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

DOCTRINE OF THE SPHERE. 13
Joouer Circles, because the arcs of the equator intercepted between them are used as measures of time.
3 1. 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.
32. 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, near 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.
33. The Ecliptic is a great circle in which the earth performs
its annual revolution around the sun. It passes through the
center of the earth and the center 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, but that it is turned about
231 degrees out of a perpendicular direction, making an angle
with the plane itself of 661~. 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 23j~
(230 27' 40"). It is particularly important for the learner to
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.
34. The Equinoctial Points, or Equinoxes,* are the intersections of the ecliptic and equator. The time when the sun
crosses the equator in returning northward is called the vernal,
and in going southward the autumnal equinox. The vernal
equinox occurs about the 21st of MIarch, and the autumnal the
22d of September.
T* he term Equinoxes strictly denotes the times when the sun arrives at the
equinoctial points, but it is also frequently used to denote those points themsclves.