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Chapter 1 Periods of Western Astronomy Prehistoric Astronomy Prehistoric Astronomy Stonehenge the Celestial Sphere

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Chapter 1 Periods of Western Astronomy Prehistoric Astronomy Prehistoric Astronomy Stonehenge the Celestial Sphere Periods of Western Astronomy • Western astronomy divides into 4 periods – Prehistoric (before 500 B.C. ) • Cyclical motions of Sun, Moon and stars observed • Keeping time and determining directions develops Chapter 1 – Classical (500 B.C. to A.D. 1400) • Measurements of the heavens • Geometry and models to explain motions History of Astronomy – Renaissance (1400 to 1650) • Accumulation of data led to better models • Technology (the telescope) enters picture – Modern (1650 to present) • Physical laws and mathematical techniques • Technological advances accelerate Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prehistoric Astronomy Prehistoric Astronomy • The heavens have been – Many astronomical phenomena are cyclic on a studied for thousands of years. day-to-day and year-to-year basis and consequently gave prehistoric people: • Early astronomers noted the obvious: • Methods for time keeping – Rising of the Sun in the • Ability to predict and plan future events eastern sky and its setting • Incentive to build monumental structures such as in the west Stonehenge – Changing appearance of – Modern civilization no longer relies on direct the Moon astronomical observations for time keeping and – Eclipses – Planets as a distinct class planning of objects different from – Studying the night sky provides link to past the stars Stonehenge The Celestial Sphere • Vast distances to stars prevent us from sensing their true 3-D arrangement • Naked eye observations treat all stars at the same distance, on a giant celestial sphere with the Earth at its center 1 Models and Science Constellations • The celestial sphere is a model, which does not necessarily match physical reality • Models provide a means to enhance our understanding of nature • Constellations are fixed arrangements of stars that resemble animals, objects, and mythological figures • Stars in a constellation are not physically related Constellations Diurnal vs. Annular Motion • Diurnal Motion • Annual Motion – “Daily Motion” – “Yearly Motion” – Sun, Moon, planets, – Due to the Earth’s and stars rise in the revolution east and set in the west – Due to the Earth’s – Is the sky different rotation from day to day? – Month to month? • Positions of stars change very • Origin of the ancient – Ancient astronomers – Year to year? slowly; constellations will constellations is unknown took all celestial look the same for thousands although they probably served motion to be diurnal of years as mnemonic tools for tracking – The Celestial Sphere! seasons and navigation Diurnal Motion Annual Motion • Daily motion can be explained by the rotation of the celestial sphere about the north and south celestial poles located directly above the Earth’s north and south poles • The celestial equator, which lies directly above the Earth’s equator, • For a given time (say 10:00 PM ), as the months proceed, provides another constellations do not appear in the same part of the sky astronomical reference marker 2 Annual Motion The Ecliptic • A given star rises 3 minutes 56 seconds • The path of the Sun earlier each night through the stars on the celestial sphere is • This annual motion is called the ecliptic caused by the Earth’s motion around the Sun, • The ecliptic is a the result of projection projection of the Earth’s orbit onto the • The ancients used the periodic annual motion celestial sphere and is to mark the seasons tipped relative to the celestial equator The Seasons The Seasons • The Earth is closest to the Sun in January, which is • The rotation axis of the Earth maintains nearly the same winter in the northern hemisphere tilt and direction from year to year • Therefore, the seasons cannot be caused by the • The northern and southern hemispheres alternate Sun’s proximity to the Earth receiving (on a yearly cycle) the majority of direct light • The Earth’s rotation axis is tilted 23.5º from a line from the Sun perpendicular to the Earth’s orbital plane • This leads to the seasons! The Seasons Seasons and The Ecliptic • The tilt of the Earth’s rotation axis causes the ecliptic not to be aligned with the celestial equator • Sun is above celestial equator in June when the Northern Hemisphere is tipped toward the Sun, and is below the equator in December when tipped away • Tilting explains seasonal altitude of Sun at noon, highest in summer and lowest in winter 3 The Ecliptic’s Tilt Solstices and Equinoxes • Points on horizon where Sun rises and sets changes periodically throughout year • In summer months of Northern hemisphere, the Sun rises north of east and sets north of west • In winter months of Northern hemisphere, the Sun rises south of east and sets south of west • The solstices (about June 21 and December 21) are when the Sun rises at the most extreme north and south points • The equinoxes (equal day and night and about March 21 and September 23) are when the Sun rises directly east • Ancients marked position of Sun rising and setting to determine the seasons (e.g., Stonehenge) Solstices and Equinoxes Planets and the Zodiac • The planets (Greek for • Motion and location of the “wanderers”) do not follow planets in the sky is a the same cyclic behavior of combination of all the planets’ the stars orbits being nearly in the • The planets move relative to same plane and their relative the stars in a very narrow speeds about the Sun band centered about the ecliptic and called the zodiac Planets and the Zodiac Retrograde Motion • Apparent motion of planets is usually from west to east relative to the stars , although on a daily basis, the planets always • Occasionally, a planet will move from east to west relative to the rise in the east stars; this is called retrograde motion • Explaining retrograde motion was one of the main reasons astronomers ultimately rejected the idea of the Earth being located at the center of the solar system 4 The Moon The Phases of the Moon • During a period of • Rises in the east and about 30 days, the sets in the west Moon goes through a • Like the planets and complete set of Sun, the Moon phases: new, waxing moves from west to crescent, first quarter, east relative to the waxing gibbous, full, stars (roughly the waning gibbous, third width of the Moon in quarter, waning one hour) crescent The Phases of the Moon Lunar Rise and Set Times • The Moon rises roughly 50 minutes later each day – The phase cycle is the origin of the month (derived from the word moon) as a time period – The phases of the Moon are caused by the relative positions of the Sun, Earth, and Moon Eclipses Solar Eclipse from Space • An eclipse occurs when the Sun, Earth, and Moon are directly in line with each other • A solar eclipse occurs when the Moon passes between the Sun and Earth, with the Moon casting its shadow on the Earth causing a midday sky to become dark as night for a few minutes 5 Lunar Eclipses Eclipse Periods • Eclipses do not occur every 30 days since the Moon’s orbit is tipped relative to the Earth’s • A lunar eclipse occurs when the Earth passes orbit between the Sun and Moon, with the Earth • The tipped orbit allows the shadow of the casting its shadow on the Moon giving it a dull Earth (Moon) to miss the Moon (Earth) red color Ancient Greek Astronomers Early Ideas: Pythagoras • Through the use of models and observations, they were the first to use a • Pythagoras taught as careful and systematic manner to explain early as 500 B.C. that the workings of the heavens the Earth was round, • Limited to naked-eye observations, their based on the belief idea of using logic and mathematics as tools that the sphere is the for investigating nature is still with us today perfect shape used • Their investigative methodology is in many by the gods ways as important as the discoveries themselves Early Ideas: Aristotle Early Ideas: Aristotle • By 300 B.C. , Aristotle presented naked-eye – He also noted observations for the that a traveler Earth’s spherical moving south shape: will see stars previously – Shape of Earth’s hidden by the shadow on the Moon southern horizon during an eclipse 6 Early Ideas: The Size of the Earth Early Ideas: The Size of the Earth • He measured the • Eratosthenes (276- shadow length of a stick set vertically in 195 B.C. ) made the the ground in the town first measurement of Alexandria on the of the Earth’s size summer solstice at noon, converting the • He obtained a value shadow length to an of 25,000 miles for angle of solar light the circumference, incidence, and using the distance to Syene, a value very close a town where no to today’s value shadow is cast at noon on the summer solstice Early Ideas: Distance and Size of Early Ideas: Distance and Size of the Sun and Moon the Sun and Moon • The sizes and distances of the Sun and Moon relative to Earth were determined by Aristarchus about 75 years before Eratosthenes measured the Earth’s size • Once the actual size of the Earth was determined, the absolute sizes and • These relative sizes were based on the angular size of distances of the Sun objects and a simple geometry formula relating the and Moon could be object’s diameter, its angular size, and its distance determined Early Ideas: Distance and Size of Early Ideas: The Geocentric Model the Sun and Moon • Because of the general east to west motion of objects in the sky, geocentric theories were developed to explain the motions • Eudoxus (400-347 B.C. ) proposed a geocentric model in which each celestial object was mounted on its own revolving transparent sphere with its own separate tilt • The faster an object
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