notions of time.

*in order to measure time, you must have a regularly recurring phenomenon in nature.  the standard technique is to find something that happens regularly, and then define the unit of time in terms of the reappearance and recurrence of the phenomenon.  for example, one unit of time is the "day"- the time between two successive sunrises.  all systems for measuring time depend, ultimately, on the recurring phenomenon that is chosen to define the basic standard.
throughout most of human history the passage of time has been measured in terms of the day (which is related to the time that it takes the earth to turn once on its axis) and the year (the time it takes the earth to go once in its orbit around the sun).

*the egyptians defined the hour to be one-twelfth of the time between sunrise & sunset.  this meant that for the egyptians the length of the hour was different from one day to the next, and was not the same during the day as it was at night.

*the first exercise in the measurement of time was the production of the calendar.  when human beings began to develop agriculture, it became necessary for them to mark important events like the planting time for particular crops.  in other words, they had to have a calendar.  the calendar is really a clock that "ticks" once a year and therefore keeps track of where the earth is in its orbit around the sun.  it is this position that determines the seasons.
the basic problem in constructing a calendar is that the number of days in a year is not an even number.  the following calendars represent successive approximations to the true length of the year:

*egyptian calendar.... this calendar consisted of twelve months of thirty days each, followed by a five-day party.  the problem with the egyptian calendar arose from the fact that there are approximately 365 1/4 days in a year, not 365.  this meant that the calendar would "slip" a quarter day every year.  these slippages built up, and, if you had followed it, it would have led to a situation where you had the egyptian equivalent of snow in "august".

*our modern new year's eve parties trace their way back to the egyptian end-of-year bash.  it was a time that didn't really belong in the year, hence a time when nothing really counted.  anything went.  we many have a more modern calendar these days, but we seem to have managed to retain the truly important part of this old one.

*julian calendar.... the calendar introduced by julius caesar tried to bring some order into time keeping in the roman empire.  it solved the problem of the extra quarter day by introducing the leap year.  every four years the year is one day longer, and this makes up for most of the slippage that appeared in the egyptian calendar.  it didn't catch all of it, though, because the year is 11 minutes 14 seconds shorter than 365 1/4 days.  these errors started to accumulate (they amount to 7 days every 1000 years) until they began to mess up the observance of easter.  this led to the...

*gregorian calendar... the gregorian calendar was introduced by pope gregory in 1582 to deal with the accumulated slippage in the julian calendar.  it works by dropping leap years when they fall on centennials except when the centennial is divisible by four.  thus, 2000 will retain its leap year while 1700, 1800, and 1900 did not.  the gregorian calendar is the one we use today and the one with which you are familiar.

*the rotation of the earth is actually a very poor time standard.  if you look at the rotation closely enough, it is quite unsteady.  the gravitational pull of the moon and the planets, the effects of the tides, earthquakes, and even the motion of the winds cause the rotation to slow down and speed up erratically.  these changes aren't huge - they're on the order of milliseconds per day, but if you define the second to be a particular fraction of the length of a day (which is the way it used to be done), the second will change from one year to the next.

*the second is now defined in terms of the motion of an electron in an atom.  in 1967, the international commission on weights and measures redefined the second in terms of the time it takes for an electron to spin on its own axis inside an atom of cesium.  this is the standard that is used today.
the so-called atomic clock can measure the length of a second to an accuracy of thirteen decimal places.

*we now keep our calendars accurate by inserting a leap second into the year.  here's how it works: there are a number of clocks at various national laboratories around the world.  when a majority of the clocks agree that the rotation of the earth has gotten out of line by half a second, a "leap second" is inserted into a chosen day at midnight.  this is done every few years, most recently on dec 31, 1990.

-from 1001 things everyone should know about science by james trefil