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Suppose the circle ABC completed, and that E is the point on the circumferenceopposite B, so that BE equals the diameter of the earth (=2 R). By geometry, BD:BA = BA:BE, By geometry, whence BE = BA^2/BD, or R = BA^2/2BD. Now BA is one mile, and BD = 2/3 of a foot, or 1/7920 of a mil... |
The two stations beingon the same meridian, all that is necessary is to measure their latitudesby any of the methods which have been given in CHAPTERIVChapter IV. and takethe difference. This will be the angle wanted. Three hundred and sixty times this would be the circumference ofthe earth, a little less than 25,000 ... |
The first really valuable measure of the arc of a meridian was that madeby Picard in Northern France in 1671—the measure which served Newton sowell in his verification of the idea of gravitation. The Rotation of the Earth.—At the time of Copernicus theonly argument in favor of the earth's rotation[The word rotate deno... |
1. The Eastward Deviation of Bodies falling from a GreatHeight.—The idea that such a deviation ought to occur was firstsuggested by Newton. Evidently, since the top of a tower, situatedanywhere but at the pole of the earth, describes everyday a larger circle than its base, it must move faster. illo045Fig. 45illustrate... |
The latter obtained afree fall of 520 feet, and from the mean of 160 trials, the eastern deviationobserved was 1.12 inches, while theory would make it 1.08. The experimentalso gave a southern deviation of 0.17 of an inch, unexplained by theory. It seems to indicate the probable error of observation. The balls in falli... |
From the dome of the Pantheonin Paris he suspended a heavyiron ball about a foot in diameterby a wire more than 200 feet long(illo046Fig. 46). In fact, the floor of the Pantheon was soon turning underthe plane of the pendulum's vibration. The experiment createdgreat enthusiasm at the time, and has since been very freq... |
In any other latitude the effect will be intermediate,and the time required for the pendulum to complete the revolutionof its plane will be twenty-four hoursdivided by the sine of the latitude. The northernedge of the floor of a room (in the northernhemisphere) is nearer the axis of the earththan its southern edge, an... |
Now it is easy to prove that the angle of this sectorequals 360�� sinϕ (ϕ being the latitude). The circumference of the parallel AB (illo047Fig. 47)= 2π� Rcosϕ, since R�cosϕ=AD, which isthe radius of the parallel, R being AC, the radiusof the globe, and the angle ACD equal to 90�-ϕ.(2) At the pole the cone produced b... |
This is best near the pole, theother at the equator.illo049Fig. 49.—Foucault's Gyroscope. 3. By the Gyroscope, an experiment also due to Foucaultand proposed and executed soon after the pendulum experiment. The instrument shown in illo049Fig. 49 consists of a wheel so mounted ingimbals that it is free to turn in ever... |
In thenorthern hemisphere the wind ina cyclone moves spirally towardsthe centre of the storm, whirlingcounter clock-wise, while in thesouthern, the spiral motion is with the hands of a watch. The Ordinary Law of Wind-change; that is, in the northern hemispherethe north wind, under ordinary circumstances, changes to a ... |
Theoretically itmust almost necessarily do so. Then geologicalchanges, the elevation and subsidence of continents, and the transportationof matter by rivers, act, some one way, some the other. Atpresent it can only be said that the change, if any has occurred sinceastronomy became accurate, has been too small to be de... |
The flattening at the poles is the necessary consequence of theearth's rotation, and might have been cited in the preceding sectionas proving it. There are two ways of determining the form of the earth:one, by measurement of distances upon its surface in connection withthe latitudes and longitudes of the points of obs... |
The geodetic work consists in measuringtheir distance from each other in miles, feet, or metres, and it is thispart of the work which consumes the most time and labor. Theprocess is generally that known as triangulation. Two stations are selected for the extremities of abase line six or seven miles long, and the groun... |
Knowing one distance and all theangles in this system, it is possible to compute with great accuracy the exactlength of the line 1 5 and its direction. The sides of the triangles are usually from twenty-five to thirtymiles in length, though in a mountainous country not infrequentlymuch longer ones are available. Gener... |
Radii of Curvature of the Meridian.149. Each measurement of a degree of latitude gives the “radius of curvature,”as it is called, of the meridian at the degree measured. Next produce the line Ba to b,making Bb the radius of curvature ofthe second degree, and draw this seconddegree-arc; and so proceed until thewhole ... |
The ellipticity of an ellipse must not be confounded with itseccentricity. The latter is e = √(A^2 - B^2)/A,and is always a much larger numerical quantity than the ellipticity. In the case of the earth's meridian, it is 1/12.1 as against 1/295. Itssymbol is usually e.151. Arcs of longitude are also available for de... |
Extensive surveys of this sort have been made, and are still in progress,and it is found that the force of gravity at the pole exceeds that atthe equator by about 1/190 part. In other words, a person who weighs190 pounds at the equator (by a spring balance) would, if carried tothe pole, show 191 pounds by the same bal... |
A pin attached to the end of the pendulumtouches a globule of mercury (which is momentarily raised for the purposeonce in eight or ten minutes), and so records the swing upon the chronographsheet. The observations determine the “force of gravity” (French“pesanteur”) at the station. Since V is equal tothe earth's circu... |
Now, this centrifugalforce c is not wholly effective in diminishingthe weight of a body, but onlythat portion of it (MR in illo052Fig. 52) whichis directed vertically. This is greater thenearer each station is to the centre of the earth; but unfortunatelythere is no simple relation connecting the force with the distan... |
53.Astronomical and GeocentricLatitude. The geocentric latitude is but little used except in certain astronomicalcalculations where parallax is involved. In illo053Fig. 53, the angle MOQ is the geocentric latitude of M, while MNQ isthe geographical latitude. MNQ is also the astronomical latitude, unlessthere is some l... |
In the eastern partof the United States these station errors, according to the Coast Surveyobservations, average about 11/2”. Errors of from 4” to 6” are not uncommon,and in mountainous countries, as for instance in the Caucasus and inNorthern India, these errors occasionally amount to 30” or 40”. Pendulum observation... |
We must not imagine the word “attract” to mean too much. It merelystates the fact that there is a tendency for the bodies to move toward eachother, without including or implying any explanation of the fact. So far,no explanation has appeared which is less difficult to comprehend thanthe fact itself. When, however, the... |
Two stations were chosen on the same meridian, one north and onesouth of the mountain Schehallien, in Scotland. The next operation was to observe theastronomical latitude at each station. Similarly, if C in the illo055figure is the centre of attraction of the mountain, we have f =km/d^2, m being the mass of... |
2. Much more trustworthy results, however, are obtained bythe method of the torsion balance, first devised by Michell, but firstemployed by Cavendish in 1798. A light rod, carrying two smallballs at its extremities, is suspended horizontally at its centre by along fine metallic wire. When the deflectingforce is remove... |
Wemust also measure accurately the distance, Aa' and Bb' between the centreof each of the large balls and the point of rest of the small ball whendeflected. The earth's attraction on each of the small balls of course equalsthe ball's weight. The attractive force of the large ball on the small onenear it is found direc... |
The horizontal bar was in this case only half a metre long, of aluminium,with small platinum balls at the end. For the large balls, glass globes wereused, which could be pumped full of mercury or emptied at pleasure. Thewhole was enclosed in an air-tight case, and the air exhausted by an air-pump. Cornu obtained 5.56 ... |
The mass of the mountain must be determined by a survey, just asin the Schehallien method, which makes the method unsatisfactory. When g and g' are ascertained by the pendulum experiments,E remains as the only unknown quantity, and can be readily found. Observationsof this kind were made by Carlini, in 1821, at Mt. Ce... |
At the same time our distancefrom the earth's centre has been decreased by d, the depth of the mine. At the surface g = kE/R^2, as before. At the bottom of the mine g' = kE - “shell”/(R - d)^2. At the bottom of tComparing the two equations, we find E in the terms of the shell, sinc... |
The experiment, with somemodifications, is soon to be tried again on a very large scale in Germany. There is nothing in this that mightnot have been expected. If the earth were ever fluid, it is natural tosuppose that in the solidification the densest materials would settletowards the interior. Whether the interior of... |
A year, the interval between the successivereturns of the sun to the same position, was very early found toconsist of a little more than three hundred and sixty days. (Just as two opposite teeth on a gear-wheelmove in the same angular direction, though at any momentthey are moving in opposite linear directions.) That ... |
It may be defined as thetrace of the plane of the earth's orbit upon the celestial sphere, just asthe celestial equator is the trace of the plane of the terrestrial equatoron the same sphere. The obliquity is, of course, simply equal to the sun's maximum declination,or greatest distance from the equator, which is reac... |
It was taken of that particularwidth by the ancients simply because the moon and the then knownplanets never go further than 8� from the ecliptic. Winter {[ Capricornus, ♑; Aquarius, ♒; Pisces, ... |
The two points in the heavens 90� distant from the ecliptic are calledthe Poles of the ecliptic. The northern one is in the constellationof Draco, about half-way between the stars δ and ζ Draconis. Now,suppose a set of great circles drawn, like meridians, through thesepoles of the ecliptic, and hence perpendicular to ... |
Sbeing a star, its right ascension (α) isER and its declination (δ) is SR; its longitude (λ) is EL, and its latitude(β) is SL. P and K are the poles of the equator and eclipticrespectively, and the circle KPCQ is the Solstitial Colure, so called. The student can hardly take too great care to avoid confusion of celesti... |
We proceed as follows: Inthe triangle ERS, right-angled at R, we have given ER and RS (α and δ),from which we find the hypothenuse ES and the angle RES. Next in thetriangle ELS, right-angled at L, we have the hypothenuse ES and the angleLES, which is equal to RES-LEQ (LEQ being ω, the obliquity of theecliptic). Hence... |
To find the Form of the Orbit, we may proceed thus: Takea point S for the sun and drawfrom it a line SO, illo058Fig. 58,directed towards the vernalequinox as the origin of longitudes. Weshall thus get a sort of “spider,” showing the directionsas seen from the earth on these days. Next, as to relative distances. When t... |
The variations of the sun's diameter are too small to be detectedwithout a telescope (amounting to only about three per cent), so thatthe ancients were unable to perceive them. Hipparchus, however, about150 b.c., discovered that the earth is not in the centre of the circular orbitwhich he supposed the sun to describe ... |
The actual values of p and q are 32' 36”.4 and 31' 31”.8, which givee = 0.01678: this is about 1/60, as has been stated.illo060Fig. Equable Description of Areas. To find the Law of the Earth's Motion.—By comparing themeasured apparent diameter with the differences of longitude from dayto day, we can also deduce the l... |
In such a case the“equable description of areas” is anecessary mechanical consequence. The solution of this problem, knownas “Kepler's problem,” leads to transcendental equations, and liesbeyond our scope. See Watson's “Theoretical Astronomy,” pp. 53 and 54, or any other similarwork. Anomaly and Equation of the Centre... |
At that time, the line which separates the illuminated portions ofthe earth passes through the two poles, and day and night are everywhereequal. The same is again true of the 22d of September, whenthe sun is at the autumnal equinox on the opposite side of the orbit. About the 21st of June the earth is so situated that... |
Effect of Sun's Elevation on Amountof Heat Imparted to the Soil. Sunshine lasts more than half the day. The mean elevation of the sun during the day is greater thanwhen it is at the equinoxes, since it is higher at noon, and in anycase reaches the horizon at rising and setting. Now, the more obliquelythe rays strike, ... |
For similar reasons the minimum temperature of winter occurs about Feb. 1,about half-way between the solstice and the vernal equinox. Forthis reason the southern summer, which occurs in December andJanuary, is hotter than the northern. It is, however, seven daysshorter, because the earth moves more rapidly in that par... |
2. Change of Eccentricity.—At present the eccentricity ofthe earth's orbit (which is now 0.0168) is also slowly diminishing. According to Leverrier, it will continue to decrease for about 24,000years, until it becomes 0.003, and the orbit will be almost circular. Then it will increase again for 40,000 years, until it ... |
The hour-angleof this mean sun is, as has been already explained, thelocal mean time (or clock time); while the hour-angle of thereal sun is the apparent or sun-dial time. The Equation of Timeis the difference between these two times, reckoned as plus whenthe sun-dial is slower than the clock, and minus when it is fas... |
1. The Variable Motion of the Sun in the Ecliptic, due to theEccentricity of the Earth's Orbit.—Near perihelion, which occursabout Dec. 31, the sun's motion in longitude is most rapid. Accordingly,at this time the apparent solar days exceed the siderealby more than the average amount, making the sun-dial days longerth... |
But at intermediate points, between the equinoxesand solstices, they would not be together on the same hour-circle. Itwill at once be seen that the former point, m in illo063Fig. 63,[Fig. 63 represents a celestial globe viewed from the west side, the axis beingvertical, and K, the pole of the ecliptic, on the meridia... |
The full-lined curve represents their combined effect, and is constructedby making its ordinate at each point equal to the sum(algebraic) of the ordinates of the two other curves. At the 1st ofFebruary, for instance, the distance F, 3, in the illo064figure =F, 1 + F, 2. So, also, M, 6 = M, 4 + M, 5; the components,... |
Precession of the Equinoxes.—The length of year wasfound in two ways by the ancients:—1. By the gnomon, which gives the time of the equinox and solstice;and2. By observing the position of the sun with reference to thestars,—their rising and setting at sunrise or sunset. Comparing the results of observations made by ... |
The pole ofthe ecliptic has remained practically fixed among the stars, but theother pole has changed its position materially. At present the polestar is about 11/4� from the pole. At the time of the star catalogue ofHipparchus it was 12� distant from it, and during the next centuryit will approach to within about 30'... |
The action is just whatit would be if a spheroidal ball of iron of the shape of the earth hada magnet brought near it. We then have the result of the combination of tworotations at right angles with each other, one thewhirl of the wheel, the other the “tip” which theweight tends to give the axis. (See Brackett's Physi... |
(See article, “Gyroscope,” in Johnson'sCyclop�dia.) The moon's action, on accountof her proximity, is still more powerful than that of the sun; on theaverage two and a half times as great. Now, the moon crosses the celestialequator twice every month, and at these times her action ceases. Regression of the Gyroscope Wh... |
The precession, therefore, is not uniform, but variable, almost ceasing atsome times and at others becoming rapid. The consequence is whatis called Nutation or “nodding.” We distinguish three of these nutations, (a) The Lunar Nutation,depending upon the motion of the moon's nodes. This has a period of a little less th... |
The right ascension and declination of a star are both affected. The Three Kinds of Year.—In consequence of the motionof the equinoxes caused by precession, the sidereal year and theequinoctial or “tropical” year do not agree in length. The tropical year is the year usually employed,unless it is expressly stated to th... |
The month meets with the sameobjection, and for all chronological purposes, therefore, the year isthe unit practically employed. Since the two are incommensurable, the problem is a very difficult,and indeed strictly speaking, an impossible, one. The Mohammedans still use a purely lunar calendar,having a year of twelve... |
The calendar of the phases of the moon, for instance, for 1889 is thesame as for 1870 and 1908 (except that intervening leap-years may changethe dates by one day). The “Golden number” of a year is its number in this Metonic cycle, andis found by adding 1 to the “date-number” of the year and dividing by 19. The remaind... |
He also altered the name ofthe month Quintilis, calling it “July” after himself. From that time on, the Julian calendar continued unbrokenly in use until1582; and it is still the calendar of Russia and of the Greek Church.220. The Gregorian Calendar.—The Julian calendar is not quitecorrect. As a consequence, the date ... |
(The year 1600 was a leap-year accordingto the Gregorian system as well as the Julian, but 1700 was not.) The change was made under very great opposition, and there were violentriots in consequence in different parts of the country, especially at Bristol,where several persons were killed. In Russia, however, for scien... |
The direction in which we have to point our telescope in observinga star is not the same that it would be if the earth were at rest. In illo070Fig. 70 BA'= V and AA'=u. illo071Fig. 71.—Aberration of Light. Now take the more general case. Suppose a star sending us light with avelocity V in the direction SP, illo071Fig.... |
Aberrational Orbit of a Star. The latest, and probably the most accurate, determination of thisconstant (derived from the Pulkowa Observationsby Nyr�n in 1882) is 20”.492. Aberration was discovered and explainedby Bradley, the English Astronomer Royal,in 1726. It must, therefore, appear to describe alittle circle 41” ... |
Its “constant” is only 0”.31 for an observer situated at the equator; anywhereelse it is 0”.31cosϕ, ϕ being the latitude of the observer. We pass next to a consideration of our nearest neighbor inthe celestial spaces, the moon, which is a satellite of the earth andaccompanies us in our annual motion around the sun. Sh... |
Sidereal and Synodic Revolutions.—The Sidereal revolutionof the moon is the time occupied in passing from a starto the same star again, as the name implies. 11^s.545 ± 0^s.01, or 27^d.32166. The moon's mean dailymotion among the stars equals 360� divided by this, which is13� 11' (nearly). The Synodic revolution is the... |
When the elongationis 90�, as at the half-moon, the moon is in “Quadrature.”231. The earliest authentically recorded eclipse is one that was observedat Nineveh in the year 763 b.c. between 9 and 10 o'clock on themorning of June 15th. But themonth is a little shorter now than it was 2000 years ago. Relation of Sidereal... |
In the firstcase the moon's declination will change during the month by 57� 12',from - 28� 36' to + 28� 36'. In the other case it will change only by 36� 40',so that at different times the difference in the behavior of the moon inthis respect is very striking.235. Interval between Moon's Transits.—On the average t... |
In the latitude of New York the least possible daily retardationof moon-rise is 23minutes, and the greatest is 1 hour and 17 minutes. In higher latitudes the variation is greater yet. Moons.—The variations in the retardationof the moon's rising attract most attention when they occur at the timeof the full moon. When t... |
The extremities of the major axis of the moon's orbit are calledthe perigee and apogee (from perì gh and >apò gh). The line of apsides, which passes through these two points, movesaround towards the east once in about nine years, also on accountof perturbations. This gives inthe quadrilateral BOCM the twoangles at B a... |
we have BC and the two anglesCBM and MCB, from which we can find the two sides BM and CM. Finally, in the triangle OBM, we now know the sides OB and BM and theincluded angle OBM, so that the side OM can be computed, which is thedistance of the moon from the earth's centre. Knowing this, the horizontalparallax KMO, or ... |
But the motion of the moon must be allowedfor, as the observations to be compared are necessarily separated byconsiderable intervals of time, and this complicates the calculation. A third, and a very accurate, method is by means of occultations ofstars, observed at widely separated points on the earth. The mean parall... |
Diameter of the Moon.—The mean apparent diameter of themoon is 31' 7”. This gives it a real diameter of 2163 miles (plus orminus one mile), which equals 0.273 of the earth's diameter. Mass of the Moon.—This is about 1/80 of the earth's mass,different authorities giving the value from 1/75 to 1/85. It is not easyto det... |
Apparent Displacement of Sun at First andThird Quarters of the Month. The explanation of this method cannotbe given until we have further studied the motion of bodies under the lawof gravitation. It is not susceptible of great accuracy. (3) Still another method is by comparing the tides produced by the moonwith these ... |
A man on the mooncould jump six times as high as he could on earth and could throwa stone six times as far. This is a fact to be remembered in connectionwith the enormous scale of the surface-structure of the moon. Volcanic forces, for instance, upon the moon would throw the rejectedmaterials to a vastly greater dista... |
The pivot turns underneathit as the crank whirls, but the compassneedle does not rotate, maintaining alwaysits own direction with the marked end north. In every rotating body, one suchline can be so drawn that the circle described by it in the sky becomes infinitelysmall, This is the axis of the body. Librations of th... |
Now the rotation is uniform. For in the quarter-month next following the perigee,the moon will travel to a point M, considerably more than half-way toapogee. But the point a will have made only one quarter-turn, whichis not enough to bring it to the line ME. We shall therefore see a littlearound the western edge. Simi... |
The moon's motions have reference to the earth's centre. The agreement between the moon's time of rotation and ofher orbital revolution cannot be accidental. It is probably due to theaction of the earth on some slight protuberance on the moon's surface,analogous to a tidal wave. This subject will be resumedlater. illo... |
Between the half moon and the full, during the second and thirdquarters of the lunation, we see more than half of the moon's illuminatedside, and have what is called the “gibbous” phase. It is sometimes incorrectly attempted to represent the crescent formby a construction like illo083Fig. 83, B (where asmaller circle ... |
The moon's atmosphere, if it has any at all, is extremely rare, probablynot producing a barometric pressure to exceed 1/25 of an inchof mercury, or 1/750 of the pressure at the earth's surface. Theevidence on this point is twofold. (a) The telescopic appearance. There is no sensible twilight at the cusps of the moon; ... |
Now if themoon had any perceptible atmosphere (or the star any sensible diameter)the disappearance would be gradual. The star would changecolor, become distorted, and fade away more or less gradually. The spectroscope adds its evidence in the same direction. Certain Greenwich observations apparently show a difference,... |
Theearth's core is supposed to be now too intensely heated to absorb much gas;but if it goes on cooling, it will absorb more and more, and in time it mayrob the surface of the earth of all its air. There are still other hypotheses,which we can not take space even to mention.258. Water on the Moon's Surface.—Of course ... |
According to this, if the wholevisible hemisphere were packed with full moons, we should receivefrom it about one-eighth part of the light of the sun. It is found, also, that the half moon does not give even nearly half asmuch light as the full moon. Z�llner has calculated that an average angle of 52� for these elevat... |
The first sensible effect was obtained by Melloni, in 1846,with the then newly invented thermopile, by a series of observationsfrom the summit of Vesuvius. Since then several physicists haveworked upon the subject with more or less success, but the most recentand reliable investigations are those of Lord Rosse and Pro... |
At the end of the long lunar night of fourteen days the temperaturemust fall appallingly low, certainly 200� below zero. The whole amount of heat sent by the full moon to the earth isestimated by Rosse as about one eighty-thousandth part of that sent bythe sun. Lunar Influences on the Earth.—The moon's attraction coop... |
Now it requires only a very slight prepossession in favor of a belief inthe effectiveness of the moon's changes to make one forget a few of theweather changes that occur too far from the proper time. Coincidencesenough can easily be found to justify a pre�xisting belief. Even to the naked eye the moon is a beautiful o... |
Those who have seen such a landscape knowhow little is to be made out with the naked eye at that distance. The Moon's Surface Structure.—The moon's surface for themost part is extremely uneven and broken, far more so than that ofthe earth. The structure, however, is not like that of the earth'ssurface. On the earth th... |
Moreover, on the earth, volcanoes necessarily require theaction of air and water, which do not now exist on the moon. The moon's surface appearsto be absolutely quiescent—still in death. On some portions of the moon these craters stand very thickly. This is especially the case near the moon'ssouth pole. It is noticeab... |
It is not at all too much to say thatour maps of the earth's surface do not, on the whole, compare in fulness andaccuracy with our maps of the moon. [16]illo086Fig. 86.—Archimedes and the Apennines (Nasmyth). Other Lunar Formations.—The craters and mountains arenot the only interesting formationson the moon's surface.... |
We do not know whether they are like the so-called “trap-dykes”on the earth,—fissures which have been filled up from belowwith some light-colored material,—or whether they are mere surfacemarkings. No satisfactory explanation has ever been given. The most remarkable system of “rays” of this kind is the oneconnected wi... |
It was observedby Schroeter very early in the century, and is figured and described byBeer and Maedler as being about five and a half or six miles in diameter, quitedeep and very bright. In 1866 Schmidt, who had several times observed itbefore, announced that it had disappeared. A few months later it wasvisible again,... |
As time passes, the bright spot becomeslarger as the light extends lower down the mountain side, untilthe terminator reaches and passes it. Nowthe angle BAD = GBE” = 90� - S'CN, so that AB, or b, = AD (or a)�sinS'CN. Knowing b, and the radius of the moon r, we get (r + h)^2 = r^2 + b^2,in which h is the height of t... |
A few of the lunar mountainsreach the height of 22,000 or 23,000 feet, but there are none whichattain the elevation of the very highest terrestrial mountains. Heightsranging from 10,000 to 20,000 feet are common. Certain features, however, as hasbeen before mentioned, are then best seen, as, for instance, the streaks ... |
The SUN is simply a star; a hot, self-luminous globe ofenormous magnitude as compared with the earth and the moon,though probably only of medium size among its stellar compeers. But to the earth and the other planets which circle around it, it isthe grandest of all physical objects. Its attraction confines itsplanets ... |
Perhaps the simplest is thatdrawn from the motion of a railway train. Such a train going 1000miles a day (nearly forty-two miles an hour, and faster than theChicago Limited on the Pennsylvania Railroad) would take 2541/3years to make the journey. A cannon-ballmoving unretarded, at the rate of 1700 feet per second, wou... |
Its distance from the sun on thatscale would be just about 220 feet, and the nearest star (still on thesame scale) would be eight thousand miles away, at the antipodes. If we were to place the earth in the centre of the sun, supposingit to be hollowed out, the sun's surface would be 433,000 miles awayfrom us. illo090F... |
To find f we havefrom Mechanics (Physics, p. 62), f = V^2/R, bthis being the expression for the “central force” in the case of abody revolving in a circle. (We may neglect the eccentricity of theearth's orbit in a merely approximate treatment of the problem.) V is the orbital velocity of the earth, which is found b... |
In travelling eighteen and one-half miles the deflection isonly one-ninth of an inch. The Sun's Density.—This density[The determination of the sun's density does not necessarily involve its parallax. Put ρ for the sun's radius, and Ds for its density: also let De be the earth's meandensity. Substitute in equation (c),... |
The Sun's Rotation.—The sun's surface often shows spotsupon it, which pass across the disc from east to west. Different observers get slightly differentresults. Carrington finds 25^d.38; Spoerer, 25^d.23. Position of the Sun's Axis.—On watching the spots withcare as they cross the disc, it appears that they usually de... |
The earth rotates as a whole, every point on its surface makingits diurnal revolution in the same time; so also with the moon andwith the planet Mars. Of course it is necessarily so with any solidglobe. But this is not the case with the sun. Thus spots near the sun's equator give T = 25 days; atsolar latitude 20�, T= ... |
It can be shown that if the matter formingthe spots had thus fallen from an elevation of about 20,000 miles, it wouldaccount for their apparent acceleration. Matter so falling would have anapparent eastward motion, just as do bodies on the earth when falling fromthe summit of a tower (Art.138). There is no decisive ob... |
Onpointing the instrument to the sun andproperly adjusting the focus, a distinct image is formed on thescreen, which shows the main features very fairly. It is, however,much more satisfactory to look at it directly, with a proper eye-piece. With larger instruments, it is necessary to useeye-pieces especially designed ... |
It is well to have the shade-glassmade wedge-shaped,—thinner at oneend than at the other—so that one canchoose the particular thickness which isbest adapted to the magnifying poweremployed. The polarizing eye-pieces are still better when well made. Inthese the light is reflected twice at plane surfaces of glass at the... |
With a low power there is no objection to reducing the amount ofheat admitted into the telescope tube in that way, but with the higherpowers the whole aperture should always be used. 93.—Polarizing Helioscope. Photography.—In the study of the sun's surface photographyis for some purposes very advantageous and much use... |
They represent the surface as it happened to be at the momentwhen the plate was uncovered. The two most important of these solar orastro-physical observatories, are the observatory at Meudon and the so-called“Sonnenwarte” at Potsdam. There ought to be one in this country. General Views.—Before passing to a discussion ... |
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