of the Bodies. Apparent point coincide both with each other, and with the Motions greater axis of the ellipse. After passing the apogee the radius vector of the point gets before that of the sun, Heavenly and forms with it angles exactly equal to the angles formed by the same lines in the former half of the ellipse, at the same distance from the perigee. At the perigee, the radius vector of the sun and of the point again coincide with each other, and with the greater axis of the ellipse. The angle which the radius vector of the sun makes with that of the point, which indicates how much the one precedes the other, is called Equation of the equation of the centre. It is always greatest when the centre the motions of the point and of the sun are equal, and explained. it vanishes altogether when there is the greatest difference between these motions. The angular motion of the point is called the mean motion, and that of the sun the real motion. The place of the point in the orbit is called the mean place. Now, if to the mean place in the orbit, we add or subtract the equation of the centre, it is obvious that we have the sun's real place for any given time. The angular motion of the point is known with precision for a given time, a day for instance, by ascertaining the exact length of time which the sun takes in making a complete revolution round its orbit. For if we ascertain how many days that revolution requires, we have only to divide the whole orbit by that number to prove the portion of it traversed by the point in one day. The equation of the centre can only be found by approximation. Its maximum in the year 1750 was 10.9268. 52 Signs of the zodiac. 53 Orbit varies In computations we begin always at that part of the orbit where the motion of the sun is slowest. The distance of the imaginary point from that part, is called the mean anomaly. A table is made of the equation of the centre, corresponding to each degree of the mean anomaly. By adding or subtracting these equations from the mean anomaly, we obtain the true anomaly or place of the sun for any given time. The ecliptic is usually divided, by astronomers, into 12 equal parts, called signs, each of which of course contains 30 degrees. They are usually called the signs of the zodiac; and beginning at the equinox, where the sun intersects and rises above the equator, have these names and marks: Aries Y, Taurus 8, Gemini II, Cancer, Leo, Virgo mg, Libra, Scorpio m, Sagittarius, Capricornus, Aquarius, Pisces X. Of these signs, the first six are called northern, lying on the north side of the equator; the last six are called southern, being situated to the south of the equator. The signs from Capricornus to Gemini are called ascending, the sun approaching or rising to the north pole while it passes through them; and the signs from Cancer to Sagittarius are called descending, the sun, as it moves through them, receding or descending from the north pole. The longitude of the sun is his distance in the ecliptic from the first point of Aries. His right ascension is the arch of the equator intercepted between the first point of Aries, and the meridian circle which passes through his longitude. The distance of the sun from the equator, measured upon a meridian circle, is called his declination, and it is either north or south according to the situation of the sun. It has been observed that the position of the larger in position. axis of the elliptical orbit of the sun, is not constant. 2. The angular distance of the perigee from the vernal Apparent equinox, counted according to the sun's movement, was 278°.6211 at the beginning of 1750; but it has, relative to the stars, an annual motion of about 11".89 in the same direction as the sun. The orbit of the sun is gradually approaching to the equator. Its obliquity diminishes in a century at the rate of about 1".50. The precision of modern astronomers has enabled them to ascertain small irregularities in the sun's elliptical motion, which observation alone would scarcely have been able to bring under precise laws. These irregularities will be considered afterwards. Motions of the Heavenly Bodies. 54 To determine the distance of the sun from the earth, Distance o has always been an interesting problem to astronomers, the sun. and they have tried every method which astronomy or geometry possesses in order to resolve it. The amplest and most natural, is that which mathematicians employ to measure distant terrestrial objects. From the two extremities of a base whose length is known, the angles which the visual rays from the object, whose distance is to be measured, make with the base, are measured by means of a quadrant; their sum subtracted from 180o, give the angle which these rays form at the object where they intersect. This angle is called the parallax, and when it is once known, it is easy, by means of trigonometry, to ascertain the distance of the object. Let AB, in fig. 6. be the given base, and C the object whose distance we wish to ascertain. The angles CAB and CBA, formed by the rays CA and CB with the base, may be ascertained by observation; and their sum subtracted from 180° leaves the angle ACB, which is the parallax of the object C. It gives us the apparent size of the base AB as seen from C. When this method is applied to the sun, it is necessary to have the largest possible base. Let us suppose two observers on the same meridian, observing at the same instant the meridian altitude of the centre of the sun, and his distance from the same pole. The difference of the two distances observed, will be the angle under which the line which separates the observers will be seen from the centre of the sun. The position of the observers gives this line in parts of the earth's radius. Hence, it is easy to determine, by observation, the angle at which the semidiameter of the earth would be seen from the centre of the sun. This angle is the sun's parallax. sun's parallax. But it is too small to be determined with precision by that method. We can only conclude from it, that the sun's distance from the earth is at least equal to 10,000 diameters of the earth. We shall find afterwards, that other methods have been discovered for finding the parallax with much greater precision. It amounts very nearly to 8".8: hence it follows, that the distance of the sun from the earth amounts to 23,405 semi-diameters of the earth. SECT. II. Of the Division of Time. Motion is peculiarly adapted for measuring time. For, as a body cannot be in different places in the same time, it can only arrive from one part to another, by passing successively through all the intermediate spaces. And if it be possible to ascertain, that in every point of the line which it describes it is actuated by the very same force, we can conclude with confidence, that it will of the Bodies. is the sun's motion for that day referred to the equator; Apparent and the time which that arc takes to pass the meridian Motions is equal to the excess of the astronomical day above the of the sidereal. But it is obvious, that at the equinoxes, the Heavenly will describe the line with a uniform motion. Of course Apparent Motions the different parts of the line will be a measure of the time employed to traverse them. When a pendulum Heavenly at the end of every oscillation is precisely in the same circumstances, the length of the oscillations is the same, and time may be measured by their number. We might employ also, for the same purpose, the revolutions of the heavenly sphere, which appear perfectly uniform. But all nations have agreed to employ the revolutions of the sun for that purpose. 55 Astronomical day. 56 Sidereal day. 57 In common language, the day is the interval of time which elapses from the rising to the setting of the sun; the night is the interval that the sun continues below the horizon. The astronomical day embraces the whole interval which passes during a complete revolution of the sun. It is the interval of time which passes from 12 o'clock at noon, till the next succeeding noon. It begins when the sun's centre is on the meridian of that place. It is divided into 24 hours, reckoning in a numerical succession from 1 to 24: the first 12 are sometimes distinguished by the mark P. M. signifying post meridiem, or after noon; and the latter 12 are marked A. M. signifying ante meridiem, or before noon. But astronomers generally reckon through the 24 hours. from noon to noon; and what are by the civil or common way of reckoning, called morning hours, are by astronomers reckoned in the succession from 12, or midnight, to 24 hours. Thus 9 o'clock in the morning of February 14th, is, by astronomers, called February the 13th at 21 hours. An astronomical day is somewhat greater than a complete revolution of the heavens, which forms a sidereal day. For if the sun cross the meridian at the same instant with a star, the day following it will come to the meridian somewhat later than the star, in consequence of its motion eastward, which causes it to leave the star; and after a whole year has elapsed, it will have crossed the meridian just one time less than the star. A sidereal day is less than the solar day, for it is measured by 360°, whereas the mean solar day is measured by 360° 59' 8" nearly. If an astronomical day be1, then a sidereal day is 0.997269722; or the difference between the measures of a mean solar day, and a sidereal day, viz. 59′ 8′′, reduced to time, at the rate of 24 hours to 360°, gives 3' 56"; from which we learn that a star which was on the meridian with the sun on one noon, will return to that meridian 3′ 56′′ previous to the next noon: therefore, a clock which measures mean days by 24 hours, will give 23 h. 56 m. 4 sec. for the length of a sidereal day. Days vary Astronomical or solar days, as they are also called, in length. are not equal. Two causes conspire to produce their inequality, namely, the unequal velocity of the sun in his orbit, and the obliquity of the ecliptic. The effect. of the first cause is sensible. At the summer solstice, when the sun's motion is slowest, the astronomical day approaches nearer the sidereal, than at the winter sol stice when his motion is most rapid. To conceive the effect of the second cause, it is necessary to recollect that the excess of the astronomical day above the sidereal is owing to the motion of the sun, referred to the equator. The sun describes every day a small arch of the ecliptic. Through the extremities of this arch suppose two meridian great circles drawn, the arc of the equator, which they intercept, arc of the equator is smaller than the corresponding arc of the eclptic in the proportion of the cosine of the obliquity of the ecliptic to radius: at the solstices, on the contrary, it is greater in the proportion of radius to the cosine of the same obliquity. The astronomical day is diminished in the first case, and lengthened in the second. Bodies. 58 To have a mean astronomical day, independent of Mean asthese causes of inequality, astronomers have supposed a tronomical second sun to move uniformly on the ecliptic, and today. pass over the extremities of the axis of the sun's orbit, at the same instant with the real sun. This removes the inequality arising from the inequality of the sun's motion. To remove the inequality arising from the obliquity of the ecliptic, astronomers suppose a third sun passing through the equinoxes at the same instant with the second sun, and moving along the equator in such a manner that the angular distances of the two suns at the vernal equinox shall be always equal. The interval between two consecutive returns of this third sun to the meridian forms the mean astronomical day. Mean time is measured by the number of the returns of this third sun to the meridian; and true time is measured by the returns of the real sun to the meridian. The arc of the equator, intercepted between two meridian circles drawn through the centres of the true sun, and the imaginary third sun, reduced to time, is what is called the equation of time. This will be rendered plainer by the following diagram. Let Zx (fig. 7.) be the earth; ZFR≈ its axis; abcde, &c. the equator; ABCDE, &c. the northern half of the ecliptic from V to, on the side of the globe next the eye; and MNOP, &c. the southern half on the opposite side from W to . Let the points at A, B, C, D, E, F, &c. quite round from to v again bound equal portions of the ecliptic, gone through in equal times by the real sun; and those at a, b, c, d, e, f, &c. equal portions of the equator described in equal times by the fictitious sun; and let Z be the meridian. As the real sun moves obliquely in the ecliptic, and the fictitious sun directly in the equator, with respect to the meridian; a degree, or any number of degrees, between and F on the ecliptic, must be nearer the meridian Z, than a degree, or any corresponding number of degrees, on the equator from to f; and the more so, as they are the more oblique: and therefore the true sun comes sooner to the meridian every day whilst he is in the quadrant F, than the fictitious sun does in the quadrant f; for which reason, the solar noon precedes noon by the clock, until the real sun comes to F, and the fictitious to f; which two points, being equidistant from the meridian, both suns will come to it precisely at noon by the clock. Whilst the real sun describes the second quadrant of the ecliptic FGHIKL from Cancer to, he comes later to the meridian every day than the fictitious sun moving through the second quadrant of the equator from ƒ to; for the points at G, H, I, K, and L, being farther from the meridian, their corresponding points at g, h, i, and, must be later of coming to it; D 2 and Apparent and as both suns come at the same moment to the point Motions W, they come to the meridian at the moment of noon of the by the clock. Heavenly Bodies. In departing from Libra, through the third quadrant, the real sun going through MNOPQ towards at R, and the fictitious sun through mnopq towards r, the former comes to the meridian every day sooner than the latter, until the real sun comes to O, and the fictitious to r, and then they come both to the meridian at the same time. Lastly, As the real sun moves equably through STUVW, from towards ; and the fictitious sun through stu vw, from r towards, the former comes later every day to the meridian than the latter, until they both arrive at the point, and then they make it noon at the same time with the clock. Having explained one cause of the difference of time shown by a well-regulated clock and a true sun-dial, supposing the sun, not the earth, as moving in the ecliptic; we now proceed to explain the other cause of this difference, namely, the inequality of the sun's apparent motion; which is slowest in summer, when the sun is farthest from the earth, and swiftest in winter when he is nearest to it. If the sun's motion were equable in the ecliptic, the whole difference between the equal time as shown by the clock, and the unequal time as shown by the sun, would arise from the obliquity of the ecliptic. But the sun's motion sometimes exceeds a degree in 24 hours, though generally it is less and when his motion is slowest, any particular meridian will revolve sooner to him than when his motion is quickest; for it will overtake him in less time when he advances a less space than when he moves through a larger. Now, if there were two suns moving in the plane of the ecliptic, so as to go round it in a year; the one describing an equal arc every 24 hours, and the other describing sometimes a less arc in 24 hours, and at other times a larger, gaining at one time of the year what it lost at the opposite; it is evident, that either of these suns would come sooner or later to the meridian than the other, as it happened to be behind or before the other; and when they were both in conjunction, they would come to the meridian at the same moment. As the real sun moves unequably in the ecliptic, let us suppose a fictitious sun to move equably in a circle coincident with the plane of the ecliptic. Let ABCD (fig. 8.) be the ecliptic or orbit in which the real sun moves, and the dotted circle a b c d the imaginary orbit of the fictitious sun; each going round in a year according to the order of letters, or from west to east. Let HIKL be the earth turning round its axis the same way every 24 hours; and suppose both suns to start from A and a, in a right line with the plane of the meridian EH, at the same moment: the real sun at A, being then at his greatest distance from the earth, at which time his motion is slowest; and the fictitious sun at a, whose motion is always equable, because his distance from the earth is supposed to be always the same. In the time that the meridian revolves from H to H again, according to the order of the lettersHIKL, the real sun has moved from A to F; and the fictitious with a quicker motion from n to f, through a large arc therefore, the meridian EH will revolve. 3 sooner from H to h under the real sun at F, than from Apparent HE to k under the fictitious sun at f; and consequent- Motions ly it will then be noon by the sun-dial sooner than by of the Heavenly Bodies. the clock. As the real sun moves from A towards C, the swiftness of his motion increases all the way to C, where it is at the quickest. But notwithstanding this, the fictitious sun gains so much upon the real, soon after his departing from A, that the increasing velocity of the real sun does not bring him up with the equally-moving fictitious sun till the former comes to C, and the latter to c, when each has gone half round its respective orbit; and then being in conjunction, the meridian EH, revolving to EK, comes to both suns at the same time, and therefore it is noon by them both at the same mo ment. But the increased velocity of the real sun now being at the quickest, carries him before the fictitious one; and therefore, the same meridian will come to the fictitious sun sooner than to the real: for whilst the fictitious sun moves from a to g, the real sun moves through a greater arc from C to G: consequently the point K has its noon by the clock when it comes to k, but not its noon by the sun till it comes to . And although the velocity of the real sun diminishes all the way from C to A, and the fictitious sun by an equable motion is still coming nearer to the real sun, yet they are not in conjunction till the one comes to A and the other to a, and then it is noon by them both at the same mo ment. True time is obtained by adding or subtracting this equation to the mean time. The mean and apparent solar days are never equal, except when the sun's daily motion in right ascension is 59' 8"; this is nearly the case about April 15th, June 15th, September 1st, and December 24th: on these days the equation is nothing, or nearly so; it is at the greatest about November 1st, when it is 16 m. 14 sec. 59 The return of the sun to the same equinox marks the Year. years, in the same way as his return to the same meridian indicates the days. It has been ascertained, that before the sun returns again to the same equinox, an interval of 365.242222 days elapses, or 365 days, 5 hours, 48 minutes, and 47 seconds. This is called the tropi cal year: The sun takes a larger interval of time to return again to the same star. The sidereal year is the interval which the sun employs to return from one star to another. It is greater than the tropical year by 0.014162 days, or 20 m. 23 sec.; therefore the length of the sidereal year is 365 days, 6 h. 9 m. and 10 sec. From this it follows, that the equinoxes do not retain the same place in the ecliptic, but that they have a retrograde motion, or contrary to that of the sun, in consequence of which they describe every year an arc equal to the mean space which the sun passes over in 20′ 23′′, or about 50"; so that they would make a complete revolution in 25972 years. This is called the precession of the equinoxes. бо the equa Dr Maskelyne has invented a rule for computing Method of the equation of time, in which the precession of the computing equinoxes, as well as the two causes mentioned above, tion of are included. Let APLQ, fig. 9. be the ecliptic, time. ALQ the equator, A the first point of Aries, P the point where the sun's apparent motion is slowest, 8 any place of the sun; draw Sv perpendicular to the equator, Heavenly Bodies. Apparent equator, and take A n=AP. When the sun begins Motions to move from P, suppose a star to begin to move from of the n, with the sun's mean motion in right ascension or longitude, viz. at the rate of 59′ 8′′ in a day, and when n passes the meridian let the clock be adjusted to 12. Take n m Ps, and when the star comes to m, if the sun moved uniformly with his mean motion, he would be found at s; but at that time let S be the place of the sun. Let the sun S, and consequently v, be on the meridian; and then as m is the place of the imaginary star at that instant, m v must be the equation of time. The sun's mean place is at s, and as An=AP, and nm=P s, we have Am=AP s, consequently mv Av—Am=Av-APs. Let a be the mean equinox, or the point where it would have been if it had moved with its mean velocity, and draw a z perpendicular to AQ; then Am = A x + x m = A a X cosine ≈ Aa+≈ m: or because the cosine of Aa the obliquity of the ecliptic, 23° 28′, is == II 12 II 12 cension, ≈m the mean right ascension or mean longitude; and A a (viz. A x) is the equation of the equinoxes in right ascension; therefore the equation of time is equal to the difference of the sun's true right ascension and his mean longitude, corrected by the equation of the equinoxes in right ascension.-When Am is less than Av, mean or true time precedes apparent; when it is greater, apparent time precedes mean. That is, when the sun's true right ascension is greater than bis mean longitude corrected as above shewn, we must add the equation of time to the apparent to obtain the mean time; and when it is less, we must subtract. To convert mean time into apparent, we must subtract in the former case, and add in the latter. 61 Smaller divisions of time. Tables of the equation of time are computed by this rule, for the use of astronomers: they are either calculated for the noon of each day, as given in the Nautical and some other almanacks; or for every degree of the sun's place in the ecliptic. But a table of this kind will not answer accurately for many years, on account of the precession and other causes, which render a frequent revisal of the calculations necessary. The smaller divisions of time were anciently meaeured by the phases of the moon. It is well known that the moon changes once every 29 or 30 days, and that the interval from one new moon to another is called a lunation, or, in common language, a month. There are about twelve lunations in a year. Hence the year was divided into twelve months. In ancient times people were placed upon eminences on purpose to watch the first appearance of the new moon, when their month began. It was customary for these persons to proclaim the first appearance of the moon. Hence the first day of every month was called Culenda; from which term the word calendar is derived. Almost all nations have divided the year into twelve months, because the seasons nearly return in that period. But they soon perceived that twelve lunar months were far from making a complete year or revolution of the sun. They were anxi Va of the ous, however, to be able to divide the solar year into Apparent a precise number of lunar months, because many of Motions the feasts depended upon particular new moons. Heavenly rious contrivances were fallen upon for this purpose Rodies. without much success, till at last Meton, a Greek philosopher, announced that 19 years contained exactly 235 lunations: an affirmation which is within 24 hours of being exact. To make every year correspond as nearly as possible to the lunar, he divided the year into 12 months, consisting alternately of 30 and 29 days each; at the end of every three years an intercalary month of 30 days was added, and at the end of the 19th year there was added an intercalary month of 29 days. So that at the end of 19 years the solar and lunar years began again on the same day their cycle of 19 years. This discovery of Meton appeared so admirable to the Greeks, that they engraved it in letters of gold in their public places. Hence the number which denotes the current year of that cycle is denominated golden number. As the moon changes its appearance in a very remarkable degree every seven days, almost all nations have subdivided the month into periods of seven days, called weeks; the ancient Greeks were almost the only people who did not employ that division. year com 62 The Roman year in the time of Romulus consisted Roman length was 304 days only. Numa added 50 days to of 10 months only, of 30 or 31 days each, so that its year. that year, and thus made it 354 days; and he added two additional months of 29 and 28 days by shortening some of the ancient months. He made the mence on the first of January. Numa's year was still more than 11 days shorter than a complete revolution of the sun. To make it correspond with the seasons, it was necessary to intercalate three days; and these intercalations being left entirely to the priests, were converted into a state engine; being omitted, inserted, altered, and varied, as it suited the purposes of those magistrates whose views they favoured. The consequence was, what might have been expected, the most complete confusion and want of correspondence between the year and the seasons. 63 Julius Caesar undertook to remedy this inconvenience. He was both dictator and high pontiff, and of course the by Julins reformation of the calendar was his peculiar province. Cesar, That the undertaking might be properly executed, he invited Sosigenes, an Egyptian mathematician, to come to his assistance. It was agreed upon to abandon the motions of the moon altogether, and to make the year correspond with those of the sun. The reformation was made in the year 47 before the Christian era. Ninety days were added to that year, which was from that circumstance called the year of confusion, consisting of 445 days. Instead of 354 days, the year of Numa, Sosigenes made the year to consist of 365 days, dispersing the additional days among those months which had only 29 days. As the revolution of the sun employs nearly six hours more than 365 days, an additional day was intercalated every fourth year, so that every such year was to consist of 366 days. The additional day was inserted after the 23d of February, or the 7th before the calends of March; the day before the annual feast celebrated in commemoration of the flight of Tarquin from Rome. That feast was held the 6th before the calends of March. The Apparent The intercalated day was also called the 6th before the Motions calends of March. So that every fourth year there. of the were two days denominated the 6th before the calends Heavenly of March. Hence that year was called bissextile. In Bodies. 64 and by Gregory XIII Britain it is denominated leap year. After the death of Julius Cæsar there was a degree of confusion respecting the intercalations, from the ignorance of the priests. Augustus corrected the mistake, and after that time the Julian period went on without any interruption. It is obvious that the Julian year, though a great improvement upon the ancient Roman, was still imperIt went on the supposition that the revolution of the sun occupied precisely 365 days and 6 hours, which is about II minutes more than the truth. This error in the interval which elapsed between the reformation of Julius Cæsar and the year 1582, had accumulated till it amounted to 10 days; of course the year began 10 days later than it ought to have begun; and the same error had taken place respecting the seasons and the equinoctial points. Various attempts had been made to correct this error; at last it was corrected by Pope Gregory XIII. The Gregorian calendar commenced in the year 1582; the changes which he intro duced were two in number. He ordered, that after the 4th of October 1582, ten days should be omitted, so that the day which succeeded the 4th was reckoned not the 5th but the 15th of the month. This corrected the error which had crept into the Julian year. To prevent any such error from accumulating again, he ordered that the secular years 1700, 1800, 1900, should not be bissextile but common years; that the secular year 2000 should be bissextile, the next three secular years common, the fourth again bissextile, and so on, as in the following table. 1600 bissextile. 1700 common. 2000 bissextile. 2100 common. 2300 ib. 2600 common. 2700 ib. 2800 bissextile. 2400 bissextile. 2900 common. 2500 common. 3000 ib. In short these secular years only are bissextile whose number, omitting the cyphers, is divisible by 4. The Gregorian calendar is sufficiently exact for the purposes of common life, though it does not correspond precisely with the revolution of the sun. The error will amount to a day in 3600 years, so that in the year 5200 it will be necessary to omit the additional day which ought to be added according to the rule laid down above. The Gregorian calendar was immediately adopted by all the Roman Catholic kingdoms in Europe, but the Protestant states refused at first to accede to it. It was adopted by most of them on the continent about the beginning of the 18th century; but in England the change did not take place till 1752. From that year 11 days were omitted; the omission of the additional day in 1700 having made the difference between the Julian and Gregorian calendar amount to 11 days. The Julian calendar is called the old style, the Gregorian, the new style. At present the difference between them is 12 days, in consequence of the omission of the additional day in 1800. SECT. III. Of the Nature of the Sun. The smallness of the sun's parallax is a demonstration of its immense size. We are certain that at the distance at which the sun appears to us under an angle of 0.53424, the earth would be seen under an angle not exceeding o°.009. Now, as the sun is obviously a spherical body as well as the earth; and as spheres are to each other as the cubes of their diameters, it follows from this, that the sun is at least 200,000 times bigger than the earth. By the exactest observations it has been ascertained, that the diameter of the sun is nearly 883,000 miles. Apparen Motion of the Heaven! Bodies. Dark spots are very frequently observed upon the surface of the sun. These were entirely unknown be- • fore the invention of telescopes, though they are sometimes of sufficient magnitude to be discerned by the naked eye, only looking through a smoked glass to prevent the brightness of the luminary from destroying the 65 sight. The spots are said to have been first discovered Solar spots in the year 1611; and the honour of the discovery is when first disputed betwixt Galileo and Scheiner, a German Jesuit discovered. at Ingolstadt. But whatever merit Scheiner might have in the priority of the discovery, it is certain that Galileo far exceeded him in accuracy, though the work of Scheiner has considerable merit, as containing observations selected from above 3000, made by himself. Since his time the subject has been carefully studied by all the astronomers in Europe. 66 There is great variety in the magnitudes of the Dr Long's solar spots; the difference is chiefly in superficial ex-account of tent of length and breadth; their depth or thickness them. is very small; some have been so large, as by computation to be capable of covering the continents of Asia and Africa; nay, the whole surface of the earth, or even five times its surface. The diameter of a spot, when near the middle of the disk, is measured by comparing the time it takes in passing over a cross hair in a telescope, with the time wherein the whole disk of the sun passes over the same hair; it may also be measured by the micrometer; and by either of these methods we may judge how many times the diameter of the spot is contained in the diameter of the sun. Spots are subject to increase and diminution of magnitude, and seldom continue long in the same state. They are of various shapes; most of them having a deep black nucleus surrounded by a dusky cloud, whereof the inner parts near the black are a little brighter than the outskirts. They change their shapes, something in the manner that our clouds do; though not often so suddenly; thus, what is of a certain figure to-day, shall to-morrow, or perhaps in a few hours, be of a different one; what is now but one spot, shall in a little time be broken into two or three; and sometimes two or three spots shall coalesce, and be united into one. Dr Long, many years since, while he was viewing the image of the sun through a telescope cast upon white paper, saw one roundish spot, by estimation not much less than the diameter of our earth, break into two, which receded from one another with prodigious velocity. This observation was singular at the time; for though several writers had taken notice of this after it was done, none of them had been making any observation at the time it was actually doing. The |