Time seems a simple system to keep track of the days, bedtimes and dates, but once you start thinking about it, it's a rather complex concept. There are different calendars, like the Jewish and Chinese, but the calendar explained below is the one used in the Western world.

Firstly, the calendar is based upon two factors:

• The 365.256 day period it takes the Earth to circulate around the Sun.

• The 27.32 day period it takes the Moon to circulate around the Earth.

Years

Based on the circulation period of the Earth around the Sun, a year is made up of 365 days, which is the approximate time it takes the Earth to travel around the Sun. There is, since the Earth actually takes 365 days and a quarter to rotate around the sun, a leap year once every four years. This means that the year has 366 days instead of 365, and the extra day is 29 February.

Seasons

A year is divided in four seasons; winter, spring, summer and autumn. Each season lasts three months and they are mostly based on the changing of the weather.

For countries in the Northern Hemisphere the seasons are as follows:

• Winter starts on 21 December and finishes on 21 March.
• Spring starts on 21 March and finishes on 21 June.
• Summer starts on 21 June and finishes on 21 September.
• Autumn starts on 21 September and finishes on 21 December.

For countries in the Southern Hemisphere the seasons are as follows:

• Winter starts on 21 June and finishes on 21 September.
• Spring starts on 21 September and finishes on 21 December.
• Summer starts on 21 December and finishes on 21 March.
• Autumn starts on 21 March and finishes on 21 June.

Generally, it is cold in the winter and warm in the summer but this, of course, depends on the local climate. Additionally, as you get closer to the equator there is less distinction between seasons.

Months

A year is also divided in 12 months. Each month has its own name; January, February, March, April, May, June, July, August, September, October, November and December. A month is either 30 or 31 days long, February being the only exception, with 28 days in a normal year or 29 in a leap year.

The moon takes slightly less than a month to circle around the Earth. The result is that the phases of the moon and the tides of the ocean do not occur on the same days each month.

Weeks

Then there are weeks, with roughly 52 weeks in a year, or roughly four weeks in a month. A week is seven days long. The days are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday. Saturday and Sunday (and usually Friday evening too) are know as the weekend.

The week, unlike the rest of the time periods, has no relevance to the movement of the sun or moon. Tradition in western Christian and Jewish countries dictates that God set aside one day in seven as a day of rest (usually Saturday or Sunday), giving rise to the human weekly cycle of life.

Days

A day is the time the Earth takes to turn around once. This means that, if you are standing on the equator, the sun lights the scene for a period of 12 hours, and the other 12 hours are dark. These are known respectively as day and night. Away from the equator, the boundaries are blurred.

Hours

There are 24 hours in a day, often divided into morning, afternoon, evening and night. There are no determined time periods for these, as it varies from place to place, and varies through the year. This division of the day is often used in greetings (eg good morning).

Minutes

There are 60 minutes in an hour which are then further divided into seconds. There are 60 seconds in a minute. A second is the smallest standard measure of time; anything smaller is measured in fractions of a second. A second is also the universal standard unit for measuring time.

In short...

A non-leap year has;

• 4 seasons
• 12 months
• 52 weeks
• 365 days
• 8,760 hours
• 525,600 minutes
• 31,536,000 seconds

Telling the Time

Daily time can be told by either using the 12 hour clock or 24 hour clock method.

Twelve Hour Clocks

12 hour clocks are more traditional, due to the fact that a circular clock face divided into 12 partitions is easier to read, and can show hours, minutes per hour, and seconds per minute. The day is divided into two 12 hour partitions, one for the morning (midnight to midday) and one for the evening (midday to midnight). The time is told as the time that has lapsed since start of the current partition, while stating whether it is morning or afternoon.

For example, 'ten past five in the morning' would mean five hours and ten minutes since midnight.

'12.30 in the afternoon' would mean half an hour (30 minutes) past midday. Note that in this case 'twelve hours' is an exception to the case and is read as zero hours.

'Quarter to two o'clock' means the time is quarter of an hour (fifteen minutes) before it becomes two hours since either midday or midnight.

Morning and afternoon are often represented as 'am' and 'pm' respectively, as in '8.30pm'. The term 'am' comes from the Latin ante meridiem meaning before noon/midday. The term 'pm' similarly comes from the Latin post meridiem meaning after noon/midday.

24 Hour Clocks

24 hour time has increased in popularity since the invention of digital clocks, which display the time in figures rather than on a dial. The time is told as four numbers, the first two being the hours since midnight, and the last two being the minutes past the hour.

For example, '15.20 hours' would be 20 minutes past three in the afternoon, or 15 hours and 20 minutes since midnight.

'00.00 hours' is midnight.

Time Zones

On 1 November, 1884, at the International Meridian Conference in Washington DC, USA, the time zones were designed. The 24 standard meridians, every 15° longitude East and West of 0° at Greenwich, England, were designated the centres of the zones.

The International Dateline, the point at which all dates start, was drawn to generally follow the 180° meridian in the Pacific Ocean. Because some countries, islands and states did not want to be divided into several zones, the zones' boundaries tend to wander considerably from the straight North - South line.

Daylight Saving Time

This system was, as the name says, designed to save daylight. When summer starts you set your clock one hour back, and when the winter sets in you set your clock one hour ahead, to avoid having to get up when it's still dark in the winter¹. Unfortunately there's no international date for the resetting of the clock, so confusingly, the US does it one week later than Europe.

History of Earth Time