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History of Astronomy
Early Ideas of the Heavens
 The Shape of the Earth
Pythagoras taught as early as 500 B.C. that
the Earth was round, based on the belief that
the sphere is the perfect shape used by the
gods
 By 300 B.C., Aristotle presented naked-eye
observations for the Earth’s spherical shape:
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Shape of Earth’s shadow on the Moon during an
eclipse
 A traveler moving south will see stars previously
hidden by the southern horizon
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 Earth is Round clip
History of Astronomy
Early Ideas of the Heavens
 Eratosthenes
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Eratosthenes (276-195 B.C.) made the first
measurement of the Earth’s size
He obtained a value of 25,000 miles for the
circumference, a value very close to today’s value
His method entailed measuring the shadow length of
a stick set vertically in the ground in the town of
Alexandria on the summer solstice at noon,
converting the shadow length to an angle of solar
light incidence, and using the distance to Syene, a
town where no shadow is cast at noon on the
summer solstice
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Astronomy in the Renaissance
 Nicolaus Copernicus (1473-1543)
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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.
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Planet
Copernican Distance
Real Distance
Mercury
0.38 AU
0.39 AU
Venus
0.75 AU
0.72 AU
Earth
1.00 AU
1.00 AU
Mars
1.52 AU
1.52 AU
Jupiter
5.22 AU
5.20 AU
Saturn
9.17 AU
9.54 AU
Astronomy in the Renaissance
 Johannes Kepler (1571-1630)
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Kepler’s Three Laws:
I. Planets move in elliptical orbits with the Sun at
one focus of the ellipse
II. The orbital speed of a planet varies so that a
line joining the Sun and the planet will sweep
out equal areas in equal time intervals
III.The amount of time a planet takes to orbit the
Sun is related to its orbit’s size, such that the
period, P, squared is proportional to the
semimajor axis, a, cubed:
P2 = a3
where P is measured in years and a is
measured in AU
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Astronomy in the Renaissance
 Johannes Kepler (continued)
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Consequences of Kepler’s laws:
Second law implies that the closer a planet is to
the Sun, the faster it moves
 Third law implies that a planet with a larger
average distance from the Sun, which is the
semimajor axis distance, will take longer to
circle the Sun
 Third law hints at the nature of the force holding
the planets in orbit
 Third law can be used to determine the
semimajor axis, a, if the period, P, is known, a
measurement that is not difficult to make
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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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Astronomy in the Renaissance
 Galileo (1564-1642)
Contemporary of Kepler
 First person to use the telescope to study
the heavens and offer interpretations
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The Moon’s surface has features similar to that
of the Earth  The Moon is a ball of rock
 The Sun has spots  The Sun is not perfect,
changes its appearance, and rotates
 Jupiter has four objects orbiting it  The objects
are moons and they are not circling Earth
 Milky Way is populated by uncountable number
of stars  Earth-centered universe is too simple
 Venus undergoes full phase cycle  Venus
must circle Sun
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Astronomy in the Renaissance
 Galileo (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
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Isaac Newton & Birth of Astrophysics
 Isaac Newton (1642-1727) was born the
year Galileo died
 He made major advances in mathematics,
physics, and astronomy
 He pioneered the modern studies of
motion, optics, and gravity and discovered
the mathematical methods of calculus
 It was not until the 20th century that
Newton’s laws of motion and gravity were
modified by the theories of relativity
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Big Bang Theory
 Hubble’s Law (1920’s) – All other galaxies
objects of the universe are moving away
from our galaxy
 Redshift vs. Blueshift?
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https://www.youtube.com/watch?v=th_9ZR
2I0_w&index=95&list=PL908547EAA7E4A
E74
Nebular Hypothesis
 Kant and Laplace (1800’s)
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Theory that our Solar System originated
from the collapse of a Nebula
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http://csep10.phys.utk.edu/astr161/lect/sol
arsys/nebular.html
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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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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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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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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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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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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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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Why we see retrograde motion. (Object sizes and distances are exaggerated for clarity.)
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(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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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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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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(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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