Page:The American Cyclopædia (1879) Volume XI.djvu/203

 MARS 191 so much as the god of war, as of the tumult, horror, and carnage of battle. He is the im- personation of physical strength, delighting in the slaughter of men and the sack of towns. Though worshipped in all parts of Greece, no Greek city regarded him as its tutelary deity. According to the ancients, he was worshipped, among the warlike tribes of Thrace and the barbarians of Scythia. He had a temple at Athens containing a statue of him by Alca- menes, and at Sparta there was a statue of him in chains, signifying that the martial spirit would never leave the city. Women were not allowed to participate in his worship. MARS, the fourth planet in order of distance from the sun, and the nearest to us of the su- perior planets, that is, of the planets whose or- bits lie outside that of the earth. Mars travels around the sun in a mean sidereal period of 686-9797 days, on an orbit inclined 1 51' to the plane of the ecliptic, at a mean distance of 139,311,000 m. from the sun; but this orbit is considerably eccentric, insomuch that his great- est distance, 152,304,000 m., exceeds his least, 126,318,000 m., by more than 26,000,000 m. He returns to opposition at intervals separated by a mean period of 779-936 days, which is there- fore the planet's mean synodical period. The earth's mean distance being 91,430,000 m., the mean distance of Mars from the earth at the time when the two planets are in conjunction is about 48,000,000 m. But at a conjunction when Mars is near his perihelion, the distance is much reduced, more indeed than by the 13,000,000 m. by which the perihelion distance of Mars is less than his mean distance. For the perihelion of the orbit of Mars lies in Ion. 333|, while the aphelion of the earth's orbit lies in Ion. 280^, so that the earth is about 53 from aphelion when in the same longitude as the perihelion of Mars, and is therefore at a distance from the sun considerably exceeding her mean distance. Without entering at any length, however, into niceties of the sort (which would be idle, since conjunctions do not hap- pen exactly when Mars is in perihelion), we may say that at a conjunction near the peri- helion of Mars the distance between the two planets amounts to about 35,000,000 m., while at a conjunction near the aphelion of Mars the two planets are separated by about 61,000,000 m. Accordingly, Mars is studied under very different conditions when he is in opposition (to the sun that is, in conjunction with the earth) near perihelion and near aphelion. For not only is his apparent disk larger in the for- mer than in the latter case in the proportion of about (6 1) 2 to (35) 2, or more than 3 to 1, but furthermore the planet is more brilliantly illuminated (and will therefore better bear tel- escopic magnifying) in the former case in the proportion of about (152) 2 to (126) 2, or nearly as 3 to 2. On the whole, therefore, the planet can be more favorably studied in the former case than in the latter in the proportion of about 9 to 2, measuring the conditions by the 534 VOL. XL 13 amount of magnifying due to proximity and to the telescopic powers practically available. To ordinary observation, the effect of the con- siderations just pointed out is that Mars when in opposition near perihelion looks about 4% times brighter than when in opposition near aphelion. The opposition of the year 1877 will illustrate this; for during the oppositions which have recently occurred Mars has been far from perihelion, the opposition of 1869 oc- curring when Mars was nearly in aphelion, and those of 1871 and 1873 being little more fa- vorable. In the opposition of 1875 he will be nearer, but still some 65 from his perihelion. But in 1877 the place of opposition will have been carried more nearly to the perihelion (somewhat past that point) than previously for more than 30 years. He will therefore pre- sent a very distinguished appearance in the heavens, being little inferior in brightness to the planet Jupiter. It is indeed a noteworthy circumstance that theoretically Mars should then be brighter than Jupiter. That is, com- paring the size of his disk with that of Jupiter, and the amount of light received by the two planets from the sun, Mars should be brighter when he is in opposition near perihelion than Jupiter ever is ; but owing either to the infe- rior reflective quality of the surface of Mars, or more probably to the fact that most of the light of Jupiter is reflected from cloud mass- es, and that a portion of that light is inherent, Jupiter in opposition is always brighter than Mars ever is. The diameter of Mars is vari- ously estimated by different observers, but is probably about 4,400 m. The volume of his globe is therefore less than the earth, in the proportion of about 168 to 1,000, or the earth is nearly six times as large as Mars. The mass of the planet is however even smaller in proportion ; for his density is estimated at seven tenths of the earth's, and his mass, though less than hers in the proportion of about 118 to 1,000, is nearer one ninth than one eighth of the earth's mass. This small planet rotates on an axis inclined. about 28 to the orbit, so that the seasons have a greater range than those of the earth, whose inclina- tion to her orbit is but 23. The rotation has been determined with a near approach to ac- curacy. Cassini, who was one of the first to study the telescopic aspect of Mars, assigned to the planet a rotation period of 24h. 40m. Sir W. Herschel in 1777 attacked the problem with less success than usually attended his work as an observer. He unfortunately missed count of one rotation in a synodical revolution of Mars, and this error, distributed among all the rota- tions of the revolution (corresponding to about 24h. divided into 750 parts), amounted to nearly two minutes, so that his estimate of the ro- tation period, 24h. 39m. 25s., was about two minutes too fast. Madler, from observations extending over the years 1830-'37, deduced a period of 24h. 37m. 23 -8s. Kaiser of Ley den, combining his own observations with those by