Kids Research Express

Developments of modern physics have forced a modification of the concept of simultaneity. As Albert Einstein demonstrated in his theory of relativity , when two observers are in relative motion, they will necessarily arrange events in a somewhat different time sequence. As a result, events that are simultaneous in one observer's time sequence will not be simultaneous in some other observer's sequence. In the theory of relativity, the intuitive notion of time as an independent entity is replaced by the concept that space and time are intertwined and inseparable aspects of a four-dimensional universe, which is given the name space-time . One of the most curious aspects of the relativistic theory is that all events appear to take place at a slower rate in a moving system when judged by a viewer in a stationary system. For example, a moving clock will appear to run slower than a stationary clock of identical construction. This effect, known as time dilation, depends on the relativ

The belief in time as an absolute has a long tradition in philosophy and science. It still underlies the common sense notion of time. Isaac Newton, in formulating the basic concepts of classical physics, compared absolute time to a stream flowing at a uniform rate of its own accord. In everyday life, we likewise regard each instant of time as somehow possessing a unique existence apart from any particular observer or system of timekeeping. Inherent in the concept of absolute time is the assumption that the simultaneity of two given events is also absolute. In other words, if two events are simultaneous for one observer, they are simultaneous for all observers.

Time, sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological. Physical Time and Its Measurement The accurate measurement of time by establishing accurate time standards poses difficult technological problems. In prehistory, humans recognized the alternation of day and night, the phases of the moon, and the succession of the seasons; from these cycles, they developed the day, month, and year as the corresponding units of time. With the development of primitive clocks and systematic astronomical observations, the day was divided into hours, minutes, and seconds. Any measurement of time is ultimately based on counting the cycles of some regularly recurring phenomenon and accurately measuring fractions of that cycle. The earth rotates on its axis at a very nearly constant rate, and the angular positions of celestia

Sidereal time (ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian . Because the earth moves in its orbit about the sun, the sidereal day is about 4 min shorter than the solar day (see solar time ). Thus, a given star will appear to rise 4 min earlier each night, so that different stars are visible at different times of the year. The local sidereal time of an observer is equal to the hour angle of the vernal equinox.

Vertical circle, in astronomy, the great circle on the celestial sphere that passes from the observer's zenith through a given celestial body. In the altazimuth coordinate system the altitude of a body is measured along its vertical circle.

Zenith, in astronomy, the point in the sky directly overhead; more precisely, it is the point at which the celestial sphere is intersected by an upward extension of a plumb line from the observer's location. Its position in the sky thus depends on the direction of the earth's gravitational field at the observer's location. The zenith is a reference point in the altazimuth coordinate system ; its altitude above the celestial horizon is 90°. The angular distance from the zenith to a celestial body is called the zenith distance. The nadir, directly opposite the zenith, has a zenith distance of 180°; the celestial horizon has a zenith distance of 90°.

Celestial meridian, vertical circle passing through the north celestial pole and an observer's zenith . It is an axis in the altazimuth coordinate system .