Transit (astronomy)

“ This is the more unaccountable, as a small equatorial may be purchased for a moderate sum; and as there is no single instrument so well adapted for illustrating all the operations of Practical Astronomy. Where very great accuracy is not required, it may occasionally be made to serve the general purposes of a transit instrument for observing the passages of the sun and stars across the meridian. ”

“ The transit of Mercury or Venus is its passage across the sun's disk, as the moon passes over it in a solar eclipse. The planet is seen projected on the sun's disk in a small, black, round spot, moving slowly over [238] the face of the sun. As the transit takes place only when the planet is in inferior conjunction, at which time her motion is retrograde, it is always from left to right; and, on account of its motion being retarded by the motion of the earth, (as was explained by Fig. 51, page 232,) it remains sometimes a long time on the solar disk. ”

“ The transit circle is the most important instrument of astronomy, and such is the perfection of the Greenwich instrument that nothing could be more unfortunate for astronomy than that that instrument should be in any way damaged. And though many of us are admirers of physical astronomy, we have yet to find the instrument that is as important to physical astronomy as the transit circle at Greenwich is to astronomy of position. ”

“ That century also witnessed the combination of the transit instrument and the mural quadrant or circle in one instrument-the transit or meridian circle. While the necessity of following the sun, moon and planets as regularly as possible increased the daily work of observatories, other branches of astronomy were opened and demanded other observations. ”

“ The interval of time will depend entirely on the inclination of the meridian at these two places. If, then, we erect a transit instrument in one place, and another transit instrument at another place, and compare the times at which the same star passes the two transit instruments, we have the means of seeing how much the planes of the meridian are inclined. Its makes no difference whether we suppose the earth to turn round so as to bring the plane of PL to pass through S, or suppose the star to turn round the earth, so as to make S pass through the plane of PL ”

“ As a recreation, I have for some time time past dabbled in astronomy, physical astronomy, I should say, as I am no mathematician. I began it in Westland with a small telescope, about the time of the Transit of Venus in 1874. With the help of an old friend there, J. O'Connor, a clever Engineer and Surveyor, we tackled the general facts of the problem, and planned to observe the transit of the planet across the sun, which occurs once in a hundred years, and eight years later again takes place. ”

“ This instrument is used to measure the declination of stars, or angular Page 64 distance north or south of the equator. Thus a star's place in two directions is exactly fixed. When the telescope is mounted on two pillars instead of the face of a wall, it is called a transit instrument. This is used to determine the time of transit of a star over the meridian, and if the transit instrument is provided with a graduated circle it can also be used for the same purposes as the mural circle. Man's capacity to measure exactly is indicated in his ascertainment of the length of waves of light. ”

“ It is employed in observing the passage of the heavenly bodies across the observer’s meridian. To note accurately by means of the astronomical clock the exact instant of time at which a celestial body crosses the centre of the field of view is the essential part of a transit observation. Small transit instruments are employed for taking the time and [Pg 55] for regulating the observatory clock, but large instruments are used for delicate and exact observations of Right Ascensions and Declinations of stars of different magnitudes. ”

“ Transits of Jupiter's Satellites.—Whenever one of Jupiter's moons passes in front of the planet, it is said to make a transit across his disk. When a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from Fig. ”

“ The actual meteoric train is known to travel in an elongated ellipse, the far end of which lies near the confines of the solar system, while at a point near the hither end the earth's orbit runs slantingly athwart it, forming, as it were, a level crossing common to the two orbits, the earth taking some five or six hours in transit. ”

“ In this way then the transit instrument enables us, having true time, to determine the right ascension of a heavenly body as it transits the meridian, and, knowing the right ascension of a heavenly body, we have only to watch its transit in order to know the true time; so if the observer knows at what time a known star ought to transit, he has an opportunity of correcting his clock. ”

“ A transit is the passing of a primary planet across the Sun, or of a secondary planet (i.e. satellite) across its primary, whilst an occultation is the concealment of a star by the Moon, or of a secondary planet (i.e. satellite) by its primary. A little thought given to this definition will make it [236] clear that a transit is essentially the same in principle as an eclipse of the Sun by the Moon—one body comes in front of another, and the former conceals in succession parts of the latter. ”

“ Obviously, if the planets by their transits effect anything whatsoever, the double transit of major planets must have [Pg 133] a correspondingly greater effect. The careful student of Astrology will institute a number of tests in order to find what effects are due to the combined action of the planets when in conjunction at transit, and also when in opposition or quadrature. The chief points to be noticed in connection with the transits of the planets are the Midheaven, Ascendant, and the places of the Sun and Moon, as already mentioned in Chapter II of this section. ”

“ At every transit of the planet through the -axis, a light signal shall be traveling to the sun. Let be the orbital period. When the sun is at rest, the time between the upper and lower transit will amount to (at a motion being presupposed as circular) ”

“ You do not see the stars moving as you do the trees and houses when you travel along in a carriage. But in the transit instrument, so sensible is the motion, that it is a most nervous business to note the precise moment when the star passes across the various wires ”

“ We see that our Voyage in Space will take us over big distances; 240,000 miles to the Moon, and 93 million miles to the Sun. It hardly seems worth while in starting to bother about the first few miles. But perhaps you have noticed, when you have taken
Horrox observing a Transit of Venus for the first time. ”

“ The present transit is the fourth which the Observatory has possessed, and its three predecessors, known as Halley's, Bradley's, and Troughton's, respectively, are still preserved and hang on the walls of the transit room, affording by their comparison an interesting object-lesson in the evolution of a modern astronomical instrument.
The watcher who wishes to observe the passing of a star must note two things: he must know in what direction to point his telescope, and at what time to look for the star. ”

“ Consequently there would be at least two transits of Vulcan [Pg 45] every year, and there would generally be four transits; the average number of transits would be about eleven in three years. With a wider orbit and a greater inclination transits would be fewer; but even with the widest orbit and the greatest inclination that can possibly be allowed, there would be at least one transit a year on the average. ”

“ Supposing, finally, that our train were started from the sun, and made to run straight out to the known boundaries of the solar system, that is to say, as far as the orbit of Neptune, it would take over 5000 years to traverse the distance.
That sixty miles an hour is a very great speed any one, I think, will admit who has stood upon the platform of a country station while one of the great mail trains has dashed past. ”

“ I suppose it would cause the astronomer royal to laugh could he see the first transit instrument used by us at Dorchester, a strip of brass nailed to the east end of the house, with a hole in it to see a fixed star and note its transit ”

“ Interesting, though of somewhat rare occurrence, are the transits of Mercury, when the planet comes between us and the sun, and passes as a black circular dot across the bright solar surface. The first occasion on which such a phenomenon was observed was November 7, 1631. The occurrence of this transit was predicted by Kepler four years in advance; and the transit itself was duly observed by Gassendi, though five hours later than Kepler's predicted time. ”

“ The consequence is that when another eight years have elapsed, the planet passes a little above the sun or a little below the sun, and thus a third occurrence of the transit is avoided for more157 than a century. ”

“ Suppose the whole of that hemisphere of the earth on which the sun is shining when the transit is about to begin were covered with observers waiting for the event. As Venus sweeps rapidly onwards to the critical part of her path, it is clear that some of these observers will get an earlier view of the commencement of the transit than others will; just as at a boat-race, persons variously placed round a projecting corner of the course see the leading boat come into view at different times. ”

“ Occasionally Mercury passes at inferior conjunction between us and the disc of the sun, appearing like a black spot against the sun, and thus makes what we call a transit. Because the planet is so small, his transit across the sun cannot be seen with the naked eye; but it is an interesting phenomenon to those who can view it with a telescope, though, apparently, astronomers do not regard it as having any great scientific importance. ”

“ The double observation with the transit circle gives us, then, the place in the sky where the star appeared to be at the moment of observation, not its true place; to find that true place we have to calculate how much refraction had displaced the star at the particular height in the sky, and at the particular temperature and atmospheric pressure at which the observation was made.
THE MURAL CIRCLE.
The transit circle is a comparatively recent instrument. ”

“ Yes, we are racing through space with another motion, too, besides those of rotation and revolution, for our earth keeps up with its master attractor, the sun. It is difficult, no doubt, to follow this, but if you think for a moment you will remember that when you are in a railway-carriage everything in that carriage is really travelling along with it, though it does not appear to move. And the whole solar system may be looked at as if it were one block in movement. As in a carriage, the different bodies in it continue their own movements all the time, while sharing in the common movement. ”

“ But perhaps the following illustration may serve sufficiently, yet simply, to indicate the qualities of the two leading methods of using a transit. Imagine a bird flying in a circle round a distant globe in such a way that, as seen from a certain window (a circular window suppose) , the bird will seem to cross the face of the globe once in each circuit. Suppose that though the distance of the globe is not known, the window is known to be exactly half as far again from the globe as the bird's path is, and that the window is exactly a yard in diameter. ”

“ When I meet the engine with its train of cars moving off with planetary motion,—or, rather, like a comet, for the beholder knows not if with that velocity and with that direction it will ever revisit this system, since its orbit does not look like a returning curve,—with its steam cloud like a banner streaming behind in golden and silver wreaths, like many a downy cloud which I have seen, high in the heavens, unfolding its masses to the light,—as if this travelling demigod, this cloud-compeller, would ere long take the sunset sky for the livery of his train ”

“ The stage from the one orbit to that of the other is about one-third of the whole journey to the sun. In a railway train, running at the ordinary speed, this distance could not be done under less than six thousand years; so that, if the train started at the creation of man, it would not have yet reached this first station on the way to the sun. ”

“ In a modern transit observation with the use of the chronograph it will be seen at once that before the observer can register a star-transit on the chronograph, he has to perceive with his eye that the star has reached the wire, he has to mentally recognize the fact, and consciously or unconsciously to exert the effort of will necessary to bring his finger down on the button. ”

“ Onward, inward, we flashed, veering here and there to avoid some star's far-swinging planets, dipping or rising to keep clear of the masses of traffic that were jamming the space-lanes leading inward, racing on at the same unvarying, tremendous velocity while we three in the pilot room, and the remainder of our crew beneath, strove to remain awake and conscious against the utterly crushing oppression of fatigue that pressed down upon us. ”

“ The various assistants are now in training at Greenwich, Poltava, and Washington, with the very instruments which they will use on the expeditions.
At Washington and Poltava an apparatus for the representation of the transit is in use. A disk representing Venus is caused to travel over an illuminated space which is representative of the sun, and the circumstances of the transit are then observed.
In this way it is hoped to obtain an idea of the personal error of each observer in watching contacts, so that, in reducing the observations of the transit, all personality may be eliminated. ”