CONVERSATION XXXIII.

Of the Equation of Time.

TUTOR. You are now, I presume, acquainted with the motions peculiar to this globe on which we live?

Charles. Yes it has a rotation on its axis from west to east every 24 hours, by which day and night are produced, and also the apparent diurnal motion of the heavens from east to west.

James. The other is its annual revolution in an orbit round the sun, likewise from west to east, at the distance of about 95 millions of miles from the un.

Tutor. You understand also, in what

manner this annual motion of the earth, combined with the inclination of its axis, is the cause of the variety of seasons.

We will therefore proceed to investigate another curious subject, viz. the equation of time, and to explain to you the difference between equal and apparent time.

Charles. Will you tell us what you mean by the words equal and apparent, as applied to time?

Tutor. Equal time is measured by a clock, that is supposed to go without any variations, and to measure exactly 24 hours from noon to noon. And apparent time is measured by the apparent motion of the sun in the heavens, or by a good sun-dial. Charles. And what do you mean, Sir, by the equation of time?

Tutor. It is the adjustment of the difference of time, as shown by a well-regulated clock and a true sun-dial.

James. Upon what does this difference depend?

Tutor. It depends first. upon the inclination of the earth's axis. And secondly

upon the elliptic form of the earth's orbit; for, as we have already seen, the earth's orbit being an ellipse, its motion is quicker when it is in perihelion, or nearest to the sun; and slower when it is in aphelion, or farthest from the sun.

Charles. But I do not yet comprehend what the rotation of the earth has to do with the going of a clock or watch.

Tutor. The rotation of the earth is the most equable and uniform motion in nature, and is completed in 23 hours, 56 minutes, and 4 seconds; this space of time is called a sidereal day, because any meridian on the earth will revolve from a fixed star, to that star again in this time, But a solar or natural day, which our clocks are intended to measure, is the time which any meridian on the earth will take in revolving from the sun to the sun again, which is about 24 hours, sometimes a little more, but generally less.

James. What occasions this difference betwen the solar and sidereal day? Tutor. The distance of the fixed stars

is so great, that the diameter of the earth's orbit, though 190 millions of miles, when compared with it, is but a point, and therefore any meridian on the earth will revolve from a fixed star to that star again in exactly the same time, as if the earth had only a diurnal motion, and remained always in the same part of its orbit. But with respect to the sun, as the earth advances almost a degree eastward in its orhit, in the same time that it turns eastward round its axis, it must make more than a complete rotation before it can come into the same position with the sun that it had the day before. In the same way, as when both the hands of a watch or clock set off together as twelve o'clock, the minute-hand must travel more than a whole circle before it will overtake the hour-hand, that is, before they will be in the same relative position again. Thus the sidereal days are shorter than the solar ones by about four minutes, as is evident from observation :

Watch with nice eye the earth's diurnal way
Marking her solar and sidereal day;

Her slow nutation, and her varying clime,
And trace with mimic art the march of time.

BOTANIC GARDEN.

Charles.

Still I do not understand the reason why the clocks and dials do not agree.

Tutor. A good clock is intended to measure that equable and uniform time which the rotation of the earth on its axis exhibits. Whereas the dial measures time by the apparent motion of the sun, which, as we have explained, is subject to variation. Or thus though the earth's motion on its axis be perfectly uniform, and consequently the rotation of the equator, the plane of which is perpendicular to the axis, or of any other circle parallel to it, be likewise equable, yet we measure the length of the natural day by means of the sun, whose apparent annual motion is not in the equator, or any of its parallels, but in the ecliptic, which is oblique to it.