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* * * CHAPTER 4 The Sky as Observed from the Rotating, Revolving Earth 411. Name several common uses for The Astronomical Almanac. 412. The "trace" of the Sun on a star chart during the course of a year is called (a) the celestial equator; (b) the vernal equinox; (c) the galactic equator; (d) the ecliptic. 413. The vernal equinox is the point in the star field occupied by the Sun on about (a) 21 September; (b) 21 March; (c) 21 June; (d) 21 April. 414. What is a stellar constellation? 415. What is the collective name of the twelve constellations through which the Sun passes during a year (as of 2,000 years ago)? 416. Of the 88 constellations, 12 are called "signs of the Zodiac". Why were 12 so designated? 417. What is the astronomical meaning of the term "sign of the Zodiac"? 418. There are __ "signs" of the Zodiac. 57 58 / The Sky as Observed from the Rotating, Revolving Earth 419. Why is Polaris one of the favorite stars of navigators and surveyors ? 420. Which one of the following constellations is not one of the "signs of the Zodiac"? (a) Capricornus; (b) Pisces; (c) Aquarius ; (d) Pegasus; (e) Sagittarius. 421. During what month of the year is the Sun primarily in the constellation Virgo (present epoch)? 422. What astronomical event occurs every year on or about the 22nd of September? 423. The Sun is at the vernal equinox on the following date (± 2 days): (a) 21 March; (b) 21 December; (c) 21 September; (d) 21 June. 424. Give approximate values for the right ascension and declination of the Sun on 21 December. 425. Criticize the following claim: All stars in a particular constellation (e.g., Orion) are at approximately the same distance from the Sun. 426. Explain why it is impossible to answer the following question: "What is the distance, in miles, between two stars five degrees apart in the sky?" 427. Cite several lines of evidence-physical rather than geometricthat the Earth is rotating. 428. As viewed from a northern mid-latitude station, what is the sense of rotation of circumpolar stars? 429. The period of rotation of the Earth is commonly said to be 24 hours. What, precisely, is the reference object with respect to which this period is measured? The Sky as Observed from the Rotating, Revolving Earth / 59 430. Although the rotation of the Earth has been the classical basis for time-keeping it has been supplanted in modern precision work by (a) radioactive sources; (b) atomic clocks; (c) the revolution of the Galilean satellites of Jupiter; (d) lasers. 431. Write a one sentence definition of the sidereal day. 432. Define (a) the vernal equinox and (b) sidereal time. 433. The difference between the lengths of the solar and the sidereal day is principally due to (a) precession of the equinoxes; (b) perturbations of the Earth's orbit by the Moon; (c) gradual slowdown of the Earth's rotation; (d) orbital motion of the Earth about the Sun. 434. What is the equation of time? 435. A common unit of angular measure in scientific work is the radian. Define it and calculate the number of degrees in a radian. 436. An observer at ground level notes that the Washington Monument , whose height is 555 feet, subtends an angle of six degrees. How far is the base of the monument from the observer? 437. The principle of parallax is essential to the classical method of finding the (a) ratio of the Earth-Moon distance to the diameter of the Earth; (b) angular diameter of the Moon; (c) angle between two stars; (d) motion of the Moon with respect to the star field. 438. Two persons on the equator of the Earth and at nearly 180 degrees difference in longitude observe the Moon's position on the star field at the same moment. What is the difference in right ascension between the two apparent positions if the declination of the Moon is zero? 439. What is meant by the parallax of a star? 60 / The Sky as Observed from the Rotating, Revolving Earth 440. How can one distinguish a planet from a star by observation with the unaided eye? 441. Estimate the total number of stars that can be seen by the unaided eye on a clear, moonless night by counting those that you can see within a specified fraction of the sky. 442. How many stars can be seen by the unaided eye? 443. Given a...


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