faces the windows: why do they appear of a spherical form?

Because the light falls on the most prominent parts of the globes, leaving the other parts more or less in shadow.

Now, observe what I say: when a globular body is near enough to appear of its true figure, this appearance is owing to the light, and sha dow being both obvious. But when a body is almost all luminous, or when it is removed to such a distance, that these distinctions are not observable, the sphere no longer appears of its true shape, but looks like a flat circle; thus it is with the sun; it appears to us like a bright flat circle, which flat circle is termed the sun's disk. That the sun is of a spherical form, you will not doubt when I inform you, that it turns around its axis in rather more than twenty-five days.

The distance of the sun seems so inaccessibly great, that I should like to know, how this motion has been ascertained.

The proportional mean distance of the earth from the sun, is one hundred thousand miles, of which immense distance you will be able to form a more adequate idea, when I tell that you its diameter is eight hundred and ninety thou sand miles, while the diameter of the earth is not more than seven thousand nine hundred and seventy miles; so that the sun is about one million three hundred and ninety-two thousand

five hundred times larger than the earth; the number of solid miles of which it consists, are three hundred and sixty-nine thousand, one hundred and twenty-one billions of solid miles and hence it follows, that its distance must be extremely great, since it appears not much larger than the moon, but, notwithstanding this, its rotation on its axis is capable of proof. I long to hear how.

By the motion of a certain number of dark spots on its surface, and these are found to have a motion from east to west; and their quicker motion, when they are at the centre than when they are near the edge, evinces that the sun is a spherical body, that he turns on his axis in a contrary direction, or from west to east, that he is twenty-five days in performing this revolution, and that his axis is inclined to the plane of the ecliptic, seven degrees and a half.

Do these spots, through succeeding revolutions, always present the same aspect?

No, far from it: the spots are sometimes of a comparatively moderate size, while at other times a single spot has been seen much larger than the circumference of the whole earth; and they are so changeable in their situation and figure, as frequently to vary, even during the time of observation.

Have these spots any effect on the sun's light?

It is probable, they have. The page of history informs us, that there have been periods, when the sun has been deficient in its accustomed measure of brightness, and has shone with a dim light for the space of a year. This might be the effect of the magnitude, or number, of these spots, though the cause at the time might not be known.

What number of spots have been seen at once?

About the time, when they were first observed by Galileo, in 1611, forty, or fifty, of them were seen at a time, but at present we seldom observe more than thirty, and there have been periods of seven or eight years, in which none could be discerned.

What you say leads me to suppose, then, that there must be an appearance, and a disappearance of these spots, totally different from that which arises from the rotation of the sun on its axis, which creates a movement from east to west.

That there certainly is, and at the disappearance of a dark spot it is generally converted into an illuminated fæcula, or luminous spot, which appears much brighter than the other parts of the sun.

Of what do these spots consist?

Of a nucleus, or central part, which appears to be much darker than the rest, and to be surrounded by a kind of mist or smoke.

E