The secondaries to the equator,--that is, the great circles passing through the poles of the equator,--are called _meridians_, because that secondary which passes through the zenith of any place is the meridian of that place, and is at right angles both to the equator and the horizon, passing, as it does, through the poles of both. These secondaries are also called _hour circles_ because the arcs of the equator intercepted between them are used as measures of time. The _latitude_ of a place on the earth is its distance from the equator north or south. The _polar distance_, or angular distance from the nearest pole, is the complement of the latitude. The _longitude_ of a place is its distance from some standard meridian, either east or west, measured on the equator. The meridian, usually taken as the standard, is that of the Observatory of Greenwich, in London. If a place is directly _on_ the equator, we have only to inquire, how many degrees of the equator there are between that place and the point where the meridian of Greenwich cuts the equator. If the place is north or south of the equator, then its longitude is the arc of the equator intercepted between the meridian which passes through the place and the meridian of Greenwich. The _ecliptic_ is a great circle, in which the earth performs its annual revolutions around the sun. It passes through the centre of the earth and the centre of the sun. It is found, by observation, that the earth does not lie with its axis at right angles to the plane of the ecliptic, so as to make the equator coincide with it, but that it is turned about twenty-three and a half degrees out of a perpendicular direction, making an angle with the plane itself of sixty-six and a half degrees. The equator, therefore, must be turned the same distance out of a coincidence with the ecliptic, the two circles making an angle with each other of twenty-three and a half degrees. It is particularly important that we should form correct ideas of the ecliptic, and of its relations to the equator, since to these two circles a great number of astronomical measurements and phenomena are referred. The _equinoctial points_, or _equinoxes_, are the intersections of the ecliptic and equator. The time when the sun crosses the equator, in going northward, is called the _vernal_, and in returning southward, the _autumnal_, equinox. The vernal equinox occurs about the twenty-first of March, and the autumnal, about the twenty-second of September. The _solstitial points_ are the two points of the ecliptic most distant from the equator. The times when the sun comes to them are called _solstices_. The Summer solstice occurs about the twenty-second of June, and the Winter solstice about the twenty-second of December. The ecliptic is divided into twelve equal parts, of thirty degrees each, called _signs_, which, beginning at the vernal equinox, succeed each other, in the following order: