1-6 Eclipses occur only during rarely observed events when our sun, moon, and earth are perfectly aligned

From time to time the Sun, Earth, and Moon all happen to lie along a straight line. When this occurs, Earth’s shadow can fall on the Moon or the Moon’s shadow can fall on Earth. Such phenomena are called eclipses. They are perhaps the most dramatic astronomical events that can be seen with the naked eye.

A lunar eclipse occurs when the Moon passes through Earth’s shadow. This occurs when the Sun, Earth, and Moon are in a straight line, with Earth directly between the Sun and Moon so that the Moon is at full phase (position E in Figure 1-21). At this point in the Moon’s orbit, the face of the Moon seen from Earth would normally be fully illuminated by the Sun. Instead, it appears quite dim because Earth casts a shadow on the Moon.

A solar eclipse occurs when Earth passes through the Moon’s shadow. As seen from Earth, the Moon moves in front of the Sun. Once again, this can happen only when the Sun, Moon, and Earth are in a straight line. However, for a solar eclipse to occur, the Moon must be between Earth and the Sun. Therefore, a solar eclipse can occur only at new moon (position A in Figure 1-21).

CAUTION

Both new moon and full moon occur at intervals of 29½ days. Hence, you might expect that there would be a solar eclipse every 29½ days, followed by a lunar eclipse about two weeks (half a lunar orbit) later. But in fact, there are only a few solar eclipses and lunar eclipses per year. Solar and lunar eclipses are so infrequent because the plane of the Moon’s orbit and the plane of Earth’s orbit are not exactly aligned, as Figure 1-25 shows. The angle between the plane of Earth’s orbit and the plane of the Moon’s orbit is about 5°. Because of this tilt, new moon and full moon usually occur when the Moon is either above or below the plane of Earth’s orbit. When the Moon is not in the plane of Earth’s orbit, the Sun, Moon, and Earth cannot align perfectly, and an eclipse cannot occur.

Figure 1-25: The Inclination of the Moon’s Orbit This drawing shows the Moon’s orbit around Earth (in yellow) and part of Earth’s orbit around the Sun (in red). The plane of the Moon’s orbit (shown in brown) is tilted by about 5° with respect to the plane of Earth’s orbit, also called the plane of the ecliptic (shown in blue). These two planes intersect along a line called the line of nodes.

22

In order for the Sun, Earth, and Moon to be lined up for an eclipse, the Moon must lie precisely in the same plane as Earth’s orbit around the Sun. As we saw in Section 1-4, this plane is called the ecliptic plane because it is the same as the plane of the Sun’s apparent path around the sky, or ecliptic (see Figure 1-17). Thus, when an eclipse occurs, the Moon appears from Earth to be on the ecliptic—which is how the ecliptic gets its name.

The planes of Earth’s orbit and the Moon’s orbit intersect along a line called the line of nodes, shown in Figure 1-26. The line of nodes passes through Earth and is pointed in a particular direction in space. Eclipses can occur only if the line of nodes is pointed toward the Sun—that is, if the Sun lies on or near the line of nodes—and if, at the same time, the Moon lies on or very near the line of nodes. Only then do the Sun, Earth, and Moon lie in a line straight enough for an eclipse to occur.

Figure 1-26: Conditions for Eclipses Eclipses can take place only if the Sun and Moon are both very near to or on the line of nodes. Only then can the Sun, Earth, and Moon all lie along a straight line. A solar eclipse occurs only if the Moon is very near the line of nodes at new moon; a lunar eclipse occurs only if the Moon is very near the line of nodes at full moon. If the Sun and Moon are not near the line of nodes, the Moon’s shadow cannot fall on Earth and Earth’s shadow cannot fall on the Moon.

Anyone who wants to predict eclipses must know the orientation of the line of nodes. But the line of nodes is gradually shifting because of the gravitational pull of the Sun on the Moon. As a result, the line of nodes rotates slowly westward. Astronomers calculate such details to fix the dates and times of upcoming eclipses.

There are at least two—but never more than five—solar eclipses each year. The last year in which five solar eclipses occurred was 1935. The least number of eclipses possible (two solar, zero lunar) happened in 1969. Lunar eclipses occur just about as frequently as solar eclipses, but the maximum possible number of eclipses (lunar and solar combined) in a single year is seven.

23

Question

ConceptCheck 1-13: Why don’t lunar eclipses occur each time the Moon reaches full moon phase?

Lunar Eclipses

The character of a lunar eclipse depends on exactly how the Moon travels through Earth’s shadow. As Figure 1-27 shows, the shadow of Earth has two distinct parts. In the umbra, the darkest part of the shadow, no portion of the Moon’s surface can be seen. A portion of the Moon’s surface is visible in the penumbra, which therefore is not quite as dark. Most people notice a lunar eclipse only if the Moon passes into Earth’s umbra. As this umbral phase of the eclipse begins, a bite seems to be taken out of the Moon.

Figure 1-27: Three Types of Lunar Eclipses People on the nighttime side of Earth see a lunar eclipse when the Moon moves through Earth’s shadow. In the umbra, the darkest part of the shadow, the Sun is completely covered by Earth. The penumbra is less dark because only part of the Sun is covered by Earth. The three paths show the motion of the Moon if the lunar eclipse is penumbral (Path 1), partial (Path 2), or total (Path 3). The inset shows these same paths, along with the umbra and penumbra, as viewed from Earth.

The inset in Figure 1-27 shows the different ways in which the Moon can pass into Earth’s shadow. When the Moon passes through only Earth’s penumbra (Path 1), we see a penumbral eclipse. During a penumbral eclipse, Earth blocks only part of the Sun’s light and so none of the lunar surface is completely shaded. Because the Moon still looks full but only a little dimmer than usual, penumbral eclipses are easy to miss. If the Moon travels completely into the umbra (Path 2), a total lunar eclipse occurs. If only part of the Moon passes through the umbra (Path 3), we see a partial lunar eclipse.

If you were on the Moon during a total lunar eclipse, the Sun would be hidden behind Earth. But some sunlight would be visible through the thin ring of atmosphere around Earth, just as you would see sunlight through a person’s hair whose head was between your eyes and the Sun. As a result, a small amount of light reaches the Moon during a total lunar eclipse, and so the Moon does not completely disappear from the sky as seen from Earth. Most of the sunlight that passes through Earth’s atmosphere is red, and thus the eclipsed Moon glows faintly in reddish hues, as Figure 1-28 shows.

Figure 1-28: RIVUXG A Total Lunar Eclipse This sequence of nine photographs was taken over a three-hour period during the lunar eclipse of January 20, 2000. The sequence, which runs from right to left, shows the Moon moving through Earth’s umbra. During the total phase of the eclipse (shown in the center), the Moon has a distinct reddish color.

Lunar eclipses occur at full moon, when the Moon is directly opposite the Sun in the sky. Hence, a lunar eclipse can be seen at any place on Earth where the Sun is below the horizon (that is, where it is nighttime). A lunar eclipse has the maximum possible duration if the Moon travels directly through the center of the umbra. The Moon’s speed through Earth’s shadow is roughly 1 kilometer per second (3600 kilometers per hour, or 2280 miles per hour), which means that totality—the period when the Moon is completely within Earth’s umbra—can last for as long as 1 hour and 42 minutes.

Eclipses can only occur at new moon or full moon AND only if the Sun and Earth are in perfect alignment with the Moon’s position.

On average, two or three lunar eclipses occur in a year. Table 1-1 lists all 10 lunar eclipses from 2014 to 2018. Of all lunar eclipses, roughly one-third are total, one-third are partial, and one-third are penumbral.

Date Type Where visible Duration of totality (h = hours, m = minutes)
2014 April 15 Total Australia, Pacific, Americas 1h 18m
2014 October 8 Total Asia, Australia, Pacific, Americas      59m
2015 April 4 Total Asia, Australia, Pacific, Americas       5m
2015 September 28 Total Americas, Europe, Africa, western Asia 1h 12m
2016 March 23 Penumbral Asia, Australia, Pacific, western Americas     —
2016 September 16 Penumbral Europe, Africa, Asia, Australia, western Pacific     —
2017 February 11 Penumbral Americas, Europe, Africa, Asia     —
2017 August 7 Partial Europe, Africa, Asia, Australia     —
2018 January 31 Total Asia, Australia, Pacific, western North America 1h 16m
2018 July 27 Total South America, Europe, Africa, Asia, Australia 1h 43m
Table : Table 1-1: Lunar Eclipses, 2014–2018

Question

ConceptCheck 1-14: Why does the eclipsing Moon spend more time in the penumbral shadow than the umbral shadow?

24

Solar Eclipses

As seen from Earth, the angular diameter of the Moon is almost exactly the same as the angular diameter of the far larger but more distant Sun—about 0.5°. Thanks to this coincidence of nature, the Moon just “fits” over the Sun during a total solar eclipse.

A total solar eclipse is a dramatic event. The sky begins to darken, the air temperature falls, and winds increase as the Moon gradually covers more and more of the Sun’s disk. All nature responds: Birds go to roost, flowers close their petals, and crickets begin to chirp as if evening had arrived. As the last few rays of sunlight peek out from behind the edge of the Moon and the eclipse becomes total, the landscape around you is bathed in an eerie gray or, less frequently, in shimmering bands of light and dark. Finally, for a few minutes the Moon completely blocks out the dazzling solar disk and not much else (Figure 1-29a). The Sun’s thin, hot outer atmosphere, which is normally too dim to be seen—blazes forth in the darkened daytime sky (Figure 1-29b). It is an awe-inspiring sight.

Figure 1-29: RIVUXG A Total Solar Eclipse (a) This photograph shows the total solar eclipse of August 11, 1999, as seen from Elâzğ, Turkey. The sky is so dark that the planet Venus can be seen to the left of the eclipsed Sun. (b) When the Moon completely covers the Sun’s disk during a total eclipse, the solar corona is revealed.

CAUTION

If you are fortunate enough to see a solar eclipse, keep in mind that the only time when it is safe to look at the Sun is during totality, when the solar disk is blocked by the Moon and only the Sun’s outermost atmosphere is visible. Viewing this magnificent spectacle cannot harm you in any way. But you must never look directly at the Sun when even a portion of its intensely brilliant disk is exposed. If you look directly at the Sun at any time without a special filter approved for solar viewing, you will suffer permanent eye damage or blindness.

To see the remarkable spectacle of a total solar eclipse, you must be inside the darkest part of the Moon’s shadow, also called the umbra, where the Moon completely blocks the Sun. Because the Sun and the Moon have nearly the same angular diameter as seen from Earth, only the tip of the Moon’s umbra reaches Earth’s surface (Figure 1-30). As Earth rotates, the tip of the umbra traces an eclipse path across Earth’s surface. Only those locations within the eclipse path are treated to the spectacle of a total solar eclipse. The inset in Figure 1-30 shows the dark spot on Earth’s surface produced by the Moon’s umbra.

Figure 1-30: RIVUXG The Geometry of a Total Solar Eclipse During a total solar eclipse, the tip of the Moon’s umbra reaches Earth’s surface. As Earth and the Moon move along their orbits, this tip traces an eclipse path across Earth’s surface. People within the eclipse path see a total solar eclipse as the tip moves over them. Anyone within the penumbra sees only a partial eclipse. The inset photograph was taken from the Mir space station during the August 11, 1999, total solar eclipse (the same eclipse shown in Figure 1-29). The tip of the umbra appears as a black spot on Earth’s surface. At the time the photograph was taken, this spot was 65 mi (105 km) wide and was crossing the English Channel at 1900 mi/h (3000 km/h).

Immediately surrounding the Moon’s umbra is the region of partial shadow called the penumbra. As seen from this area, the Sun’s surface appears only partially covered by the Moon. During a solar eclipse, the Moon’s penumbra covers a large portion of Earth’s surface, and anyone standing inside the penumbra sees a partial solar eclipse. Such eclipses are much less interesting events than total solar eclipses, which is why astronomy enthusiasts strive to be inside the eclipse path. If you are within the eclipse path, you will see a partial eclipse before and after the brief period of totality.

25

The width of the eclipse path depends primarily on the Earth-Moon distance during totality. The eclipse path is widest if the Moon happens to be at perigee, the point in its orbit nearest Earth. In this case the width of the eclipse path can be as great as 270 kilometers (170 miles). In most eclipses, however, the path is much narrower.

Question

ConceptCheck 1-15: Why can a total lunar eclipse be seen by people all over the world but total solar eclipses can only been seen from a very limited geographic location?

Annular Solar Eclipses

In some eclipses the Moon’s umbra does not reach all the way to Earth’s surface. This can happen if the Moon is at or near apogee, its farthest position from Earth. In this case, the Moon appears too small to cover the Sun completely. The result is a third type of solar eclipse, called an annular eclipse. During an annular eclipse, a thin ring of the Sun is seen around the edge of the Moon (Figure 1-31). The length of the Moon’s umbra is nearly 5000 kilometers (3100 miles) less than the average distance between the Moon and Earth’s surface. Thus, the Moon’s shadow often fails to reach Earth even when the Sun, Moon, and Earth are properly aligned for an eclipse. Hence, annular eclipses are slightly more common—as well as far less dramatic—than total eclipses.

Figure 1-31: RIVUXG An Annular Solar Eclipse This composite of nine photographs was taken from the small Spanish town of Carrascosa del Campo on October 3, 2005. At maximum coverage, the Sun is still too bright to observe without special eye protection, even though 90% of the disk is covered. This type of eclipse occurs when the Moon happens to be too far from Earth to completely cover the Sun’s disk.

Even during a total eclipse, most people along the eclipse path observe totality for only a few moments. Earth’s rotation, coupled with the orbital motion of the Moon, causes the umbra to race eastward along the eclipse path at speeds in excess of 1700 kilometers per hour (1060 miles per hour). Because of the umbra’s high speed, totality never lasts for more than 7½ minutes. In a typical total solar eclipse, the Sun-Moon-Earth alignment and the Earth-Moon distance are such that totality lasts much less than this maximum.

The details of solar eclipses are calculated well in advance. They are published in such reference books as the Astronomical Almanac and are available on the World Wide Web. Figure 1-32 shows the eclipse paths for all total solar eclipses from 1997 to 2020. Table 1-2 lists all the total, annular, and partial eclipses from 2014 to 2018, including the maximum duration of totality for total eclipses.

Figure 1-32: Eclipse Paths for Total Eclipses, 2001–2025 This map shows the eclipse paths for all total solar eclipses occurring from 2001 through 2025. In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface.

26

Date Type Where visible Duration of totality
2014 April 29 Annular southern Indian Ocean, Australia, Antarctica
2014 October 23 Partial northern Pacific, North America 81% eclipsed
2015 March 20 Total Iceland, Europe, northern Africa, northern Asia 2m 47s
2015 September 13 Partial southern Africa, southern Indian Ocean, Antarctica 79% eclipsed
2016 March 9 Total eastern Asia, Australia, Pacific Ocean 4m 09s
2016 September 1 Annular Africa, Indian Ocean
2017 February 26 Annular southern South America, Atlantic Ocean, Africa
2017 August 21 Total North America, northern South America 2m 40s
2018 February 15 Partial southern South America, Antarctica 60% eclipsed
2018 July 13 Partial southern Australia 34% eclipsed
2018 August 11 Partial northern Europe, northeast Asia 74% eclipsed
Table : Table 1-2: Eclipses, 2014–2018

Ancient astronomers achieved a limited ability to predict eclipses. In those times, religious and political leaders who were able to predict such awe-inspiring events as eclipses must have made a tremendous impression on their followers. One of three priceless manuscripts to survive the devastating Spanish Conquest shows that the Mayan astronomers of Mexico and Guatemala had a fairly reliable method for predicting eclipses. The great Greek astronomer Thales of Miletus is said to have predicted the famous eclipse of 585 b.c.e., which occurred during the middle of a war. The sight was so unnerving that the soldiers put down their arms and declared peace.

In retrospect, it seems that what ancient astronomers actually produced were eclipse “warnings” of various degrees of reliability rather than true predictions. Working with historical records, these astronomers generally sought to discover cycles and regularities from which future eclipses could be predicted.

Question

ConceptCheck 1-16: If you had a chance to observe a total solar eclipse and a total lunar eclipse, in general, how much longer would you expect one type to last than the other?