meridian, of the earth

Is an imaginary great circle passing through the zenith and the poles, and cutting the equator at right angles. When the sun is on the meridian of any place, it is mid-day there, and at all places situated under the same meridian.

♦ First meridian is that from which the longitude is reckoned. Magnetic meridian is not a great circle but a wavy line uniting those poles. In common acceptation, a meridian is any line supposed to be drawn from the north to the south pole; therefore a place being under the same meridian as another place, is either due north or south of it.

♦ Plane of the meridian is the plane of this great circle, and its intersection with the sensible horizon is called the meridian line.

The meridian transit of a heavenly body is the act of passing over the said plane, when it is either due north or south of the spectator.

♦ Ante meridiem, or A.M., before noon.

♦ Post meridiem, or P.M., after noon.