From the ancient sky-watchers of Babylon to space-age cosmology, the story of astronomy - Copernicus, Kepler, Galileo, Newton, Hubble.
 

Outline:

Astronmy is born of the need to measure time - motions of the heavens indicate the date, the season, when to sow, when to prepare for winter.  Some 6,000 years ago, the Babylonians chart celestial movements, group stars into patterns and observe "wandering stars" - in reality, the five nearest planets.  The Chinese map the cosmos and record comets

The Egyptians divide the year into 365 days, while the Maya make a calendar of 584 days.  The Greeks place the Sun, the Moon, their five known planets, and the background stars in crystal spheres that orbit Earth.  Eratosthenes shows that the Earth is round.  Hipparchus maps the sky and works out the relative distance to the Sun and Moon.

Ptolemy believes the planets orbit Earth and develops a theory to fit his observations.  He's wrong, but his ideas persist for nearly 1,500 years.  Then Copernicus puts the Sun at the centre of our planetary system and Johannes Kepler devises his three laws of planetary motion.

The start of modern astronomy as Galileo turns his telescope on the sky.  Isaac Newton works out gravity, splits light, and improves the telescope.  William Herschel discovers Uranus, builds the world's biggest reflecting telescope, catalogues the stars and identifies the shape of our galaxy, the Milky Way.  The Earl of Rosse builds a larger telescope and sees the spiral structure of galaxies.

The 19th century yields two key tools - spectroscopy and photography.  In the 1920s, Edwin Hubble employs both to record vast numbers of galaxies and the fact they are all racing outwards.  The advent of radio astronomy and ever more powerful telescopes peer back to the beginnings of space and time.
 
 

Sub-chapters:

First Astronomers

*  The need to measure time - to know the date, the season, when to sow, when to prepare for winter.

*  In ancient times, people were familiar with the motions of the heavens.  The establishment of calendars.

*  The Babylonians, about 6,000 years ago, chart celestial movements, group the stars into constellations and create the signs of the Zodiac.  The "wandering stars" observed by the ancients were, in fact, the five brightest planets.

*  Meanwhile, Chinese astronomers map the cosmos and carefully record phenomena such as comets.
 

Calendars and Spheres

*  The Egyptians use the dawn rising of Sirius to foretell the annual flooding of the Nile.  Although believing their gods "control" heavenly cycles, the Egyptians devise a 365-day calendar.

*  Much later, in Central America, the Maya invent a 584-day calendar based on the cyclical movements of Venus.

*  Early ideas that "explain" the mechanics of the heavens - an intriguing Hindu view.

*  The Greeks place the Sun, the Moon, the five known planets and the background stars in a series of concentric crystal spheres that surround the Earth.
 

Measuring and Observing

*  In 200 BC, Eratosthenes shows that our world is round by using geometry to work out the circumference of the Earth.

*  Hipparchus compiles the first stellar map, records the brightnesses of stars and, by observing eclipses, calculates the relative distance to the Moon and Sun.

*  Ptolemy believes that the planets orbit Earth.  He devises complex explanations to fit his observations - even the strange little "reverses" made by some planets.  Although he's wrong, Ptolemy's ideas persist for almost 1,500 years.
 

Planetary Orbits

*  Nicolaus Copernicus, the 16th century Polish cleric, reasons that Earth and other planets orbit the Sun and that the Moon orbits Earth.  But the orbits, he believes, are perfect circles.

*  In the early 17th century, Johannes Kepler devises three laws of planetary motion.

*  Kepler uses the precise observations of Tycho Brahe to show that the planets orbit the Sun in ellipses, not circles.  Kepler demonstrates that a planet moves quickest when closest to the Sun and slowest when farthest away.  He also reveals that the revolution periods of planets nearer to the Sun are shorter than those farther away.

*  Importantly, Kepler correctly explains why some planets appear to make a backwards loop in the sky as they are overtaken by Earth.
 

Through the Telescope

*  Early in the 17th century, Galileo is first to turn a telescope on the sky.  Among many discoveries, he sees craters on the Moon, moons around Jupiter's and spots on the Sun.

*  Christiaan Huygens discovers Saturn's largest moon and that the Saturnian rings are detached from the planet.  Giovanni Cassini finds a gap in the rings, now called the Cassini Division.

*  Isaac Newton revolutionises astronomy.  He works out gravity, splits light and shortens the telescope with the use of mirrors.

*  In the 18th century, William Herschel discovers Uranus, builds better reflecting telescopes, catalogues the stars and recognises that our galaxy, the Milky Way, is a flattened disk.
 

Spectroscopy and Photography

*  In Ireland, in the mid-19th century, the Third Earl of Rosse builds an even larger telescope.  He glimpses galaxies beyond our own and sketches their spiral structure.

*  The 19th century yields two new astronomical tools - photography and spectroscopy.  The latter is the analysis of light to work out the nature of objects in space.

*  In the 1920s, Edwin Hubble uses both tools to record galaxy after galaxy.  He discovers they are all racing outward.

*  Where optical telescopes cannot see, radio telescopes detect ever more distant objects.

*  Today, increasingly powerful telescopes penetrate back towards the beginnings of time and space.
 
 

Background:

The First Telescopes

It is not clear who invented the telescope.  Indeed, it may have been invented and re-invented many times.  By the beginning of the 17th century, spectacle lenses had been in use in Europe for about 300 years.  During that time, on several occasions, two spectacle lenses and a tube must surely have been arranged, purely by chance, to form a telescope.

By 1608 a Dutch spectacle maker, Hans Lippershey, had built an eyeglass that could make distant objects appear much closer.  He called it a "device for seeing at a distance" - which is what "teleskopos" means in Greek.  Hence, our modern word telescope.

The Italian astronomer, Galileo Galilei, heard about the invention.  The following year, in1609, he constructed a telescope of his own.  He used an organ pipe and two lenses - one convex and one concave.  Later, he built a number of improved telescopes.  They were the finest in the world.  Resting in a cradle on a stand, Galileo's best telescope could bring the heavens 30 times closer.
 

Galileo's Discoveries

Galileo first turned his telescope on the sky in 1610.  He saw craters and mountains on the Moon, previously believed to be smooth.  By measuring shadows cast by the lunar mountains, he calculated that some of them must be as much as six kilometres high.  Suddenly, thousands of stars, never before seen by human eyes, were discerned through Galileo's telescope.  Although magnified 30 times, they still appeared as mere points of light.  Galileo reasoned that they must be a very great distance.

On January 7, 1610, when observing Jupiter, Galileo noted three bright star-like objects close to the planet.  The following night, their positions had shifted.  On subsequent nights, he saw them move again and again.  As he sketched the planet and the attendent objects, Galileo  concluded that they must be moons orbiting Jupiter, just as our Moon orbits Earth.  He was right.

Galieo also observed that Venus appeared quite different to Jupiter.  Venus had no moons and it appeared to change size and shape as it moved across the sky night by night - much like our  Moon.  At times Venus would be a large, thin crescent.  A few weeks later it would be a smaller "half Venus".  And just before disappearing into the glare of the Sun, Venus would appear as a tiny, fully illuminated disk.  Galileo realised that this could be explained only if Venus moved around the Sun - and not around Earth as was previously believed.

In his solar observations, Galileo discovered dark sunspots and recognised that the Sun rotated.
 

Isaac Newton and the Spectrum

The English genius Isaac Newton  (1642-1727) first investigated how coloured light could be made from white light.  In 1665, Newton was doing some experiments with lenses.  He noticed that the images formed by the lenses - which he had made himself - were not clear.  They seemed blurred and surrounded by a narrow fringe of coloured light.  Newton made more lenses, taking great care when polishing them.  But he always met the same problem.  Finally, he concluded that the fault was not with the lenses.  His hunch was that it was something to do with the refraction of light itself.

Newton projected a narrow beam of sunlight - about 8 mm across - from a hole in a window-blind across a darkened room.  About five metres from the window, the beam produced an image of the Sun on a white screen.  When he placed a triangular glass prism in the beam, the rays bent upwards.  Newton observed that the image on the screen was stretched out into a broad band.  It was coloured at either end.  Subsequent experiments, using a narrow slit, revealed that the image was actually made up of a number of overlapping coloured patches.

Newton had discovered what we now call a spectrum.  The colours were red, orange, yellow, green, blue, indigo and violet.  By separating each of the seven main colours from the rest, Newton showed that the colours themselves could not be changed by refraction through a further prism.

Newton then allowed the whole spectrum to fall on another prism.  This was placed the opposite way up to the first prism.  A white image was obtained.  If just one colour was removed from the spectrum before passing it into this second prism, white light was not produced.  Newton realised that sunlight, or white light, was a mixture of seven different colours.

So why is white light separated into its main colours by a prism?  Each colour of light travels as a wave and each has a different wavelength.  The wavelength of red light, for instance, is seven ten-thousandths of a millimetre.  The wavelength of violet light is four ten-thousandths of a millimetre.  When passed into the glass prism, the movement of the waves is hindered.  They travel more slowly in glass than in air.  As a result, each colour is bent or refracted.  The colour with the longest wavelength - red - bends the least.  That with the shortest wavelength - violet - bends the most.  This is because violet light waves travel more slowly through glass than do red light waves.  The more slowly the coloured wave travels through the prism, the more it is bent or refracted.
 

Spectroscopy

The key to determining the composition and conditions of a moon or a planet or a star or a galaxy - indeed of any astronomical object - is its spectrum.  The technique used to capture and analyse such a spectrum is called spectroscopy.

In spectroscopy, the light emitted or reflected by an object - or, more correctly, its electromagnetic radiation -  is collected by a telescope.  The light is then spread into its component colours to form a spectrum.  By studying the spectrum of a particular object, scientists can work out what it's made of - among other things.

That is because light is emitted from atoms when the electrons within the atoms shift between orbits.  An atom of hydrogen, for instance, will emit a different light to an atom of helium.  This enables astronomers to search for the "signature" of different elements in an object by measuring how much light is present at each wavelength of the object's spectrum.

Spectroscopy is a a vastly important tool for astronomers.
 

Newton's Law of Gravity

Isaac Newton is towering figure.  As well as splitting sunlight into its component colours, he shortened and improved the telescope by using mirrors.  He formulated three laws of motion.  And even today, he influences scientific investigation.  Newton began the practice of comparatively testing theories by experiment and then refining ideas as necessary.

Perhaps his greatest contribution to science was his work with gravity.  Newton discovered that gravity operates the same way throughout the Universe.  Gravity abides by a universal law.  On the basis of his studies, Newton concluded that every mass exerts a force of attraction on every other mass.  He showed, moreover, that the strength of the force is directly proportional to the product of the two masses, divided by the square of the distance between them.  If m and M are the masses of any two bodies, and the distance between their two centres is r, then the strength of the attractive force, F, between them is given by:

F = GMm/r2

The factor G in this equation is a constant of proportionality whose value is found by measuring the force between two bodies of known mass and separation very precisely.  If M and m are measured in kilogrammes and r is measured in metres, and F in SI units, then G = 6.67 x 10-11 m3 kg-1 s-2.
 

Kepler's Laws of Planetary Motion

The German mathematician Johannes Kepler (1571-1630) formulated three laws of planetary motion.  These laws were based on empirical evidence taken from Tycho Brahe's detailed and very precise visual observations of the motions of the planets, particularly Mars.  Kepler's Laws are:

1.  The orbit of each planet is an ellipse - with the Sun at one focus of the ellipse.

2.  As each planet orbits the Sun, an imaginary line connecting the Sun and the planet - known as the radius vector - sweeps out equal areas in equal intervals of time.  This means that the speed of a planet in an elliptical orbit will vary as the planet orbits the Sun.  The planet will be moving fastest when it is closest to the Sun - a position known as perihelion - and slowest when farthest away - a position known as aphelion.

3.  The squares of the sidereal periods of the planets are proportional to the cubes of their mean distances from the Sun.

Although Kepler's Laws vastly increased knowledge of planetary motion and behaviour, the physical basis of the laws was not understood until Isaac Newton formulated his Law of Gravity.
 
 

Links for Further Information:

Detailed page explaining Newton's Universal Law of Gravitation - recounting the ideas Newton formulated and how he tested them.  Includes images and the equations used by Newton - plus the application of theories on weight and gravity to planets and stars.
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

Johannes Kepler's Laws of Planetary Motion.  Explains elliptical orbits and Kepler's three laws - with images.
http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

The Hubble Constant explained - the rate at which the Universe is expanding from the time of the Big Bang.
http://csep10.phys.utk.edu/guidry/violence/hubble_constant.html

Galileo's observations through the telescope and his theories on the law of motion.  Includes images illustrating how Galileo's observations altered theories on the structure of the Solar System.
http://csep10.phys.utk.edu/astr161/lect/history/galileo.html

The Universe of Aristotle and Ptolemy - explaining their theories on the structure of an Earth-centred Universe, complete with illustration.
http://csep10.phys.utk.edu/astr161/lect/retrograde.aristotle.html

Calendars through history - from the Roman Lunar Calendar to the Gregorian Calendar.
http://csep10.phys.utk.edu/astr161/lect/time/calendars.html

Interesting page explaining how the motion of the sky can be used to record time.
http://csep10.phys.utk.edu/astr161/lect/time/timekeeping.html
 
 

Questions and Activities for the Curious:

1.  Describe the method used by Eratosthenes to measure the diameter of the Earth.

2.  Outline Kepler's three laws of planetary motion.

3.   What are the main astronomical discoveries made by Galileo?

4.   Explain, with a diagram, why, each year, planets that are farther from the Sun than the Earth make a loop in the night sky.

5.  What are the major difference between the planetary systems of Ptolomy and Copernicus?

6.  Describe three of William Herschel's major contributions to astronomy.

7.  Some people still believe that the Earth is flat.  What proof is there that our planet is round?

8.  Tycho Brahe argued that the Sun orbited the Earth but that other planets orbited the Sun. Could Tycho's model explain the phases of Venus as observed by Galileo?