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* * * CHAPTER 7 Radiations and Telescopes 809. Only one of the following is a form of electromagnetic radiation. Which one? (a) cosmic ray; (b) proton; (c) neutron; (d) gamma ray. 810. Which one of the following is not a type of electromagnetic radiation ? (a) X ray; (b) ultraviolet; (c) cosmic ray; (d) gamma ray. 811. What are cosmic rays? 812. The first measurement of the speed of light was made by (a) Fizeau; (b) Roemer; (c) Michelson; (d) Galileo. 813. Describe the essential elements of the determination of the speed of light by observation of a satellite of Jupiter. 814. The apparent periods of revolution of the satellites of Jupiter vary cyclically during the course of a year. This effect is attributed to the (a) precession of their orbits; (b) tilt of the axis of the planet to the plane ofits orbit; (c) finite speed of light; (d) mutual gravitational perturbations. 107 108 / Radiations and Telescopes 815. The true sidereal period of revolution of 10 (one of the satellites of Jupiter) is 42h 28m • What is its apparent period to an observer on the Earth when the Earth is approaching Jupiter at a relative speed of 30 km s-l? 816. What is the magnitude of the speed of light? 817. Describe an experimental method for measuring the speed of light. 818. What is the time of transit of a light pulse from the Sun to the Earth? 819. Noting that a light signal requires 8.32 minutes to travel from the Sun to the Earth, calculate the number of astronomical units in a light year. 820. From the fact that a light signal travels from the Sun to the Earth in about 8.3 minutes, find the ratio of the speed of light to the orbital speed of the Earth. 821. How does the intensity of light vary with distance from a point source? 822. The brightness of sunlight at the Earth is ____ times as great as at Pluto (40 AU from the Sun). 823. The intensity of sunlight at the Earth is ____ times as great as it is at Neptune (30 AU from the Sun). 824. What is the relative brightness of sunlight at the Earth and at Jupiter (5.2 AU from the Sun)? 825. A spacecraft at the orbit of Uranus (20 AU from the Sun) uses a parabolic mirror to collect sunlight for powering its electronic equipment. Assuming that 200 watts of electrical power are required and that the conversion of sunlight to electrical power is 10 percent efficient, find the necessary diameter of the mirror. At the Earth, 1,360 watts of sunlight strike one square meter. Radiations and Telescopes / 109 826. A spacecraft at the orbit of Pluto (40 AU from the Sun) uses a parabolic mirror to collect sunlight for powering its electronic equipment. Assuming that 200 watts of electrical power are required and that the conversion of sunlight to electrical power is 10 percent efficient, find the necessary diameter of the mirror. At the Earth, 1,360 watts of sunlight strike one square meter. 827. Consider two identical stars A and B. A is four times as far away from the Earth as is B. What is the ratio of their apparent brightnesses? 828. At perihelion the planet Mercury is 0.31 AU from the Sun. How much brighter is sunlight there than at the Earth? 829. When the spacecraft Pioneer 10 was 52 AU from the Sun, its instruments were at a temperature of 260 degrees Kelvin; but Saturn at 10 AU has a surface temperature of only 75 degrees Kelvin. How could the Pioneer 10 instruments be so warm? 830. The temperature of an inert object in interplanetary space is dependent primarily on (a) the amount of sunlight that it absorbs ; (b) its speed through the interplanetary medium; (c) the temperature of the gas in the solar wind; (d) the amount of dust with which it collides. 831. The radiative equilibrium temperature of an inert body in space varies directly as the fourth root of the amount of radiation that it absorbs from external sources. If the intensity of starlight is 10-8 of that of sunlight at 1 AU, what would be the temperature of a body in "deep space" (very remote from the Sun)? 832. Barnard's star has a proper motion of 10 seconds of arc per year and is at a distance of 1.8 parsecs (pc). What is the component of its space...