book, published in 1543, in which he had announced his views, had been condemned by the Papal “Congregation of the Index;” and therefore Tycho might have had as a further motive a desire to curry favour with the authorities of the Church of Rome, and to gratify his own vanity at the same time. With these explanations it will no longer be misleading if, for convenience sake, I speak of a certain great circle of the heavens as apparently traversed by the Sun every year, owing to the revolution of our Earth round that body. This circle is called the “Ecliptic,” and its plane is usually employed by astronomers as a fixed plane of reference. It must be distinguished from that other great circle called the “celestial equator,” which is the plane of the Earth’s equator extended towards the stars. The plane of the equator is inclined to the ecliptic at an angle of about 23½°, which angle is known as the “obliquity of the ecliptic.” It is this inclination which gives rise to the seasons which follow one another in succession during our annual journey round the Sun. The two points where the celestial equator and the ecliptic intersect are called the “equinoxes,” of spring or autumn as the case may be; the points midway between these being the “solstices,” of summer or winter as the case may be. These words need but little explanation, at any rate, as regards those persons who are able to trace the Latin origin of the words. “Equinox” is simply the place occupied by the Sun twice every year (namely about March 20 and September 22), when day and night are theoretically equal throughout the world, when also the sun rises exactly in the east and sets exactly in the west. The “solstices” represent the standing still of the sun at the given times and places, and are the neutral points where the Sun attains its greatest northern or southern declination. This usually occurs about June 21 and December 21. It must not be forgotten by the way, that the above application of the words “summer” and “winter” to the solstices is only correct so far as concerns places in northern terrestrial latitudes—Europe and the United States, for instance. In southern terrestrial latitudes—for instance, when speaking of what happens at the Cape of Good Hope and in Australia—the words must be reversed.