ON PRACTICAL ASTRONOMY. 431 



altitudes observed at different times in the same day, noticing accurately 

 the interval of time that elapses between the observations. This method 

 has a great advantage in cloudy weather, when it is not possible to ensure 

 an observation of a meridian altitude. 



The longitude of a place, or the relative position of its meridian, is by no 

 means so readily determined. For this purpose it becomes necessary to 

 ascertain the time that elapses between the passages of a given point in the 

 heavens over its meridian and some other meridian which serves as a 

 standard of comparison. Thus, if the sun arrives three hours later at the 

 meridian of any place than at the meridian of London, that place must 

 necessarily be 45 degrees west of London, or in 45 west longitude : and if 

 we know, when it is noon at the given place, that it is three o'clock in the 

 afternoon at Greenwich, we may be certain that we are in some part of a 

 meridian 45 west of that of Greenwich. Had we perfect timekeepers, we 

 might easily adjust them to the time of our first meridian, and then, by 

 comparison with the usual determinations of time in any other place, to 

 which they might be carried, the longitude of this place might be found 

 with perfect accuracy. Such timekeepers as we have are indeed suffi- 

 ciently correct, to be of considerable utility, but it is necessary to compare 

 them frequently with astronomical observations of phenomena, which occur 

 at times capable of a correct calculation. Sometimes the transits of Mer- 

 cury and Venus, or the eclipses of the moon, are employed for this purpose, 

 but more usually the eclipses of the satellites of Jupiter ; these, however, 

 cannot be well observed without a more powerful telescope than can be 

 employed at sea ; and the theory of the moon's motion, has of late years 

 been so much improved, that her distance from the sun or from a fixed 

 star can be calculated, with sufficient accuracy, for determining the time in 

 London or at Paris without an error of one third of a minute ; so that 

 supposing the observation could be rendered perfectly correct, the longitude 

 might be thus ascertained within about one twelfth of a degree, or at most 

 five nautical miles. 



The observed parallax of the sun and moon may be employed for the 

 determination of their distances from the earth ; but in the case of the sun, 

 the simple comparison of his calculated with his apparent altitude is in- 

 sufficient for ascertaining the magnitude of the parallax with accuracy. 

 Sometimes the parallax of Mars, which is considerably greater than the 

 sun's, has been directly measured ; but the most correct mode of ascer- 

 taining the actual dimensions of the solar system is, to observe a transit 

 of Venus over the sun's disc, at two places situated in opposite parts of 

 the earth's surface. For, since the diurnal motion of some parts of the 

 earth is directed the same way with the motion of Venus in her orbit, 

 and that of others the contrary way, the different effects of these motions 

 must furnish a mode of comparing the rotatory velocity of the earth, 

 with the progressive velocity of Venus, and consequently of inferring, 

 from the known velocity with which the earth's surface revolves, the 

 actual velocity of Venus, and her distance from the sun ; whence the 

 distances of all the other planets may be readily deduced. (Plate XXXV. 

 Fig. 514.) 



