in the recurrence of eclipses.

A lunar, or synodic, month is the period from one new Moon to the next, about 29.53 days. The draconic month is the time required for the Moon to complete one cycle of crossing and recrossing the ecliptic plane (from south to north and from north to south), about 27.21 days. The coincidence of these two cycles produces eclipses, so the pattern starts again when the two cycles return to the same relative matchup. This happens every 223 lunar months (equaling 6,585.32 days or 242 draconic months or a little more than eighteen years) in a repeating pattern called the Saros cycle. The cycle lasts eighteen years and 11.32 or 10.32 days, depending on whether four or five leap years occur during that time. The extra third of a day (or about eight hours) means that eclipses 223 lunar months apart, although similar in overall geometry, will occur about 120° farther west in longitude because of the Earth’s rotation. After three such Saros cycles, about fifty-six years and one month, eclipses repeat in nearly the same part of the Earth again.

These cycles, at least as they applied to lunar eclipses, were known to Babylonian astronomers by around the eighth century BCE and may have been known to some peoples long before that (based on disputed interpretations of a circle of fifty-six pits around the neolithic monument at Stonehenge in England). Knowledge of these cycles enabled the Babylonians to predict the relative motions of the Sun and Moon in the sky. The Saros cycle was of limited use in predicting solar eclipses because the path of totality is so narrow. Precise knowledge of the relative motions of the bodies involved was required. This was only possible after Sir Isaac Newton developed his laws of motion and gravity and calculus in the seventeenth century. One of the first tests he applied to his new methods was the calculation of the orbit of the Moon.

Centuries of refinement, mathematical methods, and measurements of the positions of the Moon, Earth, and Sun relative to one another were necessary to achieve modern accuracy in eclipse predictions. Modern astronomers can calculate eclipses, including exact times and paths of totality, many years into the future with almost total precision. However, even these calculations are limited by residual uncertainties in the motions of the bodies involved when extrapolations hundreds of years in the past or future are attempted. With three bodies gravitationally interacting, no exact solution for the orbits is possible, although modern approximation methods are excellent. Furthermore, the rate of rotation of the Earth has varied over time. It continues gradually to slow, complicating the calculation of eclipse times and locations far back into the past or forward into the future.

A total solar eclipse is perhaps the most spectacular natural event that can be seen. During a total solar eclipse, the Moon appears as a dark disk that slowly moves across and covers the Sun's bright disk. Just before the Sun is completely covered, the remaining bright crescent narrows until it becomes a chain of bright spots along the edge of the Moon. These spots, called Baily's beads, represent a last glimpse of the Sun’s photosphere between mountains at the edge of the Moon. Then, for a few seconds, the Sun’s chromosphere (a thin layer of transparent gas above the photosphere) can be seen as a red fringe along the leading edge of the Moon. At about this time, rapidly moving shadow bands, striations of light and dark a few centimeters across, can be seen rippling across the ground and along walls. These are believed to be due to atmospheric refraction. The sky turns dark during totality, but not completely dark; some light is scattered into the umbra from outside the region of totality. The darkness at totality produces uneasiness among some animals, and birds are sometimes seen to go to roost, as at sunset. The solar corona, the Sun’s outer atmosphere of hot ionized gases, is seen as a glowing white halo around the dark silhouette of the Moon. Smaller, fiery red solar prominences are often observed around the edge of the Moon. As totality ends, the shadow bands, chromosphere, and Baily’s beads can be seen briefly again.

It is important never to look directly or through optical instruments, such as telescopes, binoculars, or camera viewfinders, at the uneclipsed or partially eclipsed Sun without using suitable filters manufactured for this purpose. Standard sunglasses are insufficient to prevent severe, painful, and permanent eye injuries, which can occur nearly instantaneously. However, during totality, when the Moon completely covers the Sun’s photosphere, and only the corona is visible, no filter is needed. Furthermore, no filters are required to watch lunar eclipses. A telescope or at least binoculars generally are necessary to enlarge the view of most occultations and transits.

Context

Eclipses, occultations, and transits have been powerful tools for deriving useful information, making discoveries, and testing and confirming various theories and predictions. One of the oldest uses can be traced to the ancient Greeks. They understood how lunar and solar eclipses occurred, and the circular outline of the Earth’s shadow seen on the Moon during lunar eclipses was cited by Aristotle (384-322 BCE) and other Greek philosophers as evidence that the Earth must be spherical.

In 1675, the Danish astronomer Ole Rømer studied the orbital motion of Jupiter’s four largest moons (its Galilean satellites) by carefully timing their eclipses in Jupiter’s shadow. He discovered that the eclipses occurred later than expected when Earth and Jupiter were farther apart and earlier than expected when Earth and Jupiter were closer. He realized that this phenomenon could be explained if light did not travel instantaneously but took the time (about 16.6 minutes by his measurements) to cross the Earth’s orbit (a distance of two astronomical units). Since the length of the astronomical unit was not known accurately then, he never calculated the speed of light in common units, but this was the first demonstration that light traveled at a finite speed.

Eclipses of the satellites of Jupiter and much less frequent lunar and solar eclipses helped seafarers find their location at sea and map the Earth. Although latitudes can be determined easily by measurements of the maximum altitude above the horizon reached by the Sun in the daytime or specific stars at night, longitudes can only be determined astronomically with a time reference. Pendulum clocks did not run accurately at sea because of the ship's motion, but the calculated times of eclipses provided the necessary time reference and permitted reliable navigation and the construction of accurate maps.

Solar eclipses have helped resolve some of the most important questions in science. They provided an infrequent opportunity for observing some of the Sun’s features—such as its corona, chromosphere, and prominences—which, before the development of modern instruments, were otherwise hidden most of the time by the brightness of the Sun. The element helium (from Helios, the Greek Sun god) was first discovered in the flash spectrum of the chromosphere during a total solar eclipse in 1868.

An observational test of general relativity was carried out first during the 1919 total solar eclipse. In 1915, Albert Einstein published his then-controversial general theory of relativity. One of its predictions was that light passing by a massive object, such as the Sun, would be deflected by gravity. By photographing the star field around the eclipsed Sun and comparing the apparent positions of the stars near the edge of the Sun to a photograph of the same star field when the Sun was in a different part of the sky, astronomers verified this prediction of general relativity. It has been confirmed repeatedly at several total solar eclipses since then. Radio interferometry observations of a shift in the position of quasar 3C273, when it is occulted by the Sun, have provided even more accurate confirmation.

In 1977, astronomers observed the occultation of a star by the planet Uranus to study its atmosphere by the way it absorbed light from the star. They were surprised when the star faded and brightened several times before and after being occulted by Uranus itself. This was attributed to a set of rings around Uranus and was the first discovery of rings around a planet other than Saturn.

Bibliography

Baker, Robert H. Astronomy. 7th ed. Van Nostrand, 1959.

Boles, Dana. "2024 Total Eclipse." NASA, Mar. 2024, science.nasa.gov/eclipses/future-eclipses/eclipse-2024/. Accessed 25 Mar. 2024.

Brewer, Bryan. Eclipse. 2nd ed. Earth View, 1991.

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Addison-Wesley, 2008.

"Eclipses." NASA, solarsystem.nasa.gov/eclipses/home. Accessed 20 Sept. 2023.

Freedman, Roger A., and William J. Kaufmann III. Universe. 6th ed. W. H. Freeman, 2019.

Schneider, Stephen E. Pathways to Astronomy. 6th ed. McGraw-Hill, 2020.

Stevenson, F. Richard. “Historical Eclipses.” Scientific American, vol. 247, Oct. 1982, pp. 170-183.

Taff, Laurence G. Celestial Mechanics. Wiley-Interscience, 1985.

Zirker, Jack B. Total Eclipses of the Sun. Expanded ed. Princeton University Press, 1995.