Precession

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Your star maps will be useful to you for the rest of your life. You may be interested to know, however, that they will finally go out of date hundreds of years from now.

Earth's axis of rotation shifts extremely slowly around a cone in space once about every 25,800 years. This slow motion of Earth's axis, caused mainly by the tug of the gravity of the Sun and Moon on Earth's equatorial bulge, is called precession.

Earth's axis always tilts 23.5° to its orbital plane, so precession causes the north celestial pole to circle among the stars. After thousands of years, the polestar changes (Figure 1.14).

The vernal equinox, the zero point of right ascension, drifts westward around the ecliptic at a rate of about 50 seconds a year. It drifts 30°, a whole zodiac constellation, in 2150 years. Then all star charts are out of date. (Astronomers revise their precise star charts regularly.)

In astrology today, each sign of the zodiac bears the name of the constellation for which it was originally named but with which it no longer coincides due to precession of the equinoxes.

Refer to Figure 1.14. The present polestar is Polaris, and the vernal equinox is located in the constellation Pisces. (a) What was the polestar in the year 3000 b.c.?_(b) What will it be in the year a.d. 14,000?_

Figure 1.14. Precession. Earth's axis very slowly traces out a cone in space, so eventually the polestar changes.

SELF-TEST

This self-test is designed to show you whether or not you have mastered the material in Chapter 1. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test.

1. For each of the following references used on a terrestrial globe, list the corresponding name on the celestial sphere:

2. Refer to Table 1.1. Which of the five brightest stars in the sky are above the celestial equator, and which are below?_

3. Refer to Table 1.1. Which of the five brightest stars never appear above the horizon at latitude 40° (about New York City)?_

4. Match where you might be on Earth with the correct description of the stars:

(a) The stars seem to move along circles around sky parallel to your horizon.

(b) The stars rise at right angles to the horizon in the east and set at right angles to the horizon in the west.

(c) Vega practically crosses your zenith.

(d) Acrux is always above your horizon.

(e) Polaris appears about 30° above your horizon.

5. Why do the stars appear to move along arcs in the sky during the night?

6. Why do some different constellations appear in the sky each season?

7. What is the zodiac?

8. Where on Earth would you have to be to have the Sun pass directly across your zenith at the time of the (a) vernal equinox?_

(b) summer solstice?_(c) winter solstice?_

9. If a star rises at 8 p.m. tonight, at approximately what time will it rise a month from now?_

10. Why is a solar day about 4 minutes longer than a sidereal day?_

11. Arrange the following stars in order of decreasing brightness: Antares (magnitude 1); Canopus (magnitude -1); Polaris (magnitude 2); Vega (magnitude 0)._

12. Why will the polestar and the location of the vernal equinox on the celestial sphere be different thousands of years from now, causing your star maps finally to go out of date?_

ANSWERS

Compare your answers to the questions on the self-test with the answers given below. If all of your answers are correct, you are ready to go on to the next chapter. If you missed any questions, review the sections indicated in parentheses following the answer. If you missed several questions, you should probably reread the entire chapter carefully.

1. (a) Celestial equator. (d) Declination.

(b) North celestial pole. (e) Right ascension.

(c) South celestial pole. (f) Vernal equinox. (Sections 1.1, 1.8, 1.9)

2. Above: Arcturus, Vega. Below: Sirius, Canopus, Rigil Kentaurus. (Sections 1.9, 1.10)

3. Canopus, Rigil Kentaurus. (Sections 1.10, 1.13, 1.14)

4. (a) 4; (b) 2; (c) 5; (d) 1; (e) 3. (Sections 1.10, 1.13 through 1.15)

5. Because of Earth's rotation. (Sections 1.1, 1.12, 1.14)

6. Because of Earth's revolution around the Sun. (Section 1.16)

7. A belt about 16° wide around the sky centered on the ecliptic, containing 12 constellations. (Section 1.17)

8. (a) Equator; (b) 23.5°N (Tropic of Cancer); (c) 23.5°S (Tropic of Capricorn). (Sections 1.19 through 1.21)

10. Because, while Earth rotates on its axis, it also moves along in its orbit around the Sun. Earth must complete slightly more than one whole turn in space before the Sun reappears on your meridian. (Section 1.23)

11. Canopus, Vega, Antares, Polaris. (Section 1.7)

12. Because of the precession of Earth's axis. (Section 1.24)

LIGHT AND TELESCOPES

Curiosity is one of the permanent and certain characteristics of a vigorous mind.

Samuel Johnson (1709-1784) The Rambler

Objectives

^ Describe the wave nature of light, including how it is produced and how it travels.

^ Name the major regions of the electromagnetic spectrum from the shortest wavelength to the longest.

^ State the relationship between wavelength and frequency.

^ State the relationship between the color of a star and its temperature.

^ List the three windows (spectral regions) in Earth's atmosphere in order of their importance to observational astronomy.

^ Explain how refracting and reflecting telescopes work.

^ Define light-gathering power, resolving power, and magnification with respect to a telescope.

^ State the two most important factors in telescope performance.

^ State the purpose of a spectrograph.

^ Explain how radio telescopes work, and list some interesting radio sources.

^ Explain why infrared telescopes are located in very high, dry sites, and list some objects they observe.

^ Explain why ultraviolet, X-ray, and gamma ray telescopes must operate above Earth's atmosphere, and list some objects they study.

Source Receiver

Figure 2.1. Visualizing a light wave.

Source Receiver

Figure 2.1. Visualizing a light wave.

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