The Earth rotates on its axis about once
every twenty four hours. Seen from a point in the sky
above the Earth's North Pole, almost everything in the solar system
rotates anti-clockwise so, because of the rotation of the Earth, the
Sun appears to rise approximately in the East, get higher in
the sky, reach its highest point (its zenith) at about mid-day,
then get lower in the sky, and finally set approximately in
the West. Of course the Sun reaches its zenith in different
places on the Earth's surface at different times: when the Sun
is at its zenith in London it has not even risen
in New York. (See also Note.)
The earliest people measured the passing of time by counting Sunsets - a Sunset is a very definite ending. Once they had developed ways of measuring the apparent movement of the Sun they discovered that the time from Sunrise to Sunset, the direction of the Sunrise and Sunset, and the height of the Sun above the horizon at its zenith all varied with the seasons. But they also found that the Sun is always in the same direction when it is at its zenith, and that the time from one zenith to the next is always about the same - these do not depend upon the season of the year. In today's terminology we say that the Sun is due South at its zenith and that the time from one zenith to the next is (about) twenty four hours. (Advanced readers only might like to read the page on sidereal time, now or later.)
A circle going round the Earth and passing through both North and South Poles is called a meridian. We refer to the meridian on which we are standing as our meridian. A meridian is always a North-South line. The Greenwich, or Prime, Meridian (see Note), from which all lines of longitude are measured, passes through The Old Royal Observatory at Greenwich.
The Sun crosses the meridian on which we are standing as it reaches its zenith. From Sunrise until then it is before the meridian, or ante-meridian (am) and from then until Sunset it is after the meridian, or post-meridian (pm). It is due South and at its zenith when it is on the meridian.
Our word meridian was once written meridiem and that came from the Latin word medidiem which means the middle of the day. So some books say that am is ante meridiem.
In (and before) the Middle Ages the period between Sunrise and Sunset was divided into twelve 'hours' - the 'hours' were of course longer in the summer than the winter. Every day the monks had to sing many services in their abbey church. Three of these services were Terce (the third hour, sung halfway between Sunrise and the Sun's zenith), Sext (the sixth hour, sung at the zenith), and None (pronounced known, the ninth hour, originally sung halfway between the zenith and Sunset). Under the monastic Rule the main meal of the day could not be taken before the monks had sung None. By the late Middle Ages the monks were singing None immediately after Sext, so that they could have their main meal immediately after the middle of the day, but still after None so as not to break the Rule. Hence any time after the middle of the day is now called afternoon! But afternoon has no astronomical significance.