Читать книгу Exploring the Solar System - Peter Bond - Страница 40

Observation of the Sun usually requires specialized equipment and proper eye protection. However, there are rare occasions when anyone can stare at the Sun (or at least its outer regions) for several minutes without any protection. These spectacular apparitions are total solar eclipses.

Solar eclipses occur when the Moon passes directly in front of the Sun, covering its disk. This is possible because of a mathematical coincidence: the diameter of the Sun is some 400 times greater than the Moon's diameter, but, at this particular epoch of time, the Moon is approximately 400 times closer to Earth than the Sun.

Figure 2.5 Once every four weeks, the Moon moves between the Sun and Earth. This is the lunar phase known as New Moon. Occasionally, the Moon passes precisely in front of the Sun, so that its shadow (the umbra) just reaches Earth. This results in a total solar eclipse for anyone located within the umbra. As the shadow travels across the surface of the rotating Earth, it traces a narrow path of totality, where the Sun's disk is hidden from view for up to 7½ minutes.

(ESA)

Solar eclipses can only happen at the time of New Moon. If the Moon travels directly between the Sun and Earth, it is clear that our planet will pass through the lunar shadow and an eclipse will occur. However, such alignments do not take place every 4 weeks, partly because the plane of the Moon's orbit is inclined around 5° to the plane of Earth's orbit (see Chapter 4). Moreover, although the Moon crosses the plane of Earth's orbit twice every four weeks, one of these intersection points (nodes) must intersect a line joining the Sun and Earth for a solar eclipse to occur.

This means that, although the Moon passes between the Sun and Earth during each New Moon phase, the objects are usually not precisely aligned. As a result, there are only two to five total eclipses per year, when the Moon completely covers the Sun's disk.

At such times, the Moon's cone‐shaped shadow (the umbra) stretches across space and grazes the Earth. For an observer situated inside the umbra, the eclipse is total. For someone located in the outer part of the shadow (the penumbra), only part of the Sun is masked, so the eclipse is partial.

As the planet rotates, the tip of the umbra travels over the surface at about 1,600 km/h, tracing a path no more than 270 km wide. Within the path of totality the sky goes dark, enabling the stars and planets to appear for up to 7½ minutes.²

Just before the Moon completely covers the photosphere, the Sun's light shines through gaps in lunar mountains to produce a glittering “diamond ring” effect. As totality sets in, the normally invisible solar atmosphere – the corona – appears as a pearl‐white ring around the black Moon. (The corona is not usually visible in daylight because its luminosity is only about one millionth that of the photosphere.) A number of reddish, flame‐like prominences may rise above the lunar limb, held in place by powerful magnetic fields.

The visible structure of the corona is related to the density of electrons in the solar atmosphere that are available to reflect light from the photosphere. Its appearance varies considerably. Near times of solar maximum, when sunspots are most numerous, the corona displays numerous bright “helmet” streamers that emanate all around the solar disk. When the Sun is less active, these streamers are fewer in number, and often missing altogether from the polar regions.

Sometimes, when the Moon is near its apogee, the lunar disk does not completely cover the Sun. During such an annular eclipse, the Moon's shadow does not reach Earth, so the dark circle of the Moon is surrounded by a bright ring of the Sun's surface. (Annular comes from annulus, the Latin word for ring.) Annular eclipses are slightly more frequent than total eclipses.

Partial eclipses are seen over a much larger area than total eclipses, so they are much more frequent in any given location on Earth. At these times, only part of the Sun is covered by the Moon, resembling a bite taken out of its disk.

Figure 2.6 The total solar eclipse of August 21, 2017, was seen across the United States. This image shows the Sun's modest activity as it neared solar minimum. In addition to the bright coronal streamers, three red prominences are visible (right).

(David H. Hathaway)

Figure 2.7 The Moon's shadow consists of two cone‐shaped areas, known as the umbra and the penumbra. For an observer standing within the umbra, the eclipse is total. If Earth is slightly further away from the Moon, the outer part of the solar disk is not covered and the eclipse is annular (ring‐like). For an observer standing in the penumbra, only part of the Sun is masked: the eclipse is partial.

(ESA)

Occasionally, major outbursts on the Sun, involving the interaction of high‐energy electrons, protons, and atomic nuclei, have been found to produce gamma rays – a form of electromagnetic radiation with extremely low wavelengths (high frequencies) and energies up to one million electron volts.³

Since most of the high‐frequency radiation is screened out by Earth's atmosphere (fortunately for our health), studies of the high energy end of the spectrum generally require instruments to be lofted above the blanket of air, on balloons, sounding rockets, or satellites.

Considerable information on the Sun's composition and temperature can be obtained by spreading out the visible spectrum in a spectrograph (Figure 2.9). By passing light through a series of fine slits or a diffraction grating, it is possible to see hundreds of black lines in the spectrum. These are called absorption lines because they are created when different ionized atoms in the solar atmosphere absorb light at certain wavelengths.⁴ The wavelength of each line indicates a specific ionized element in the Sun, while the darkness of each line shows its relative abundance.

This spectral fingerprinting makes it possible to unravel the chemical composition of the solar atmosphere. It turns out that the Sun is mainly composed of the two lightest elements, hydrogen and helium – both of which are rare on Earth.

71% of the Sun (by mass) consists of hydrogen, although this element actually accounts for 92% of the atoms in the Sun. Helium, the second‐most abundant element in the Sun, is so rare on Earth that it was first discovered by studying the solar spectrum during the eclipse of 1868. Helium accounts for 27% of the Sun's mass and 7.8% of its atoms. This means that heavier elements make up only 0.1% of the atoms in the Sun.

Oxygen, carbon, and nitrogen are the Sun's three most abundant “metals,” i.e. elements heavier than helium. It also has traces of neon, sodium, magnesium, aluminum, silicon, phosphorus, sulfur, potassium, and iron. Altogether some 67 elements have been detected in the solar spectrum.

Figure 2.8 The electromagnetic spectrum ranges from extremely short wavelength (high frequency) gamma rays to extremely long wavelength (low frequency) radio waves. The shorter wavelength radiation (gamma rays, X‐rays, and ultraviolet) is associated with high temperatures and high energy processes.

(NASA)

Figure 2.9 The solar spectrum in the visible, or white light, region. Spectrograms like this split the light up into different wavelengths (colors). Dark bands superimposed on the colors are absorption lines, created when atoms in the Sun's outer regions absorb light at certain wavelengths. These lines (sometimes called Fraunhofer lines) indicate specific ionized chemical elements in the Sun.

(AURA/NOAO/NSF)