Practical astronomy, a textbook for engineering schools and a manual of field methods

303 Pages·1925·12.171 MB·English

by  George L Hosmer, James Melvin Robbins

Frontispiece
Observation onPolarisforAzimuth
PRACTICAL ASTRONOMY
A TEXTBOOK FOR ENGINEERING SCHOOLS
AND
A MANUAL OF FIELD METHODS
BY
GEORGE HQSMER
L.
AssociateProfessorofGeodesy,MassachusettsInstituteofTechnology
THIRD EDITION
NEW YORK *
JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED
TA-5C1
CCflPYRIGHT, 1910, 1917 AND 1925
BY
GEORGE HOSMER
L.
PREFACE
THE purpose of this volume is to furnish a text in Practical
Astronomy especially adapted to the needs of civil-engineering
students who can devote but little time to the subject, and who
are not likely to take up advanced study of Astronomy. The
text deals chiefly with the class of observations which can be
made with surveying instruments, the methods applicable to
t
astronomical and geodetic instruments being treated b$t briefly.
It has been the author's intention to produce a book%hich is
intermediate between the text-book written for the student of
Astronomy or Geodesy and the short chapter on the subject
generally given in text-books on Surveying. The subject has
therefore been treated from the standpoint of the engineer, who
is interested chiefly in obtaining results, and those refinements
have been omitted which are beyond the requirements of the
work which can be performed with the engineer's transit. This
has led to the introduction of some rather crude mathematical
processes, but it ishoped that these are presentedin such a way
as to aid the student in gaining a clearer conception of the prob-
lem without conveying wrong notions as to when such short-cut
methods can properly be applied. The elementary principles
have been treated rather elaborately but with a view to making
these principles clear rather than to the introduction of refiner
ments. Much space has been devoted to the Measurement of
Time because this subject seems to cause the student more
difficulty thar \y other branch of Practical Astronomy. The
attempt has Iv{J made to arrange the text so that it will be a
convenient reference book for the engineer who is doing field
work.
For convenience in arranging a shorter course those subjects
ill
iv PREFACE
which are most elementary are printed in large type. The mat-
ter printed in smaller type may be included in a longer course
and will be found convenient for reference in field practice, par-
X
ticularly that contained in Chapters to XIII.
The author desires to acknowledge his indebtedness to those
who have assisted in the preparation of this book, especially to
Professor A. G. Robbins and Mr. J. W. Howard of the Massa-
chusetts Institute of Technology and to Mr. F. C. Starr of the
George Washington Universityforvaluablesuggestions and crit-
icisms of the manuscript.
G. L. H.
BOSTON, June, 1910.
PREFACE TO THE THIRD EDITION
THE adoption of Civil Time in the American Ephemeris and
Nautical Almanac in place of Astronomical Time (in effect in
1925) necessitated a complete revision of this book. Advantage
has been taken of this opportunity to introduce several improve-
ments, among which may be mentioned: the change of the no-
tation to agree with that now in use in the principal textbooks
and government publications, a revision of the chapter on the
different kinds of time, simpler proofs of the refraction and
parallax formulae, the extension of the article on interpolation
to include two and three variables, the discussion of errors by
means of differentiation of the trigonometric formulae, thein-
troduction of valuable material from Serial 166, U. S. Coast
and Geodetic Survey, a table of convergence of the meridians,
and several new illustrations. In the chapter on Nautical As-
tronomy, which has beenre-written, tfeemethodbfMarcqSaint-
Hilaire and the new tables (H. O. 201 and 203) for laying down
Sumner lines are briefly explained. An appendix on Spherical
Trigonometry is added for convenience of reference. The size
PREFACE V
of the bookhasbeen reduced tomake it convenient forfield use.
This has been done without reducing the size of the type.
In this book an attempt has been made to emphasize the
great importance to the engineer of using the true meridian and
true azimuth as the basis for all kinds of surveys; the chapter
on Observations for Azimuth is therefore the most important
one from the engineering standpoint. In this new edition the
chapter has been enlarged by the addition of tables, illustrative
examples and methods of observing.
Thanks are due to Messrs. C. L. Berger & Sons for the use
of electrotypes, and to Professor Owen B. French of George
Washington University (formerly of the U. S. Coast and Geo-
detic Survey) for valuable suggestions and criticisms. The
author desires to thank those who have sent notices of errors
discovered in the book and asks their continued cooperation.
G. L. H.
CAMBRIDGE,MASS.,June, 1924.

CONTENTS
CHAPTER
I
THE CELESTIAL SPHERE REAL AND APPARENT MOTIONS
CHAPTER II
DEFINITIONS POINTS AND CIRCLES OP REFERENCE
10. Definitions................................................... 14
Vertical Line Zenith Nadir Horizon Vertical Circles
Almucantars Poles Equator Hour Circles Par-
allels of Declination Meridian Prime Vertical Eclip-
tic Equinoxes Solstices.
CHAPTER
III
SYSTEMS OF COORDINATES ON THE SPHERE
11. SphericalCoordinates.......................................... 18
12. TheHorizon System........................................... 19
13. TheEquatorSystems......................................... 19
15. CoordinatesoftheObserver..................................'. . 22
16. RelationbetweentheTwo Systemsof Coordinates................ 23
CHAPTER IV
RELATION BETWEEN COORDINATES
17. Relationbetween AltitudeofPole andLatitudeofObserver........ 27
18. RelationbetweenLatitudeofObserverand the DeclinationandAlti-
tudeofaPointontheMeridian............................... 30
vii
CONTENTS
Viii
ART. PAGE
19. TheAstronomicalTriangle..................................... 31
20. Relation between RightAscensionandHourAngle................ 36
CHAPTER V
MEASUREMENT OF TIME
21. TheEarth'sRotation.......................................... 4<
22. TransitorCulmination........................................ 4c
23. SiderealDay.................................................. 40
24. SiderealTime................................................. 41
25. SolarDay.................................................... 41
26. SolarTime................................................ 41
27. EquationofTime............................................ 42
28. ConversionofMeanTimeintoApparentTimeandviceversa........ 45
29. AstronomicalTime CivilTime................................ 46
30. RelationbetweenLongitudeandTime.......................... 46
31. Relationbetween Hoursand Degrees............................ 49
32. StandardTime................................................ 50
33. Relation between SiderealTime, Right Ascension and HourAngle of
anyPointata GivenInstant................................. 52
34. Staron theMeridian.......................................... 53
35. Mean Solarand SiderealIntervalsofTime....................... 54
36. Approximate Corrections...................................... 56
37. Relationbetween SiderealandMeanTimeatanyInstant. ......... 57
38. TheDateLine.............................................. 61
39. The Calendar................................................ 62
CHAPTER VI
THE AMERICAN EPHEMERIS AND NAUTICAL ALMANAC STAR
CATALOGUES INTERPOLATION
40. TheEphemeris................................................ 6^
41. Star Catalogues............................................... 6*
42. Interpolation................................................. 7,
43. DoubleInterpolation.......................................... 7;
CHAPTER VII
THE EARTH'S^ FIGURE CORRECTIONS TO OBSERVED ALTITUDES
44. TheEarth'sFigure. . .......................................... 7
45. TheParallaxCorrection........................................ 8.
46. TheRefraction Correction............".......................... 84
47. Semidiameters............................................... 87
48. DipoftheSeaHorizon........................................ 88
49. SequenceofCorrections. ....................................... 89
CONTENTS IX
CHAPTER VIII
DESCRIPTION OF INSTRUMENTS OBSERVING
RT. PAGE
50. TheEngineer'sTransit 91
51. EliminationofErrors 92
52. Attachments to the Engineer'sTransit Reflector 95
-53. PrismaticEyepiece 96
54. Sun Glass ' 96
55. ThePortableAstronomicalTransit 96
56. TheSextant 100
57. ArtificialHorizon 103
58. Chronometer 104
/59. Chronograph 105
60. TheZenithTelescope 105
61. Suggestionsabout Observing 107
62. ErrorsinHorizontalAngles 108
CHAPTER IX
THE CONSTELLATIONS
i
63. The Constellations no
64. MethodofNamingStars no
65. Magnitudes 111
66. Constellationsnearthe Pole nr
67. ConstellationsneartheEquator 112
68. The Planets 114
CHAPTER X
OBSERVATIONS FOR LATITUDE
"69. Latitude bya CircumpolarStarat Culmination 115
70. LatitudebyAltitudeofthe Sun atNoon 117
71. LatitudebytheMeridianAltitudeofa Southern Star 119
72. AltitudesNeartheMeridian 120
73. Latitude by Altitude of Polaris when the Time is Known 122
74. PreciseLatitudes Harrebow-Talcott Method 125
CHAPTER XI
OBSERVATIONS FOR DETERMINING THE TIME
75. ObservationsforLocalTime 127
76. TimebyTransitofaStar 127
77. ObservationswithAstronomicalTransit 130