The equator is a great circle cutting the axis of the earth at right angles. Hence, the axis of the earth is the axis of the equator, and its poles are the poles of the equator. The intersection of the plane of the equator with the surface of the earth constitutes the terrestrial, and its intersection with the concave sphere of the heavens, the celestial, equator. The latter, by way of distinction, is sometimes denominated the equinoctial.
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:
| 1. Aries, ♈ | 7. Libra, ♎ |
| 2. Taurus, ♉ | 8. Scorpio, ♏ |
| 3. Gemini, ♊ | 9. Sagittarius, ♐ |
| 4. Cancer, ♋ | 10. Capricornus, ♑ |
| 5. Leo, ♌ | 11. Aquarius, ♒ |
| 6. Virgo, ♍ | 12. Pisces. ♓ |
The mode of reckoning on the ecliptic is by signs, degrees, minutes, and seconds. The sign is denoted either by its name or its number. Thus, one hundred degrees may be expressed either as the tenth degree of Cancer, or as 3s 10°. It will be found an advantage to repeat the signs in their proper order, until they are well fixed in the memory, and to be able to recognise each sign by its appropriate character.
Of the various meridians, two are distinguished by the name of colures. The equinoctial colure is the meridian which passes through the equinoctial points. From this meridian, right ascension and celestial longitude are reckoned, as longitude on the earth is reckoned from the meridian of Greenwich. The solstitial colure is the meridian which passes through the solstitial points.