Download Introduction to Astronomy, Lecture 2

Introduction to Astrophysics
Lecture 2: Landmarks in the history of astronomy
NB: The material in this lecture is covered in almost
any introductory astronomy textbook, so it is not really
necessary to take notes. The slides will also appear on
the WWW.
Prehistoric astronomy
There is endless evidence of prehistoric attempts to measure
various astronomical phenomena.
The main motivation would be to use the sun and the moon to
judge times, for example for planting of harvests.
Many civilisations were very sophisticated, eg in China in
2159BC two astronomers were executed for failing to predict
an eclipse!
The Ancient Greeks
By far the most famous early astronomers are the ancient
Greeks. Between about 500BC and 100BC, they built a
picture of the Universe which dominated for over 1000 years.
Thales (624 - 545 BC)
Realised that celestial objects were at different
distances, that the Earth was spherical, and that the
light of the moon was reflected sunlight.
Pythagoras (582 - 500 BC)
Produced the first geocentric model of the
Universe, with everything making perfectly circular
orbits around the Earth.
Plato (428 - 347 BC)
Invented the idea of epicycles, later `perfected’ by Ptolemy.
Aristarchus (310-230 BC)
 He created the first heliocentric cosmology; that is, he was the
first to propose that the Earth, and the other planets, went around
the Sun. 1,750 years later Copernicus will claim the credit.
 Aristarchus is also famous for devising a way of measuring the
size of the moon in terms of the size of the Earth.
 Hipparchus later used this to conclude that the moon had about
one quarter the radius of the Earth (modern value 0.27) and that its
distance was about 60 Earth radii away (modern value 60.4).
However, his calculations did not give absolute values; only those
relative to the unknown radius of the Earth.
 Aristarchus also measured the relative distances of the Sun and
Moon. He underestimated that badly, but even so realised that the
Sun was bigger than the Earth.
Eratosthenes (276 - 195 BC)
Measured the circumference of the Earth with amazing
accuracy. He did so with a particularly powerful piece
of observational technology, namely a long stick.
Hipparchus (190 - 120 BC)
Powerful insights into many aspects of astronomy.
Invented the magnitude scale, worked out the size of the
moon, developed star and eclipse catalogues.
Ptolemy (127 - ??? AD)
Perfected the geocentric model with epicycles. It will
go unchallenged for 1300 years.
An example of an epicycle.
Nicolaus Copernicus (1473-1543)
Credited with the heliocentric
model of the Solar System.
He divided the planets into the
`inferior’ ones closer to the Sun
than the Earth, and the `superior’
ones outside the Earth’s orbit.
Only planets out to Saturn were
known at that time.
The orbits were all taken as
circular.
Finally we shall place the Sun himself at the centre of the Universe. All this
is suggested by the systematic procession of events and the harmony of the
whole Universe, if only we face the facts, as they say, `with both eyes open'.
Nicolaus Copernicus
Tycho Brahe (1546 - 1601)
Almost as famous for his silver nose
(he lost the original in a duel) as for
his observations. With the support of
the King of Denmark, he developed
instruments of unprecedented quality,
capable of positional accuracies of
one arcminute.
He is less remembered for his
cosmological model, an attempted
compromise in which the Sun goes
round the Earth but the planets round
the Sun.
Johannes Kepler (1571-1630)
Painstaking studies of Tycho’s
observations led him to the nowaccepted conclusion that planets
moved not on circles but on
ellipses.
He went on to formulate his
three laws of planetary motion,
which we will be studying in a
later lecture.
Galileo Galilei (1564 -1642)
Didn’t invent the telescope (and
might well not have dropped
cannonballs off the leaning tower
of Pisa either).
But he is considered the inventor of
the modern scientific method, with
its emphasis on experimental
verification of theoretical models.
Galileo was the first to properly exploit the telescope for
astronomical purposes.
Considering that his original telescope had a magnification power
of only three, he made amazing new discoveries, including the four
large moons of Jupiter, sunspots, and the rings of Saturn.
GALILEO
NOW
His belief in Copernicus’s heliocentric Universe nearly led
him to a nasty end.
Isaac Newton (1643 - 1727)
Widely acknowledged as one of the
greatest scientific geniuses of all
time. Co-inventor of calculus and
founder of the modern theories of
dynamics and optics, he also
developed a theory of gravity.
His theory explains Kepler’s Laws,
and shows that the gravity we feel
on Earth is the same as the gravity
that governs planetary orbits.
Despite the development of the telescope, it would be many years
before the remaining planets of the solar system were discovered.
Uranus was discovered in
1781 by William Herschel.
Neptune was first observed in
1846, after its position was
predicted by Adams and
Leverrier by analysing
perturbations to the motion of
Uranus...
… while Pluto was not
discovered until 1930,
and its moon Charon
only in 1978.
Next on `Introduction to
Astrophysics’
There will be no class
tomorrow morning.
The next lecture will be on
Monday at 11:00.