Astronomy of Today, Cecil Dolmage
The following passage on astronomy was written by scientist Cecil Dolmage
1. Had the telescope never been invented our knowledge
2. of astronomy would be trifling indeed.
3. Prior to the year 1610, when Galileo first turned the
4. new instrument upon the sky, all that men knew of
5. the starry realms was gathered from observation with
6. their own eyes unaided by any artificial means. In
7. such researches they had been very much at a disadvantage.
8. The sun and moon, in their opinion, were no doubt the
9. largest bodies in the heavens, for the mere reason that
10. they looked so! The mighty solar disturbances, which
11. are now such common-places to us, were then quite undreamed
12. of. The moon displayed a patchy surface, and that was all;
13. her craters and ring-mountains were surprises as yet in store
14. for men. Nothing of course was known about the surfaces
15. of the planets. These objects had indeed no particular
16. characteristics to distinguish them from the great host of
17. the stars, except that they continually changed their positions
18. in the sky while the rest did not. The stars themselves
19. were considered as fixed inalterably upon the vault of
20. heaven. The sun, moon, and planets apparently moved about
21. in the intermediate space, supported in their courses by strange
22. and fanciful devices. The idea of satellites was as yet unknown.
23. Comets were regarded as celestial portents, and meteors as
24. small conflagrations taking place in the upper air.
25. In the entire absence of any knowledge with regard to the
26. actual sizes and distances of the various celestial bodies,
27. men naturally considered them as small; and,
28. concluding that they were comparatively near, assigned
29. to them in consequence a permanent connection with
30. terrestrial affairs. Thus arose the quaint and erroneous
31. beliefs of astrology, according to which the events which
32. took place upon our earth were considered to depend upon
33. the various positions in which the planets, for instance,
34. found themselves from time to time.
35. It must, however, be acknowledged that the study of astrology,
36. fallacious though its conclusions were, indirectly performed
37. a great service to astronomy by reason of the accurate observations
38. and diligent study of the stars which it entailed.
39. We will now inquire into the means by which the distances and
40. sizes of the celestial orbs have been ascertained, and see how it
41. was that the ancients were so entirely in the dark in this matter.
42. There are two distinct methods of finding out the distance
43. at which any object happens to be situated from us.
44. One method is by actual measurement.
45. The other is by moving oneself a little to the right or left, and
46. observing whether the distant object appears in any degree
47. altered in position by our own change of place.
48. One of the best illustrations of this relative change of position
49. which objects undergo as a result of our own change of
50. place, is to observe the landscape from the window of a
51. moving railway carriage. As we are borne rapidly along we
52. notice that the telegraph posts which are set close to the line
53. appear to fly past us in the contrary direction; the trees,
54. houses, and other things beyond go by too, but not so fast;
55. objects a good way off displace slowly; while some spire,
56. or tall landmark, in the far distance appears to
57. remain unmoved during a comparatively long time.
58. Actual change of position on our own part is found
59. indeed to be invariably accompanied by an apparent
60. displacement of the objects about us, such apparent
61. displacement as a result of our own change of position
62. being known as "parallax." The dependence between
63. the two is so mathematically exact, that if we know the
64. amount of our own change of place, and if we observe
65. the amount of the consequent displacement of any
66. object, we are enabled to calculate its precise distance from
67. us. Thus it comes to pass that distances can be measured
68. without the necessity of moving over them; and the breadth
69. of a river, for instance, or the distance from us of a ship at sea,
70. can be found merely by such means.
71. It is by the application of this principle to the wider field of
72. the sky that we are able to ascertain the distance of celestial
73. bodies. We have noted that it requires a goodly change of
74. place on our own part to shift the position in which some
75. object in the far distance is seen by us. To two persons
76. separated by, say, a few hundred yards, a ship upon the
77. horizon will appear pretty much in the same direction.
78. They would require, in fact, to be much farther apart
79. in order to displace it sufficiently for the purpose of estimating
80. their distance from it. It is the same with regard to the
81. moon. Two observers, standing upon our earth, will require to
82. be some thousands of miles apart in order to see the
83. position of our satellite sufficiently altered with regard to the
84. starry background, to give the necessary data upon which
85. to ground their calculations.
86. The change of position thus offered by one side of the earth's
87. surface at a time is, however, not sufficient to displace any
88. but the nearest celestial bodies. When we have occasion to
89. go farther afield we have to seek a greater change of place.
90. This we can get as a consequence of the earth's movement
91. around the sun. Observations, taken several days apart, will
92. show the effect of the earth's change of place during the
93. interval upon the positions of the other bodies of our system.
94. But when we desire to sound the depths of space beyond, and
95. to reach out to measure the distance of the nearest star, we
96. find ourselves at once thrown upon the greatest change
97. of place which we can possibly hope for; and this we get
98. during the long journey of many millions of miles which
99. our earth performs around the sun during the course of each
100. year. But even this last change of place, great as it seems
101. in comparison with terrestrial measurements, is insufficient to
102. show anything more than the tiniest displacements in a paltry
103. forty-three out of the entire host of the stars.
Dolmage, Cecil. Astronomy of Today, a Popular Introduction in Non-technical Language. 3d ed. London: Seeley, 1910. WorldCat. Web. 23 Mar. 2016.
In line 85, the term "calculations" is referring to