comparison of 240 thousand miles, the distance of the moon from us,

Tutor. Perhaps three yards bear a much greater proportion to 240 thousand miles, than 190 millions of miles bear to our distance from the polar star.

CONVERSATION XII.

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 first 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 sun.

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 or mean, and apparent time.

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

Tutor. Equal or mean time is

*The inclination of an axis or orbit is merely relative, because we compare it with some other axis or orbit which is supposed to have no inclination. Thus the horizon at London, or at any other place where we live, being level to us, we regard it as having no inclination; but if we set out and travel 90 degrees from that place, our horizon then will be perpendicular to the horizon of London.

measured by a clock, that is supposed to go without any variation, 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 sundial.

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 dif ference 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 oftener less.

James. What occasions this dif