Time FAQs

  • Why isn’t the earliest sunrise on the longest day and the latest sunrise on the shortest?
  • Which years are leap years? When was the first leap year?
  • What are (my) latitude and longitude?
  • What is a meridian? What is the Prime Meridian?
  • When and where did the new Millennium officially start, and why?
  • Is noon 12 a.m. or 12 p.m.?
  • Why are there 12 months in the year, 60 minutes in an hour etc?

Why are there twelve months in the year?

Julius Caesar's astronomers explained the need for 12 months in a year and the addition of a leap year to synchronize with the seasons. At the time, there were only ten months in the calendar while there are just over 12 lunar cycles in a year.

The months of January and February were added to the calendar and the original fifth and sixth months were renamed July and August in honour of Julius Caesar and his successor Augustus.

These months were both given 31 days to reflect their importance, having been named after Roman leaders.

Who divided the day into 24 hours?

The Ancient Egyptians were the first to use 24 hours to divide the day. They divided the day into 12 hours from sunrise to sunset, and the night into a further 12 hours from sunset to sunrise.

Why are minutes and hours divided into 60?

When the hour was divided into 60 minutes, consisting of 60 seconds, the number 60 was probably chosen for its mathematical convenience. It is divisible by a large number of smaller numbers without a remainder: 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30.

The longest and shortest days of the year are those with the greatest and least amount of daytime. They occur at the time of the solstices, either on or around 21 June and 21 December in the UK and most of the northern hemisphere. (The winter solstice is the time when the apparent position of the Sun reaches its most southerly point against the background stars.)

Many people notice that the time of sunrise continues to get later after the winter solstice. The reason for this has to do with the slight variation in the length of 'natural' days throughout the year, which results from a combination of the elliptical shape of the Earth’s orbit around the Sun and the tilt of the rotation axis of the Earth. (The longest natural day is about 51 seconds longer than the shortest.)

However, for a clock to work, all days measured need to have a fixed, equal length. Each is therefore fixed at the average length of a natural day (this is where the ‘mean’ in Greenwich Mean Time comes from). By averaging out the length of each day like this, the clock time at which the sun reaches its highest point slowly drifts backwards and forwards as the months progress. There is a knock-on effect on the times of sunrise and sunset. The earliest sunrise occurs a number of days before the longest day and the latest a number of days after the shortest.

In London, the latest sunrise occurs about three minutes later than the sunrise on the shortest day.

To be a leap year, the year number must be divisible by four – except for end-of-century years, which must be divisible by 400. (So the year 2000 was a leap year, although 1900 was not.)

The first leap year

The first leap year in the modern sense was 1752, when 11 days were 'lost' from the month September with the adoption of the Gregorian calendar by Britain and her colonies. After 1752 we adopted the system still in use today where an additional day is inserted in February in years wholly divisible by four, other than years ending in 00 with the exception of those divisible by 400 which are still leap years (like 2000). This is certainly not the first use of leap years; the Julian calendar we used before 1752 had a simpler system of leap years, and remember, no calendar is universal.

What are latitude and longitude?

Latitude and longitude are imaginary lines encircling the Earth in a grid-like pattern, used to measure positions in degrees (°), minutes (') and seconds ("). There are 60 seconds to a minute and 60 minutes to a degree.

Lines of latitude run parallel to the equator and are measured in degrees north or south (0-90° N or S) of the equator. A minute of latitude corresponds to one nautical mile.

Lines of longitude run through the North and South Poles, dividing the Earth like the segments of an orange. They are measured in degrees east or west (0-180° E or W) of the Prime Meridian, Longitude Zero (0° 0' 0"), which is measured from the cross-hairs of Airy's Transit Circle, a special telescope at the Royal Observatory, Greenwich.

The Earth turns on its axis at the equivalent of 1° of longitude in four minutes, or 15° an hour. A complete turn of 360° takes approximately 24 hours, or one day.

What are the latitude and longitude of my location?

You can find out the latitude and longitude of named places using the Getty Theasurus of Geographic Names Online.

You can also use a handheld GPS (Global Positioning System) receiver to find your latitude, longitude and altitude.

Two female tourists on the Meridian LineTwo female tourists on the Meridian Line c. 1931. (Copy of original photograph.) Repro ID: D5568 ©National Maritime Museum, Greenwich, London A meridian is an arbitrary north-south line used by an astronomer as a zero point from where to take measurements. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the night sky.

The meridian line in Greenwich represents the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on the Earth is measured in terms of its angle east or west from this line. To stand astride the line is to have one foot in the eastern and one foot in the western hemisphere of the earth – just as the Equator divides the northern and southern hemispheres.

The Prime Meridian at Greenwich passes through a massive special telescope called a transit circle. The transit circle was built by Sir George Airy, the seventh Astronomer Royal, in 1850. The cross-hairs seen in the eyepiece of this transit circle define Longitude 0° for the world.

The 'universal day' is measured from the Prime Meridian. It is the average of a year's worth of 'natural' days and is a scientific time scale used irrespective of time zones.

When did the new Millennium start?

The Meridian Line at the Royal Observatory at duskThe Meridian Line at the Royal Observatory at dusk. Repro ID: D6854 ©National Maritime Museum, Greenwich, LondonThe first day of the new Millennium was 1 January 2001. The year 2000 was actually the final year of the previous Millennium. So, despite the fact that it was the year 2000, it was officially the 20th century for one more year.

Where did the new Millennium start and why?

The new Millennium officially started at the Prime Meridian measured from the Royal Observatory, Greenwich.

At the 1884 International Meridian Conference, a resolution was agreed which established the formulation of a 'universal day' calculated in terms of mean solar time, i.e. the average of a year's worth of days. This is a scientific time scale used irrespective of time zones. The universal day is measured from the Prime Meridian in Greenwich, which is therefore the point from whence each new millennium will begin.

Trafalgar at NoonTrafalgar at Noon; Nicolas, P H [artist]. 2nd Lt Paul Nicolas’s drawing of the approach of the British fleet at Trafalgar. Repro ID: PU8607 ©National Maritime Museum, Greenwich, London12 noon is neither a.m. nor p.m.

To avoid confusion, the correct designation for 12 o'clock is 12 noon or 12 midnight. Alternatively, the 24-hour clock system may be used.

The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither 'ante-' nor 'post-'.