CONVERSATION IX.

Of the Annual Motion of the Earth.

diurnal

TUTOR. Besides the motion of the earth, by which the succession of day and night is produced, it has another, called its annual motion, which is the journey it performs round the sun in 365 days, 5 hours, 48 minutes, and 49 seconds.

Charles. Are the different seasons to be accounted for by this motion of the earth?

Tutor. Yes, it is the cause of the different lengths of the days and nights, and consequently of the dif

ferent seasons, viz. Spring, Summer, Autumn, and Winter.

It shifts the seasons, months, and days,
The short-liv'd offspring of revolving time;
By turns they die, by turns are born.
Now cheerful Spring the circle leads

And strews with flowers the smiling meads;
Gay Summer next, whom russet robes adorn,
And waving fields of yellow corn;

Then Autumn, who with lavish stores the lap of
Nature spreads;

Decrepit Winter, laggard in the dance

(Like feeble age oppress'd with pain),
A heavy season does maintain,

With driving snows and winds and rain;
Till Spring, recruited to advance,

The various year rolls round again.

HUGHES,

James. How is it known that the earth makes this annual journey round the sun?

Tutor. I told you yesterday, that, through the shaft of a very deep mine, the stars are visible in the day as well

as in the night. They are also visible in the day time, by means of a telescope properly fitted up for the purpose; by this method, the sun and stars are visible at the same time. Now if the sun be seen in a line with a fixed star to-day at any particular hour, it will, in a few weeks, by the motion of the earth, be found considerably to the east of him: and if the observations be continued through the year, we shall be able to trace him round the heavens to the same fixed star from which we set out; consequently, the sun must have made a journey round the earth in that time, or the earth round him.

Charles. And the sun being a million of times larger than the earth, you will say that it is more natural, that the smaller body should go round the larger, than the reverse.

Tutor. That is a proper argument but it may be stated in a much stronger manner. The sun and earth mutually attract one another, and since they are in equilibrio by this attraction, you know, their mo menta must be equal, therefore the earth being the smaller body, makes out by its motion what it wants in the quantity of its matter, and, of course, it is that which performs the journey.

James. But if you refer to the principle of the lever, to explain the mutual attraction of the sun and earth, it is evident, that both bodies must turn round some point as a common centre.

Tutor. They do; and that is the common centre of gravity of the two

See Vol. I. Conversation XIV

bodies. Now this point between the earth and sun is within the surface of the latter body.

Charles. I understand how this is: because the centre of gravity between any two bodies, will be as much nearer to the centre of the larger body than to that of the smaller, as the former contains a greater quantity of matter than the latter.

Tutor. You are right: but you will not conclude that, because the sun is a million of times larger than the earth, therefore it contains a quantity of matter a million of times greater than that contained in the earth.

James. Is it then known, that the earth is composed of matter more dense than that which composes the body of the sun?

Tutor. The earth is composed of