Eclipses of Jupiter's Satellites.

There are nearly as many eclipses of Jupiter's first and second satellites this month, as there was last, but none of them are visible at the Royal Observatory.

Form of Saturn's Ring.

April 1st

Transverse diameter

Conjugate diameter

1.000

-0.035

By comparing this result with that for January, it will be perceived that the plane of the ring has passed through the Earth between these two epochs, as there is a change of sign from plus to minus; and consequently that ring was wholly invisible during a part of this interval. This happened at the end of February and the beginning of March, as may readily be verified by performing the calculation for that time, when it will be found that the conjugate diameter for the 1st of March was only 0.001; and therefore much too small to be perceived, even by the best telescopes.

Other Phenomena.

Mercury will attain his greatest elongation on the 13th of this month, and he will be stationary on the 22d. Mars and Saturn will be in conjunction at 20 m. past 2 in the morning of the 26th, at which time Mars will be 45' north of Saturn. The Moon will be in conjunction with the star Pollux at 44 m. past 9 in the morning of the 3d; with Spica, in Virgo, at 46 m. after 3 in the afternoon of the 10th; with Antares at 17 m. after midnight of the 13th; with Mercury at 3 m. past 11 in the morning of the 24th; with ẞ in Taurus, at 41 m. after 5 in the morning of the 28th; and with y in Gemini, at 33 m. past 2 in the afternoon of the 29th.

On TIME and its APPLICATION.

[Continued fromp. 77.]

In addition to the divisions of time stated in the preceding part of this article, astronomers frequent

ly refer to two others, respecting which, a brief explanation appears to be necessary. The two opposite points which constitute the intersections of the equator and ecliptic, are called the equinoctial points; and the two, where the ecliptic touches the tropics, the solstitial points: the time elapsed between two consecutive passages of the Sun through the same equinoctial or solstitial points, is called a tropical year, which the observations of astronomers have found to contain 365 d. 5 h. 48 m., and 45 s. In a similar manner, the sidereal year is determined by the returning of the Sun to the same fixed star. If, for example, the Sun be observed to coincide with any star at a given time, the interval of duration which elapses before he returns again to the same star, constitutes the sidereal year; the length of which is 365 d. 6h. 9 m., and 14 s. The sidereal year is therefore 20 m. 291s. longer than the solar or tropical year, and 9 m. 144 s. longer than the civil year, which is 365 d. 6h. Hence the civil year is nearly a mean between the sidereal and tropical years.

But for astronomers, to whom uniformity of motion is indispensable to the simplicity and facility of their calculations, the use of apparent solar time, which answers all the purposes of civil life, is not practicable. They consequently make use of a third species of time, which is altogether of an artificial nature. For this purpose they suppose the Sun to move along the equator with a uniform motion, by which means he would describe equal arcs of that circle in equal times. This is the time that is shown by well regulated clocks and watches; and as they suppose the mean motion of the Sun to be adopted as the measure of this species of duration, it is denominated mean time. The origin of this species of time, as well as that of apparent solar time, is placed at the vernal equinox, so that, as they both commence at the same point, the variation between mean time,

and that actually shown by the real motion of the Earth on its axis and in its orbit, and which is indicated by a good sun-dial, is never very great; the latter oscillating about the former without ever departing much from it.

The difference between solar and mean time arises from several causes, but especially from two, which we hope the following explanation will render familiar to our youthful readers. These causes are the unequal motion of the Earth in its orbit, or, according to appearances, of the Sun in the ecliptic; and the inclination of the ecliptic to the equator.

As the Earth's axis is perpendicular to the plane of the equator, and the motion of the Earth on its axis uniform, equal portions of this circle would pass over the meridian in equal times. But as the ecliptic is inclined to the equator, this cannot be the case with respect to it. The daily motion of the Earth therefore carries unequal portions of the ecliptic over the same meridian in equal times, the difference being always proportional to the obliquity, which varies from 0 to nearly 23° 28', and hence these differences are unequal to each other. This may be familiarly illustrated in the following manner.

'Suppose, for example, that the Sun and a star were to set out together from one of the equinoctial points, and to move continually through equal arcs in equal times, the star in the equator, and the Sun in the ecliptic; then it is evident that the star, moving in the equator, would always return to the meridian exactly at the end of every 24 hours, as measured by a well regulated clock, that keeps equal time; but the Sun, moving in the ecliptic, would come to the meridian sometimes sooner than the star, and sometimes later, according to their relative situations; and they would never be found upon that circle exactly together but on four days of the year; namely, on the 20th of March, when the Sun enters Aries; on the 21st of June, when he enters Cancer; on the

23d of September, when he enters Libra; and on the 21st of December, when he enters Capricorn.'

This part of the equation of time may be made still more familiar by means of a globe; for if a small black patch be put upon every tenth or fifteenth degree, both of the equator and ecliptic, beginning at the point Aries, and the globe be turned round slowly to the westward, all the patches from Aries to Cancer, and from Libra to Capricorn, will come to the meridian sooner than their corresponding patches on the equator; and all those from Cancer to Libra, and from Capricorn to Aries, will come to the meridian later than their corresponding patches on the equator: while the patches at the beginning of Aries, Cancer, Libra, and Capricorn, being on or even with those of the equator, show that the Sun and star will either meet there, or are even with each other, and for that reason must come to the meridian at the same time.'

The following table exhibits the relation of solar and mean time at certain intervals, as resulting from this cause, and they relate to the period when the sun-dial is faster than the clock; that is, the difference of the arrival of the Sun and the star, as supposed above, at the same meridian, will be on

The other principal cause of the difference between solar and mean time, as already stated, is the unequal motion of the Earth in its orbit, arising from the unequal action of the Sun upon it at different times of the year. The apparent motion of the Sun is slowest in summer, when he is farthest from the Earth, and

swiftest in winter, when he is nearest to it. This is shown by his being longer by about eight days on the northern than in the southern half of the ecliptic; from which circumstance alone the motion of the Sun could not be a true measure of time. This motion sometimes exceeds a degree in 24 h., and at others is less than that quantity; and consequently when it is slowest, any particular meridian will come sooner to the Sun than when his motion is quickest; so that, if there were no other cause of difference, the days cannot be equal to each other.

If two bodies, therefore, were to move in the plane of the ecliptic, so as to go exactly round the Earth in a year; the one describing an equal arc every 24 h., and the other describing sometimes a greater and at others a less arc in the same period, gaining in one part of the year what it lost in another; one of these bodies would obviously come sooner or later to the meridian than the other, according to their respective situations; and when they were both in conjunction, they would come to the meridian at the same instant.

To illustrate this conception more fully, let it be supposed that the Sun and a star commence their annual motions together from that part of the ecliptic in which the Sun is at the greatest distance from the Earth, the former moving with a variable and the latter with a constant velocity, but both completing the whole orbit in exactly the same time, that is, in the space of a year. As the Sun is supposed to be in apogee when the motion commences, his motion is the slowest; and as that of the star is always uniform, it is evident that the star will describe a larger arc in the same time than the Sun will; and consequently, as the Earth turns on its axis from west to east, it will be noon by the Sun sooner than by the star. But as the Sun moves from his apogee where his distance is greatest, to his perigee where it is