ECLIPSES

Eclipses are among the most spectacular natural phenomena. During a lunar eclipse, the brilliant full Moon often darkens to a rusty red. A lunar eclipse occurs when the Moon passes through Earth’s shadow. This can happen only when the Sun, Earth, and the Moon are in a straight line at full Moon. During a solar eclipse, broad daylight is transformed into an eerie twilight, as the Sun seems to be blotted from the sky. A solar eclipse occurs when the Moon’s shadow moves across Earth’s surface. As seen from Earth, the Moon moves in front of the Sun—a new Moon.

1-11 Eclipses do not occur during every new or full Moon phase

At first glance, it would seem that eclipses should happen at every new and full Moon, but in fact, they occur much less often because the Moon’s orbit is tilted 5° from the plane of the ecliptic (Figure 1-23). Consequently, the new Moon and full Moon usually occur when the Moon is either above or below the plane of Earth’s orbit. In such positions, a true alignment between the Sun, Moon, and Earth is not possible. Hence, Earth and the Moon do not enter each others’ shadows and an eclipse cannot occur. This tilt also explains why, as mentioned earlier, the new Moon is often seen as a thin crescent—we see “under” or “above” it to a tiny sliver of the sunlit side.

Figure 1-23: Conditions for Eclipses The Moon must be very nearly on the ecliptic at new Moon for a solar eclipse to occur. A lunar eclipse occurs only if the Moon is very nearly on the ecliptic at full Moon. When new Moon or full Moon phases occur away from the ecliptic, no eclipse is seen because the Moon and Earth do not pass through each other’s shadows.

Indeed, because its orbit is tilted 5° from the ecliptic, the Moon is usually above or below the plane of our orbit around the Sun. The Moon crosses the ecliptic at what is called the line of nodes (see Figure 1-23). An eclipse takes place only when the Moon crosses the plane of the ecliptic during its new or full phase. By calculating the number of times a new Moon takes place on the line of nodes, we find that at least two and no more than five solar eclipses occur each year. Lunar eclipses occur just about as frequently as solar eclipses, with the maximum number of eclipses (solar plus lunar) possible in a year being seven.

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1-12 Three types of lunar eclipses occur

Earth’s shadow has two distinct parts, as shown in Figure 1-24a. The umbra is the part of the shadow where all direct sunlight is blocked by Earth. If you were in Earth’s umbra looking at Earth, you would not see the Sun behind it at all. The penumbra of the shadow is where Earth blocks only some of the sunlight. If you were in Earth’s penumbra looking at Earth, you would see a crescent Sun behind it. The Moon has an analogous umbra and penumbra.

Depending on how the Moon travels through Earth’s shadow, one of three kinds of lunar eclipse may occur. A penumbral eclipse, when the Moon passes through only Earth’s penumbra, is easy to miss. The Moon still looks full, just a little dimmer than usual and sometimes slightly reddish in color (path 1 in Figure 1-24a).

When just part of the lunar surface passes through the umbra, a bite seems to be taken out of the Moon, and we see a partial eclipse (path 2 in Figure 1-24a). When the Moon travels completely into the umbra, we see a total eclipse of the Moon (path 3 in Figure 1-24a). Total lunar eclipses with the maximum duration, lasting for up to 1 hour and 47 minutes, occur when the Moon is closest to Earth and is traveling directly through the center of the umbra. Table 1-2 lists all the total and partial lunar eclipses from 2015 through 2018.

Figure 1-24: Three Types of Lunar Eclipses (a) People on the nighttime side of Earth see a lunar eclipse when the Moon moves through Earth’s shadow. The umbra is the darkest part of the shadow. In the penumbra, only part of the Sun is covered by Earth. The inset shows the various lunar eclipses that occur, depending on the Moon’s path through Earth’s shadow. (b) This sequence of nine photographs was taken over a 3-hour period during the total lunar eclipse of January 20, 2000. During the total phase, the Moon has a distinctly reddish color.

Date

Visible from

Type

Duration of totality (h:min)

2015 Apr 04

Asia, Australia, Pacific, Americas

Total

  :05

2015 Sep 28

Europe, Africa, west Asia, Americas

Total

1:12

2016 Mar 23

Asia, Australia, Pacific, western Americas

Penumbral

 

2016 Sep 16

Europe, Africa, Asia, Australia

Penumbral

 

2017 Feb 11

Europe, Africa, Asia, Americas

Penumbral

 

2017 Aug 07

Europe, Africa, Asia, Australia

Partial

 

2018 Jan 31

Asia, Australia, western North America

Total

1:16

2018 Jul 27

South America, Europe, Africa, Asia, Aus.

Total

1:43

TABLE 1-2 LUNAR ECLIPSES, 2015–2018

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Focus Question 1-7

Why does the new Moon sometimes appear as a crescent?

Even during a total eclipse, the Moon does not completely disappear. A small amount of sunlight passing through Earth’s atmosphere is bent into Earth’s umbra. The light deflected into the umbra is primarily red and orange, and thus the darkened Moon glows faintly in rust-colored hues (Figure 1-24b). Everyone on the side of Earth over which a lunar eclipse occurs can see it, provided that clouds do not obscure the event. Lunar eclipses are perfectly safe to watch with the naked eye.

1-13 Three types of solar eclipses also occur

Figure 1-25: The Geometry of a Total Solar Eclipse During a total solar eclipse, the tip of the Moon’s umbra traces an eclipse path across Earth’s surface. People inside the eclipse path see a total solar eclipse, while people inside the penumbra see only a partial eclipse. The photograph in this figure shows the Moon’s shadow on Earth. It was taken from the Mir space station during the August 11, 1999, total solar eclipse. The Moon’s umbra appears as the very dark spot on the eastern coast of the United States. The umbra is surrounded by the penumbra.

The different physical diameters of the Sun and Moon and their different distances from us combine so that they both have nearly the same angular diameter as seen from Earth—about ½°. When the Moon completely covers the Sun, the result is a total solar eclipse. You can see in Figure 1-25 that only the tip of the Moon’s umbra ever reaches Earth’s surface. As Earth turns and the Moon orbits, the tip traces an eclipse path across our planet. Only people within these areas are treated to the spectacle of a total solar eclipse, which is why cruise lines love solar eclipses. During those few precious moments, hot gases (the solar corona) surrounding the Sun can be observed and photographed (Figure 1-26). By studying the light from the outer layers of the Sun, astronomers have been able to learn about its atmospheric temperature, chemistry, and gas activity.

Viewing the Sun directly for more than a moment at any time without an approved filter causes permanent eye damage. It is only safe to look at a total solar eclipse without a filter during the brief time when the Moon completely blocks the Sun. At all other times during the eclipse, you must view it either through an approved filter or in the image made on a flat surface by a telescope or a pinhole camera.

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Earth’s rotation and the orbital motion of the Moon cause the umbra to race along the eclipse path at speeds in excess of 1700 km/h (1050 mi/h). For this reason, complete blockage of the Sun (totality) during a total eclipse never lasts for more than 7½ minutes at any one location on the eclipse path, and it usually lasts for only a few moments.

Figure 1-27: An Annular Eclipse of the Sun This composite of five exposures taken at sunrise in Costa Rica shows the progress of an annular eclipse of the Sun that occurred on December 24, 1974. Note that at mid-eclipse the edge of the Sun is visible around the Moon.
Figure 1-26: A Total Eclipse of the Sun During a total solar eclipse, the Moon completely covers the Sun’s disk, and the solar corona can be photographed. This halo of hot gases extends for millions of kilometers into space. This gorgeous image was taken in southwestern Mongolia during the August 1, 2008, solar eclipse.

The Moon’s umbra is also surrounded by a penumbra (see Figure 1-25). The photograph in Figure 1-25 shows the dark spot produced by the Moon’s umbra and the less dark penumbral ring surrounding it on Earth’s surface during a total solar eclipse. During a solar eclipse, the Moon’s penumbra extends over a large portion of Earth’s surface. When only the penumbra sweeps across Earth’s surface, as happens in high latitude regions, the Sun is only covered by the Moon. This circumstance results in a partial eclipse of the Sun. Similarly, people in the penumbra of a total eclipse see a partial eclipse. In either case, the Sun looks crescent-shaped as seen from Earth. Never look directly at a partial solar eclipse.

The Moon’s orbit around Earth is oval shaped. Therefore, the distance between Earth and the Moon, which averages 384,400 km (238,900 mi), varies by 14% as the Moon moves around Earth. Consequently, the angular size of the Moon in our sky varies by that amount. The width of the eclipse path depends primarily on the Earth–Moon distance during an eclipse. The eclipse path is widest—up to 270 km (170 mi)—when the new Moon happens to be at the point in its orbit nearest Earth. Usually, however, the path is much narrower.

Focus Question 1-8

Why aren’t there any annular lunar eclipses?

If what would otherwise be a total solar eclipse occurs when the Moon is relatively far from Earth, then the Moon’s umbra falls short of Earth and no one sees a total eclipse. From Earth’s surface, the Moon then appears too small to cover the Sun completely, and a thin ring or “annulus” of light is seen around the edge of the Moon at mid-eclipse. This type of eclipse is called an annular eclipse (Figure 1-27). The length of the Moon’s umbra is nearly 5000 km (3100 mi) shorter than the average distance between the Moon and Earth’s surface. Thus, the Moon’s shadow often fails to reach Earth, making annular solar eclipses slightly more common than total solar eclipses. Table 1-3 lists all the total, partial, and annular solar eclipses from 2015 through 2018. Figure 1-28 shows the paths over Earth of all the total and annular eclipses through 2020. Please remember: It is never safe to look directly at a partial or annular eclipse.

Date

Type

Visible from

Total/annular eclipse time (min:sec)

2015 Mar 20

Partial/Total

Partial: Iceland, Europe, northern Africa, northern Asia Total: north Atlantic Ocean

2:47

2015 Sep 13

Partial

Southern Africa, Antarctica

 

2016 Mar 09

Partial/Total

Partial: eastern Asia, Australia Total: Pacific Ocean, Borneo

4:09

2016 Sep 01

Annular

Atlantic Ocean, central Africa, Indian Ocean

3:06

2017 Feb 26

Annular

Pacific Ocean, Chile, Argentina, Atlantic Ocean, Africa

0:44

2017 Aug 21

Partial/Total

Partial: northern South America, all of North America except United States Total: United States

2:40

2018 Feb 15

Partial

southern South America

 

2018 Jul 13

Partial

southern Australia

 

2018 Aug 11

Partial

northern Europe, northeast Asia

 

TABLE 1-3 SOLAR ECLIPSES, 2015–2018
Figure 1-28: Eclipse Paths for Total and Annular Eclipses 2001–2020 This map shows the eclipse paths for the 14 total solar and 13 annular eclipses occurring between 2001 and 2020. In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface.

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