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PSCI 1414 GENERAL
ASTRONOMY
LECTURE 3: THE HISTORY OF ASTRONOMY
PART 2: THE COPERNICAN REVOLUTION
ALEXANDER C. SPAHN
THE COPERNICAN REVOLUTION
• The Greeks and other ancient peoples developed many important scientific ideas, but what
we now think of as science arose during the European Renaissance.
• Within a half century after the fall of Constantinople in 1453, Polish scientist Nicholas
Copernicus began the work that ultimately overturned the Earth-centered Ptolemaic model
(geocentric model).
THE COPERNICAN REVOLUTION
Copernicus (1473 – 1543)
Nicholas Copernicus was born in Poland on February 19, 1473.
His family was wealthy and he received an education in mathematics,
medicine, and law. He began studying astronomy in his late teens.
By that time, tables of planetary motion based on the Ptolemaic model had
become noticeably inaccurate. However, few people were willing to
undertake the difficult calculations required to revise the tables.
THE COPERNICAN REVOLUTION
Copernicus (1473 – 1543)
In his quest for a better way to predict planetary positions, Copernicus
decided to try Aristarchus’s Sun-centered idea, first proposed more than
1700 years earlier.
He had read of Aristarchus’s work, and recognized the much simpler
explanation for retrograde motion offered by a Sun-centered system.
But he went far beyond Aristarchus in working out mathematical details of
the model. Through this process, Copernicus discovered simple geometric
relationships that allowed him to calculate each planet’s orbital period
around the Sun and its relative distance from the Sun in terms of the EarthSun distance.
THE COPERNICAN REVOLUTION
Copernicus (1473 – 1543)
The model’s success in providing a geometric layout for the solar system
convinced him that the Sun-centered (Heliocentric) model is correct.
Copernicus was hesitant to publish his work, fearing that his suggestion
that Earth moved would be considered absurd. However, he discussed his
system with other scholars, including high-ranking officials of the Catholic
Church, who urged him to publish a book.
Copernicus saw the first printed copy of his book, De Revolutionibus
Orbium Coelestium (“Concerning the Revolutions of the Heavenly
Spheres”), on the day he died—May 24, 1543.
THE COPERNICAN REVOLUTION
Copernicus (1473 – 1543)
Publication of the book spread the Sun-centered idea widely, and many
scholars were drawn to its aesthetic advantages.
Nevertheless, the Copernican model gained relatively few converts over
the next 50 years, for a good reason: It didn’t work all that well.
The primary problem was that while Copernicus had been willing to
overturn Earth’s central place in the cosmos, he had held fast to the ancient
belief that heavenly motion must occur in perfect circles.
THE COPERNICAN REVOLUTION
The heliocentric model accounts
for retrograde motion
The Earth travels around the Sun
more quickly than Mars.
Consequently, as Earth overtakes
and passes this slower-moving
planet, Mars appears for a few
months (points 4-6) to fall behind
and move backward with respect
to the background stars.
CONCEPT CHECK 4.4
In the heliocentric model, could an imaginary observer on the surface of the Sun look
out and see planets moving in retrograde motion?
No. A planet only appears to move in retrograde motion if seen from another planet if
the two planets move at different speeds and pass one another. An imaginary observer
on the stationary Sun would only see planets moving in the same direction as they orbit
the Sun.
THE COPERNICAN REVOLUTION
The heliocentric model helped determined the arrangement of the planets
Because Mercury and Venus are always observed fairly near the Sun in the sky, their orbits
must be smaller than the Earth’s. Planets in such orbits are called inferior planets.
The other visible planets (Mars, Jupiter, and Saturn) are sometimes seen on the side of the
celestial sphere opposite the Sun, so these planets appear high above the horizon at midnight
when the Sun is far below the horizon. When this happens, Earth must lie between the Sun
and those planets. Thus, it was concluded that their orbits were larger than the Earth’s and
these planets are referred to as superior planets.
THE COPERNICAN REVOLUTION
The heliocentric model explains why planets appear in different pars of the sky on different dates
Elongation – The angle between the Sun and a planet as viewed from Earth
Greatest eastern elongation – When a planet’s position in the sky is as far east of the Sun as possible
Greatest western elongation - When a planet’s position in the sky is as far east of the Sun as possible
Inferior conjunction – When an inferior planet is between the Earth and the Sun, moving into the
morning sky
Superior conjunction – When an inferior planet is on the opposite side of the Sun, moving into the
evening sky
Opposition – When a superior planet lies directly behind the Earth from the Sun and is brightest in
the night sky
Conjunction – When a superior planet lies on the opposite side of the Sun and is only up in daytime
THE COPERNICAN REVOLUTION
When and where in the
sky a planet can be seen
from Earth depends on
the size of its orbit and
its location on that
orbit. The inferior
planets cycle between
being visible in the west
after sunset and in the
east before sunrise.
CONCEPT CHECK 4.5
How many times is Mars at inferior conjunction during one orbit around the Sun?
Mars has an orbit around the Sun that is larger than Earth’s orbit. As a result, Mars
never moves to a position between the Earth and the Sun, so Mars never is at inferior
conjunction.
THE COPERNICAN REVOLUTION
The heliocentric model showed that there are rules relating the motion of one planet to another
Copernicus found a correspondence between
the time a planet takes to complete one orbit –
that is, its period – and the size of the orbit.
Synodic period – the time that elapses
between two successive identical
configurations as seen from Earth (for example:
from one opposition to the next or from one
conjunction to the next)
Sidereal period – the true orbital period of a
planet, the time it takes the planet to complete
one full orbit of the Sun relative to the stars
CONCEPT CHECK 4.7
Why is Jupiter’s sidereal period longer than its synodic period?
Jupiter moves slowly and does not move very far in the time it takes for Earth to pass by
Jupiter, move around the Sun, and pass by Jupiter again, giving Jupiter a synodic period
similar to the Earth’s orbital period of one year. However, slow moving Jupiter takes
more than a decade to move around the Sun back to its original position, giving it a
large sidereal period.
THE COPERNICAN REVOLUTION
The heliocentric model led to the determination of the relative distances of the planets from the Sun
Copernicus devised a straightforward geometric
method of determining the relative distances
using trigonometry. His answers turned out to be
remarkably close to modern values.
The distances in this table are given in terms of
the astronomical unit (AU), which is the average
distance of the Earth from the Sun.
1 AU = 1.496 x 108 km
THE COPERNICAN REVOLUTION
By comparing the two tables, it is clear that the farther
a planet is from the Sun, the longer it takes to travel
around its orbit. This is so for two reasons:
1) The larger the orbit, the farther a planet must
travel to complete an orbit
2) The larger the orbit, the slower a planet moves
THE COPERNICAN REVOLUTION
Tycho (1546 - 1601)
Part of the difficulty faced by astronomers who sought to improve either
the Ptolemaic or the Copernican system was a lack of quality data. The
telescope had not yet been invented, and existing naked-eye observations
were not very accurate. Better data were needed, and they were provided
by the Danish nobleman Tycho Brahe.
In 1563, Tycho decided to observe a widely anticipated alignment of Jupiter
and Saturn. To his surprise, the alignment occurred nearly 2 days later than
the date Copernicus had predicted. Resolving to improve the state of
astronomical prediction, he set about compiling careful observations of
stellar and planetary positions in the sky.
THE COPERNICAN REVOLUTION
Tycho (1546 - 1601)
Tycho’s fame grew after he observed what he called a nova,
meaning “new star,” in 1572. By measuring its parallax and
comparing it to the parallax of the Moon, he proved that the
nova was much farther away than the Moon (Today, we know
that Tycho saw a supernova—the explosion of a distant star).
Parallax – A phenomenon in which the apparent position of
an object changes because of the motion of the observer
THE COPERNICAN REVOLUTION
Tycho (1546 - 1601)
King Frederick II of Denmark decided to sponsor Tycho’s ongoing
work, providing him with money to build an unparalleled
observatory for naked eye observations.
Over a period of three decades, Tycho and his assistants compiled
naked-eye observations accurate to within less than 1 arcminute—
less than the thickness of a fingernail viewed at arm’s length.
Because the telescope was invented shortly after his death, Tycho’s
data remain the best set of naked-eye observations ever made.
FOR NEXT TIME…
• Read box 4.2, section 4.5, and the first several sections of chapter 2.
• Homework 2: Chapter 4 Questions 6, 8, 9, 12 (Due Friday by 4:00 p.m. in my office)