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Home , Celestial sphere, History of astronomy

Periods of Western

• Western astronomy divides into 4 periods – Prehistoric (before 500 B.C. ) • Cyclical motions of , and observed • Keeping time and determining directions develops Chapter 1 – Classical (500 B.C. to A.D. 1400) • Measurements of the heavens • and models to explain motions – (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

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Prehistoric Astronomy Prehistoric Astronomy • The heavens have been – Many astronomical phenomena are cyclic on a studied for thousands of . -to-day and -to-year basis and consequently gave prehistoric people: • Early noted the obvious: • Methods for time keeping – Rising of the Sun in the • Ability to predict and plan future events eastern and its setting • Incentive to build monumental structures such as in the west – Changing appearance of – Modern no longer relies on direct the Moon astronomical observations for time keeping and – – as a distinct class planning of objects different from – Studying the sky provides link to past the stars

Stonehenge The Celestial • 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 with the at its center

1 Models and Science

• 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 are not physically related

Constellations Diurnal vs. Annular 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 from day to day? – 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! and

Diurnal Motion Annual Motion • Daily motion can be explained by the rotation of the celestial sphere about the and south celestial poles located directly above the Earth’s north and south poles • The celestial , which lies directly above the Earth’s equator, • For a given time (say 10:00 PM ), as the proceed, provides another constellations do not appear in the same part of the sky astronomical reference marker

2 Annual Motion The • A given 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

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 from the Sun perpendicular to the Earth’s orbital • 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 and

• 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

• 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 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

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 from Space

• An eclipse occurs when the Sun, Earth, and Moon are directly in line with each other • A 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 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

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 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: 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 • (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 at noon, converting the • He obtained a value shadow length to an of 25,000 miles for 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 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 moved in the sky, the smaller was its corresponding sphere • This simple geocentric model could not explain • Aristarchus, realizing the Sun was very large, retrograde motion without appealing to clumsy and proposed the Sun as center of the Solar System, but the unappealing contrivances lack of argued against such a model

7 Early Ideas: The Geocentric Model of Alexandria • Ptolemy of Alexandria improved the geocentric model by assuming each planet moved on a small circle, which in turn had its center move on a much larger circle centered on the Earth • The small circles were called epicycles and were incorporated so as to explain retrograde motion

Ptolemy of Alexandria Non-Western Contributions • Ptolemy’s model was able to • Islamic Contributions predict planetary motion with – Relied on celestial phenomena to set its religious fair precision – Created a large vocabulary still evident today (e.g., , • Discrepancies remained and ) this led to the development of – Developed algebra and Arabic numerals very complex Ptolemaic • Asian Contributions models up until about the 1500s – Devised constellations based on Asian mythologies – Kept detailed records of unusual celestial events (e.g., • Ultimately, all the geocentric eclipses, comets, , and sunspots) models collapsed under the weight of “Occam’s razor” – Eclipse predictions and the heliocentric models prevailed

Astronomy in the Renaissance Astronomy in the Renaissance • • Heliocentric models (1473-1543) explain retrograde motion – Could not reconcile as a natural consequence centuries of data with of two planets (one being Ptolemy’s geocentric the Earth) passing each model other – Consequently, Copernicus • Copernicus could also reconsidered Aristarchus’s derive the relative heliocentric model with the distances of the planets Sun at the center of the solar system from the Sun

8 Astronomy in the Renaissance Astronomy in the Renaissance • However, problems remained: • (1546- – Could not predict planet 1601) positions any more – Designed and built accurately than the model of Ptolemy instruments of far greater accuracy than – Could not explain lack of parallax motion of stars any yet devised – Conflicted with – Made meticulous Aristotelian “common measurements of the sense” planets

Astronomy in the Renaissance Astronomy in the Renaissance

• Tycho Brahe (1546-1601) • (1571- – Made observations 1630) (supernova and comet) that suggested that the heavens – Upon Tycho’s death, were both changeable and his data passed to more complex than Kepler, his young previously believed assistant – Proposed compromise – Using the very precise geocentric model, as he data, Kepler observed no parallax motion! showed the orbit to be an ellipse

Kepler’s 1 st Law Kepler’s 2nd Law • The orbital speed of a planet varies so that a line joining the Sun • Planets move in and the planet will elliptical orbits with sweep out equal areas the Sun at one focus in equal time intervals of the ellipse • The closer a planet is to the Sun, the faster it moves

9 Kepler’s 3 rd Law Kepler’s 3 rd Law

• The amount of time a • This law implies that planet takes to orbit a planet with a larger the Sun is related to average distance from its orbit’s size the Sun, which is the semimajor axis • The square of the distance, will take period, P, is longer to circle the proportional to the Sun cube of the semimajor • Third law hints at the axis, a nature of the holding the planets in orbit

Kepler’s 3 rd Law Astronomy in the Renaissance

• Third law can be • Galileo (1564-1642) used to determine – Contemporary of Kepler the semimajor axis, – First person to use the a, if the period, P, telescope to study the is known, a heavens and offer measurement that interpretations is not difficult to • The Moon’s surface has features similar to that of make the Earth ⇒ The Moon is a ball of rock

Astronomy in the Renaissance Evidence for the Heliocentric Model

– The Sun has spots ⇒ The Sun is not perfect, changes its appearance, and rotates – has four objects orbiting it ⇒ The objects are and they are not circling Earth – is populated by uncountable number of stars ⇒ Earth-centered is too simple • undergoes full phase cycle ⇒ Venus must circle Sun

10 Astronomy in the Renaissance

• Credited with originating the • Isaac Newton (1642- experimental method for 1727) was born the studying scientific problems year Galileo died • Deduced the first correct • He made major “laws of motion” advances in • Was brought before the mathematics, , Inquisition and put under and astronomy house arrest for the remainder of his life

Isaac Newton The Growth of • He pioneered the modern • New Discoveries studies of motion, optics, – In 1781, Sir discovered ; and gravity and he also discovered that stars can have companions discovered the mathematical methods of – Irregularities in Uranus’s orbit together with law of calculus gravity led to discovery of Neptune • It was not until the 20 th • New Technologies century that Newton’s – Improved optics led to bigger telescopes and the laws of motion and discovery of nebulas and gravity were modified by – allowed the detection of very faint the theories of relativity objects

The Growth of Astrophysics The Growth of Astrophysics

• The Kelvin Scale • The Nature of Matter and Heat – An object’s temperature is directly related to its – The ancient Greeks introduced the idea of the atom energy content and to the speed of molecular (Greek for “uncuttable”), which today has been motion modified to include a nucleus and a surrounding – As a body is cooled to zero Kelvin, molecular cloud of electrons motion within it slows to a virtual halt and its – Heating (transfer of energy) and the motion of energy approaches zero ⇒ no negative atoms was an important topic in the 1700s and – Fahrenheit and Celsius are two other temperature 1800s scales that are easily converted to Kelvin

11 The Kelvin Temperature Scale

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