You will be surprised to hear, that if time is calculated by the stars instead of the sun, the irregularity which we have just noticed does not occur, and that one complete rotation of the earth on its axis, brings the same meridian back to any fixed star. _Emily._ That seems quite unaccountable; for the earth advances in her orbit with regard to the fixed stars, the same as with regard to the sun. _Mrs. B._ True, but then the distance of the fixed stars is so immense, that our solar system is in comparison to it but a spot, and the whole extent of the earth's orbit but a point; therefore, whether the earth remain stationary, or whether it revolved in its orbit during its rotation on its axis, no sensible difference would be produced with regard to the fixed stars. One complete revolution brings the same meridian back to the same fixed star; hence the fixed stars appear to go round the earth in a shorter time than the sun by three minutes fifty-six seconds of time. _Caroline._ These three minutes fifty-six seconds is the time which the earth takes to perform the additional three hundred and sixty-fifth part of the circle, in order to bring the same meridian back to the sun. _Mrs. B._ Precisely. Hence the stars gain every day three minutes fifty-six seconds on the sun, which makes them rise that portion of time earlier every day. When time is calculated by the stars it is called sidereal time; when by the sun, solar, or apparent time. _Caroline._ Then a sidereal day is three minutes fifty-six seconds shorter, than a solar day of twenty-four hours. _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. [Illustration: PLATE XI.] _Emily._ What does that mean? I thought the equinoctial points, were fixed points in the heavens, in which the equator cuts the ecliptic.