the unusual
conditions which exist in the sun; and so we are unable to trace them
with the spectroscope, the experience of which rests on laboratory
experiments conducted, at best, in conditions which obtain upon the
earth.

[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford
University, noted a prominence which rose to a height of 324,600 miles.

CHAPTER XIV

THE INFERIOR PLANETS

Starting from the centre of the solar system, the first body we meet
with is the planet Mercury. It circulates at an average distance from
the sun of about thirty-six millions of miles. The next body to it is
the planet Venus, at about sixty-seven millions of miles, namely, about
double the distance of Mercury from the sun. Since our earth comes next
again, astronomers call those planets which circulate within its orbit,
_i.e._ Mercury and Venus, the Inferior Planets, while those which
circulate outside it they call the Superior Planets.[11]

In studying the inferior planets, the circumstances in which we make our
observations are so very similar with regard to each, that it is best to
take them together. Let us begin by considering the various positions of
an inferior planet, as seen from the earth, during the course of its
journeys round the sun. When furthest from us it is at the other side of
the sun, and cannot then be seen owing to the blaze of light. As it
continues its journey it passes to the left of the sun, and is then
sufficiently away from the glare to be plainly seen. It next draws in
again towards the sun, and is once more lost to view in the blaze at
the time of its passing nearest to us. Then it gradually comes out to
view on the right hand, separates from the sun up to a certain distance
as before, and again recedes beyond the sun, and is for the time being
once more lost to view.

To these various positions technical names are given. When the inferior
planet is on the far side of the sun from us, it is said to be in
_Superior Conjunction_. When it has drawn as far as it can to the left
hand, and is then as east as possible of the sun, it is said to be at
its _Greatest Eastern Elongation_. Again, when it is passing nearest to
us, it is said to be in _Inferior Conjunction_; and, finally, when it
has drawn as far as it can to the right hand, it is spoken of as being
at its _Greatest Western Elongation_ (see Fig. 11, p. 148).

The continual variation in the distance of an interior planet f