An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

12 THE EARTH.
The Altitude of a body is its elevation above the horizon
measured on a vertical circle.
The Azimuth of a body is its distance measured on the horizon
from the meridian to a vertical circle passing through the body.
The Amplitude of a body is its distance on the horizon from
the prime vertical to a vertical circle passing through the body.
Azimuth is reckoned 90~ from either the north or south point;
and amplitude 90~ fi-om either the east or west point. Azimuth
and amplitude are mutually complements of each other. When
a point is on the horizon it is only necessary to count the number of degrees of the horizon between that point and the
meridian, in order to find its azimuth; but if the point is above
the horizon, then its azimuth is estimated by passing a vertical
circle through it, and reckoning the azimuth from the point
where this circle cuts the horizon.
The Zenith D)istance of a body is measured on a vertical circle passing through that body. It is the complement of the
altitude.
28. The Axis of the Earthl is the diameter on which the
earth is conceived to turn in its diurnal revolution. The same
line continued until it meets the starry concave, constitutes the
axis of the celestial sphere.
The Poles of the Earth are the extremities of the earth's axis:
the Poles of the Heavens, the extremities of the celestial axis.
29. 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 with the concave sphere of
the heavens, the celestial equator. The latter, by way of distinction, is sometimes denominated the euirnoctial.
30. The secondaries to the equator, that is, the great circles
passing through the poles of the equator, are called lfMeridians, 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. (Art. 21.) These secondaries are also called