In the study of astronomy, keeping accurate time is critical. Astronomical time keeping has to do with the celestial meridian. Every point on earth has a celestial meridian. It is a line on the celestial sphere, an imaginary sphere containing the earth, which goes from the north celestial pole to the south celestial pole and intersects the zenith, the point directly above a point on earth. The celestial poles are extensions of the terrestrial poles into space. A picture of the zenith and celestial meridian for a spot on earth is shown below.
|Figure 1: This image shows the zenith and celestial meridian for a spot on earth.|
There are two common ways of keeping time, in relation to the rotation of the earth, and in relation to a phenomenon which has a constant period. The first method is not completely accurate because of the variations in the speed of the earths rotation, but is useful for astronomical purposes. Solar and sidereal times fall into this category. The second method uses a phenomenon which has a constant period such as the vibration of a certain material. Pendulum, quartz and atomic clocks fall into this category.
Because of the counterclockwise, or eastward, rotation of the earth, a point fixed in space as seen from earth moves in a westward direction. This accounts for the sun rising in the east and setting in the west. A day is always divided into 24 hours.
For every method of measuring time explained here, a day is divided into 24 hours, an hour into 60 minutes, and a minute into 60 seconds.
Apparent Solar Time
Apparent solar time is measured in relation to the apparent position of the sun. An apparent solar day is the amount of time the sun takes to pass through the celestial meridian twice. Apparent noon is the moment when the sun crosses the celestial meridian of a point on earth. Because of the variations in the rotation speed of the earth and because the earth speeds up and slows down on its way around the sun, apparent solar days vary in length and are not an accurate method of keeping time.
Mean Solar Time
Mean solar time is in relation to the mean sun. The mean sun is a point which moves uniformly around the earth along the plane of the ecliptic, but is usually not is the same position as the real sun. The real sun, as viewed from earth, does not move uniformly because of the elliptical nature of the earths orbit and because of the slight variations in the earth's rotation period. A mean solar day contains 24 solar hours, the time it takes for the mean sun to be on a points celestial meridian, the mean noon, twice. It is measured from midnight to midnight with midnight being hour 0. A solar day is divided into 24 solar hours, each solar hour is divided into 60 solar minutes, and solar minutes are divided into 60 solar seconds.
A solar year is the time the earth takes to make a revolution around the sun, from one vernal equinox to the next. A solar year is sometimes called a tropical year and is equal to 365 days, 5 hours, 48 minutes, and 45.51 seconds, or 365.24219 days, in solar time. A solar year is affected by the precession of the equinox.
A solar month is one twelfth of a solar year. The moon also creates an interval of time known as a synodic, or lunar, month. A lunar month is the time between two new moons and is equal to about 29.53 days.
Sidereal time is measured in relation to the stars or the vernal equinox. A sidereal day is the time the celestial meridian takes to intersect the vernal equinox twice. Sidereal noon is the instant when the vernal equinox is on the celestial meridian.
There are two types of sidereal time, mean and apparent. Mean sidereal time is measured with the mean vernal equinox, apparent sidereal time is measured with the true vernal equinox. Mean sidereal time is more uniform than apparent sidereal time because of the small constant changes in the position of the equinoxes. Either way, the length of a sidereal day is affected by the precession of the equinoxes.
A sidereal rotation period is the time the celestial meridian takes to pass through a certain star twice. It is not affected by the precession of the equinoxes.
A mean sidereal day is equal to 23 hours, 56 minutes, and 4.091 seconds of mean solar time. The earths sidereal rotation period is equal to 23 hours, 56 minutes, and 4.099 seconds of mean solar time. The difference in length of the sidereal day versus a sidereal rotation period is accounted for by the precession of the equinoxes. Sidereal days are divided into sidereal hours, minutes, and seconds like solar days, but the length of a sidereal hour is slightly shorter than a solar hour. 366.2422 mean sidereal days are equal to 365.2422 mean solar days because of the extra rotation the earth gets in relation to the sun as it orbits the sun. This is illustrated below.
|Figure 2: This image shows the direction the celestial meridian is pointing at noon of sidereal time (left) and solar time (right). Notice that the sidereal noon hour circle always points in the same direction in relation to stars, but the solar noon hour circle changes positions in relation to the stars as the earth orbits the sun. The direction the solar noon hour circle points changes in relation to the stars because it always points to the sun, thus making the solar day longer than the sidereal day. In other words, the solar noon hour circle has to catch up to the sun because the earth has moved about one degree in its orbit since the last solar noon which makes it look as if the sun has moved one degree to the east. This accounts for the small difference in lengths of the sidereal and solar days.|
A sidereal year is the time the earth takes to be in the same location in relation to a background star twice. In other words, it is the time it takes for a planet to be between the sun and a certain star twice. It is about 20 minutes longer than the mean solar year because of the precession of the equinoxes. It is equal to 365.25636 mean solar days. A sidereal year, unlike a solar year, is not affected by the precession of the equinoxes.
A sidereal month is the time the moon takes to make one trip around the earth in relation to the stars. In other words, it is the time the moon takes to be between the earth and a certain star twice. A sidereal month is equal to about 27.3 days.
A sidereal period is the time a secondary, such as a planet or moon, takes to complete one complete revolution around its primary in relation to the stars. This is different from the sidereal rotation period. The earths sidereal period is the sidereal year. The moons sidereal period is the sidereal month.
Sidereal time is used with the equatorial coordinate system. If a certain hour circle is on the celestial meridian, one sidereal hour later the next hour circle will be on the celestial meridian. The celestial sphere completes one rotation in relation to the earth every sidereal day.
Universal time is the precise measurement of time used as the basis for civil time keeping. It is also usually used for recording the precise time of an astronomical observation. The abbreviation for universal time is UT. Universal time is a precise form of mean solar time, time which is in relation to the mean sun, but is calculated from, and thus related to, local sidereal time at the Greenwich meridian. Universal time is solar time at the Greenwich meridian and thus is sometimes called Greenwich mean time. It should be noted, however, that UT is measured from midnight to midnight, with midnight being hour 0. Prior to 1925, Greenwich mean time began at noon. To eliminate confusion, it is recommended that Universal time be used instead of Greenwich mean time when referring to this system.
Universal time is calculated from precise astronomical observations. The form of universal time calculated directly from astronomical observations is called UT0 and suffers from all of the irregular motions of the earth. UT0 corrected for the Chandler wobble is called UT1. UT1 is used for many celestial navigation applications because it accurately accounts for the long term uniform changes in the earths rotation rate. UT1 corrected for the annual and semiannual changes in the earths rotation rate is known as UT2.
Coordinated Universal Time, UTC, is kept by atomic clocks and is more uniform than UT1. UTC is based on International Atomic Time, which defines a second as 9,192,631,770 periods of radiation of the ground state of cesium-133. UTC is used for all civil time keeping. Because it is desirable to keep UTC time close to the actual rotation speed of the earth, it was internationally agreed that UTC would be kept within 0.9 seconds of UT1. Leap seconds are introduced every year or so into UTC to keep it within 0.9 seconds of UT1. These leap seconds are always introduced after the last second of a month, usually June 30 or December 31. A leap second can be either negative or positive but so far only positive leap seconds have been necessary. Most references to time are in relation to UTC.
The difference in UTC and UT1 is that any atomic clock can be made to keep UTC, however, the only clock capable of keeping UT1 is the earth itself.
The National Institute for Standards and Technology (NIST) provides UTC and UT1 time services such as time of day radio broadcasts. For more information, visit their Time and Frequency Division home page at http://www.boulder.nist.gov/timefreq/index.html
The Time Sphere
The current time for every point on earth can be thought of as being carried on a giant sphere around earth which is rotating in a clockwise, or westward, direction on the earths axis. The speed at which this sphere rotates is defined by the method of measuring time you are using. For sidereal time, this sphere moves slightly faster than for solar time. This sphere is illustrated in the following animation. It has twenty-four sections, or hours, each taking up 15 degrees of the sphere. The sphere has an hour circle for noon or midnight (0 hours) which is anchored to the point of reference. For solar time, this point is the sun, for sidereal time, this point is the vernal equinox. Time on earth is measured by how much time has elapsed since the last time the reference point was on the celestial meridian, thus, the time elapsed since the last time the reference point was overhead gets greater in a counterclockwise direction on this sphere. The different lengths of the sidereal and solar days is because of the constant relative motion of the two reference points.
|Figure X: This animation shows how the noon hour (yellow line) rotates around the earth and how it takes the current times around earth with it.|
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