Part 45

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, 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, among the signs of the zodiac; that is to say, instead of being at 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 the true time cannot be measured by the apparent place of the sun. A perfectly correct clock, 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.

Questions

1. (Pg. 92) What does the line A B, (fig. 2 plate 8.) represent, and what are its extremities called?

2. (Pg. 92) What is meant by the equator, and how is it situated?

3. (Pg. 92) There are two hemispheres; how are they named and distinguished?

4. (Pg. 92) What are the circles near the poles called?

5. (Pg. 92) What do the lines I K, and L M, represent?

6. (Pg. 92) What circle is in part represented by the line L K?

7. (Pg. 92) Against what mistake must you guard respecting this line?

8. (Pg. 92) What is meant by a plane, and how could one be represented?

9. (Pg. 93) Describe what is intended by the plane of the earth's orbit.

10. (Pg. 93) Extending this plane to the fixed stars, what circle would it form, and among what particular stars would it be found?

11. (Pg. 93) What is fig. 1. plate 9, designed to represent?

12. (Pg. 93) The ecliptic does not properly belong to the earth, for what purpose then is it described on the terrestrial globe?

13. (Pg. 93) What does the obliquity of the ecliptic to the equator serve to show?

14. (Pg. 93) Within what limits do you find the torrid zone?