xlvlii INTRODUCTION TO ASTRONOMY. 



to the earth ; and yet, though both the longest days and the most per- 

 pendicular rays are on the 21st of June, the greatest heat prevails in July 

 and August. To account for this, you must reflect that those parts of the 

 earth which are once heated retain the heat for a considerable length of 

 time ; and the additional quantity they receive occasions an elevation of 

 temperature, although the days begin to shorten, and the sun's rays to 

 fall more obliquely. For the same reason, we have generally more heat 

 at three o'clock in the afternoon than at twelve, when the sun is on the 

 meridian. As long as the sun continues to communicate more heat than 

 the earth parts with in a given time, so long the heat of the earth will 

 increase, even though the rate at which it receives new heat from the sun 

 is diminished. 



The vicissitudes of seasons of the other planets vary according as their 

 axes deviate more or less from the perpendicular to the plane of their 

 orbits. The axis of Jupiter is nearly perpendicular to the plane of his 

 orbit; those of Mars and of Saturn are each inclined at angles of about 

 60 degrees; whilst the axis of Venus is believed to be elevated only 15 or 

 20 degrees above her orbit: the vicissitudes of her seasons must therefore 

 be considerably greater than ours. 



There is one more observation to make relative to the earth's motion, 

 which is, that although we have but 365 days and nights in the year, she 

 performs 366 complete revolutions on her axis during that time. This is 

 owing to the progressive motion of the earth in its orbit whilst it revolves 

 on its axis : as it advances almost a degree westward in its orbit, in the 

 same time that it completes a revolution eastward on its axis, it must 

 revolve nearly one degree more in order to bring the same meridian back 

 to the sun. These small daily portions of rotation are each equal to the 

 three hundred and sixty-fifth part of a circle, which at the end of the year 

 amounts to one complete rotation. If the earth, then, had no other than 

 its diurnal motion, we should have 366 days in the year ; or rather, we 

 should have 366 days in the same period of time that we now have 365 ; 

 for if we did not revolve round the sun, we should have no natural means 

 of computing* years. If time be calculated by the stars instead of the sun, 

 the irregularity which we have just noticed does not occur, and that one 

 complete rotation of the earth on its axis brings the same meridian back 

 to any fixed star ; and yet the earth's advance in her orbit must change 

 her position with regard to the fixed stars, as well as with regard to the 

 sun ! This difficulty is explained by the distance of the fixed stars, which 

 is so immense, that our solar system is in comparison to it but a spot, 

 and the whole extent of the earth's orbit but a point ; therefore, whether 

 the earth remained stationary, or whether it revolved in its orbit during 

 its rotation on its axis, no sensible difference would be produced with 

 regard to the fixed stars. One complete revolution brings the same 

 meridian back to the same fixed star : hence the fixed stars appear to go 

 round the earlh in a shorter time than the sun by three minutes sixty- 

 six seconds of time, the time which the earth takes to perform the addi- 

 tional three hundred and sixty-fifth part of the circle, in order to bring 

 the same meridian back to the sun. Hence the stars gain every day three 

 minutes fifty-six seconds on the sun, which makes them rise that portion 

 of time earlier every day. 



When time is calculated by the stars, it is called sidereal time ; when 

 by the sun, solar or apparent time ; and a sidereal day is three minutes 

 fifty-six seconds shorter than a solar day of twenty-four hours. The 

 difference of the solar and the sidereal year must also be explained : the 



