sun and earth.-The excentricity of the orbit of Mars being about o 0933, the distance from the sun is subject to a variation, the extreme amount of which is less than one-tenth of its mean The extreme distances are

value.

152 millions of miles in aphelion,

126 millions of miles in perihelion.

It appears, therefore, that the mean distances of the planet from the earth are

In opposition

In conjunction

In quadrature

47 millions of miles,

2303 millions of miles,

104 millions of miles,

These distances are subject to variation, whose extreme limit is about 15 millions of miles, owing to the combined effects of the excentricities of the two orbits. Although the mean distance of the planet in opposition from the earth is about half the distance of the sun, it may in certain positions of the orbit come within a distance of 35 hundredths of the sun's distance. In the opposition which took place in September, 1830, the distance of the planet was only 38 hundredths of the sun's mean distance.

350. Scale of orbit relatively to that of the earth. If s, fig. 62, represent the position of the sun, and s м the distance of Mars, the orbit of the earth will be represented by E E'E' 'E'.

351. Division of the synodic period. - The earth is at E"" when Mars is in conjunction, at E' when in quadrature west of the sun, at E when in opposition, and at E" when in quadrature east of the sun.

The angle of elongation s E'M being 90°, and the mean value of S M being 152, that of S E' being expressed by 1, it follows that the angle E'SM will be about 48°, and therefore E'SE"" =180°-48°=132°.

Since the synodic period is 780 days, the mean time between quadrature and opposition will be

and the mean time between quadrature and conjunction will be

352. Apparent motion. -The yarious changes of the apparent positions of the planet and sun during the synodic period may, therefore, be easily explained. At conjunction the earth being at E"", the planet and sun pass the meridian together. In this case, the planet being above the horizon only during the day, is not visible. After conjunction, the planet passes the meridian in the forenoon, and is therefore visible above the eastern horizon before sunrise. Before conjunction it passes the meridian in the afternoon, and is therefore visible above the western horizon after sunset.

At the time of the western quadrature, the earth being at E', the planet passes the meridian about 6 A.M., and at the time of western quadrature, the earth being at E", it passes the meridian about 6 P.M. The planet has these positions about 286 days, more or less, after and before its conjunction.

At the time of opposition, the earth being at E, the planet passes the meridian at midnight; and is therefore above the horizon from sunset till sunrise. Before opposition it passes the meridian before midnight, and is above the horizon chiefly during the later part of the night, and after opposition it passes the meridian after midnight, and is therefore above the horizon chiefly during the earlier part of the night.

The interval during which it is visible more or less in the absence of the sun, being that during which it passes from western to eastern quadrature through opposition is, in the case of Mars, 208 days.

353. Stations and retrogression. -The elongations at which Mars is stationary, and the lengths of his arc of retrogression, vary to some extent with the distances of the planet from the sun and earth, which distances depend on the ellipticity of the two orbits, and the direction of their major axes. In 1860, Mars was in opposition on the 17th of July, and was stationary on the 17th of June and 18th of August. The right ascension on these days was, 17th of June R.A. 20h 13m 49° 18th of August

=

R.A.

=

19 27 19

It follows, therefore, that the extent of retrogression in right

ascension at this opposition was 46m 30', which reduced to angular magnitude is 11° 37′ 30′′.

354. Phases. At opposition and conjunction the same hemisphere being turned to the earth and sun, the planet appears with a full phase. In all other positions the lines drawn from the planet to the earth and sun, making with each other an acute angle of greater or less magnitude, the phase will be deficient of complete fulness, and the planet will be gibbous, more so the nearer it is to its quadrature, in which position the lines drawn to the earth and sun make the greatest possible angle, which being the complement of E's M, fig. 62, will be 90°-48° 42°. Of the entire hemisphere presented to the earth, 138° will therefore be enlightened and 42° dark. The corresponding form of the disk, as can easily be deduced from the common principles of projection, will be that which is represented in fig. 63, the dark part being indicated by the dotted line. The gibbosity will be less, the nearer the planet approaches to opposition or conjunction.

355. Apparent and real diameter.—The apparent diameter of Mars in opposition varies between rather wide limits, in consequence of the variation of its distance from the earth in that position, arising from the causes explained above. When at its mean distance at opposition the apparent

magnitude does not exceed 16", and at conjunction it is reduced to 3"-7.

In 1830, soon after opposition, when its distance from the earth was 38.4 millions of miles, it exhibited a diameter of 233"; the linear value of 1" at that distance being 185.7 miles, which gives for the real diameter 4363 miles.

The mean

356. Solar light and heat. distance of the earth from the sun being

Fig. 63.

less than that of Mars in the ratio of 10 to 15, the apparent diameter of the sun as seen from Mars will be less than its diameter

as seen from the earth in the same ratio. If E, fig. 64, represent the apparent disk of the sun as seen from the earth, м will represent its apparent disk as seen from Mars.

Since the density of the solar radiation decreases as the square of

Fig. 64.

the distance increases, its density at Mars will be less than at the

earth in the ratio of 4 to 9.

So far as the illuminating and heating powers of the solar rays depend on their density, they will, therefore, be less in the same proportion.

357. Rotation.-There is no body of the solar system, the moon alone excepted, which has been submitted to such rigorous and successful telescopic examination as Mars. Its proximity to the earth in opposition, when it is seen on the meridian at midnight with a full phase, affords great facility for this kind of observation.

By observing the permanent lineaments of light and shade exhibited by the disk, its rotation on its axis can be distinctly seen, and has been ascertained to take place in 24h 37m 23, the axis on which it revolves appearing to be inclined to the plane of the planet's orbit at an angle of 28° 27'. The exact direction of the axis is, however, still subject to some uncertainty.

358. Days and nights.- It thus appears that the days and nights in Mars are nearly the same as on the earth, that the year is diversified by seasons, and the surface of the planet by zones and climates not very different from those which prevail on our globe. The tropics, instead of being 23° 28', are 28° 27′ from the equator, and the polar circles are in the same proportion more extended.

359. Seasons and climates.-The year consists of 668 Martial days and 16 hours, the Martial being longer than the terrestrial day in the ratio of 100 to 97.

Owing to the excentricity of the planet's orbit, the summer on the northern hemisphere is shorter than on the southern in the ratio of 100 to 79, but owing to the greater proximity of the sun, the intensity of its light and heat during the shorter northern summer is greater than during the longer southern summer in the ratio of 145 to 100. From the same causes, the longer northern winter is less inclement than the shorter southern winter in the same proportion.

There is thus a complete compensation in both seasons in the two hemispheres.

The duration of the seasons in Martial days in the northern hemisphere is as follows:-spring 192, summer 180, autumn 150, winter 147.

360. Observations and researches of Messrs. Beer and Mädler. It is mainly to the persevering labours of these eminent observers that we are indebted for all the physical information we possess respecting the condition of the surface of this planet. Their observations, commenced at an early epoch, were regularly organised at the time of the opposition of 1830, with a view to