CHAPTER XIX.

OF THE HARVEST MOON.

As the moon moves towards the east at the rate of about 13o a day, the time of her rising is about 50 minutes later on an average every successive day. At the equator this is uniformly the difference in the time of her rising; but in places of considerable latitude, the intervals between her times of rising vary from 17 minutes to 1 hour 15 minutes. This is owing to the different angles which the moon's path makes with the horizon; for as the moon's path nearly coincides with the ecliptic, it will be seen, by inspeeting the globe, that that part of her path which is adjacent to Aries, makes a much more acute angle with the eastern horizon, than that which is adjacent to Libra. The consequence of this is, that the former part rises much more rapidly than the latter; and therefore the same advance of the moon in the former part makes a much less difference in the time of her rising than in the latter. To illustrate this, let small patches of paper be fixed at the distance of every 13 degrees on the ecliptic of a celestial globe, to represent the moon's situation on successive days. Then, the globe being rectified for the latitude of the place, by turning it westward, it will be seen that the marks in Pisces and Aries appear above the eastern side of the horizon

in very quick succession, while those in Virgo and Libra appear with more considerable intervals, which shows that the same advance of the moon in her orbit makes a much smaller difference in the successive times of her rising in the former situation than in the latter. Now the former is the situation of the full moons which happen in autumn, because the sun being then in the signs Virgo or Libra, the moon at the time of her opposition will be in Pisces or Aries. The full moon thus rising for several successive evenings nearly at the same hour, that is, at intervals of a quarter of an hour, during the period of harvest, is called the Harvest Moon. The same circumstance in her time of rising must happen, be it observed, every month, because every month she passes through the signs Pisces and Aries; but as it is only in autumn that it coincides with the time of full moon, it is only then that the phenomenon is particularly remarked.

CHAPTER XX.

OF THE SUN.

THAT body in the system which next claims our attention, is the Sun. It has already been shown that this great luminary is situated in the centre of the system to which our planet belongs, and is the source of light and heat to the whole; and its apparent diurnal and annual motions have been accounted for by the real diurnal and annual motions of the earth. It only now remains, therefore, to state some particulars relating to its bulk, and its real motion upon its axis. Its diameter is estimated at about 883,000 miles, which is more than 111 times the diameter of the earth. In solid dimensions, therefore, it is more than a million times larger than the earth.* It revolves upon its axis in about 25 days 10 hours. This fact has been determined by means of certain spots which have been observed on the sun's disk, which first make their appearance on the eastern side, then by degrees advance towards the middle, and so pass on till they reach the western edge, and then disappear. When

* Spheres and all other regular and similar solids are to each other as the cubes of their diameters. The diameters, therefore, of the earth and sun being as 1 to 111, their solid dimensions will be as 1 to 1,367,631.

they have been absent for nearly the same period during which they were visible, they appear again as at first, finishing their apparent circuit in 27d. 7h. 37′, which, allowing for the earth's advance during that period in the same direction, gives 25d. 10h. for the time of their real revolution. The sun's axis makes an angle of 7° 20′ with a perpendicular to the plane of the earth's orbit.

Various have been the speculations of astronomers respecting the cause and nature of those spots which have been mentioned as appearing occasionally on his disk. The opinion of Dr. Herschel is, that this great luminary, which we are prone to consider as a vast mass of fire, is an opaque body, like the earth and planets, but surrounded by a sort of luminous atmosphere, which is the source of light and heat to the planetary system, and that the spots in question are nothing else than openings in this luminous atmosphere, which afford us a view of the sun's dark body underneath. But this opinion, though sanctioned by so high a name, and therefore entitled to some attention, can be considered only as a conjecture; and perhaps the subject itself is one of those on which we must for ever remain in ignorance.

CHAPTER XXI.

OF THE INFERIOR PLANETS.

THE planets Mercury and Venus are called inferior planets, because their orbits are included within that of the earth. This is evident from their appearances and motions; for they are never seen in opposition to the sun, nor even in quadrature, but accompany the sun in his annual course, moving alternately to the east and west of him, but never exceeding a certain limit, which is called their greatest elongation. When they are to the eastward of the sun, they rise and set after him, and, if sufficiently distant from him, may be seen in the evening a little after sunset: they are then called evening stars. When they have attained their greatest elongation eastward, they appear for a time stationary, and then begin to retrograde or move backward towards the west. When they have passed the sun, and reached a sufficient distance to the west, they rise and set before him, and being visible only in the morning before sunrise, are then called morning stars. Having reached the same elongation or distance from the sun in this direction, they again seem stationary, till they have resumed their direct motion towards the east, when they again pass the sun, and appear eastward of him as before. These appearances will be easily explained by referring to Fig. 17. Let S re