Time

Apparent Solar Time:

Our daily lifecycle is based on the position of the Sun in the sky, called apparent solar time (also called synodic time). When the Sun is on the meridian it is called noon. When the Sun is 180° away, it is midnight. As the Earth rotates on its axis the Sun and stars appear to move across the sky from East to West. Sundials tell solar time based on the shadow of the Sun falling on a marked surface.

Thus, we define solar time as the hour angle of the Sun plus 12 (so that when the Sun is on the meridian, i.e. the HA is zero, it is noon = 12H). Of course, local time varys depending on where on the Earth's surface you are. Thus, we set the time at zero longitude (through Greenwich, England) as an international reference or what is called Universal Time (UT) or Greenwich Mean Time (GMT). At the beginning of the 20th century, the Earth was divided into 24 time zones spaced approximately 15° apart in longitude (1 hour). For the continental U.S., the four time zones are called Eastern, Central, Mountain and Pacific.

Note that within each zone your clock will read some time, but your apparent solar time will not match your clock unless you live exactly in the middle of the zone. Since this is less than 30 minutes on either side, only a careful observer notices the difference. Note also that the line separating the time zones at 180 longitude is called the International Date Line, the point where a given day first begins. Crossing the Date Line east to west subtracts one day from your calendar, west to east adds one day (which caused Magellan's sailors puzzlement when they arrived home one day late).

Solar time is not a stable measure of the Earth's rotation for two reasons. One, the orbit of the Earth is an ellipse. Thus, at different parts of the year the Earth is moving faster or slower making the motion of the Sun in the sky vary. Second, the Sun does not move along the equator (because the Earth's pole is tilted with respect to the plane of the solar system) and, therefore, its declination varys during the year. Different declination means different rates of change in hour angle (think of walking 10° around the equator versus 10° near the North Pole). The result of these two variations is that the real time (called solar mean time) varys from apparent solar time by up to 15 minutes during the year. To correct apparent solar time to mean solar time, one uses the equation of time displayed graphically below.

Sidereal Time:

A `day' is defined by the rotation of object in question. For example, the Moon's `day' is 27 Earth days.

A `year' is defined by the revolution of object in question. For example, the Earth's year is 365 days divided into months; whereas, Pluto's `year' is 248.6 Earth years.

Typically we use solar time, with respect to the Sun, in our everyday life. For example, noon, midnight, twilight are all examples of synodic time based on where the Sun is in the sky. Thus, one solar day is the time between successive passages of the Sun over the observer's meridian (i.e. the Sun has passed through 24 hours of angle) after correction for the equation of time.

Astronomers, on the other hand, use sidereal time, which means time with respect to the stars. One sidereal day is the passage of a particular star twice across the meridian. Since the Earth moves around the Sun once every 365 days, the Sun's apparent position in the sky changes from day to day and the result is that solar days do not equal sidereal days.

This means that a solar day will differ from a sidereal day by being slightly longer. In the above diagram, we see that the Earth's solar day is 4 mins longer than its sidereal day.

Much like our selecting a zero longitude through Greenwich, the zero hour for right ascension and sidereal time is chosen to pass through the Point of Aries, the point where the path of the Sun (the ecliptic) crosses the celestial equator in the spring. Thus, we define the local sidereal time as the right ascension of your local meridian or the hour angle of the Point of Aries.

To find out the LST right now, click here.

Atomic Time:

Ephemerides are tables that list the positions of Sun, Moon, planets and their respective moons at different times. Formerly, the positions were given as a function of Greenwich Mean Time (GMT) which lead to recurring problems, in particular with predictions for the Moon's motion. Finally (at about 1930), it was realized that Earth's rotation is irregular and that any timescale derived from it must be erratic.

In the Systeme Internationale of units of measurements, the second is defined as the duration of 9,192,631,770 cycles of a particular hyperfine structure transition in the ground state of Cesium-133. This definition was chosen to match as best as possible the length of the ephemeris second that was used before. To obtain a timescale of practical usability a device is required that attempts to realize the SI-second. Such a device is called an atomic clock.

Real-word atomic clocks do not agree fully with each another. Therefore, the weighted mean of many atomic clocks -- distributed over various laboratories on the whole Earth -- is used to define the Atomic Time TAI (french Temps Atomique International). TAI is currently the best realization of a timescale based on the SI-second, with a relative accuracy of +/- 2*10^-14 (as of 1990).

Equinox and Solstice:

The projection of the Sun's path across the sky during the year is called the ecliptic. The points where the ecliptic crosses the celestial equator are the vernal and autumnal equinox's. The point were the Sun is highest in the northern hemisphere is called the summer solstice. The lowest point is the winter solstice.

Days are longest in the summer for the northern hemisphere due to tilt of the Earth's axis allowing for more sunlight to be projected onto surface. Note also the reason for the "midnight" sun at the North Pole in summer. Longest day of the year is at the summer solstice

For opposite reasons, days are short and nights long in the winter.

Seasons:

Due to the Earth's tilt, the Earth's surface is divided into five zones, Torrid, North temperate, South temperate, Arctic and Antarctic. The boundaries are 23^{^o} 27^' and 66^{^o} 33^'. The Torrid zone is bounded by the Tropics of Cancer and Capricorn. The Arctic zones are bounded by the Arctic and Antarctic circles.

If an observers latitude is numerically less than 23^{^o} 27^', there will be two occasions during the year when the Sun passes through the zenith. Likewise, within the Arctic zones, there will be times of the year when the Sun remains below the horizon for a full 24 hours and 6 months later will remain above the horizon for 24 hours (the midnight Sun). Of course, the landmasses have shifted over time due to continental drift.

The seasons are caused by the angle the sun's rays make with the ground. Higher Sun angle means more luminosity per square meter. Low Sun angle produces fewer rays per square meter. More intensity means more heat and, therefore, higher temperatures.

Note that, due to the fact that our oceans store heat, the actual changes in mean Earth temperature are delayed by several weeks, i.e. the hottest days of summer are usually in late July, over a month from the summer solstice.

Calendars:

Time keeping and construction of calendars are among the oldest branches of astronomy. The most common definition in the western world of the year is based on the revolution of the Earth around the Sun and is therefore called a `Solar Year'. However, there are several possibilities to define beginning and end of one revolution and thus also several kinds of solar years:

A tropical year is the interval between two successive passages of the Mean Sun through the mean vernal equinox and lasts 365.242199 days UT. The name refers to the changes of seasons (greek `tropai', the turning points) which are fixed in this kind of year. It is for this reason that the tropical year is of great importance in the construction of calendars.
A sidereal year is the interval between two successive passages of the Mean Sun at the same (fixed) star. It lasts 365.256366 days UT.
An anomalistic year is the interval between two successive passages of the Earth through the perihelion (the point closest to the Sun) of its orbit and it lasts 365.259636 days UT.

The Julian calendar is based on a solar year with originally 365 days. To account for the fact that the tropical year is longer than 365 days by about a quarter day, a leap day is inserted at the end of month of February in every fourth year. This simple leap year rule was already known in late Egypt. It was in fact an Alexandrian scholar named Sosigenes who advised Julius Caesar during the introduction of the calendar into the Roman empire in the year 46 BC. The calendar is named after Julius Caesar.

The Julian day number, or simply the Julian day, is a continuous count of days, starting with the day 0 that began on the 1st of January, 4713 BC at 12 o'clock noon. Consequently, a new Julian day always begins at 12 o'clock noon that originally gave European astronomers the advantage that all observations of any particular night happened at the same Julian day. The Julian day count can easily be extended to a precise measure of time by appending the fraction of the day elapsed since 12 o'clock noon. For instance, JD 2,451,605 signifies the day that will begin on March 1, 2000, 12 o'clock noon whereas JD 2,451,605.25 means the point of time at 18 o'clock of the same day. This extension is called the Julian date in many texts (as for example in the Astronomical Almanac).

The Julian year with its duration of 365.25 days was too long by 0.0078 days or 11 minutes 14 seconds with respect to the tropical year. Although this difference was not perceptible within a few years, it acculumated over the centuries. Astronomers first noticed that the true beginning of spring (when the Sun passes through the Vernal equinox) moved away from the nominal start of spring on March 21. This nominal date had been decreed by the Roman church in the connection with the Easter date. At the beginning of the 16th century the date in the Julian calendar already lagged 10 days behind the true position of Earth in its orbit and the Easter date began to lose its intended connection with the Jewish feast of Passover (that is tied to the true start of spring). To solve this problem, pope Gregor XIII in AD 1582 ordered a calendar reform for the domain of the Catholic church. It consisted of three parts:

Omission of 10 calendar days, the 4th of October 1582 was followed directly by the 15th of October 1582 in the new calendar. This brought the start of spring back to the 21th of March. The reckoning of week days was not changed.
Introduction of a new leap year rule according to which no leap days occur in years that are divisible by 100 but not by 400. This reduces the error in the year length and slows down the accumulation of this error. The leap day is inserted at the end of February as in the Julian calendar.
Modification of the Easter rule to accommodate the new calendar.

The leap year rule described under 2. is the basis for the Gregorian calendar still in use today. It results in a mean year length of 365.2425 days. The remaining difference with respect to the tropical year is small enough to require the insertion of an extra leap day only after 3333 years.