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Introduction to Astronomy
Part 2 — Pale Blue Dot
Dr Adrian Jannetta FRAS
Northumberland Astronomical Society
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Summary
This presentation deals with history of astronomy from
antiquity to the start of the modern age (the era of Newton’s
discovery of the laws motion and gravity).
Antiquity Before 500BC. Periodic motion of the Sun and
Moon. Calendars and time keeping developed.
Classical 500BC to 1400AD. Measurements of the sky.
Mathematics, geometry and models of the universe.
Renaissance 1400 - 1650. Collection of data, refinement of
models. Invention of the telescope.
Modern 1650 - present. Discovery of physical laws
governing the universe.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Astronomy in antiquity
Early cultures identified celestial objects with supernatural gods
and spirits. Prehistoric astronomy was concerned with:
Daily motion of the Sun, moon and stars.
Seasonal variations of constellations.
Phases of the moon.
Eclipses.
It is likely that earliest astronomers were ‘priests’ and that their
understanding of the world around them was linked with
interpreting the movements in the heavens and therefore the
intentions of the gods and spirits.
Astronomy and astrology were synonymous.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Early observatories
Some ancient structures, such as Stonehenge, show a distinct
north-south alignment. These ancient structures probably
fulfilled dual astronomical and religious purposes.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Devising a calendar
Lunar and solar calendars
Reliable calendars became an essential feature in many
agricultural civilisations. Common calendar systems have
included:
Solar The year is divided into 365 or 366 days. This is
the period repetition of the seasons, or between the
Sun completing one complete cycle across the sky.
Lunar The year is divided into 12 lunar months of 354 or
355 days, with a month being the period between
new moons. This calendar slips by 11 or 12 days
per year with respect to the seasons.
Luni-solar A lunar calendar with some years containing 13
lunar months to stop the seasons getting out of
sync.
The Islamic Calendar is based on a lunar calendar. Each month
begins with a sighting of the hilal (young crescent moon after
new moon).
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Devising a calendar
The thirteen stone towers at Chankillo, Peru were erected around 2,400
years ago. The first and last towers mark the positions of the sun at the
summer and winter solstices.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Devising a calendar
Heliacal rising
Many stars in the sky are visible for
only part of the year. At other times
they are too close to the Sun.
A heliacal rising of a star is the day
it becomes visible in the dawn sky,
just before sunrise, after a period of
invisibility.
Dr Adrian Jannetta FRAS
The heliacal rising of Sothis (now
called Sirius) meant the annual
flooding of the Nile was about to
start.
This event, although inaccurate,
marked the start of the new year in
the Egyptian calendar.
Introduction to Astronomy
Classical astronomy
Greece
Greek astronomers made
observations of the sky. They
devised models and made
predictions. Logic and mathematics
were tools; it was the beginning of
the scientific method.
Pythagoras (c500BC) believed the
Earth was round because a sphere
was deemed to be a perfect shape.
Aristotle (c300BC) saw that the
shadow of the Earth on the Moon
during a total eclipse was always
curved. This implied the Earth was
spherical.
Eratosthenes (c240BC) estimated to
the circumference of the spherical
Earth to within 2% of its modern
value.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Classical astronomy
The size of the Sun and Moon
Aristarchus of Samos attempted to calculate the sizes of the Sun and Moon
their distances from the Earth. For example:
At half-moon (first or last quarter) then a right-angle (90 degrees) is formed
by the Sun-Moon-Earth line.
Aristarchus estimated the angle ϕ to be 87 degrees. This implied the Sun
to be nearly 20 times more distant than the Moon.
The Sun and Moon are roughly the same size in the sky, the implication
was also that the Sun was 20 times the diameter of the Moon and also
much larger than the Earth.
Such measurements gave credence to the Sun-centred (heliocentric) model
of the solar system.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Classical astronomy
Ptolemy’s Almagest
A significant amount of
astronomical knowledge from Greek
astronomy was embodied in the
work of Ptolemy’s Almagest.
Ptolemy advocated the geocentric
system and an unmoving Earth.
The Almagest was a hugely
influential collection of books. It
was adopted later by Islamic
astronomers and by the Catholic
Church.
Claudius Ptolemy (c85 — c165)
Dr Adrian Jannetta FRAS
Scribes stopped copying other
works; knowledge of earlier
astronomers was lost.
Introduction to Astronomy
Classical astronomy
Geocentric model
The geocentric model of the solar
system flourished because it
successfully accounted for the
motion of the Sun, Moon and
planets.
Stars were embedded in a distant,
outer celestial sphere.
It became increasingly
complicated as it attempted to
more accurately match the real
positions of the planets.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Classical astronomy
Retrograde motion of the planets
The planets generally travel eastwards among the constellations (prograde
motion).
The planets also stop to perform a loop in the sky, temporarily moving west
(retrograde) before continuing eastwards again.
Retrograde motion is particularly noticeable for Mars and Venus, but all
planets display it. Any model of the solar system had to account for this
motion.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
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Courtesy of UNL Astronomy Education
Classical astronomy
Islamic astronomy
Islamic astronomers salvaged
many of the manuscripts from
earlier eras (8th — 15th
centuries). These were later
passed back to western cultures
with the spread of Islam.
Great advances in
mathematics and science.
Many astronomical terms and
star names and terms come
from this era (Altair,
Aldebaran, azimuth, etc).
Persian astronomer Ulugh Beg
built an impressive observatory
at Samarkland for observing
the heavens.
Ulugh Beg (1394 — 1449)
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Renaissance astronomy
Rebirth of a sun-centred system
Copernicus was a Polish monk
(and a diplomat,
mathematician, economist,
artist and physician).
He revived the notion of a
sun-centred solar system
(heliocentrism) with planets in
circular orbits.
Heliocentric system was
simpler and accounted for the
observed motion of the planets
fairly well.
Nicolaus Copernicus (1473 — 1543)
Dr Adrian Jannetta FRAS
Heliocentric system was not a
perfect fit with observed
positions; discrepancies
remained because planetary
orbits weren’t actually circular.
Introduction to Astronomy
Renaissance astronomy
Heliocentric model
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Renaissance astronomy
Religion v Science
A sun-centred solar system conflicted with the scriptures. For
example:
Then spake Joshua to the LORD in the day when the LORD
delivered up the Amorites before the children of Israel, and he
said in the sight of Israel, Sun, stand thou still upon Gibeon;
and thou, Moon, in the valley of Ajalon.
Joshua 10:12
Copernicus delayed publication of his life’s work until he was on his
deathbed.
The first printed editions of his book contained a preface, not
authored by Copernicus, which attempted to deflect controversy. It
stated that the work probably wasn’t true; it merely provided a
practical method for calculating the positions of the planets.
As the ‘Copernican’ system spread it received a hostile reception from
religious authorities.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Renaissance astronomy
Tycho Brahe was a Danish
nobleman and astronomer.
An man of eccentric character!
A fervent supporter of
Ptolemy’s geocentric system.
Tycho built an observatory
(Uraniborg) on the island of
Hven.
Tycho Brahe (1546 — 1601)
Dr Adrian Jannetta FRAS
He was a prolific observer of
the planets - perhaps the
greatest observer prior to the
invention of the telescope.
Tycho extensively observed a
supernova (an exploding star)
in 1572.
Introduction to Astronomy
Renaissance astronomy
Tycho observed the stars and planets. He hoped his observations
would disprove the heretical ideas of Copernicus. Tycho was assisted
at Uraniborg by his German assistant, Johannes Kepler.
Renaissance astronomy
Inconstant stars
Tycho observed the appearance of a
new star in Cassiopeia in 1572. It
became as bright as Venus before
slowly fading from view. Tycho
referred to the star as ‘Nova Stella’.
David Fabricius noticed a new star
whilst observing Mercury in 1596.
The star brightened and then faded
from view. Unlike Tycho’s star, it
eventually reappeared months later.
It was named Mira, meaning
‘wonderful’ or ‘astounding’.
Another ‘nova’ was seen in 1604. These discoveries may have motivated
astronomers to make accurate star catalogues more systematic observations.
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Renaissance astronomy
Fall of the geocentric system
The geocentric system was discarded because of the contributions of Kepler
and Galileo in the early 17th century.
Johannes Kepler (1571 — 1630)
Dr Adrian Jannetta FRAS
Galileo Galilei (1564 — 1642)
Introduction to Astronomy
Renaissance astronomy
Kepler’s laws of planetary motion
Kepler used Tycho’s observations of the planets as evidence of
the heliocentric system of Copernicus.
His attempts to fit the data to circular orbits failed. Elliptical
orbits fit perfectly.
Kepler formulated three laws of planetary motion.
The orbits of the planets are ellipses. The Sun is at one
focus of the ellipse; the other is empty.
The radius vector of the planet sweeps out equal areas in
equal time periods.
The cube of the radius vector r of the planet is proportional
to the square of the orbital period T .
So that r3 ∝ T 2 .
The need for epicycles and deferents was abandoned in his
simplified version of the solar system.
Renaissance astronomy
Kepler’s laws of planetary motion
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Renaissance astronomy
Galileo and the telescope
Galileo used a telescope to
make astronomical
observations. He discovered:
The moons of Jupiter
Mountains and craters on
the Moon
The Milky Way is
comprised of countless
stars
The phases of Venus
Sunspots on the Sun
Many of his observations
were contradictory to the
geocentric system.
Galileo published his findings as support for the heliocentric solar
system but was later forced to recant his testimony by Church and
spent the last part of his life under house arrest.
Renaissance astronomy
Galileo’s observations
Records of Galileo’s observations of Jupiter from Sidereus Nuncius
(‘Starry Messenger’) in 1610. The four largest moons are still referred
to as ‘Galilean moons’.
Modern astronomy
Isaac Newton and gravity
Isaac Newton developed new
mathematics (calculus) to
understand problems in
physics connected with motion.
He identified the rule
governing the behaviour of
gravity and demonstrated that
it accounted for Kepler’s laws.
Newton studied light and
optics - paving the way for the
new discoveries based on the
analysis of light.
Isaac Newton (1642 — 1727)
Dr Adrian Jannetta FRAS
Newton’s work heralded a new
age of explanation, prediction
and discovery.
Introduction to Astronomy
Pale Blue Dot
A portrait of the planets of the solar system composed from images taken
by the Voyager 1 spacecraft in 1990. At that time Voyager 1 was 4 billion
miles from Earth and 32 degrees above the plane of the ecliptic.
American astronomer Carl Sagan described the Earth as a pale blue dot.
This image highlights a significant reduction in our perception of the size of
the Earth in comparison to earlier eras. Alternatively, it can be taken a
step in a growing realisation that the universe is much larger than earlier
astronomers could possibly comprehend!
Dr Adrian Jannetta FRAS
Introduction to Astronomy
Further reading
P Aughton.
The Story of Astronomy.
Quercus, 2011.
A Aveni.
People and the Sky: Our Ancestors and the Cosmos.
Thames & Hundson, 2008.
H Couper and N Henbest.
The History of Astronomy.
Cassell Illustrated, 2007.
M Hoskin, editor.
The Cambridge Illustrated History of Astronomy.
Cambridge University Press, 1997.
P Moore.
Patrick Moore’s History of Astronomy.
Harpercollins, 1984.
Dr Adrian Jannetta FRAS
Introduction to Astronomy