The Guinness Encyclopedia

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Extensible Markup Language | 1995-08-15 | 6KB | 34 lines

<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE card PUBLIC "-//Apple, Inc.//DTD card V 2.0//EN" "" > <card> <id>5344</id> <filler1>0</filler1> <cantDelete> <false /> </cantDelete> <showPict> <true /> </showPict> <dontSearch> <false /> </dontSearch> <owner>5472</owner> <link rel="stylesheet" type="text/css" href="stylesheet_3106.css" /> <content> <layer>background</layer> <id>25</id> <text>ime (1 of 3)Time forms the basis of many scientific laws, but time itself is very difficult to define. Time, like distance, separates objects and events and for this reason can be regarded as the fourth dimension. However, time cannot be measured directly. We must make do with measuring the way in which the passage of time affects things.Time always moves forwards. This is demonstrated by the fact that there are many processes that once done cannot be undone. Although we measure time as if it passes at a regular rate, time can appear to move at different rates depending upon what one is doing. Someone enjoying themselves may find that time appears to pass very quickly. Conversely, a person laboring at a monotonous job may find that time appears to pass slowly. This is known as subjective time.Time systemsThe Earth's orbit is not circular but elliptical, so the Sun does not appear to move against the stars at a constant speed. Most everyday time systems are therefore based on a hypothetical `mean Sun', which is taken to travel at a constant speed equal to the average speed of the actual Sun.A day is the time taken for the Earth to turn once on its axis. A sidereal day is reckoned with reference to the stars and is the time taken between successive passes of the observer's meridian by the same star. (The meridian is an imaginary line from due north to due south running through a point directly above the observer.) One sidereal day is 23 hours 56 minutes 4 seconds. A solar day is calculated with respect to the mean Sun. The mean solar day is 24 hours long.A year is the time taken for the Earth to complete one orbit of the Sun. The Earth's true revolution period is 365 days 6 hours 9 minutes 10 seconds, and this is known as a sidereal year. However, the direction in which the Earth's axis points is changing due to an effect known as precession. The north celestial pole now lies near the star Polaris in the constellation Ursa Minor, thus Polaris is also known as the Pole Star. By the year AD 14 000, the Earth's axis will point in a different direction and the bright star Vega in Lyra will be near the pole. This effect also means that the position of the Sun's apparent path across the sky is changing with respect to the stars. A tropical year compensates for the effects of precession and is 365 days 5 hours 48 minutes 45 seconds long. It is the tropical year which is used as the basis for developing a calendar.The SI unit of time is the second, which was originally defined as 1/86 400 of the mean solar day. However, as we have seen, the Earth is not a very good timekeeper, so scientists no longer use it to define the fundamental unit of time. Thesecond is now defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium-133 atom.Greenwich Mean Time (GMT) is the local time at Greenwich, England. The Greenwich Meridian is the line of 0 deg longitude which passes through Greenwich Observatory. The mean Sun crosses the Greenwich Meridian at midday GMT. Also known as Universal Time (UT), GMT is used as a standard reference time throughout the world. Sidereal time literally means `star time'. It is reckoned with reference to the stars and not the Sun.</text> </content> <content> <layer>background</layer> <id>26</id> <text>n Aztec calendar woodcut. In the inner circle are the symbols of the twenty Aztec days, each of which is associated with a particular god. The Aztec year of 365 days was made up of 18 months of 20 days each, with an additional 5 unlucky days. At the center of the woodcut is the date on which the Aztecs believed the world would be destroyed by an earthquake, together with the dates on which they believed similar destructions had occurred previously.</text> </content> <content> <layer>background</layer> <id>23</id> <text>ΓÇó THE UNIVERSE AND COSMOLOGYΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE INNER PLANETSΓÇó QUANTUM THEORY AND RELATIVITYΓÇó THE REVOLUTIONARY CALENDAR</text> </content> <content> <layer>background</layer> <id>36</id> <text>4101226425</text> </content> <name>p006-1</name> <script></script> </card>