Mrs. B. I must also explain to you what is meant by a sidereal year.

The common year, called the solar or tropical year, containing 365 days, five hours, forty-eight minutes, and fifty-two seconds, is measured from the time the sun sets out from one of the equinoxes, or solstices, till it returns to the same again; but this year is completed before the earth has finished one entire revolution in its orbit.

Emily. I thought that the earth performed one complete revolution in its orbit every year; what is the reason of this variation ?

Mrs. B. It is owing to the spheroidal figure of the earth. The elevation about the equator produces much the same effect as if a similar mass of matter, collected in the form of a moon, revolved round the equator. When this moon acted on the earth in conjunction with or in opposition to the sun, variations in the earth's motion would be occasioned, and these variations produce what is called the precession of the equinoxes.

Emily. What does that mean? I thought the equinoctial points, or nodes, were fixed points in the heavens, in which the equator cuts the ecliptic.

Mrs. B. These points are not quite fixed, but have an apparently retrograde motion, that is to say, instead of being every revolution in the same place, they move backwards. Thus if the vernal equinox is at A, (fig. 1. plate XI.) the autumnal one will be at B instead of C, and the following vernal equinox at D instead of at A, as would be the case if the equinoxes were stationary at opposite points of the earth's orbit.

Caroline. So that when the earth moves from one equinox to the other, though it takes half a year to perform the journey, it has not travelled through half its orbit.

Mrs. B. And, consequently, when it returns again to the first equinox, it has not completed the whole of its orbit. In order to ascertain when the earth has performed an entire revolution in its orbit, we must observe when the sun returns in conjunction with any fixed star; and this is called a sidereal year. Supposing a fixed star situated at E, (fig. 1. plate XI.) the sun would not appear in conjunction with it till the earth had returned to A, when it would have completed its orbit.

Emily. And how much longer is the sidereal than the solar year?

Mrs. B. Only twenty minutes; so that the variation of the equinoctial points is very inconsiderable. I have given them a greater extent in the figure in order to render them sensible.

In regard to time, I must further add, that the earth's diurnal motion on an inclined axis, together with its annual revolution in an elliptic orbit, occasions so much complication in its motion, as to produce many irregularities; therefore, true equal time cannot be measured by the sun. A clock, which was always perfectly correct, would in some parts of the year be before the sun, and in other parts after it. There are but four periods in which the sun and a perfect clock would agree, which is the 15th of April, the 16th of June, the 23d of August, and the 24th of December.

Emily. And is there any considerable difference between solar time and true time?

Mrs. B. The greatest difference amounts to between fifteen and sixteen minutes. Tables of equation are constructed for the purpose of pointing out and correcting these differences between solar time and equal or mean time, which is the denomination given by astronomers to true time.

14*

CONVERSATION IX.

ON THE MOON.

Of the Moon's Motion-Phases of the Moon.-Eclipses of the Moon.-Eclipses of Jupiter's Moons. Of the Latitude and Longitude.-Of the Transits of the Inferior Planets.-Of the Tides.

MRS. B.

WE shall to-day confine our attention to theʼmoon, which offers many interesting phenomena.

The moon revolves round the earth in the

space of

about twenty-nine days and a half, in an orbit nearly parallel to that of the earth, and accompanies us in our revolution round the sun.

Emily. Her motion then must be rather of a complicated nature; for as the earth is not stationary, but advances in her orbit whilst the moon goes round her, the moon must proceed in a sort of progressive circle. Mrs. B. That is true; and there are also other circumstances which interfere with the simplicity and