Solar and Lunar Eclipses 521 Eclipse Phenomena

Few heavenly sights are as spectacular as a total solar eclipse, and the awe in which it was held in antiquity provided both a powerful drive to study its predictability and a potent political instrument for those who knew something about it. Lunar eclipses too have inspired awe. Assyrian examples of both types will be given in §7; here, we present a few examples from ancient Greece.

Herodotus (b. Halicarnassos ~484 b.c.) mentions an event predicted by Thales in connection with a battle between the Medes and the Lydians (see §5.2.2 below) in which day was "changed into night."

Thucydides [Athens; ~460—400 b.c.] cites a lunar eclipse in connection with the invasion of Syracuse by the Athenians in 413 b.c. and their subsequent disastrous defeat.

Prior to the fateful battle of Arbela (Gaugemela), in 331 b.c., between the armies of Alexander the Great and Darius, a lunar eclipse caused panic among the Macedonian troops. Quintus Curtius Rufus in The History of Alexander (Yardley tr. 1984, p. 73) wrote,

First the moon lost its usual brightness, and then became suffused with a blood-red colour which caused a general dimness in the light it shed. Right on the brink of a decisive battle the men were already in a state of anxiety, and now this struck them with a deep religious awe which precipitated a kind of panic.

Alexander consulted Egyptian seers, whom he regarded as astronomical experts, about the meaning of the phenomenon. Curtius Rufus (Yardley tr. 1984, p. 73) says of them,

Figure 5.7. The basic geometry of solar and lunar eclipses: Eclipses are due to the incursion of the Moon into the cone defined by the cross-sectional outlines of the Sun and the Earth. Diagram and slide, courtesy Dr. D.J.I. Fry.

Figure 5.7. The basic geometry of solar and lunar eclipses: Eclipses are due to the incursion of the Moon into the cone defined by the cross-sectional outlines of the Sun and the Earth. Diagram and slide, courtesy Dr. D.J.I. Fry.

They were well aware that the annual cycle follows a pattern of changes, that the moon is eclipsed when it passes behind the earth or is blocked by the sun (sic), but they did not give this explanation which they themselves knew, to the common soldiers. Instead they declared that the sun represented the Greeks and the moon the Persians, and that an eclipse of the moon predicted disaster and slaughter for those nations. They then listed examples from the history of Persian kings whom a lunar eclipse had demonstrated to have fought without divine approval.

Most lunar eclipses are not as impressive, especially when they are deprived of ascribed astrological significance, but no one can forget the sight of a total solar eclipse. It demonstrates the existence of powerful forces at work in the heavens.

In both lunar and solar eclipses, one body passes within the shadow cone of another (see Figure 5.7). Both for lunar and solar eclipses, also, the extent of the eclipse is called the magnitude of the eclipse. For lunar eclipses, it is the fraction of diameter of the Moon obscured by the shadow of the Earth. For solar eclipses, it is the fraction of the Sun obscured by the Moon. Because the geometries of the eclipses are slightly different, we describe them separately.

5.2.1.1. Solar Eclipses

Well before a solar eclipse reaches the stage of totality, the light at the horizon is noticeably different, as the distant shadow makes its way over the landscape. The Sun begins to disappear as a dark lune or crescent encroaches on it from the west: The dark lune waxes as the Sun's disk wanes. As the eclipse progresses, the air feels suddenly chilled, even on a warm day. The wind may come up, and clouds may form. As the sky darkens, birds stop singing and fly to their nests, dogs howl, and people in many cultures around the world make noise to frighten away whatever it is that menaces the Sun. As the solar crescent thins, a bright point on the limb may create the "Diamond Ring" effect. The last gleams of sunlight ("Bailey's Beads") appear to sparkle at the Moon's eastern limb, and the landscape seems to be filled with flickering shadows (the "shadow bands"). Suddenly, only a halo of light—the solar corona—can be seen surrounding a black disk. At this time, the stars, planets, and perhaps even one or more comets will be visible. Within minutes, the Sun's light emerges from the western limb, and the process is reversed. The solar crescent waxes as the dark disk wanes and the Sun returns to warm the world of the viewer. The reasons for the terror that a solar eclipse evokes involve not only the physical effects, which are evidently shared by other species, but also cultural cosmological frameworks. We will discuss these further on a culture by culture basis in Part II.

The outer atmosphere of the Sun is revealed during an eclipse. The visible disk of the uneclipsed Sun reveals the photosphere, a physically thin region of the solar atmosphere from which the bulk of the optical light is contributed. It is overlain by two other regions—the chromosphere, an even narrower, reddish region seen prominently during total solar eclipses and the appearance of which has been likened to that of a prairie fire, and the corona, a region extending to several photospheric radii. The corona is composed of very hot gases, ions, and dust; it provides the pearly white light and plumed extensions so familiar in eclipse pictures. See Figure 5.8.

The intensity of the remaining light is ~10-6 to 10-7 of the unobstructed Sun. When totality does not occur, so that the Moon does not cover the Sun completely, the chromosphere and the corona are not seen. A classic reference may provide a description of the appearance of these layers: Plutarch [46-120 a.d.] noted that "Even if the Moon ... does sometime cover the Sun entirely, the eclipse does not have duration or extension; but a kind of light is visible about the rim

Figure 5.8. The solar eclipse of October 24, 1995, seen in Lopburi, Thailand, revealing the outer atmosphere of the Sun. A 70-mm lens photo by E.F. Milone.

which keeps the shadow from being profound and absolute" (The Face on the Moon, in Moralia, XII, tr. Cherniss, in Cherniss and Helmbold 1957, p. 121). Cherniss (p. 11) challenges the interpretation of this passage as a description of the upper solar atmosphere. He suggests instead that if Plutarch was describing an observed phenomenon at all, the passage is more likely to refer to an annular eclipse. If that is so, the choice of words in the phrase, "does sometime cover the Sun entirely," would seem to require some explanation.

Plutarch also discusses eclipses as portents in his Lives, usually arguing that the eclipses were nothing of the kind (Brenk 1977, pp. 28-48) and using such accounts to deprecate astrology and to extol astronomy. Plutarch may have witnessed an eclipse (The Face on the Moon, p. 117), identified by Ginzel (1899) as that of Mar. 20, 71 a.d., visible in Plutarch's town of Chaeronea. However, Sandbach (1929), assuming it to be in either Rome or Alexandria, identified the eclipse with that of Jan. 5, 75 a.d. or Dec. 28, 83 a.d., respectively. All fail the condition cited in this dialogue by Lucius, as starting "just after noonday." Stephenson and Fatoohi (1998), using AT values of Stephenson and Morrison (1995) and of Stephenson (1997), conclude that the 71 a.d. eclipse, with magnitude 99.5% at Athens, is the most likely.

Solar eclipses occur when sunlight is blocked out by the Moon: when the observer comes to be inside the Moon's shadow. Figure 5.9 illustrates the geometry and shows the distinction between total and partial phases as viewed at the same site but at different instances of time. A solar eclipse will be total where the disk of the Sun is completely covered by the Moon. Owing to the closeness in size of their apparent or angular diameters, at any instant, this can occur only over a narrow region on Earth, from zero to several hundred kilometers across. Partial eclipses are seen outside the umbral zone, and at all points along the path of totality as well except during the moments of totality. The penumbral zone or area of partial eclipse can be 3000 km or more on

Figure 5.8. The solar eclipse of October 24, 1995, seen in Lopburi, Thailand, revealing the outer atmosphere of the Sun. A 70-mm lens photo by E.F. Milone.

Figure 5.9. Solar eclipses occur only when the Moon is new and moves across the line joining the Sun and the Earth. Illustration courtesy Dr. D.J.I. Fry.

either side. The totally eclipsed area at any instant is oval, not circular, because of the projection of the shadow cone onto the curved surface of the Earth.

Seen from a particular site, a total eclipse begins with the instant of "first contact"—the onset of the eclipse, when the Moon starts to obscure the western limb of the Sun. The partial eclipse state that follows extends for as much as an hour or more. The instant of second contact is the onset of the total phase—when the Sun's disk is completely obscured. Third contact marks the end of the total phase; this is followed by a lengthy partial phase until fourth contact—the end of the eclipse, when the eastern limb of the Sun becomes fully visible. Around the moments of second and third contacts, a series of bright and dark shadows, the shadow bands,8 sweep over the landscape at high speed.

The speed at which the shadow moves across the landscape is surprisingly large. The motion of the Moon in its orbit is more than 3000km/hr eastward, the same direction in which the Earth is moving beneath it at up to 1500km/hr.9 The net motion of the shadow, with respect to a point on the ground, is eastward at more than 1500km/hr. This means that the duration of the eclipse is short, less than 71/2 minutes at most, and usually much less.10

The shadow cast by the Moon must, however, reach the Earth for the eclipse to be seen as total. If the Moon is too far away, the zone of totality (the umbra) does not reach Earth. In some cases, even on the central track, totality is not achieved because the Moon is far enough from the Earth that it has a smaller angular diameter than does the Sun. Such an eclipse is called an annular eclipse, because the result is to produce an annulus or ring at maximum obscuration. If the annular eclipse is central, there are four times of contact analogous to those for total eclipses.

Solar eclipses must occur when the Moon is new, but they do not occur every month because of the character of the

8 To EFM, observing the solar eclipse of March 1970, the shadow bands seemed to coincide with the flickering of the "diamond ring" and "Bailey's Beads"—features on the Moon's limb caused by sunlight shining through lunar valleys. Unfortunately, on that occasion, the available film proved insufficiently sensitive to capture the rapidly varying but low-contrast shadow bands so that the degree of time-correlation with the flickering, which was captured on high-speed film, could not be determined. The experiment is worth repeating.

9 The linear velocity of a point on the Earth varies with latitude: the circumference of a latitude circle/Protn, where Protn is the rotation period of the Earth, namely, 0^99727 in units of the mean solar day. Because any arc on a small circle is equal to the corresponding large circle arc times the cosine of the latitude angle, v = 2pR-cosf/Protn.

10 The totality phase of the eclipse of Feb. 26,1979, which both authors viewed, lasted only 2m15s at Great Falls, Montana, the site of a joint gathering by eclipse observers from the University of Calgary and the College of Great Falls. The reason for the brevity is that the duration depends on the angular rate of motion of the Moon on the sky, and this depends on Moon's speed in its orbit (see §4.1.1.2) and on its distance from the Earth. During this eclipse, the Moon was near perigee;so the orbital motion (eastward) was greater, and its angular motion appeared large because of its proximity. These effects were more than enough to offset the larger angular size of the Moon (also due to its proximity), which by itself would tend to lengthen the eclipse.

lunar orbit. If the line joining the centers of the Sun and Moon does not intersect the Earth's surface at all, the eclipse can only be partial. If the line does intersect the Earth's surface but the Moon is far enough away so that the umbral shadow does not reach the Earth's surface, the eclipse will be annular. The partial eclipse zone or penumbra on the Earth's surface is always more extensive than is the umbra, which is confined to a narrow track, and may be absent altogether. Next, we discuss the determining conditions of an eclipse.

In §§3 and 4, we discussed the nature of the lunar orbit and the different periodicities of the Moon's motions. The inclination of the Moon's orbit to the ecliptic means that eclipses can occur only when the Sun and Moon are close to a node, or crossover point, where the Moon's orbit crosses the ecliptic (Figure 5.10).

An eclipse will be seen somewhere on Earth if the Sun is within a certain range of the node on the ecliptic, the ecliptic limits. The solar ecliptic limits are ~±17° for partial and ~±11° for total solar eclipses. These values vary because the angular sizes of the Sun and Moon vary with the distance to the Earth and because the inclination of the Moon's orbit varies slightly. See Figure 5.11 for illustrations of the eclipse limits and one source of their variations. The largest ("major") and smallest ("minor") values of the partial solar ecliptic limits are ±18°31' and ±15°21', whereas the major and minor total (or central) solar ecliptic limits are ±11°50' and ±9°55', respectively. The limits mean that a solar eclipse may occur within 18 days of the passage of the Sun through a node and must take place if the new moon occurs within 10 days of this solar node passage.

Many descriptions of solar eclipses can be found in historical records. A list of total or deep partial ancient solar eclipses of relatively high reliability is given in Table 5.1, taken primarily from Stephenson and Clark (1978). The location of the eclipse is not always explicitly known, but, in the case of Chinese eclipses, is usually assumed to be the capital, because this was also the site of the imperial observatory.

An additional eclipse, not included in Stephenson and Clark (1978), but well documented, is the Athenian eclipse of a.d. 484 Jan 14, in which the stars became visible (see Figure 4.13 for the conditions of the eclipse and the need for a AT correction). The event was associated with the death of the mathematician Proclos (or Proclus) in the following year. Stephenson (1997, pp. 367-368) has an extensive discussion of the circumstances of the eclipse, including the terrain to the east (Mt. Hymettus), which would have delayed the visibility of the eclipse in Athens.

5.2.1.2. Lunar Eclipses

Lunar eclipses arise when the Moon moves into the shadow of the Earth; Rufus was half right (cf. §5.2.1). They can occur only when the Moon is full, and a similar geometry to the solar eclipse prevails (see Figure 5.12).

There are differences in visibility and in duration, however. The lunar eclipse can be seen over a much larger geographic region than can solar ecl ipses—everywhere that the Moon is above the horizon during the times of eclipse. There

Figure 5.10. Nodes of the lunar orbit: Eclipses can occur only when both Moon and Sun are near a node. See Figure 5.11, which illustrates how near to a node the Sun must be. Illustration and slide, courtesy Dr. D.J.I. Fry.

Table 5.1. Selected records of ancient solar eclipses.

Date

b.c.

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