June it becomes vertical successively to all other parts of the earth between the equator and the tropic of Cancer, and in proportion as it becomes vertical to the more northern parts of the earth, it declines from the southern, and, consequently, to the former the days lengthen, and to the latter they shorten. From June to September the sun is again vertical successively to all the same parts of the earth, but in a reverse order.

Charles. Since it is summer to all those parts of the earth, where the sun is vertical, and we find that the sun is vertical twice in the year to the equator, and every part of the globe between the equator and tropics, there must be also two summers in a year to all those places.

Tutor. There are; and in those parts near the equator, they have two harvests every year.➡ But let your brother finish his description.

James. From September to December, it is successively vertical to all the parts of the earth situated between the equator and the tropic of Capricorn, which is also the cause of the lengthening of the days in the southern hemisphere, and of their becoming shorter in the northern.

Tutor. Can you, Charles, tell me why there is sometimes no day or night for some little time together within the polar circles?

Charles. The sun always shines upon the earth 90 degrees every way, and when he is vertical to the tropic of Cancer, which is 234 degrees north

VOL. I.-Q

1

of the equator, he must shine the same number of degrees beyond the pole, or to the polar circle, and while he thus shines, there can be no night to the people within that polar circle; and, of course, to the inhabitants at the southern polar circle, there can be no day at the same time, for as the sun's rays reach but 90 degrees every way, they cannot shine far enough to reach them.

Tutor. Tell me, now, why there is but one day and night in the whole year at the poles?

Charles. For the reason which I have just given, the sun must shine beyond the north-pole all the time he is vertical to those parts of the earth, situated between the equator and the tropic of Cancer, that is, from March the 21st, to September the 20th, during which time there can be no night at the north-pole, nor any day at the south-pole. The reverse of this may be applied to the southern pole.

James. I understand now, that the lengthening and shortening of the days, and different seasons, are produced by the annual motion of the earth round the sun; the axis of the earth, in all parts of its orbit, being kept parallel to itself. But if thus parallel to itself, how can it in all positions point to the pole-star in the heavens?

Tutor. Because the diameter of the earth's orbit Ac is nothing in comparison of the distance of the earth from the fixed stars. Suppose you draw two parallel lines at the distance of three or four yards from one another, will they

not both point to the moon when she is in the horizon?

James. Three or four yards cannot be accounted as any thing, in 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 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 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 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 between 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 orbit, 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, at twelve o'clock, the minute-hand must