ILLUSTRATED ASTRONOMY. its magnitude, compared with the earth? of the earth's magnitude. the specific gravity of the planet Mercury? out 15 times the weight of water. (15.111.) time does it revolve on its axis, or perform its daily ut 24 hours. (24 hours 5 minutes.) time does it revolve around the sun? ut 88 days. (87d. 23h. 14m. 33s.) st does it move in its orbit around the sun? ves 112,000 miles an hour. the light or heat at Mercury, compared with that of the at is its magnitude compared with the earth? s about of the earth's magnitude. at is the specific gravity of Venus? is 5 times the weight of water. (5.058.) what time does it revolve on its axis? about 23 hours. (23h. 21m.) what time does it revolve around the sun? 224 days. (224d. 16h. 41m. 27s.) ow fast does it move in its orbit around the sun? moves 75,000 miles an hour. hat is the comparative light or heat at Venus? is about double that of the earth. hat is the greatest elongation of Venus? bout 47 degrees. Then is Venus a morning star? When it is west of the sun, and rises before it. Then is it an evening star? When it is east of the sun, and sets after it. 17 Q. How long is Venus a morning or an evening star, alternately? A. About 290 days. Q. Why is Venus a morning or an evening star 66 days longer than the time of its revolution around the sun? A. Because the earth is moving around the sun the same way. [See diagram. If we suppose Venus to be in conjunction, or between the earth and sun, as they move the same way, Venus will move half around the sun, or 180 degrees, while the earth moves only 110 degrees. Venus will during this time be a morning star, and when Venus has completed its revolution around the sun, the earth will have passed through 220 degrees of its orbit, and Venus will still continue a morning star, although it has made a complete revolution around the It will therefore have to make one complete revolution and 103 degrees over, before it can be seen on the other side of the sun; it will then be an evening star for the same length of time.] sun. Question. How much is the axis of Venus inclined to that of its orbit? Answer. 75 degrees. Q. When the north pole of Venus inclines directly towards the sun, how many degrees will the axis point above the sun? A. Only 15 degrees. Q. How wide a torrid zone does this make? A. 150 degrees-75 degrees on each side of the equator. Q. The tropics are within how many degrees of the poles? Q. The polar circles are within how many degrees of the equator? Q. What is the diameter of the polar circles? Q. Has Venus any variation of seasons? A. She has two summers and two winters at the equator, and a summer and winter at each of the poles, during the year. Q. How does Venus appear when viewed with a telescope? A. Into inferior and superior. Q. When is a planet in inferior conjunction? A. When it is between the earth and sun. Q. What planets have inferior conjunction? A. Mercury and Venus; also the moon. Q. When is a planet in superior conjunction? A. When it is beyond the sun. Q. What planets have superior conjunction? A. All, except the earth. Q. When is a planet in opposition to the sun? A. When it is on the opposite side of the earth. Q. What planets have opposition? A. The superior planets. Q. What apparent motions have the planets? Л. Three; direct, stationary, and retrograde. Q. When does a planet's motion appear to be direct? A. When it appears to move from west to east among the stars. Q. When is a planet's motion said to be stationary. A. When it is moving directly towards or from the earth. Q. When is a planet's motion said to be retrograde? A. When it appears to move backwards, or from east to west among the stars. ILLUSTRATED ASTRONOMY. 19 LESSON XX. EARTH, DEFINITIONS, &c. HAT is the shape of the earth? Q. How far is terrestrial longitude reckoned? A. It is reckoned 180 degrees, or half round the earth. A. It is a great circle which separates the visible t is round like a globe or ball, a little flat- heavens from the invisible. poles. we know the earth to be round? avigators have sailed round it, by a conerly or easterly course.-2d. The top-mast ning in from the sea, always appears first. rth's shadow upon the moon, in a lunar ircular. manner do the inhabitants stand upon the earth? stand with their feet directed towards the e earth. (See Diagram.) o you understand by the terms upward and downward? rd is from the centre of the earth, downward the centre of the earth. eeps the inhabitants, &c., upon the surface of the earth? ttraction of the earth. s the axis of the earth? the straight line round which it performs its ution. are the poles of the earth? are the extremities of its axis. is the equator? a great circle, whose plane divides the earth ern and southern hemispheres. at is the plane of the equator perpendicular? perpendicular to the earth's axis, and equi om the poles. is the meridian of a place on the earth? Q. How many horizons are there? A. Two; the visible and the rational. Q. What is the visible or sensible horizon? A. It is that circle where the earth and sky appear to meet. Q. What is the rational horizon? A. It is a great circle, parallel to the visible horizon, whose plane passes through the centre of the earth. Q. Into what does it divide the earth? A. Into upper and lower hemispheres. Q. Is the rational horizon above or below the visible horizon? LESSON XXII. Q. Do all places on the earth have the same horizon? A. They do not; if we change our place on the earth, the horizon changes. Q. What are the poles of the horizon? A. The zenith and nadir. Q. What is the zenith? A. It is that point in the heavens directly over our heads. Q. Do all places have the same zenith? A. They do not; every place has a different zenith. A. It is that point in the heavens which is opposite to a great circle passing through the place, and the zenith, or directly under our feet. of the earth. Q. Are the zenith and nadir fixed points in the heavens? A. They are not; they make a complete revolution in the heavens every 24 hours. Q. What is the altitude of a heavenly body? A. It is its height or distance from the horizon. Q. What is the altitude of the star at A? (See Diagram.) A. It has no altitude, being in the horizon. Q. What is the altitude of the star at B? also at C? (See Diagram.) Q. What is the polar distance of a heavenly body? A. It is its distance from the pole. Q. What is the polar distance of the star at D? also at E, and F? (See Diagram.) Q. Who are the antipodes? |