TRANSIT

, in Astronomy, denotes the passage of any planet, just before or over another planet or star; or the passing of a star or planet over the meridian, or before an astronomical instrument.

Venus and Mercury, in their Transits over the sun, appear like dark specks.

Doctor Halley computed the times of a number of these visible Transits, for the last and present century, and published in the Philos. Trans. numb. 193. See also Abridg. vol. 1, pa. 427 &c. A Synopsis of these Transits is as follows, those of Mercury happening in the months of April and October, and those of Venus in May and November, both old-style; and if 11 days be added to the dates below, the sums will give the times for the new-style. First for Mercury, and then for Venus.

A Series of the Moments when Mercury is seen in Conjunction with the Sun, and within his Disc, with the Distances of the same Planet from the Sun's Centre.

“Those Transits which have the mark *, are but partly visible at London; but those which are marked **, are totally visible.

“Now it is to be observed, that at the ascending node of Mercury in the month of October, the diameter of the sun takes up 32′ 34″, and therefore the greatest duration of a central Transit is 5^h 29^m. But in the month of April the diameter of the sun is 31′ 54″, whence by reason of the slower motion of the planet, there arises the greatest duration 8^h 1^m. Now if Mercury approaches obliquely, these durations become shorter on account of the distance from the centre of the sun. And that the calculation may be more perfect, I have added the following Tables, in which are exhibited the half durations of these eclipses, to every minute of the distance seen from the centre of the sun. These added to or subtracted from the moment of conjunction found by the foregoing Table, give the beginning and end of the whole phenomenon.”

“Though Venus is the most beautiful of all the stars, yet (says Dr. Halley) like the rest of her sex, she does not care to appear in sight without her borrowed ornaments, and her assumed splendor. For the confined laws of motion envy this spectacle to the mortals of a whole age, like the secular games of the Ancients; though it be far the most noble among all those that astronomy can pretend to shew. Now it shall be declared hereafter, that by this one observation alone, the distance of the sun from the earth may be determined with the greatest certainty which hitherto has been included within wide limits, because of the parallax which is otherwise insensible. But as to the periods, they cannot be described so accurately as those of Mercury, since Venus has been observed within the sun's disk but once since the beginning of the world, and that by our Horrox.” Dr. Halley then exhibits the principles of calculating these Transits, from whence he infers that,

“After 18 years Venus returns to the sun, taking away 2^d 10^h (52 1/2)^m, from the moment of the foregoing Transit; and the planet proceeds in a path which is 24′ 41″ more to the south than the former.

“After 235 years adding 2^d 10^h 9^m, Venus may again enter the sun, but in a more northern path by 11′ 33″. But if the foregoing year is bissextile, 3^d 10^h 9^m must be added.

“After 243 years Venus may also pass the sun, only taking away 0^h 43^m from the time of the former; | but proceeds more southerly by 13′ 8″. Now if the foregoing year be bissextile, add 23^h 17^m.

“And in all these appulses of Venus to the sun, in the month of November, the angle of her path with the ecliptic is 9° 5′, and her horary motion within the sun is 4′ 7″. And since the semidiameter of the sun is 16′ 21″, the greatest duration of the Transit of the centre of Venus comes out 7^h 56^m.

“Then let the sun and Venus be in conjunction at the descending node in the month of May; and by the same numbers the same intervals may be computed. After 8 years let there be taken away 2^d 6^h 55′. And Venus will make her Transit in a more northern path by 19′ 58″.

“After 235 years add 2^d 8^h 18^m, or if the foregoing year be bissextile 3^d 8^h 18^m, and you will have Venus more to the South by 9′ 21″.

“Lastly, after 243 years add 0^d 1^h 23^m, or if the foregoing year be bissextile 1^d 1^h 23^m, and Venus will be found in conjunction with the sun, but in a more northerly path by 10′ 37″.

“In every Transit within the sun at this node, the angle of Venus's path with the ecliptic is 8° 28′, and her horary motion is 4′ 0″; and the semidiameter of the sun subtending 15′ 51″, the greatest duration of the central Transit comes out also 7^h 56^m, exactly the same as at the other node.

“As to the epochs, from that only ingress which Horrox observed, the sun being then just ready to set, it is concluded, that Venus was in conjunction with the sun at London in the year 1639, Nov. 24^d 6^h 37^m, and that she declined towards the south 8′ 30″. But in the month of May no mortal has seen her as yet within the sun. But from my numbers, which I judge to be not very different from the heavens, it appears that Venus for the next time will enter the sun in 1761, May 25^d 17^h 55^m, that being the middle of the eclipse, and then will be distant from his centre 4′ 15″, towards the south. Hence and from the foregoing revolutions all the phenomena of this kind will be easily exhibited for a whole millennium, as I have computed them in the following Table.

“As for the durations of these eclipses of Venus, they may be computed after the same manner as those of Mercury in respect of the centre. But since Venus's diameter is pretty large, and since the parallaxes also may bring a very notable difference as to time, a particular calculation must necessarily be made for every place.

“Now the diameter of Venus is so great, that while she adheres to the sun's limb almost 20 minutes of time will be elapsed, that is, when she applies directly to the sun. But when she is incident obliquely, she continues longer in the limb. Now that diameter, according to Horrox's observation, takes up 1′ 18″, when she is in conjunction with the sun at the ascending node, and 1′ 12″ at the other node.

“Now the chief use of these conjunctions is accurately to determine the sun's distance from the earth, or his parallax, which astronomers have in vain attempted to find by various other methods; for the minuteness of the angles required easily eludes the nicest instruments. But in observing the ingress of Venus into the sun, and her egress from the same, the space of time between the moments of the internal contacts, observed o a second of time, that is, to 1/15 of a second or 4‴ of an arch, may be obtained by the assistance of a moderate telescope and a pendulum clock, that is consistent with itself exactly for 6 or 8 hours. Now from two such observations rightly made in proper places, the distance of the sun within a 500th part may be certainly concluded, &c.” See Parallax.

Transit Instrument, in Astronomy, is a telescope fixed at right angles to a horizontal axis; this axis being so supported that the line of collimation may move in the plane of the meridian.

The axis, to the middle of which the telescope is fixed, should gradually taper toward its ends, and terminate in cylinders well turned and smoothed; and a proper weight or balance is put on the tube, so that it may stand at any elevation when the axis rests on the supporters. Two upright posts of wood or stone, firmly fixed at a proper distance, are to sustain the supporters to this instrument; these supporters are two thick brass | plates, having well smoothed angular notches in their upper ends to receive the cylindrical arms of the axis; each of the notched plates is contrived to be moveable by a screw, which slides them upon the surfaces of two other plates immoveably fixed to the two upright posts; one plate moving in a vertical direction, and the other horizontally, they adjust the telescope to the planes of the horizon and meridian; to the plane of the horizon, by a spirit level hung in a position parallel to the axis, and to the plane of the meridian in the following manner. Observe the times by the clock when a circumpolar star, seen through this instrument, Transits both above and below the pole; then if the times of describing the eastern and western parts of its circuit be equal, the telescope is then in the plane of the meridian; otherwise the notched plates must be gently moved till the time of the star's revolution is bisected by both the upper and lower Transits, taking care at the same time that the axis keeps its horizontal position.

When the telescope is thus adjusted, a mark must be set up, or made, at a considerable distance (the greater the better) in the horizontal direction of the intersection of the cross wires, and in a place where it can be illuminated in the night-time by a lanthorn near it, which mark, being on a fixed object, will serve at all times afterwards to examine the position of the telescope, by first adjusting the tranverse axis by the level.

To adjust a clock by the sun's Transit over the meridian, note the times by the clock, when the preceding and following edges of the sun's limb touch the cross wires: the difference between the middle time and 12 hours, shews how much the mean, or clock time, is faster and slower than the apparent or solar time, for that day; to which the equation of time being applied, it will shew the time of mean noon for that day, by which the clock may be adjusted.