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Global Geophysics-Lowrie-2007 LOWRIE Chs 3-4 (M827).qxd 28/2/07 11:19 AM Page 207 John John's G5:Users:john:Public:JOHN'S JOBS:10421 - CUP - LOWRIE (2nd edn): 4Earth’s age,thermal and electrical properties Earth's 4.1 GEOCHRONOLOGY orbit 4.1.1 Time star on meridian after Earth 360° rotation Time is both a philosophical and physical concept. Our to star one B awareness of time lies in the ability to determine which of additional rotation 1° day later two events occurred before the other. We are conscious of Sun on meridian 4 min later a present in which we live and which replaces continually a to star A past of which we have a memory; we are also conscious of SUN Earth a future, in some aspects predictable, that will replace the star and Sun present. The progress of time was visualized by Sir Isaac on same meridian Newton as a river that flows involuntarily at a uniform rate. The presumption that time is an independent entity Fig. 4.1 The sidereal day is the time taken for the Earth to rotate underlies all of classical physics. Although Einstein’s through 360Њ relative to a fixed star; the solar day is the time taken for a rotation between meridians relative to the Sun. This is slightly more Theory of Relativity shows that two observers moving rel- than 360Њ relative to the stars, because the Earth is also orbiting the ative to each other will have different perceptions of time, Sun. physical phenomena are influenced by this relationship only when velocities approach the speed of light. In every- trigonometry was developed dials could be accurately day usage and in non-relativistic science the Newtonian graduated and time precisely measured. Mechanical notion of time as an absolute quantity prevails. clocks were invented around 1000 AD but reliably accu- The measurement of time is based on counting cycles rate pendulum clocks first came into use in the seven- (and portions of a cycle) of repetitive phenomena. teenth century. Accurate sundials, in which the shadow Prehistoric man distinguished the differences between day was cast by a fine wire, were used to check the setting and and night, he observed the phases of the Moon and was calibration of mechanical clocks until the nineteenth aware of the regular repetition of the seasons of the year. century. From these observations the day, month and year emerged as the units of time. Only after the development 4.1.1.2 Units of time of the clock could the day be subdivided into hours, minutes and seconds. The day is defined by the rotation of the Earth about its axis. The day can be defined relative to the stars or to the Sun (Fig. 4.1). The time required for the Earth to rotate 4.1.1.1 The clock through 360Њ about its axis and to return to the same The earliest clocks were developed by the Egyptians and meridian relative to a fixed star defines the sidereal day. were later introduced to Greece and from there to Rome. All sidereal days have the same length. Sidereal time must About 2000 BC the Egyptians invented the water clock be used in scientific calculations that require rotational (or clepsydra). In its primitive form this consisted of a velocity relative to the absolute framework of the stars. container from which water could escape slowly by a The time required for the Earth to rotate about its axis small hole. The progress of time could be measured by and return to the same meridian relative to the Sun observing the change of depth of the water in the con- defines the solar day. While the Earth is rotating about its tainer (using graduations on its sides) or by collecting and axis, it is also moving forward along its orbit. The orbital measuring the amount of water that escaped. The motion about the Sun covers 360Њ in about 365 days, so Egyptians (or perhaps the Mesopotamians) are also cred- that in one day the Earth moves forward in its orbit by ited with inventing the sundial. Early sundials consisted approximately 1Њ. To return to the solar meridian the of devices – poles, upright stones, pyramids or obelisks – Earth must rotate this extra degree. The solar day is there- that cast a shadow; the passage of time was observed by fore slightly longer than the sidereal day. Solar days are the changing direction and length of the shadow. After not equal in length. For example, at perihelion the Earth 207 LOWRIE Chs 3-4 (M827).qxd 28/2/07 11:19 AM Page 208 John John's G5:Users:john:Public:JOHN'S JOBS:10421 - CUP - LOWRIE (2nd edn): 208 Earth’s age, thermal and electrical properties is moving faster forward in its orbit than at aphelion (see England. The particular ways of defining the basic units Fig. 1.2). At perihelion the higher angular rate about the of time are important for analyzing some problems in Sun means that the Earth has to rotate through a larger astronomy and satellite geodesy, but for most geophysical than average angle to catch up with the solar meridian. applications the minor differences between the different Thus, at perihelion the solar day is longer than average; at definitions are negligible. aphelion the opposite is the case. The obliquity of the ecliptic causes a further variation in the length of the 4.1.1.3 The geological timescale solar day. Mean solar time is defined in terms of the mean length of the solar day. It is used for most practical pur- Whereas igneous rocks are formed in discrete short-lived poses on Earth, and is the basis for definition of the hour, eruptions of magma, sequences of sedimentary rocks minute and second. One mean solar day is equal to exactly take very long periods of time to form. Many sedimen- 86,400 seconds. The length of the sidereal day is approxi- tary formations contain fossils, i.e., relicts of creatures mately 86,164 seconds. that lived in or near the basin in which the sediments were The sidereal month is defined as the time required for deposited. Evolution gives a particular identifiable char- the Moon to circle the Earth and return to the celestial acter to the fossils in a given formation, so that it is possi- longitude of a given star. It is equal to 27.32166 (solar) ble to trace the evolution of different fossil families and days. To describe the motion of the Moon relative to the correlate their beginnings and extinctions. These charac- Sun, we have to take into account the Earth’s motion teristics allow the formation of the host rock to be corre- around its orbit. The time between successive alignments lated with others that contain part or all of the same fossil of the Sun, Earth and Moon on the same meridian is the assemblage, and permit the development of a scheme for synodic month. It is equivalent to 29.530 59 days. dating sediments relative to other formations. Gradually The sidereal year is defined as the time that elapses a biostratigraphical timescale has been worked out, which between successive occupations by the Earth of the same permits the accurate determination of the relative age of a point in its orbit with respect to the stars. It is equal to sedimentary sequence. This information is intrinsic to any 365.256 mean solar days. Two times per year, in spring geological timescale. and autumn, the Earth occupies positions in its orbit A geological timescale combines two different types of around the Sun where the lengths of day and night are information. Its basic record is a chronostratigraphical equal at any point on the Earth. The spring occurrence is scale showing the relationship between rock sequences. called the vernal equinox; that in the autumn is the These are described in detail in stratigraphical sections autumnal equinox. The solar year (correctly called the that are thought to have continuous sedimentation and tropical year) is defined as the time between successive to contain complete fossil assemblages. The boundary vernal equinoxes. It equals 365.242 mean solar days, between two sequences serves as a standard of reference slightly less than the sidereal year. The small difference and the section in which it occurs is called a boundary (0.014 days, about 20 minutes) is due to the precession of stratotype.When ages are associated with the reference the equinoxes, which takes place in the retrograde sense points, the scale becomes a geological timescale. Time is (i.e., opposite to the revolution of the Earth about the divided into major units called eons,which are subdi- Sun) and thereby reduces the length of the tropical year. vided into eras;these in turn are subdivided into periods Unfortunately the lengths of the sidereal and tropical containing several epochs.The lengths oftime units in years are not constant but change slowly but measurably. early timescales, before ages were determined with In order to have a world standard the fundamental unit of radioactive isotopes, were expressed as multiples of the scientific time was defined in 1956 in terms of the length duration of the Eocene epoch. Modern geological of the tropical year 1900, which was set equal to timescales are based on isotopic ages, which are cali- 31,556,925.9747 seconds of ephemeris time. Even this def- brated in normal time units. inition of the second is not constant enough for the needs The geological timescale is constantly being revised of modern physics.
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