THE MINOR PLANETS. 



151 



ual fading away of the light into the darkness, produced by the band of atmo- 

 sphere illuminated by the sun which overhangs a part of the dark hemisphere, 

 and produces upon it the phenomena of twilight. 



When we examine the dark hemisphere of the planet Venus, there is ob- 

 served upon occasions a faint reddish and grayish light, which is visible on 

 parts too distant from the illuminated hemisphere to be produced by the light 

 of the sun. It is supposed that these effects are indications of the play of some 

 atmospheric phenomena in this planet similar to the aurora borealis. 



OF THE PLANET MARS. 



Proceeding outward in the solar system from the sun, the first planet which 

 we find revolving beyond the earth and including the annual path of the earth 

 within its periodical course is the planet MARS. This body makes its revolu- 

 tion round the sun at a distance of nearly one hundred and fifty millions of 

 miles from that luminary, and completes its revolution in six hundred and eighty- 

 six days, or a little less than two years. 



When the earth is between Mars and the sun, the distance of the planet 

 from the earth is less than fifty millions of miles, and as it is then seen in the 

 meridian at midnight, the circumstances are extremely favorable to telescopic 

 observation. Although its distance from the earth at that epoch is greater than 

 that of Venus when near inferior conjunction, yet as Venus in that position 

 has her dark hemisphere turned to the earth, while the enlightened hemisphere 

 of Mars is turned fully toward us, the observations made on the latter are more 

 satisfactory. 



The diameter of Mars is about half that of our globe, and it has been found 

 by the observations of Arago that its polar diameter is little less than its equa- 

 torial, and that consequently, like the earth, it is an oblate spheroid. 



As the planet includes the orbit of the earth within its periodical course 

 round the sun, the hemisphere which it presents to the sun is always very 

 nearly, although not exactly, presented to the earth ; the consequence of which 

 is that Mars is always seen with a full phase, or very slightly gibbous. It has 

 the appearance of a reddish star 



DIURNAL ROTATION OF MARS. 



On examining with a sufficiently powerful telescope the disk of Mars, it is 

 found to be characterized by features of lights and shadows, like those which 

 prevail on the other planets. These were observed at a very early period in 

 the progress of astronomical discovery. There are diagrams given in the first 

 volume of the "Philosophical Transactions," showing telescopic views of this 

 planet. 



By attentively watching these marks, they have been observed to move in 

 parallel lines east and west to disappear at one side of the disk, and to re- 

 appear after equal intervals at the other side. Hence it was discovered at a 

 very early epoch by CASSINI that Mars has a diurnal motion upon its axis in a 

 time very little different from that of the earth. Cassini's estimation of the 

 time of rotation of this planet was twenty-four hours and forty minutes. A 

 more accurate estimate proves it to be twenty-four hours thirty-nine minutes, 

 and twenty-one seconds. The axis on which it turns, and which is perpen- 

 dicular to the lines in which the marks on the disk move, is at an angle of 

 about thirty degrees from the perpendicular to its orbit. Wher it is remem- 

 bered that the earth's axis is inclined at an angle of twenty-three and a half 

 degrees, and that it is this inclination which produces the succession of sea- 



