SUN 



803 



Early observations of the sun were necessarily 

 confined to records of its motions and eclipses, of 

 which a very fair mastery was gained even in 

 Chaldtean and Egyptian times, as well as early 

 in the history of China (see ASTRONOMY). The 

 apparent motions of the sun, determining as they 

 do what part of our world shall at any time receive 

 his heat and light more or less abundantly, are so 

 regular and so important to our life that they natur- 

 ally give us our principal time measures (see DAY, 

 YEAR, SEASONS). For long the observation of 

 these formed perhaps the chief part of astronomy. 

 But when Copernicus showed that the sun was 

 really the centre of our system, and Galileo dis- 

 covered the moons of Jupiter, the idea of a com- 

 munity of nature between the sun and our world 

 the earth circling around the sun as the moons 

 around Jupiter began to take firm root in men's 

 minds. Newton's extension of the law of gravi- 

 tation to the heavenly bodies greatly aided this 

 process. The idea that the sun shone because 

 composed of mysterious fiery elements faded away, 

 and men began to ask after its real constitution, 

 and seek the secret of its stores of energy. But 

 to answer this question required much preliminary 

 investigation, and to trace this, so far as it has 

 gone, is to track some of the best and purest 

 triumphs of human patience and skill. 



( 1 ) The sun's distance was the first problem to 

 be attacked. In ancient times Aristarchus of 

 Samoa tried to solve this by measuring the angle be- 

 tween the sun and moon when the latter was in her 

 quarters (see MOON). This method, even if accur- 

 ately followed, would give no absolute measure, 

 but only the relation between the distances of the 

 sun and moon. From his attempts Aristarchus 

 concluded the sun to be eighteen times as far from 

 us as the moon. In reality his method is one which 

 can give no accurate result, though it represents 

 a great step in astronomical investigation. As 

 instruments improved, and especially when the 

 telescope was invented, new measures were made, 

 only to result in the conviction that the sun was so 

 far away that accurately to measure its distance ap- 

 peared impossible. The distance of celestial objects 

 is found oy the measurement of their Parallax 

 (q.v.). If an observer changes his own position, 

 all the objects around him appear also to shift 

 their relative positions, those nearer shift more 

 than distant ones, and by the amount of shift for 

 a known change of the observer's place their dis- 

 tance may be calculated. The greater the distance 

 between the observer's two positions the greater 

 (and therefore more easily measurable) is the 

 apparent shift of the objects before him. It was 

 found ere long that no change'of place possible on 

 our small earth, 8000 miles in diameter, was suffi- 

 cient to produce a definitely measurable change in 

 the sun's position on the celestial sphere. By an 

 opposition of Mars (see below) observed in 1672 

 by Richer at Cayenne and Cassini at Paris this 

 am'iilar change of place (or parallax) was given 

 at 9"'5 = a distance of 87,000,000 miles. Flam- 

 steed, by the same method, reached a parallax of 

 10" = 81 700,000 miles, Picard's measure was paral- 

 lax 20" = 41,000,000 miles, and Lahire's 136,000,000 

 miles. 



At last, in 1716, the English astronomer Halley 

 proposed a method of employing the transits of 

 Venus. Accordingly the transits of 1761 and 1769 

 were observed in a variety of places ; but the 

 results at first deduced were discordant and un- 

 satisfactory, until in 1824 the German astronomer 

 Encke 'discussed' the observations of 1769, and 

 arrived at a distance of about 95,300,000 miles ; 

 and this number held its place in books of as- 

 tronomy for a good many years. A transit can 

 occur only when the planet is in or near one of 



her nodes at the time of inferior conjunction, so 

 as to be in a line between the earth and the sun. 

 The coincidence of these two conditions follows 

 a rather complex law. There are usually two 

 transits within eight years of one another, and then 

 a lapse of 105 or 122 years, when another couple of 

 transits occur, with eight years between them. 

 The transit of 1874 had for its successor that of 

 1882, and there will not be another until June 

 2004. The way in which a transit is turned to 

 account may be understood by tlie help of fig. 1, 

 where E represents the earth, V Venus, and S 

 the sun. It is to be premised that the relative 

 distances of the planets from the sun are well 

 known. Their periodic times can be observed with 

 accuracy, and from these by Kepler's (q.v.) Law 

 we can deduce the proportions of the distances, 

 but not the distances themselves. _It is thus 

 known that, if the distance of the earth from the 

 sun is taken as 100, that of Venus is 72. In the 

 fig., then, AV is 28, or about one-third of Va or V6. 

 An observer at a station, A, on the northern part 

 of the earth will see the planet projected on the 

 sun as at a, while a southern observer will see it 

 at b.. The distance of the sun from Venus being 

 about three times her distance from the earth, it is 



Kg. 1. 



obvious that the distance db will be three times 

 the distance AB ; and it is a great advantage to 

 have the stations A, B, as far apart as possible, as 

 the interval ab is thus increased, and its measure- 

 ment rendered more accurate. 



But how is it measured ? For each observer sees 

 only one of the spots, and does not know where the 

 other is ; and there are no permanent marks on the 

 sun's surface to guide us. The difficulty is got over 

 in the following way. Each observer notes the 

 exact duration of the transit i.e. the time the 

 spot takes to travel from C to D, or from F to G. 

 Now as we know the rate of Venus' motion in her 

 orbit, this gives us the length of the lilies CD and 

 FG in minutes and seconds of arc. Knowing then 

 the angular diameter of the sun (32') and the 

 lengths of two chords CD and FG, we can easily, 

 by the properties of the circle, find the distance ab 

 between them. This gives us the angle aAb. In 

 the triangle AV6, then, we know the angle at A 

 and the proportion of the sides AV and V6, and 

 from that we can find the angle AftV and A6B. 

 Now this is the quantity sought, being the parallax 

 of the sun as seen from two stations on the earth. 

 Whatever the distance AB actually is, the angle is 

 reduced to correspond to a distance equal to the 

 earth's semi-diameter. The parallax deduced by 

 Encke, as above referred to, was only 8""5776. The 

 advantage of this roundabout procedure is that a 

 comparatively large angle (aAb) is measured in 

 order to deduce from it a smaller (A6B), so that 

 any error in the measurement is diminished in the 

 result. 



Meanwhile during the later part of the 18th 

 century efforts had been made by Dr Stewart of 

 Edinburgh (1763) and Mayer of Gottingen to deter- 

 mine the sun's distance by the lunar ' parallactic 

 inequality ' (see MOON). These amounted to little 

 until Laplace (q.v.) solved the problem and gave a 

 result hardly different from Encke's. In 1854 and 

 1858, however, Hansen and Leverrier found reason 

 to doubt its correctness. A favourable opposition 



