People began measuring time long before they knew that the Earth is round or that it is a planet that orbits the Sun. In ancient times, the practical need for timekeeping and navigation was one of the primary reasons for the study of astronomy.
The celestial origins of timekeeping and navigation are still evident. The time of day comes from the location of the Sun in the local sky, the month comes from the Moon's cycle of phases, and the year comes from the Sun's anuual path along the ecliptic. However, the devices used for timekeeping have evolved from the sundial to advanced atomic clocks.
Sundials and Apparent Solar Time
The Earth makes a complete rotation about its axis in 24 hours and hence, turns by 15 degrees every hour. Hence, by making a flat circular disk divided into sectors of 15 degrees and aligning the axis of the disk with the axis of Earth's rotation, one would be able to read the hour using the shadow of the vertical rod placed at the center of the disk. This is the principle behind sundials.
Sundials measure time based on the actual position of the Sun in the local sky. This time is called the apparent (or local) solar time. Noon is the precise moment when the Sun is on the meridian (which is an imaginary line passing from north to south through the zenith) and the sundial casts its shortest shadow. Before noon, when the Sun is on its way to meridian, the apparent solar time is ante meridian (a.m.) and past noon the apparent solar time is post meridian (p.m.).
Mean Solar Time
Even though the average solar day is 24 hours, the actual length of the solar day varies throughout the year due to the difference in the speed of revolution of Earth around the Sun. Due to this effect, a watch will not remain perfectly synchronized with the sundial over the year. Hence it is much more convenient to define a time in terms of the average of the apparent solar time. This is called the mean solar time and is the basis of standard time. The value of the difference between mean and apparent solar time is called the Equation of Time.
In 1800s, the time of the day was a local matter, and most cities and towns used some form of local mean solar time. Due to practical difficulties in having several times with respect to railroad travel, on November 18, 1883, railroad companies agreed to a new system which divided the United States into four standard time zones. This was established in U.S. law by the Standard Time Act of March 19, 1918. Today, the world is divided into several time zones as shown here.
In most parts of America and Europe, clocks are set to standard time for only part of the year. In The United States, between the first Sunday in April and the last Saturday in October, the time in these countries changes to daylight savings time, which is 1 hour ahead of standard time. To see the rationale behind daylight savings time, go here.
For purposes of navigation and astronomy (especially interferometry in radio astronomy), it is useful to have a single time for the entire Earth. For historical reasons, this "world" time was chosen to be the mean solar time at Greenwich, England (0 degrees longitude), and this time is called the Universal Time (UT). Due to physical processes such as mass motions in the atmosphere, the rate of rotation of the Earth varies from the mean solar time, and so a timescale called Coordinated Universal Time (UTC) has been adopted.
All astronomical objects pass across the sky through the meridian like the Sun due to the Earth's rotation. However, the Earth in addition to rotation around its axis, also revolves around the Sun. During the course of a year, due to its orbit, the Earth makes one additional rotation around the Sun. Hence relative to the stars, there is one extra rotation per year, and this amounts to a difference in the position of the stars in the sky by about four minutes of time, when viewed at the same time on two successive days.
Thus, relative to the stars, the Earth's rotation period is about 23 hours and 56 minutes (more accurately 23 hours, 56 minutes and 4.09 seconds), or 4 minutes less than 24 hours. This time period is called a sidereal day and clocks running at this rate indicate the sidereal time. The sidereal clock is defined to be 0 h at noon on Spring Equinox, and it coincides with the solar time at Autumn Equinox. The sidereal time is invaluable to amateur and professional astronomers to orient star maps to the sky and to point telescopes.
The seasons on Earth are caused by the combination of its rotation and revolution around the Sun. The axis of Earth's rotation is tilted to the plane of Earth's orbit around the Sun (called the ecliptic) by about 23.5 degrees. Because the rotation axis remains pointed in the same direction (towards the star Polaris), the orientation of Earth relative to the Sun changes during the course of the year as shown here.
On two days of the year (March 21 and September 21), both hemispheres receive equal amounts of sunlight. These days are called equinoxes. Solstices are days when the Sun reaches its farthest northern and southern declinations. Summer solstice (around June 21) is the day when the Northern hemisphere receives its most direct sunlight and has the longest period of daylight of any day of the year. It's the shortest day in the Southern hemisphere. Winter solstice (around December 21) is the day when the Southern hemisphere receives its most direct sunlight, and is the shortest day of the year in the Northern hemisphere.
Tropical and Sidereal year
The time taken for Earth to complete one orbit around the Sun relative to the stars is called a sidereal year. However, we measure a year to be the period between two successive spring equinoxes, and this period is called a tropical year. The tropical year is about 20 minutes shorter than the sidereal year.
The difference between the tropical and sidereal year arises from the precession of the Earth's axis of rotation. Like the axis of a spinning top, the Earth's axis also sweeps out a circle. The axis tilt remains close to 23.5 degrees throughtout the cycle but the orientation of the axis changes. Each cycle of Earth's precession takes about 26,000 years. Today, the axis points toward Polaris, which makes it the North Star. In about 13,000 years, the axis will point nearly in the direction of the star Vega, which will make Vega the North Star. This animation is an illustration of the precession of Earth's axis (the diagram is a view from the north pole where zenith marks the location of the north celestial pole).
Precession changes the locations in the Earth's orbit at which the equinoxes and solstices occur. For example, the spring equinox today occurs when the Sun appears in the direction of the constellation Pisces. In another 600 years, precession will move this point into the constellation Aquarius, while 2000 years ago, this point was in the constellation Aries.
- How does night and day work? (Beginner)
- Is the Sun always up for exactly 12 hours at the equator? (Beginner)
- Why is a day divided into 24 hours? (Intermediate)
- How can I calculate the position or path of the Sun for a given time and location? (Intermediate)
- How do sunrise and sunset times change with altitude? (Intermediate)
- Why doesn't the length of each day change much around the solstices? (Intermediate)
- How is the time of sunrise calculated? (Intermediate)
- How much can the location of sunset differ from due West? (Intermediate)
- Why is twilight short near the equator? (Intermediate)
- How does the location of sunrise and sunset change throughout the year? (Advanced)
- Why doesn't the earliest sunset occur on the shortest day of the year? (Advanced)
- What causes seasons? (Beginner)
- Why does the Earth tilt back and forward once a year? (Beginner)
- Will the seasons change due to precession? (Intermediate)
- What are the "dog days of summer"? (Intermediate)
- What is the significance of the Tropic of Cancer, Tropic of Capricorn, Arctic Circle and Antarctic Circle? (Beginner)
The Ask an Astronomer team's favorite links about Timekeeping:
- Information leaflets on Timekeeping: This is an excellent website on timekeeping created by the Royal Observatory Greenwich.
- Atomic Timekeeping: This National Physical Laboratory website tells you all you need to know about the most accurate way to keep time: atomic timekeeping.
- Daylight Saving Time: This site gives the history, rationale, laws and dates associated with daylight saving time.
- Reference Systems: This site describes the system of terrestrial and celestial coordinates, and the various motions of the Earth.
- Analemma: This website gives you extensive information on the Equation of Time which is the difference between the mean and apparent solar time.
- US Naval Observatory. Lots of data on sunrise/sunset and other similar tables.
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