THE MINOR PLANETS. 147 



the sun is never vertical, on the one hand, nor, on the other hand, is ever absent 

 for twenty-four successive hours. How different must be the circumstances at- 

 tending the planet Venus, if it be true, as there seems reason to believe, that the 

 axis of that planet, instead of being inclined 23^ from the perpendicular, is 

 inclined 75 from it. The polar circles would include a portion of each hem- 

 isphere, the extent of which would be five sixths of its entire breadth. Thus 

 the greater portion of such a globe would be subject to vicissitudes somewhat 

 similar to those which are incidental to our frigid zone, but the changes would ( 

 be much more complicated. Within a certain space of such a planet, the sun ] 

 would at one season of the year pass through the zenith, and the circumstances 

 of the day would resemble those between our own tropics ; while at another pe- 7 

 riod of the year, the sun would never rise for twenty-four hours. In fact, the ! 

 polar circle would overlay the tropics, and the phenomena of each zone would 

 alternately prevail at different seasons. 



The position of the axis of Mercury is not ascertained, but there is reason 

 to believe that, like that of Venus, it is inclined at a very large angle from the 

 perpendicular. 



ORBITS AND TRANSITS OF MERCURY AND VENUS. 



The motion of the planets Mercury and Venus, like that of the other bodies 

 of the system, is very nearly in the plane of the ecliptic. The orbit of Mer- 

 cury makes with the piane of the ecliptic an angle of 7, and that of Venus 

 an angle of less than 4 ; the consequence of which is, that these planets are 

 never seen much above or below the ecliptic. The apparent diameter of the 

 sun is about half a degree ; consequently the greatest distance to which Venus 

 can depart from the ecliptic, will be less than eight diameters of the sun ; and 

 the greatest distance of the planet Mercury from it will be fourteen diameters 

 of the sun. The points at which these planets are seen upon the ecliptic are 

 called the NODES of the orbits ; and if at the time they pass near these nodes 

 they happen to be in inferior conjunction, they may be directly between the 

 eye of the observer on the earth and the sun's disk. In that case, they would 

 be seen as a black spot moving in the sun's disk. In order that this remarka- 

 ble phenomenon, which is called a transit, should take place, it is obviously 

 necessary that the distance of the disk of the planet from the place of the sun's 

 centre should be less than half the sun's apparent diameter ; that is, less than 

 fifteen minutes of a degree. If, then, the distance of either of the inferior 

 planets from the ecliptic at the time they are in inferior conjunction be less than 

 fifteen minutes, there must be a transit ; and the less that distance is, the greater 

 the extent of the sun's disk over which the planet will be seen moving. If the < 

 planet be exactly in its node at the time of the inferior conjunction, then it will ) 

 / be passing directly across the centre of the sun. 



It will be evident that the part of the sun's disk in which the planet is seen j 

 i projected in a transit, will also depend on the position of the observer upon the 

 I earth. It may happen that, from some parts of the earth, the planet would not 

 | be projected upon the solar disk at all ; and, in short, at different parts of the 



! earth, the line of its projected course will necessarily be different. These 

 effects will depend on the extent of the earth, and its distance from the sun 

 and the planet. 



These phenomena have, therefore, supplied a very happy expedient by 



which the distance of the sun from the earth may be exactly ascertained. The- 



transit of Venus is especially applicable to this investigation, and has been 



used with signal success. When the transit of the planet occurred in 1765, 



) observers were sent by different European governments to the most favorable ( 



