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or decrease with the mass or volume of the body? Describe the seasons.
94. Describe the telescopic appearance. Who discovered the phases of Venus? What was Copernicus's idea?
95. What proof have we of an atmosphere? Of clouds? Has Venus any moon?
96. Earth.—Sign? What is the appearance of the earth from the other planets? Do we, then, live on a star? Is it probable that the earth was always dark and dull as it now seems to us ? * How does the size of the earth compare with that of the other planets? Form of the earth? Exact diameter? Is the equator a perfect circle?
98. Circumference? Density? Weight? What can you say of its inequalities? How do you prove the rotundity of the earth?
99. Why can we see further from the top of a hill than from its base? Why is the horizon a circle?
100. Give some illustrations of apparent motion.
101. Explain the cause of the rising and setting of the sun and stars. Who first explained it in this manner? What do you say of its simplicity?
102. Cause of day and night? Do all places on the earth revolve with equal velocity? Illustrate. At what rate do we move?
103. Why do we not perceive our motion? What would be the effect if the earth were to stop?
104-5. Is there any danger of this catastrophe? Draw the figure, and show how the stars move daily through unequal orbits and with unequal velocities. Describe the appearance of the stars at the N. Pole.
106. At the Equator. S. Pole. Describe the path of the earth about the sun. Define eccentricity. Is this stable?
107. Do we see the same stars at different seasons of the year? Why not? If we should watch from 6 P. M. to 6 A. M., what portion of the sphere could we see? What do we mean by the yearly motion of the sun among the stars? How can we see it?
109. What is the cause? What is the ecliptic? Why so called? What are the equinoxes? What do we understand
* Probably not. The earth was doubtless once a glowing star, like the sun. Its crust is only the ashes and cinders of that fearful conflagration. The rocks are all burnt bodies. The atmosphere is only the gas left over after the fuel was all consumed. Every organic object has been rescued by plants and the sunbeam from the grasp of oxygen.
when we see in the almanac "the earth is in Aries?" "The sun is in Sagittarius?"
110. How many apparent motions has the sun? Name them, and give the cause and effects of each. Has the sun anyreal motions (pp. 54 and 224)? Describe the apparent motion of the sun, N. and S. How is it that the sun in summer shines on the north side of some houses both at rising and setting, but in winter never does? Define the obliquity of the ecliptic. The parallelism of the earth's axis. What do you say of its permanence?
112. Why will a top stand while spinning, but will fall as soon as it ceases? Show how the rays of the sun strike the various parts of the earth at different angles at the same time. Show how the angles vary at different times. Is the sun really hotter in summer than in winter? Why does it seem to be?
113. Explain the cause of equal day and night at the Equinoxes. Why are our days and nights of unequal length at all other times? Why do they vary at different seasons of the year? How do the seasons, &c, in the N. Temperate Zone compare with those in the S. Temperate Zone? Describe the yearly path of the earth about the sun—(1) at the summer solstice; (2) at the autumnal equinox; (3) at the winter solstice; (4) at the vernal equinox; (5) the yearly path finished back to the starting-point. Is the division of the earth's surface into zones an artificial or a natural distinction? Who invented it?
117. How much nearer are we to the sun in the winter? Why is it not the warmest at that time? How is it in the South Temperate Zone? When do the extremes of heat and cold occur? Why not exactly at the solstices?
118. Why is summer longer than winter? Does the earth move with the same velocity in all parts of its orbit? Describe the curious appearance of the sun at the North Pole. In Greenland, at what part of the year will the midnight sun be seen due north? What is the length of the days and nights at the Equator?
119. Describe the results if the axis of the earth were perpendicular to the ecliptic.
120. If the equator were perpendicular to the ecliptic. Define precession of the equinoxes. Who discovered this? At what rate does this movement proceed? What is the amount at present?
121. What are the results? What star was formerly the Pole star?
123. Explain the cause of precession.
125. How does the spinning of a top illustrate this subject?
126. What is Nutation? Cause? How does the moon's influence compare with that of the sun?
127. What is the real path of the N. Pole through the heavens? Is the obliquity of the ecliptic invariable? What is the limit? What is the effect of this variation?
128. Are the solstices and equinoxes stationary? What is the result of this change on the seasons? When will the cycle be completed? When is the sun in perigee?
129. What do you say of the provisions made to secure permanence, so that slight changes themselves prevent greater changes?
130. What is refraction? Its effect?
131. How does it vary?
132. Are the sun and moon ever where they seem to be? Is the real day longer or shorter than the apparent one? Why do the sun and moon appear flattened when near the horizon? Why not when they are high in the heavens? Why do they appear smaller in the latter case?
133- What causes the hazy appearance of the heavenly bodies near the horizon? What is the cause of twilight? How long does it last? Is it the same at all seasons of the year?
134. At all parts of the earth? Where is it longest? Shortest? What is diffused light? What would be the effect if the atmosphere did not act in this way? Is there really any sky in the heavens? Cause of the appearance?
135. What is aberration of light? Illustrate. Give two reasons why we never see the sun where it really is.
137. The general effect of aberration? Define parallax. Illustrate.
138. Define true and apparent place. How does parallax vary? What is the practical importance of this subject (p. 300, et seq.)?
139. Define horizontal parallax. What is the sun's horizontal parallax? What is the annual parallax?
The Moon.—Signs? Describe its orbit.
140. Its distance from the earth? Illustrate. Difference between its sidereal and synodic revolution?
141. What is the real path of the moon? (Imagine a pencil fastened to the spoke of a wheel, and the wheel rolled by the side of a wall on which the pencil is constantly marking.) How often does it turn on its axis? What is the moon's diameter? Volume? How does its apparent size vary? Why does it appear larger than it really is?
142. Why does the crescent moon appear larger than the dark body of the moon? When ought the moon to appear the largest? Do all persons think the moon of the same apparent size? Explain the three librations of the moon.
143. How does moonlight compare with sunlight? Is there any heat in moonlight? Why is it generally clear at full moon? Does the centre of gravity in the moon coincide with that of magnitude? Has the moon any atmosphere? What proof have we of this?—Ans. (1) We see but slightly if any appearance of twilight in the moon. (2) When the moon passes between us and a star, it does not refract the light of the star, so that the atmosphere cannot be sufficient to support more than -rihr of an inch of the mercurial column.
144. How does the earth appear from the moon? What is the earth-shine? How is it caused? What is it called in England? Describe the path of the moon around the earth, and the consequent phases. Why is new moon seen in the west and full moon in the east? Why can we sometimes see the moon in the west after the sun rises, and in the east before the sun sets.?
147. Length of a lunar month? What do we mean by the moon's running high or low? Cause? Use?
148. What is harvest moon? Hunter's moon? Cause?
149. What are nodes? How much is the moon's orbit inclined to the ecliptic—our ideal sea-level? What is an occupation? Use?
150. Describe the seasons, heat, &c, on the moon.
152. Telescopic appearance of the moon? Are the mountains the light or dark portions? What can you say about them? The gray plains? The rills? The craters? What are the peculiar features, then, of the lunar landscapes? Are the lunar volcanoes extinct?
ECLIPSES.—When can an eclipse of the sun occur? Show how a solar eclipse may be total, partial, or annular.
156. Define umbra. Penumbra. Central eclipse. State the general principles of a solar eclipse.
158. What curious phenomena attend a total eclipse?
159. Describe the effect of a total eclipse?
160. What curious custom prevails among the Hindoos? What is the Saros? Cause?
161. Is it now of any value? What is the metonic cycle? Explain its use.
162. What is the golden number? Cause of a lunar eclipse? Draw the figure and describe it. Why are lunar eclipses seen oftener than solar ones?
163. What is the earliest account of an eclipse? How were eclipses formerly regarded?
164. What story is told of Columbus?
The Tides.*—Define ebb. Flow. How often does the tide happen? Explain the cause.
166. Why does the tide occur fifty minutes later each day? Why is there a tide on the side opposite the moon? The sun is much larger than the moon; why does it not produce the larger tide? Why is not the tide felt out at sea?
167. What is spring-tide? Neap-tide? Causes? Why does the tide differ so much in various localities? Tell about the height of the tide at different points.
168. Why is there no tide on a lake? Mars.—Definition and sign?
169. Describe its appearance. When is it brightest? Its distance from the sun? Velocity? Day? Year?
170. Distance from the earth? Peculiarity of its orbit? Diameter? Volume? Density? Mass? Force of gravity? Figure? Describe its seasons.
171. Has it any atmosphere? Moon? Appearance of our earth? Telescopic features. (The land and sea features have been so well decided that they have been named, and a Mars's globe made.)
172. Cause of its ruddy color? What are the snow-zones? Can we watch the change of its seasons?
Minor Planets (asteroids).—Give Bode's law. Tell how the first of these planets was discovered. How many are now known?—Ans. There are (Sept. 21,1870)112. Are they probably all discovered?
174. Describe these "pocket planets." Are they all found within the Zodiac? What is their origin?—Ans. According to the Nebular hypothesis, the ring of matter broke up into numberless small bodies instead of aggregating into one large planet. Give some of the names and signs.
Jupiter.—Definition and sign? Describe its appearance. Ancient views. Describe its orbit. What is its distance from the sun? Velocity? (1869. w is in T). Day? Year? Distance from the earth?
177. Diameter? Volume? Density? Centrifugal force?
* As the tidal wave does not move as rapidly as the earth does, the water has an apparent backward motion. It has been suggested that this acts as a Weak on the earth's diurnal revolution. It has been shown that the moon's true place can be best calculated if we suppose that the sidereal day is shortening, by tidal action, at the rate of of a second in 2,500 years.