ightiest telescopes cannot magnify them
sufficiently to show any breadth of surface.

Instead of saying that an object looks a certain breadth across, such
as a yard or a foot, a statement which would really mean nothing,
astronomers speak of it as measuring a certain angle. Such angles are
estimated in what are called "degrees of arc"; each degree being divided
into sixty minutes, and each minute again into sixty seconds. Popularly
considered the moon and sun _look_ about the same size, or, as an
astronomer would put it, they measure about the same angle. This is an
angle, roughly, of thirty-two minutes of arc; that is to say, slightly
more than half a degree. The broad expanse of surface which a celestial
body shows to us, whether to the naked eye, as in the case of the sun
and moon, or in the telescope, as in the case of other members of our
system, is technically known as its "disc."

CHAPTER VII

ECLIPSES AND KINDRED PHENOMENA

Since some members of the solar system are nearer to us than others, and
all are again much nearer than any of the stars, it must often happen
that one celestial body will pass between us and another, and thus
intercept its light for a while. The moon, being the nearest object in
the universe, will, of course, during its motion across the sky,
temporarily blot out every one of the others which happen to lie in its
path. When it passes in this manner across the face of the sun, it is
said to _eclipse_ it. When it thus hides a planet or star, it is said to
_occult_ it. The reason why a separate term is used for what is merely a
case of obscuring light in exactly the same way, will be plain when one
considers that the disc of the sun is almost of the same apparent size
as that of the moon, and so the complete hiding of the sun can last but
a few minutes at the most; whereas a planet or a star looks so very
small in comparison, that it is always _entirely swallowed up for some
time_ when it passes behind the body of our satellite.

The sun, of course, occults planets and stars in exactly the same manner
as the moon does, but we cannot see these occultations on account of the
blaze of sunlight.

By reason of the small size which the planets look when viewed with the
naked eye, we are not able to note them in the act of passing over stars
and so blotting them out; but such occurrences may be seen in the
telescope, for the planetary bodies then display broad discs.

There is yet ano