COMMON THINGS TIME. 



necessaiy to define what is meant by the meridional transit of th e 

 sun with more precision, and to show by what sort of observation 

 the moment of such transit can be ascertained. 



It has been agreed, that by the meridional transit of the sun, 

 that of the centre of the solar disc is to be understood.' But as this 

 centre is not marked by any visible or observable point by which 

 it can be distinguished from other points of the sun's disc, its 

 transit cannot be directly observed. 



The difficulty arising from this circumstance has been overcome 

 by a very simple expedient. 



As the solar disc enters the field of view from the east side it 

 approaches gradually the meridional wire, N s, and at length 

 touches it, as shown in fig. 2, with its western edge, w, cr 

 LIMB, as it is called by astronomers. The moment of this contact 

 is observed in the manner already described in the case of 

 a star. The solar disc then continues to move across the field 

 until it takes the position indicated by the dotted circle, in. 

 which the eastern limb touches the meridional wire, K s. The 

 moment this takes place being also observed, the middle of the 

 interval is calculated, which is the instant at which the centre of 

 the disc passed the meridian.* 



32. If the sun were stationary in the firmament, it is evident 

 that the interval between its successive meridional transits would 

 be the same as that of the successive transits of a fixed star, and 

 in that case the SIDEREAL DAY would be identical with the SOLAR 

 DAY. But it is well known that the sun is not thus fixed. On 

 the contrary, it moves constantly in the firmament, making a 

 complete circuit of the heavens in a year. If this motion were 

 uniform, the daily displacement of the sun would be 59' 8-2". 



Now let us consider what effect such a displacement, being 

 always eastward, would produce upon the interval between the 

 successive transits of the sun compared with that of the transits of 

 a star which suffers no such displacement. 



Let s (fig. 3) represent the sun at the moment its centre is on the 

 meridian, :N s, on any given day, and let o represent a fixed star 

 which is on the meridian at the same instant. After the lapse of 

 24 sidereal hours the star, o, will be again upon the meridian, N s ; 

 but during these 24 hours the sun, s, will have moved towards ];, 

 that is, eastward to the position, s', the distance, s s', being 



* In fig. 2 the motion of the celestial objects and their position is repre- 

 sented as they are seen by the naked eye, or by a terrestrial telescope. 

 But all objects are inverted and reversed by the astronomical telescope, 

 so that the top is seen at the bottom, and the east seen at the west, and 

 vice versa. It has been thought better for the present purpose to represent 

 the points and motions as they are naturally seen. 

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