text stringlengths 805 1k |
|---|
[2008/07/06][2002/09/07][2006/05/05] [2000/07/18][2001/05/26][2006/05/11] [2003/10/30][2005/09/27][2001/05/28] [2006/05/18][1999/02/16][2003/01/31] [2002/03/18][2008/11/18] 176^∘°fancy[C][R]plain [C] [R]xobeysp @processline-0.25in@line@hook@verbatimtotalleftmargin0.25in@licensingevenheadProject Gutenberg Licensi... |
Copyright, 1888, by CHARLES A. YOUNG,All Rights Reserved. Typography by J. S. Cushing & Co., Boston, U.S.A. [4cm] Presswork by Ginn & Co., Boston, U.S.A. [1]PrefacePrefaceCHAPTER: PREFACE.PREFACE. The limitations of time are such in our college course thatprobably it will not be possible in most cases for a class to... |
The author is under great obligationsto the astronomical histories of Grant and Wolf, andespecially to Miss Clerke's admirable “History of Astronomy inthe Nineteenth Century.” Many data also have been drawnfrom Houzeau's valuable “Vade Mecum de l'Astronomie.” ProfessorTrowbridge of Cambridge kindly provided the origin... |
INTRODUCTIONINTRODUCTION. Astronomy (>'astron n�mos)is the science which treats of theheavenly bodies. As such bodies we reckon the sun and moon, theplanets (of which the earth is one) and their satellites, comets andmeteors, and finally the stars and nebul�.We have to consider in Astronomy:—[(a)] * The motions of th... |
It would be impossible, for instance,to draw a line between Astronomy on one side and Geologyand Physical Geography on the other. Many problems relating tothe formation and constitution of the earth belong alike to all three. Astronomy is divided into many branches, some of which, asordinarily recognized, are the foll... |
Usually it will be best to proceed in the Euclidean order, by firststating the fact or principle in question, and then explaining itsdemonstration. The frequent references to “Physics” refer to the “ElementaryText-Book of Physics,” by Anthony & Brackett; 3d edition, 1887. Wiley & Sons, N.Y.INTRODUCTION IThe Doctrine o... |
In what follows we shall usethis conception of the celestial sphere.[To most persons the sky appears, not a true hemisphere, but a flattened vault, as if the horizon were more remote than the zenith. This is a subjective effect due mainly to the intervening objects between us and the horizon. The sun and moon when ... |
Thus, for instance, the linear semi-diameterof the sun is about 697,000 kilometers (433,000 miles),while its angular semidiametersemi-diameter is about 16', or a little more thana quarter of a degree. Obviously, angular units alone can properlybe used in describing apparent distances and dimensions in the sky. For ins... |
It may be mentioned here as a rather curious fact that most persons saythat the moon appears about a foot in diameter; at least, this seems tobe the average estimate. Vanishing Point.—Any system of parallel lines produced inone direction will appear to pierce the celestial sphere at a singlepoint. The different points... |
Both zenith and nadir are derived from the Arabic, which languagehas also given us many other astronomical terms. Horizon.—The Horīzon[ Beware of the common, but vulgar, pronunciation, H�rīzon. ]is a great circle of the celestialsphere, having the zenith and nadir as its poles: it is thereforehalf-way between them, ... |
It is not, however, a great circle, but,technically speaking, only a small circle; depressed below the truehorizon by an amount depending upon the observer's elevation abovethe water. This depression is called the Dip of the Horizon, and willbe discussed further on. Vertical Circles.—These are great circles passing th... |
The exact place of the pole may be found by observing some starvery near the pole at two times 12 hours apart, and taking the middlepoint between the two observed places of the star. The definition of the pole just given is independent of any theoryas to the cause of the apparent rotation of the heavens. If, however,w... |
It may also be defined as the intersection of the plane of the earth'sequator with the celestial sphere. It derives its name from the factthat, at the two dates in the year when the sun crosses this circle—aboutMarch 20 and Sept. 22—the day and night are equal in length. This crossing occurs twice a year, once in Sept... |
The Meridian and Prime Vertical.—The Meridian is the greatcircle passing through the pole and the zenith. Since it is a greatcircle, it must necessarily pass through both poles, and through thenadir as well as the zenith, and must be perpendicular both to theequator and to the horizon. The Prime Vertical is the Vertic... |
We may give its altitude and azimuth, or its declination and hour-angle;or, if we know the time, its declination and right ascension. Either of these pairs of co-ordinates, as they are called, will defineits place in the sky.21. Altitude and Zenith Distance (illo004Fig. 4).—The Altitude of aheavenly body is its angul... |
The word is of Arabic origin, and has the samemeaning as the true bearing in surveying and navigation.illo004Fig. 4.—The Horizon and Vertical Circles. O, the place of the Observer. M, some Star. OZ, the Observer's Vertical. ZMH, arc of the Star's Vertical Circle. Z, the Zenith; P, the Pole. TMR, ... |
would have an azimuth of 45�; in the NW., 135�; in the N., 180�; inthe NE., 225�; and in the SE. For example, to find a star whoseazimuth is 260�, and altitude 60�, we must face N. 80� E., and then lookup two-thirds of the way to the zenith. The object in this case has anamplitude of 10� N. of W., and a zenith distanc... |
The north-polar distance of a star is its angular distancefrom the North Pole, and is simply the complement of thedeclination. Declination + North-Polar Distance = 90�. The declination of a star remains always the same; at least, theslow changes that it undergoes need not be considered for ourpresent purpose. “Paralle... |
It may be defined also as the arc of the equator, interceptedbetween the vernal equinox and the foot of the star's hour-circle. It is always reckoned from the equinox toward the east; sometimesin degrees, but usually in hours, minutes, and seconds of time. The right ascension, like the declination, remains unchanged b... |
The Sidereal Time at any moment may be defined as the hour-angleof the vernal equinox. On account of the annual motionof the sun among the stars, the Solar Day, by which time is reckonedfor ordinary purposes, is about 4 minutes longer than the siderealday. The exact difference is 3^m 56^s.394 (sidereal), or just one d... |
There the pole (celestial) and zenith coincide, and any number of circlesmay be drawn through the two points, which have now become one. Thehorizon and equator coalesce, and the only direction on the earth's surfaceis due south (or north)—east and west have vanished. A single step of the observer will, however, remedy... |
Declination and Hour-Angle are the co-ordinateswhich refer the place of a star to the Pole and the Meridian;while Declination and Right Ascension refer it to the Pole and EquinoctialColure. The Altitudeof the pole, or its height in degrees above the horizon, is always equalto the Latitude of the observer. Indeed, the ... |
The usual symbol for the latitude of a place is ϕ.31. The Right Sphere.—If the observer is situated at theearth's equator, i.e., in latitude zero (ϕ = 0), the pole will be in thehorizon, and the equator will pass vertically overhead through thezenith. The stars will rise and set vertically, and their diurnal circles w... |
7.—The Oblique Sphere and Diurnal Circles. The Oblique Sphere (illo007Fig. 7).—At any station between thepole and equator the stars will move in circles oblique to the horizon,SENW in the figure. On the other hand, stars within the same distance of thedepressed pole will lie within the “Circle of Perpetual Occultation... |
Telescopes in General.—Telescopes are of two kinds, refractingand reflecting. The former were first invented, and are muchmore used, but the largest instruments ever made are reflectors. Inboth the fundamental principle is the same. In the form of telescope, however, introduced by Galileo,[In strictness, Galileo did n... |
The more exact theory of Gauss and later writers would require some slight modifications in our statements, but none of any material importance. For a thorough discussion, see Jamin, “Trait� de Physique,” or Encyc. Britannica,—Optics. 8.—Path of the Rays in the Astronomical Telescope. Similarlywith the rays which m... |
A single lens, with spherical surfaces, cannot do this very perfectly,the “aberrations” being of two kinds, the spherical aberration andthe chromatic. The former could be corrected, if it were worth while,by slightly modifying the form of the lens-surfaces; but the latter,which is far more troublesome, cannot be cured... |
9, No. 3, having the two lenses closetogether, and the adjacent surfaces of the same, or nearly the same, curvature. In small object-glasses the lenses are often cemented together withCanada balsam or some other transparent medium. In all these forms the crown glass is outside;Steinheil, Hastings, and others have cons... |
A 13-inch object-glass of this constructionat Cambridge performs admirably. Much is hoped from the new kind of glass now being made at Jena. The size of this disc-and-ring system can becalculated from the known wave-lengths of light and the dimensionsof the lens, and the results agree very precisely with observation. ... |
Usually it is best to employ for the eye-piecea combination of two or more lenses which will give a more extensivefield of view. Eye-pieces belong to two classes, the positive and the negative. Theformer, which are much more generally useful, act as simple magnifying-glasses,and can be used as hand magnifiers if desir... |
illo010a b]cHuyghenian(Negative) illo010b b]cSteinheil `Monocentric'(Positive)illo010c*Fig. 10.—Various Forms of Telescope Eye-piece. There are numerous other forms of eye-piece, each with its own advantagesand disadvantages. The erecting eye-piece, used in spy-glasses, isessentially a compound micros... |
Of course,provision must be made for illuminating either the field of view or thethreads themselves, in order to make them visible in darkness. r0pt1 illo011a2 illo011b3 illo011c*Fig. 11.—Different Forms of Reflecting Telescope. 1. The Herschellian; 2. The Newtonian; 3. With this instrument one looks di... |
At the head of thelist stands the enormous instrument of Lord Rosse, constructed in 1842, witha mirror six feet in diameter and sixty feet focal length. Relative Advantages of Refractors and Reflectors.—There hasbeen a good deal of discussion on this point, and each construction has itspartisans. In favor of the refle... |
But, on the whole, the advantages are generally considered to lie with therefractors. In their favor we mention:—First. Great superiority in light. No mirror (unless, perhaps, a freshlypolished silver-on-glass film) reflects muchmore than three-quarters of the incidentlight; while a good (single) lens transmitsover 95... |
Moreover, so far as distortions are concerned, when a lens bends a littleby its own weight, both sides are affected in a nearly compensatory manner,while in a mirror there is no such compensation. As a consequence, mirrorsvery seldom indeed give any such definition as lenses do. The lens, once made, and fairly taken c... |
But it was not untilnearly a century after Galileo's discovery that Huyghens applied itto the construction of clocks (in 1657). So far as the principles of construction are concerned, there is nodifference between an astronomical clock and any other. Of course it is constructed with extreme care inall respects. The pe... |
Graham's mercurial pendulum (illo013Fig. 13) is theone most commonly used. It consists simply of ajar (usually steel), three or four inches in diameter,and about eight inches high, containing forty or fiftypounds of mercury, and suspended at the end of asteel rod. When the temperature rises, the rodlengthens (which wo... |
This tube is itself supported at the bottomby three or four steel rods which hang from a piece attached to the pendulumspring. The standard clock at Greenwich has a pendulum of this kind.52. It varies considerably, however, with differentpendulums. In theGreenwich clock a magnet is raised or lowered by the rise or fal... |
The Chronometer.—The pendulum-clock not being portable,it is necessary to provide time-keepers that are. It usuallybeats half-seconds. It is not possible to secure in the chronometer-balanceas perfect a temperature correction as in the pendulum. Theyare simply indispensable at sea. Never turn the hands of a chronomete... |
The beginning of a new minute (the 60th sec.) is indicated eitherby a double mark as shown, or by the omission of a mark. Of course the minutes when the chronograph wasstarted and stopped are noted by the observer on the sheet, and soenable him to identify the minutes and seconds all through the record. Many European ... |
Usually a wheel on theaxis of the scape-wheel is made to give the electric signals by touching alight spring with one of its teeth every other second. Chronometers are now also fitted up in the same way, to be used with thechronograph. The signals sent are sometimes “breaks” in a continuous current, andsometimes “make... |
This axis isplaced horizontal, east and west, and turns on pivots at its extremities,in Y-bearings upon the top of two fixed piers or pillars. Asmall graduated circle is attached, to facilitate “setting” the telescopeat any designated altitude or declination. 16.—The Transit Instrument (Schematic). One ortwo wires als... |
Usually one (or both) of the pivotsis pierced, and a lamp throws light through the opening upon a small mirrorin the centre of the tube, which reflects it down upon the reticle. The Y's are used instead of round bearings, in order to prevent anyrolling or shake of the pivots as the instrument turns. Fig. 18 shows a mo... |
There are screws provided to turn the reticle alittle, so as to effect this adjustment. illo018Fig. 18.—A 3-inch Transit, with reversing apparatus. When the wires have been thus adjusted for focus and verticality, thereticle-slide should be tightly clamped and never disturbed again. The eye-piececan be moved in and ou... |
If the pier on which it is mounted is firm, the collimator cross is in allrespects as good as a star, and much more convenient. The adjustment for level is made by setting a stridinglevel on the pivots of the axis, reading the level, then reversing the level(not the transit) and reading it again. If the pivots are rou... |
This adjustment must be made by means of observations upon the stars, andis an excellent example of the method of successive approximations, whichis so characteristic of astronomical investigation. (a) After adjusting carefullythe focus and collimation of the instrument, we set it north and southby guess, and level it... |
A repetition of the operation may possibly be needed to secure all thedesired precision. The accuracy of this azimuth adjustment can then beverified by three successive “culminations” or transits of the pole star, orany other circumpolar. The final test of all the adjustments, and of the accurate goingof the clock, is... |
The instrumentmost used for this purpose is theMeridian Circle, or Transit Circle,which is simply a transit instrument,with a graduated circleattached to its axis, and revolvingwith the telescope. Sometimesthere are two circles, oneat each end of the axis. Fig. 19 represents the instrument“schematically,” showing mere... |
In acircle of forty inches diameter, 1” is a little less than 1/10000 of an inch,(20/206265 inch), so that the necessity of fine workmanship is obvious. The Reading Microscope (illo021Fig. 21).—This consists essentiallyof a compound microscope, which forms a magnified image ofthe graduation at the focus of its object... |
Thus in illo022Fig. 22, the reading of the microscope is3' 22”.1, the 3' being given by the scale in the field, the 22”.1 by thescrew-head. illo022Fig. 22.—Field of View of Reading Microscope. Method of observing a Star.—A minute or two before the starreaches the meridian the instrument is approximately pointed, so th... |
For declinations, the “zeropoint” is either the polar or the equatorial reading of the circle; i.e.,the reading of the circle when the telescope is pointed at the poleor at the equator. The “polar point” may be found by observing some circumpolarstar above the pole, and again, twelve hours later, below it. Whenthe two... |
This peculiar illuminationis commonly effected by means of Bohnenberger's “collimatingeye-piece,” shown in illo023Fig. 23. In the simplest form it is merely a commonRamsden eye-piece, with a hole in one side, and a thin glass plate insertedat an angle of 45�. A light from one side, entering through the hole, will be(p... |
It consists of a circle, carrying atelescope mounted on the face of a wall of masonry (as its name implies)and free to revolve in the plane of the meridian. The wall furnishes a convenientsupport for the microscopes.illo025Fig. 25. The Equatorial (Schematic). Altitude and Azimuth Instrument.—Since the transit instrume... |
Obviouslythe readings of these two circles, when the instrument is properlyadjusted and the zero points determined, will give the altitudeand azimuth of the body pointed on. illo024Fig. 24 represents a small instrumentof this kind.illo026Fig. 26.—The 23-inch Princeton Telescope. Sometimes, also, it iscalled the Right... |
This motion, since it is uniform,can be, and in all large instruments usually is, given by clock-work,with a continuous regulator of some kind, similar to that usedin the chronograph. The instrument once directed and clamped,and the clock-work started, the object will continue apparently immovablein the field of view ... |
The most common and most generally useful is theso-called “filar position-micrometer,” illo028Fig. 28, which is an indispensableauxiliary of every goodtelescope. It is a small instrument, muchlike the upper part of the readingmicroscope, but more complicated. It usually contains areticle of fixed wires, two orthree pa... |
Moreover, the revolving dome, which isusually erected to shelter a great telescope, is an exceedinglycumbrous and expensive affair. In the Equatorial Coud�, illo030Fig. 30, these difficulties are overcome bythe use of mirrors. The observer sits always in one fixed position,looking obliquely down through the polar axis... |
We have now to describe the instrument which, with the help ofthe chronometer, is the main dependence of the mariner. The Sextant.—The graduated limb of the sextant is carriedby a light framework, usually of metal, provided with a suitable handleX. The arc is about one-sixth of a circle, as the name implies, andis usu... |
This is merely a shallow basin of mercury,covered, when necessary to protect it from the wind, with a roof made ofglass plates having their sides plane and parallel. In this case we measure the angle between the sun's image reflected in themercury and the sun itself. But its portability andapplicability at sea render ... |
But with clock, meridian circle, and equatorial and their usualaccessories, all the fundamental observations of theoretical and sphericalastronomy can be supplied. The chronometer and sextant arepractically the only astronomical instruments of any use at sea. CHAPTERIIIIICorrections to Astronomical ObservationsCORRECT... |
From the right-angled triangle OBC we have directly cos OCB = BC/CO. Putting R for the radius of the earth, and Δ for the dip, this becomes cosΔ = R/R + h. This formula is exact, but inconvenient, because it gives the small angleΔ by means of its cosine. Since, however, 1- cosΔ = 2 sin^2 1/2Δ, we easilyobtain t... |
But since hR is a very small fraction, it may be neglected in the divisor ( 1 + hR), and the expression becomes simply, 2 sin^2 12Δ = h/R; whence sin12Δ = √(h/2R). Since Δ is a very small angle, Δ = sinΔ = 2 sin12Δ, so that Δ (in radians) = 2 √(h/2R) = √(h/1/2R). To r... |
Parallax.—In the most general sense, “parallax” is the changeof a body's direction resulting from the observer's displacement. Its position as seen from C would be determined in thesame way by producing CP to which OX is drawn parallel. Theangle POX, therefore, or its equal, OPC, is the parallax of P foran observer at... |
The “law” of the parallax is, that it varies as the sine of the zenith distancedirectly, and inversely as the linear distance (in miles) of the body. This gives us sinp = R/D ·sinζ; This gives us or, since p is always a small angle, p” = 206265” R/D ·sinζ. 83. Horizontal Parallax.—When a body is ... |
= R/D,it follows of course that D = R � sin p_h; or, (nearly) D = 206265”/p_h” � R. or, (nearly) If the sun's parallax equals 8”.8, = 206265/8.8 � R = 23439 R.85. Itis agreed to take as the standard the equatorial horizontal parallax;i.e., the earth's equatorial semi-diameter as seen from the bod... |
The qualifier, “diurnal,” is seldom used except when it isnecessary to distinguish between this kind of parallax and theannual parallax of the fixed stars, which is due to theearth's orbital motion. Smallness of Parallax.—The horizontalparallax of even the nearest of the heavenly bodies isalways small. In the case of ... |
In most cases thiswill be sensibly the same in all parts of the sky, but the moon isso near that there is quite a perceptible difference between herdiameter when in the zenith and in the horizon. A glance at illo034Fig. 34 shows that in the zenith the moon's distance isless than at the horizon, by almost exactly the e... |
illo035Fig. 35.—Atmospheric Refraction. So far asthe action is regular, theeffect is to bend the raysdirectly downwards, andthus to make the objectsappear higher in the sky. Refraction increases thealtitude of a celestial objectwithout altering theazimuth. Like parallax,it is zero at the zenithand a maximum at thehori... |
We may therefore apply the law of refraction directly at A, and writesin Z'A S' = n sin BAC ( = ZOS ), or sin( ζ +r ) = n sinζ;AC being drawnparallel to OS. Developing the first member, we have sinζcosr + cosζsinr = n sinζ. But r is always a small angle, never exceeding 40'; we may therefore takecos r... |
The refraction there is really about 37', under thecircumstances of temperature and pressure above indicated. Effect of Temperature and Barometric Pressure.—The indexof refraction of air depends of course upon its temperature and pressure. As the air grows warmer, its refractive power decreases; as it growsdenser, the... |
Lateral Refraction.—When the air is much disturbed, sometimes objectsare displaced horizontally as well as vertically. Indeed, as a generalrule, when one looks at a star with a large telescope and high power, it willseem to “dance” more or less—the effect of the varying refraction whichcontinually displaces the image.... |
In fact, atpresent our knowledge of the constants of air rests mainly on astronomicalwork.2. By Observations of Circumpolar Stars. At an observatory whoselatitude exceeds 45� select some star which passes through the zenithat the upper culmination. (Its declination must equal the latitude ofthe observatory.) It will ... |
If the star does not go exactlythrough the zenith, it is only necessary to compute each time approximaterefractions for its upper observation, as well as for the polarpoint. 3. By Observations of the Altitudes of Equatorial Stars made at an Observatorynear the Equator. We have only, then, to observe the apparent zenit... |
Then, if the sun is at S, illo036Fig. 36, it will just have set to an observer at 1,but all the air within his range of vision will still be illuminated. Nothing remains in sight on whichthe sun is shining. Duration of Twilight.—This depends upon the height of the atmosphere,and the angle at which the sun's diurnal ci... |
Probably, also, in mountainregions the clearness of the air, and its purity contribute to the effect. 1 C � 9�, or R+H = R � 9�; whence H = R ( 9� - 1) = 0.0125 R, or almost exactly fifty miles. This mustbe diminished about one-fifth part on account of the curvature ofthe lines 1 m and 5 m by refraction, making the... |
CHAPTERIIIIVProblems of Practical AstronomyPROBLEMS OF PRACTICAL ASTRONOMY, LATITUDE, TIME,LONGITUDE, AZIMUTH, AND THE RIGHT ASCENSION ANDDECLINATION OF A HEAVENLY BODY. The first of these problems is that of the§ LATITUDE. It mayalso be defined, from the mechanical point of view, as the angle betweenthe plane of the ... |
In illo037Fig. 37 the semicirclesemi-circle AQPB is the meridian, Q and P beingrespectively the equator and the pole, and Z the zenith. QZ isthe declination of the zenith, or the observer's latitude (=PB=ϕ). Suppose now that we observe Zs (=ζ_s), the zenith distance of astar s (south of the zenith), as it crosses the ... |
It would take us a littletoo far to explain the method of reduction, which is given with the necessarytables in all works on Practical Astronomy. The meridian-circle cannotbe used, as the instrument must be such as to make extra-meridianobservations possible. Usually the sextant or universal instrument isemployed. Thi... |
Evidently thelimit of accuracy depends upon the exactnesswith which the level measures the slight, butinevitable, difference between the inclinationsof the instrument when pointed on the two stars. Fifth:By the Prime Vertical Instrument (p. fig:illo020).—We observesimply the moment when a known star passes the prime v... |
By the Gnomon.—The ancients had no instrumentssuch as we have hitherto described, and of course could not use anyof the preceding methods of finding the latitude. They were, however,able to make a very respectable approximation by means of thesimplest of all astronomical instruments, the gnomon. The height AB being gi... |
It is supposed, indeed known, that many of the Egyptian obelisks wereerected primarily to serve as gnomons, and were used for that purpose. 108. Possible Variations of the Latitude.—It is an interesting questionwhether the position of the earth's axis is fixed with reference to itsmass and surface. Theoretically it i... |
Apparent Solar Time.—Just as sidereal time is the hour-angleof the vernal equinox, so at any moment the apparent solar time isthe hour-angle of the sun. It is the time shown by the sun-dial, andits noon is when the sun crosses the meridian. December23d is fifty-one seconds longer from (apparent) noon to noon thanSept.... |
Apparent solar time is scientifically unsatisfactory,because of the variation in the length of its days and hours. Andyet we have to live by the sun; its rising and setting, daylight and night,control our actions. In mean solar time we find a satisfactory compromise,an invariable time unit, and still an agreement with... |
This personal equation differs for different observers, but is reasonably(though never strictly) constant for one who has had much practice. In thecase of observations with the chronograph, it is usually less than ± 0^s.2. If mean time is wanted, it can be deduced from the sidereal timeby the data of the almanac. The ... |
The formula is sin12P = ( sin12[ ζ + ( ϕ - δ) ] sin12[ ζ - ( ϕ - δ) ] cosϕcosδ) ^1/2. In order to accuracy, it is desirable that the sun should be on theprime vertical, or as near it as practicable. It should not be near themeridian. Any slight error in the assumed latitude produces nosensible effect upon the resul... |
Beginners need tobear this in mind in using the almanac. Having now methods of obtaining the true local time, we canattack the problem of longitude, which is perhaps the most importantof all the economic problems of astronomy. The great observatoriesat Greenwich and at Paris were established simply for the purposeof f... |
Under the first head we may make use of[A] A Lunar Eclipse.—When the moon enters the shadow of theearth, the phenomenon is seen at the same moment, no matter wherethe observer may be. By noting, therefore, his own local time at themoment, and afterwards comparing it with the time at which the phenomenonwas observed at... |
[B] Lunar-Distances.—At sea it is, of course, impossible toobserve the moon with a transit instrument, but we can observe itsdistance from the stars near its path by means of a sextant. From this the longitude can bedetermined. [C] Occultations.—Occasionally, in its passage through the sky,the moon over-runs a star, o... |
If the chronometer indicates true Greenwich time, the error deducedfrom the observation will be the longitude. Usually, however, the indicationof the chronometer face requires correction for the rate andrun of the chronometer since leaving port. Chronometers are only imperfect instruments, and it is important, therefo... |
Especial care must be taken to determinewith accuracy, or to eliminate, the personal equations of the observers. It is customary to make observations of this kind on not less than fiveor six evenings in cases where it is necessary to determine the difference oflongitude with the highest accuracy. Local and Standard Ti... |
When does Monday noon become Tuesday noon? The conventionis that the change of date occurs at the 180th meridian from Greenwich. Ships crossing this line from the east skip one day in so doing. If it isMonday forenoon when the ship reaches the line, it becomes Tuesday forenoonthe moment it passes it, the intervening t... |
The methods employed are necessarily such that observationscan be made with the sextant and chronometer, the onlyinstruments available under the circumstances. The Latitude is usually obtained by observations of the sun'saltitude at noon, according to the method explained in Art.103. The Longitude is usually found by ... |
In this method, each observation of the sun's altitude, with thecorresponding chronometer time, is made to define the position of theship upon a certain line, called the circle of position. Two such observationswill, of course, determine the exact place of the vessel atone of the intersections of the two circles. At a... |
The observer must be at one ofits two intersections with the first circle—which of the two intersectionsis easily determined from the roughly observed azimuth ofthe sun. If the ship moves between the two observations, the proper allowancemust be made for the motion. The intersection with the second circle thengives th... |
And the radius of the “circle of position,”i.e., the distance from A to C—will be 50�. Again, a second observation is made three hours later, when the sun'saltitude is found to be 65�. The sub-solar point will then be at B, latitude20�, longitude 30� W., and the radius of the circle of position BC be25�, C being the s... |
The next morning by daylight the observermeasures the angle or angles between the night-signal and theobjects whose azimuth is required. This can easily be done, as the right ascension and declination ofthis star are given in the almanac for every day of the year. This will come out inhours, of course, and must be red... |
If the instrument is in exact adjustment, the sidereal timewhen the object crosses the middle wire of the reticle of the instrument isdirectly (according to Art.27) the right ascension of the object. In either case the declination can be immediatelydeduced, being the complement of the polar distance, and equal tothe l... |
Other less complicated micrometers are also in use. One of them, calledthe ring micrometer, consists merely of an opaque ring supported in the fieldof view either by being cemented to a glass plate or by slender arms ofmetal. The observations are made by noting the transits of the comparisonstar and of the object to b... |
In the actual application of themethod certain corrections are therefore necessary to take into account theearth's ellipticity. CHAPTERIVVThe EarthTHE EARTH AS AN ASTRONOMICAL BODY. The facts are broadly these:—[1.] * The earth is a great ball, about 7918 miles in diameter. * It rotates on its axis once in twenty-f... |
The fact that the sea-horizon, as seen from an eminence,is everywhere depressed to the same extent below the levelline, shows that the surface is approximately spherical. * The factthat as one goes from the equator toward the north, the elevation ofthe pole increases proportionally to the distance from the equator,... |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.