History of AstronomyHistory of Astronomy

History of Astronomy 2

Early Ideas of the HeavensEarly Ideas of the Heavens The Shape of the EarthThe Shape of the Earth

Pythagoras taught as early as 500 Pythagoras taught as early as 500 B.C.B.C. that that the Earth was round, based on the belief that the Earth was round, based on the belief that the sphere is the perfect shape used by the the sphere is the perfect shape used by the godsgods

By 300 By 300 B.C.B.C., Aristotle presented naked-eye , Aristotle presented naked-eye observations for the Earth’s spherical shape:observations for the Earth’s spherical shape:

Shape of Earth’s shadow on the Moon during an Shape of Earth’s shadow on the Moon during an eclipseeclipse

A traveler moving south will see stars previously A traveler moving south will see stars previously hidden by the southern horizonhidden by the southern horizon

Earth is Round clip

History of Astronomy

History of Astronomy 4

Early Ideas of the HeavensEarly Ideas of the Heavens EratosthenesEratosthenes

Eratosthenes (276-195 Eratosthenes (276-195 B.C.B.C.) made the first ) made the first measurement of the Earth’s sizemeasurement of the Earth’s size

He obtained a value of 25,000 miles for the He obtained a value of 25,000 miles for the circumference, a value very close to today’s valuecircumference, a value very close to today’s value

His method entailed measuring the shadow length of His method entailed measuring the shadow length of a stick set vertically in the ground in the town of a stick set vertically in the ground in the town of Alexandria on the summer solstice at noon, Alexandria on the summer solstice at noon, converting the shadow length to an angle of solar converting the shadow length to an angle of solar light incidence, and using the distance to Syene, a light incidence, and using the distance to Syene, a town where no shadow is cast at noon on the town where no shadow is cast at noon on the summer solsticesummer solstice

History of Astronomy 5

Astronomy in the RenaissanceAstronomy in the Renaissance Nicolaus Copernicus (1473-1543)Nicolaus Copernicus (1473-1543)

Sun is at the center of the universe, motionless; stars are motionless around the edge

Planets all revolve around the sun (6 total including Earth)

Moon revolves around Earth Earth rotates on axis causing apparent daily

motion of the heavens Earth revolves around sun causing sun's annual

movements Retrograde motion of planets is due to relative

planetary motions Planetary orbits are perfect circles Copernicus was the first to accurately determine

the relative distances of the planets from the sun.

PlanetPlanet Copernican DistanceCopernican Distance Real DistanceReal Distance

MercuryMercury 0.38 AU0.38 AU 0.39 AU0.39 AU

VenusVenus 0.75 AU0.75 AU 0.72 0.72 AUAU

EarthEarth 1.00 AU1.00 AU 1.00 1.00 AUAU

MarsMars 1.52 AU1.52 AU 1.52 1.52 AUAU

JupiterJupiter 5.22 AU5.22 AU 5.20 AU5.20 AU

SaturnSaturn 9.17 AU9.17 AU 9.54 9.54 AUAU

History of Astronomy 7

Astronomy in the RenaissanceAstronomy in the Renaissance Johannes Kepler (1571-1630)Johannes Kepler (1571-1630)

Kepler’s Three LawsKepler’s Three Laws::I.I. Planets move in elliptical orbits with the Sun at Planets move in elliptical orbits with the Sun at

one one focusfocus of the ellipse of the ellipseII.II. The orbital speed of a planet varies so that The orbital speed of a planet varies so that a a

line joining the Sun and line joining the Sun and the planet will sweep out the planet will sweep out equal areas in equal time intervalsequal areas in equal time intervals

III.III. The amount of time a planet takes to orbit the The amount of time a planet takes to orbit the Sun is related to its orbit’s sizeSun is related to its orbit’s size, such that the , such that the period, P, squared is proportional to the period, P, squared is proportional to the semimajor axissemimajor axis, a, cubed:, a, cubed:

PP22 = a = a33

where P is measured in years and a is where P is measured in years and a is measured in AUmeasured in AU

History of Astronomy 8

Astronomy in the RenaissanceAstronomy in the Renaissance Johannes Kepler Johannes Kepler (continued)(continued)

Consequences of Kepler’s laws:Consequences of Kepler’s laws: Second law implies that the closer a planet is to Second law implies that the closer a planet is to

the Sun, the faster it movesthe Sun, the faster it moves Third law implies that a planet with a larger Third law implies that a planet with a larger

average distance from the Sun, which is the average distance from the Sun, which is the semimajor axis distance, will take longer to semimajor axis distance, will take longer to circle the Suncircle the Sun

Third law hints at the nature of the force holding Third law hints at the nature of the force holding the planets in orbitthe planets in orbit

Third law can be used to determine the Third law can be used to determine the semimajor axis, a, if the period, P, is known, a semimajor axis, a, if the period, P, is known, a measurement that is not difficult to makemeasurement that is not difficult to make

History of Astronomy

Kepler's three laws. (A) A planet moves in an elliptical orbit with the Sun at one focus. (B) A planet moves so that a line from it to the Sun sweeps out equal areas in equal times. Thus the

planet moves fastest when nearest the Sun. (C) The square of a planet's orbital period (in years) equals the cube of the semimajor axis of its orbit (in AU), the planet's distance from the Sun if

the orbit is a circle.

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History of Astronomy 10

Astronomy in the RenaissanceAstronomy in the Renaissance Galileo (1564-1642)Galileo (1564-1642)

Contemporary of KeplerContemporary of Kepler First person to use the telescope to study First person to use the telescope to study

the heavens and offer interpretationsthe heavens and offer interpretations The Moon’s surface has features similar to that The Moon’s surface has features similar to that

of the Earth of the Earth The Moon is a ball of rock The Moon is a ball of rock The Sun has spots The Sun has spots The Sun is not perfect, The Sun is not perfect,

changes its appearance, and rotateschanges its appearance, and rotates Jupiter has four objects orbiting it Jupiter has four objects orbiting it The objects The objects

are moons and they are not circling Earthare moons and they are not circling Earth Milky Way is populated by uncountable number Milky Way is populated by uncountable number

of stars of stars Earth-centered universe is too simple Earth-centered universe is too simple Venus undergoes full phase cycle Venus undergoes full phase cycle Venus must Venus must

circle Suncircle Sun

History of Astronomy 11

Astronomy in the RenaissanceAstronomy in the Renaissance Galileo Galileo (continued)(continued)

Convicted of heresy, Galileo was placed under house arrest for the remainder of his life, a gentle punishment for any individual convicted during the Inquisition.

On 31 October 1992, 350 years after Galileo's death, Pope John Paul II gave an address on behalf of the Catholic Church in which he admitted that errors had been made by the theological advisors in the case of Galileo. The Church however never admitted that they were wrong in declaring Galileo a heretic

History of Astronomy 12

Isaac Newton & Birth of AstrophysicsIsaac Newton & Birth of Astrophysics Isaac Newton (1642-1727) was born the Isaac Newton (1642-1727) was born the

year Galileo diedyear Galileo died He made major advances in mathematics, He made major advances in mathematics,

physics, and astronomyphysics, and astronomy He pioneered the modern studies of He pioneered the modern studies of

motion, optics, and gravity and discovered motion, optics, and gravity and discovered the mathematical methods of calculusthe mathematical methods of calculus

It was not until the 20It was not until the 20thth century that century that Newton’s laws of motion and gravity were Newton’s laws of motion and gravity were modified by the theories of relativitymodified by the theories of relativity

Big Bang Theory

Hubble’s Law (1920’s) – All other galaxies objects of the universe are moving away from our galaxy

Redshift vs. Blueshift?https://www.youtube.com/watch?

v=th_9ZR2I0_w&index=95&list=PL908547EAA7E4AE74

Nebular Hypothesis

Kant and Laplace (1800’s)Theory that our Solar System originated

from the collapse of a Nebula

http://csep10.phys.utk.edu/astr161/lect/solarsys/nebular.html

History of Astronomy

The Sun hides from our view stars that lie beyond it. As we move around the Sun, those stars become visible, and the ones previously seen are hidden. Thus the constellations change with the

seasons.

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History of Astronomy

The Sun's path across the background stars is called the ecliptic. The Sun appears to lie in Taurus in June, in Cancer during August, in Virgo during October, and so forth. Note that the

ecliptic is also where the Earth's orbital plane cuts the celestial sphere.

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History of Astronomy

The Earth's rotation axis is tilted by 23.5° with respect to its orbit. The direction of the tilt remains the same as the Earth moves around the Sun. Thus for part of the year the Sun lies north of the celestial equator, whereas for another part it lies south of the celestial equator.

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History of Astronomy

These five diagrams show the Sun's position as the sky changes with the seasons. Although the Earth moves around the Sun, it looks to us on the Earth as if the Sun moves around us. Notice that because the Earth's spin axis is tilted, the Sun is north of the celestial equator half of the year (late March to late September) and south of the celestial equator for the other half of the

year (late September to late March).

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History of Astronomy

Stonehenge, a stone monument built by the ancient Britons on Salisbury Plain, England. Its orientation marks the seasonal rising and setting points of the Sun. (Courtesy Tony Stone/Rob

Talbot.)

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History of Astronomy

A planet's eastward drift against the background stars plotted on the celestial sphere. Note: Star maps usually have east on the left and west on the right, so that they depict the sky when looking

south.

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History of Astronomy

The position of Mars marked out on the background stars and showing its retrograde motion. In what constellation is Mars in October 1994? (Use the star charts on the inside covers of the

book to identify the constellations.)

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History of Astronomy

Why we see retrograde motion. (Object sizes and distances are exaggerated for clarity.)

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History of Astronomy

(A) The cycle of the phases of the Moon from new to full and back again. (B) The Moon's phases are caused by our seeing different amounts of its illuminated surface. The pictures in the dark

squares show how the Moon looks to us on Earth.

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History of Astronomy

A solar eclipse occurs when the Moon passes between the Sun and the Earth so that the Moon's shadow strikes the Earth. The photo inset shows what the eclipse looks like from Earth. (Photo

courtesy of Dennis di Cicco.)

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History of Astronomy

A lunar eclipse occurs when the Earth passes between the Sun and Moon, causing the Earth's shadow to fall on the Moon. Some sunlight leaks through the Earth's atmosphere casting a deep reddish light on the Moon. The photo inset shows what the eclipse looks like from Earth. (Photo

courtesy of Dennis di Cicco.)

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History of Astronomy

(A) During a lunar eclipse, we see that the Earth's shadow on the Moon is curved. Thus the Earth must be round. (B) As a traveler moves from north to south on the Earth, the stars that

are visible change. Some disappear below the northern horizon, whereas others, previously hidden, become visible above the southern horizon. This variation would not occur on a flat

Earth.

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