ASTRONOMY. 



bit. But at present this is done with 

 greater precision, by observing every day 

 the height of the sun when it reaches the 

 meridian, and the interval of time which 

 elapses between his passing the meridian 

 and that of the stars. The first of these 

 observations gives us the sun's daily mo- 

 tion northward or southward, in the di- 

 rection of the meridian ; and the second 

 gives us his motion eastward in the direc- 

 tion of the parallels ; and by combining 

 the two together we obtain his orbit. The 

 height of the sun from the horizon, when 

 it passes the meridian, on the arch of the 

 meridian between the sun and the horizon, 

 is called the sun's altitude. The ancients 

 ascertained the sun's altitude in the fol- 

 lowing manner: They erected an up- 

 right pillar at the south end of a meridi- 

 an line, and when the shadow of it exact- 

 ly coincided with that line, they accurate- 

 ly measured the shadow's length,and then, 

 knowing the height of the pillar, they 

 found by an easy operation in plain trigo- 

 nometry, the altitude of the sun's upper 

 limb, whence, after allowing for the appa- 

 rent semidiameter, the altitude of the 

 sun's centre was known. But the methods 

 now adopted are much more accurate. 

 In a known latitude, a large astronomical 

 quadrant, of six, eight, or ten feet radius, 

 is fixed truly upon the meridian ; the 

 limb of this quadrant is divided into mi- 

 nutes and smaller subdivisions, by means 

 of a vernier, and it is furnished with a te- 

 lescope, having cross hairs, &c. turning 

 properly upon the centre. By this instru- 

 ment the altitude of the sun's centre is 

 very carefully measured, and the proper 

 deductions made. The orbit in which the 

 sun appears to move is called the eclip- 

 tic. It does not coincide with the equa- 

 tor, but cuts it, forming with it an angle, 

 which in the year 1769, was determined 

 by Dr. Maskelyne at 23 28' 10" or 

 23.46944. This angle is called the obli- 

 quity of the ecliptic. 



It is known that the apparent motion 

 of the sun in its orbit is not uniform. 

 Observations made with precision, have 

 ascertained, that the sun moves fastest in 

 the point of his orbit situated near the 

 winter solstice, and slowest in the oppo- 

 site point of his orbit near the summer 

 solstice. When in the first point, the sun 

 moves in 24 hours 1.01943 ; in the se- 

 cond point he moves only 0.953l9. The 

 daily motion of the sun is constantly va- 

 rying in every place of its orbit between 

 these two points. The medium of the 

 two is .98632, or 59' II' 7 , which is the 

 daily motion of the sun about the begin- 



ning of October and April. It has been 

 ascertained, that the variation in the an- 

 gular velocity of the sun is very nearly 

 proportional to the mean angular distance 

 of it from the point of its orbit where its 

 velocity is greatest. It is natural to think, 

 that the distance of the sun from the earth 

 varies as well as its angular velocity. 

 This is demonstrated by measuring the 

 apparent diameter of the sun. Its dia- 

 meter increases and diminishes in the 

 same manner, and at the same time, with 

 its angular velocity, but in a ratio twice 

 as small. In the beginning of January, 

 his apparent diameter is about 32' 39", 

 and at the beginning of July it is about 

 31' 34", or more exactly, according to 

 De la Place, 32' 35" = 1955" in the'first 

 case, and 31' 18" = 1878" in the second. 

 Opticians have demonstrated, that the 

 distance of any body is always reciprocal- 

 ly as its apparent diameter. The sun must 

 follow the same law; therefore its distance 

 from the earth increases in the same pro- 

 portion that its apparent diameter dimi- 

 nishes. In that point of the orbit in which 

 the sun is nearest the earth, his apparent 

 diameter is greatest, and his motion swift- 

 est; but when lie is in the opposite point 

 both his diameter and the rapidity of his 

 motion are the smallest possible. 



To determine the distance of the sun 

 from the earth has always been an inte- 

 resting problem to astronomers, 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 mea- 

 sured by means of a quadrant ; their sum 

 subtracted from 180 gives the angle 

 which these raysform 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 ascer- 

 tain the distance of the object. Let A B, 

 in fig. 4, be the given base, and C the ob- 

 ject whose distance we wish to ascertain. 

 The angles CAB and C B A, formed by 

 the rays C A and C B with the base, may 

 be ascertained by observation ; and their 

 sum subtracted from 180 leaves the an- 

 gle A C B, which is the parallax of the 

 object C. It gives us the apparent size 

 of the base A B, as seen from C. When 

 this method is applied to the sun, it is ne- 

 cessary to have the largest possible base. 

 Let us suppose two observers on the 



