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We live on the surface of a sphere which orbits our Sun while also spinning on its axis. The Sun is a star, and the Earth is one of the planets of the solar system.

The growth of our understanding of the structure and size of the universe depends on accurate measurements and observations. Astronomers are often working at the limits of their instruments, so many measurements are estimated which may have substantial errors.

Ancient Astronomy
The idea that the Earth is flat has never been an accepted scientific theory. The Ancient Greek astronomers knew that the Earth was a sphere.
They observed that:     - the Earth’s shadow was always circular during an eclipse of                   the Moon
the north pole star was higher in the sky the further you went  north
as ships sail out to sea, the hull disappears below the horizon first, and the masts and sails later

Erastothenes
He estimated the diameter of the Earth, by measuring the length of the shadow of a vertical post at noon on midsummer day at his home in Alexandria. He knew that a similar post in Cyrene, 800km to the south, cast no shadow at that moment, because the sun was directly overhead. The difference is due to the curvature of the Earth - and from his measurements he calculated a value for the Earth’s diameter which is remarkably close to the value of 12756km that we accept today.

Everything we see in the sky seems to move around us daily. The Sun, the Moon and the stars all follow circular paths across the sky. The stars move together, so their pattern (the constellations) stay the same.

In the past, Ancient Greek astronomers observed a few object in the night sky, that appeared to move among the stars - not fast enough to see them move. They called them wanderers. Now they’ re known as planets. We can see five of them with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn.
From time to time, planets appear to move backwards and retrace its steps forward (retrograde motion).

Aristotle (384-322 BC) was the most famous philosopher of his day - and one of the most influential ever. Starting from the ideas of Eudoxus, he developed a model with 55 spheres. In his model, the Earth was in the centre. All the heavenly bodies were carried round on invisible crystalline spheres, as the sphere was regarded as the ‘perfect’ shape. The heavens, according to Aristotle, are made of a different kind of matter from that on Earth. They are perfect and unchanging whereas, on Earth, there is imperfection and change.
Even with 55 spheres, this model still couldn’t predict the exact positions of the planets.

230bc: Apollonius of Perga suggested that each planet moves in a small circle, while the centre of this moves in a larger circle centred on the Earth.



Apollonius’s idea was picked up and developed by Claudius Ptolemy (100-170). He made some modifications and added some extra circles. He described this very complex model in the Almagest. It had become so complex that most people no longer believed that these spheres and circles really existed - they were just a way of calculating the positions of the planets. It was said to ‘save the appearances’. But it worked remarkably well. For over 1000 years, it was the most accurate way of predicting the positions of the planets.

The Copernican Revolution
1260: Thomas Aquinas developed a philosophy which combined the ideas of Ptomely and Apollonius with Christian teaching. This became the accepted viewpoint of the Church. They thought it was God who kept the spheres moving.

Early in the sixteenth century, Nicolaus Copernicus wrote a book which started a revolution in thinking about the universe. He suggested that the motions of the stars and planets would look much simpler if we imagined the Sun at the centre, with the Earth and the other planets circling around it. In this model, the Earth is a planet. It spins on its axis, which causes day and night and explains wy we see all the celestial bodies moving across the sky.
In Copernicus’s model, the retrograde motion of the planets is due to the fact that the planets closer to the Sun are moving faster than those further away. As the Earth overtakes Mars, than Mars will move backwards against the background of the more distant stars.

Copernicus’s book (On the Revolutions of the Heavenly Spheres) didn’t cause an immediate stir. But in the years that followed, social and political events led to a hardening of attitued, and a greater fear of unorthodox ideas. One major factor was the Counter Reformation - the response of the Roman Catholic Church to the Protestant Reformation.

Three people played key roles in getting the heliocentric model generally accepted:
Tycho Brahe, Galileo Galilei and Johannes Kepler

Tycho Brahe
1559: 13-year-old Brahe decided to become an astronomer when he observed a partial solar eclipse. It was not the eclipse that impressed him, but the fact that the astronomers where able to predict it.

24 August 1563: something happened that was to shape the rest of his career
There was a conjunction (two planets appeared so close together in the sky that they seemed to be a single bright object) of Saturn and Jupiter. Like the eclipse, this had been predicted but the prediction was several days out.
This wasn’t good enough for Tycho, who thought that Ptolemy’s model had to be improved. To do so, he would need more accurate data. By the age of 26 he had set up the greatest observatory in Europe. He built a quadrant, which made him able to measure the positions of stars and planets to an accuracy of one minute of arc (1/60).








Two observations led him to reject Ptolemy’s model of crystalline spheres.
1572: He observed a supernova (very bright new star which appeared for a few months then gradually dimmed). Thich challenged the accepted view that the heavens were perfect and unchanging.
1577: He observed a comet and studied it over several weeks, making careful measurements to estimate its distance from Earth. He concluded that its path took it through the orbits of several planets. Planets couldn’t be mounted on crystalline spheres. Rather than switch to Copernicus’s model, he proposed a compromise.
Earth at the centre, Sun and Moon going round it, and other planets going round Sun.

This idea was later used by Johannes Kepler

Galileo Galilei:
Galileo seems to have been convinced, from his early days, of the heliocentric view. He was also a sincere Catholic and became concerned that the Church was in danger of making an error in supporting the geocentric view. At the time there was an uneasy truce between astronomy and the Church. Astronomers were allowed to use the Copernican model as a calculating model, but not to claim that it was a picture of how things really were. Galileo was convinced that evidence would eventually prove the rightness of the heliocentric view. He expressed his views very confidently.

1609: Galileo obtained one of the first telescopes. He saw how useful it could be and made a better one to observe the night sky. From 1609 till 1615, he observed mountains on the Moon and sunspots.
He noticed that:    - heavenly bodies were not ‘perfect’ spheres.
            - the moons of Jupiter showed that objects could orbit around another               centre than Earth
Venus had phases

1630: Galileo was ready to write his book: The Dialogue Concerning the Two Chief World Systems.
He believed that his theory of the tides was clinching evidence of the Earth’s movement, though we now know that it was completely wrong.

He constucted the book as a dialogue between three characters:
Sagredo: views of Galileo
Simplicio: opposed the ideas of Galileo
Salviati: open-minded and willing to listen to all arguments

Reaction of Church: All copies were seized.

Johannes Kepler
He seems to have been convinced of the heliocentric model from an early age. He thought that it made it possible to explain the movements of stars and planets, rather than just predict them.

1st January 1600: Kepler fled from Austrai to escape from conflict between Catholic and Protestant factions, and took a job as Brahe’s assistant in Prague. When Brahe died rather suddenly in 1604, Kepler inherited his data. Shortly before he died, Brahe had set Kepler the task of working out the orbit of Mars. The problem that Kepler faced was that he couldn’t get Tycho’s data to agree exactly with the predictions of the Copernican model(they were out by six minutes of arc).
After several years of trying to fit different kinds of circular orbits to the data, he came to the conclusion that the orbits of the planets were not circles, but ellipses. Using elliptical orbits, Kepler drew up a large set of tables of predicted planetary positions (Rudolphine Tables).

An explanation for the planetary motions
1664: he started thinking about, Why does the Moon orbit around the Earth at a constant distance, unlike objects near the Earth which fall towards it?
He argued that the Moon is constantly falling towards the Earth. Without the force of the Earth’s gravity, it would move in a straight line. The gravity force acting on the Moon makes it deviate from that straight line, towards the earth - and keeps it in an orbit.
To check this, Newton worked out how strong the gravitational force of the Earth should be at the distance of the Moon, compared with its strength at the Earth’s surface.
Moon (384000km) --- Object near Earth’s surface (6400km)             60x
If the gravitational force falls off as the square of the distance, it will be 1/3600x weaker at the Moon than it is on Earth. So, Moon should ‘fall’ 1/3600th as far every second as an object on Earth.

An understanding that works
The Voyager missions were planned by sciencists at NASA to take advantage of a rare arrangement of the four outer planets in the late 1970s and early 1980s. All four were lined up on the same side of the Sun, making it possible for a spacecraft to follow a path which would pass close by each. In the summer of 1977 Voyagers 1 and 2 were launched.
Voyager 1: passed close by Jupiter and Saturn
Voyager 2: passed all four outer planets and some of their moons. It took 12 years adn travelled 7128 million kilometres to reach Neptune.

How far are the stars?
During the 1640s and 1650s, the heliocentric model took hold as the accepted view. The simplicity of Kepler’s model based on elliptical orbits and the very accurate predictions were what convinced the astronomers.

When Copernicus first proposed his heliocentric model, astronomers looked for his stellar parallax but found none. Some thought that this showed the heliocentric view was wrong, others thought that it showed the stars were very far away.

Astronomers used the brightness theory (closest star will be bright) an donther ‘hunches’ to select stars to measure accurately.

New techniques
Two technological developments in the 19th century plaed a key roel i the growth of knowledge of the universe:
invention of photography. The great advantage was that observations could be recorded and studied later on. This made it much easier to spot changes.
Spectroscopy. Using a prism, Newton showed that sunlight is made up of colours, from red to violet.
1800: Thomas Young showed that two light beams produced interference, thus convincing scientists that light travels as a wave. Subsequent work showed that the colour of the visible light depended upon its wavelength.
long wavelength: red          short wavelength: violet
1814: Joseph Franhofer noticed that the spectrum of the Sun is crossed by many dark lines. These weren’t explained until 1859, following studies by Robert Bunsen and Gustav Kirchhoff. They found that the spectrum of a glowing gas consisted of bright lines of different colours, with a pattern that was characteristic of the element present. Kirchhoff realised that the lines Fraunhofer had seen were due ot the light from the Sun’s surface passing through the cooler vapour surrounding it. He published his findings: the Sun is composed mainly of hydrogen and helium, and the gases surronding it contain other heavier elements found on Earth. So he had evidence for the first time that celestial objects are made of the same elements as the Earth.

The structure of the Universe
Most objects we observe in night sky: stars
Many fuzzy objects which were, in the past, called nebulae. The most prominent is a hazy band that runs across the sky, called the Milky Way. The Magellanic Clouds, visible from southern hemisphere only, are two more.
Another smaller one: Constellation of Andromeda. These had puzzled astronemers for centuries. Two views:
they were clouds of gas and dust, relatively close to us
they were clusters of stars, a very long way away

Immanuel Kant - supporter of the second view.
He speculated that the Milky Way was really our edge-on view of our own galaxy, and that the nebulae are other galaxies beyond our own. In 1755, he wrote the Nebulae are systems of stars lying at immense distances. He called them ‘island universes’.

William Herschel - observed and documented nebulae of different kinds.
Some appeared to be stars surrounded by a halo of gas, others he took to be distant clusters of stars. He proposed a classification of five types.
Herschel’s basic idea was right - the nebulae are not all the same kind of thing.

Globular clusters and the Milky Way
With powerful telescopes, individual stars in some nebulae could be observed, and some of these were Cepheids. Harlow Shapley embarked on a programme to measure distances to many of these star clusters. He found that the cluseters seemed to form a huge sphere. He suggested that the centre of these clusters is the centre of the galaxy - and hence that the Sun is nowhere near the centre of the Milky Way.

Not all of Shapley’s conclusions turned out to be right. His distance measurements were all considerable over-estimates. As a result, his estimate of the size of the Milky Way galaxy was more than three times the size we now believe it is (100.000 light years). Because he thought the Milky Way was so large, he suggested that it was the whole universe and everything within it.

Heber Curtis argued that some nebulae were galaxies far beyond the Milky Way (‘island universes’ view). The issue was resolved not by debate (26th April 1920), but by evidence.
The evidence was provided by Edwin Hubble.

Hubble was studying the Andromede Nebula. Magnification of his telescope was sufficient to enable him to detect a few individual stars in the nebula. After taking photograpsh, Hubble noticed that one of these stars had varied in brightness. His answer: the star was 900.000 light years away, many times further than any star in our galaxy.

Over the next years, he found some more Cepheid variables in the Andromeda nebula and checked his calculations. The results were consistent. The Andromeda nebula was a separata ‘island universe’, far beyond our galaxy.

An expanding universe
Vesto Slipher reported an unexpected discovery. He had found evidence that several spiral nebulae seemed to be travelling away from us at incredible speed (up to 600 miles/second). Evidence came from measurements of their spectra and a phenomenon called the Doppler effect. Their spectra were shifted towards the red end.

Doppler Effect (sound)
A shorter wavelength means a higher frequency and so a higher pitch. The apparent wavelength of waves is shorter when the sound source is approaching you, and longer when it is going away from you.

Red Shift (light)
Source moving towards you: decreasing wavelength, more blue than usual
Source moving away from you: increasing wavelength, redder than usual
Light from a glowing gas: its spectrum will be a series of lines
light from a receding object: longer wavelengths than lines in spectrum of the same element have in a laboratory.

Around the same time as Vesto Slipher, astronomers were arguing about the imlications of Einstein’s theory of general relativity. They suggested that gravitation is due to the curvature of space by all objects that have mass. Prediction of theory: light should bend by a certain amount as it passes a massive body.

Einstein was aware that his theory had implications for cosmology. His equations led to conclusion: universe is expanding or contracting.

1922: Aleksander Friedmann published a paper which showed that Einstein’s equations concluded that the universe is expanding at a changing rate.

Edwin Hubble’s theory
Evidence for this expansion: Edwin Hubble. He used the same method as he used to measure the distance to Andromeda Galaxy (based on Cepheid variables).

He worked out the intrinsic brightness of the brightest stars in each galaxy. This enabled him to confirm that the brightest stars in every galaxy had roughly the same intrisic brightness.
He observed the brightest stars in the more remote galaxies and assumed that they too would have the same intrinsic brightness as those in the nearer galaxies.

By comparing this with their apparent brightness he was able to work out how far away they were.

Hubble measured the red shifts of these galaxies to calculate the speed at which they were moving. When he plotted this against their distance, he found a pattern.

The distance to other galaxies can be estimated by measuring the red shift, and reading their distnace off Hubble’s graph. Bigger red shift, further away.

Big Bang
Hubble’s work showed that the universe is expanding. This suggests that it started with a big bang. First some astronomers resisted this idea because it implies a creation at one moment in the past. It also implies that the first stage in the evolution of the universe cannot be explained by the normal physical laws. These astronomers argued for a ‘steady state’ theory, so the appearance of the universe would remain constant, even though it’s expanding.

Problem with big bang theory: its estimate of the age of the universe.
Hubble’s measurements of speed at which galaxies were moving apart and his measurements of their distances, he was able to measure how long ago the big bang occured. Answer: 2 billion years.
1929: estimated age of Earth: 3.4 million years.
It’s not possible that Earth is older than the universe.
Not solved until 1952: it was then shown that Hubble observed a different type of Cepheid variables than those studied by Henrietta Leavitt. Correction: Universe-age: 4 billion years.

Echo of the big bang
In 1964, Robert Wilson and Arno Penzias, were working with a radio antenna at Bell Laboratiories in New Jersey. They were studying a supernove remnant. They kept getting other radio noise, which they didn’t want.  This was constant whatever direction they looked in, so it could not be coming from a source on Earth. They thought it might be a fault in the antenna.

Meanwhile, two Russian astronomers had recently published a paper arguing that something must be wrong with the big bang theory, because the background radiation it predicted would surely have been observed by antennae such as the one at Bell Laboratories.

January 1965, Penzias happened to mention the radiation he and Wilson were observing in a phone call with another astronomer colleague. In may 1965 Penzias and Wilson published a paper on their results. They had found evidence of the big bang. Penzias and Wilson were awarded the Nobel Prize for Physics in 1978.

Dark Matter
Every bit of matter in the universe attracts every other bit, because of the gravitational force between any pair of masses.
Astronomers can estimate the mass of the matter in a galaxy by measuring its speed of rotation. Vera Rubin did this for the Andromeda galaxy, and found that their mass was almost 10x bigger than it needed to be to account for the amount of radiation they emit. This sugeest that as much of 90% of th ematter in these galaxeis is invisible - dark matter.

Dark Energy
Its existence was first proposed in 1998 to account for a surprising and unexpected finding. Two independent teams of astronomers were studying distant supernovae to try to measure the rate at which the expansion of the universe was slowing down. But their observations suggested that the expansion was in fact speeding up. They suggested that this was caused by some unknown sort of gravitationally repulsive material - dark energy.



This was the driving force pushing the universe apart at ever increasing speeds. Some astronomers question whether the evidence is strong enough to justify theories based on completely new laws of nature and on matter of a kind that has never been observed in other contexts.

Ch. 11            Are we alone in the universe?
The universe has existed for long enough for some scientists to think that life elsewhere has had time to evolve ‘intelligence’. These scientists speculate that beings in other parts of the universe may be using waves of electromagnetic radiation to communicate in the same way that we do.

Since 1927, we have been sending radio waves out from Earth. These waves have since travelled over 80 light years out into the universe. That means that they have already reached many hundreds of potentially habitable planets. So if there is intelligent life out there, they already know that we are here.

Searching for habitable planets
Of all the life that has evolved on Earth, only humans have developed the intelligence to use radio for communication.
Over the last half century groups of scientists have begun to build upon our increasing knowledge about how our universe and life on Earth evolved to speculate about what simple life could be like elsewhere. This science is multidisciplinary.

Many scientists are invlolved in the search for Earth-like planets. Each contributes to a small area of research. Some research explores life in habitats that could shed light on types of living things that could exist on another planet in extreme conditions.
study of bacteria living in sal water between cracks in Antartic ice at 20 C.
study of microorganisms that survive at 120 C beside volcanic ‘vents’ deep under the ocean

In 2007 the government chief scientific adviser invited a group of the UK’s top scientists to a meeting with the science minister.
This was not a meeting to make a policy. Instead the aim was to give the minister and senior civil servants an opportunity to:
discuss the challenge of identifying other solar systems and Earth-like planets within them
recognise areas of research that had already proved successful
understand how much we can learn about such planets using current technology

If life is common elsewhere , how will we find it?
Astronomers believe that lfie is most likely to have evolved where the conditions have some similarities to those on Earth.
They:    - look for rocky planets at temperatures and pressures in which water is lqiud
assume that the temperature extremes need to stay within the limits that micobes       can survive
look for a stable environment

Scientists predict that life is more likely to flourish if there are other bigger planets in an orbit further out from the star. The gravitational field of these outer planets protects the inner planet from hazards. Planets with these conditions might be found within the habitable zone around a star.

Searching for planets round distant stars
Suzanna Aigrain pointed out that in generel we cannot see planets outside our Solar System directly. Some powerful telescopes can detect the very slight wobble of a star. The stars are so far away that it is extremely hard to spot the disturbance to the star’s motion.

In early 2007, a team of astronomers (led by Stepahne Udry of the Geneva Observatory) announced the discovery of Gliese 581c. This planet is in orbit around a star about 20 light years away. At that time Gliese 581c was the smalles plnaet that was discovered around a distant star. Planet is about 15x closer to its star than Earth is to the sun, but the star is about 50x dimmer than the Sun. This means that the planet could just be in the habitable zone.

Unfortunately the most powerful telescopes on the Earth’s surface can only detect giant planets. IN order to detect rocky planets which might be habitable, astronomers need telescopes outside the Earth’s atmosphere that can observe one star for longer than is possible from any point on the Earth’s surface.

Detecting life on Earth-like planets
Living things leave traces of their existence because of their biological activity. They change the composition of the atmosphere.
On earth:     -  plants produce oxygen by photosynthesis
in sunlight some of the oxygen turns into ozone
all organisms give out C02 as they respire
organic matter that rots away in the absence of oxygen releases methane into the atmosphere

Astronomers use spectroscopy to investigate the chemical composition of distant plantes and stars. They can analyse the light reflected from a planet through its atmosphere to look for signatures of life. Astronomers can claim that they have found evidence of life on a planet if they detect methane and oxygen in its atmosphere, or water and ozone.

The amount of light coming through a planet’s atmosphere is tiny compared to the amount of light coming directly from its star. To solve this problem, scientists have developed a way of combining the signal from several telescopes in such a way that the light from the central star is cancelled out, leaving the much fainter planet easier to see.

Next steps
Dr Andy Longmore summarised the meeting with th eminister. In his view, we can be certain that there are habitable planets around stars other than the Sun, probably in very large numbers.

According to Longmore, scientists can be much more certain about their predictions because they have observational techniques that are sensitive enough to measure the tiny wobble of a star affected by the gravity of orbiting planets.

In his view the next key steps were to:
Establish firmly the numer and variety of habitable planets in the universe.
Directly image the nearest examples of habitable planets and analyse the light from these planets to estimate their size and surface properties
Start the search for life itself by determining the physical and chemical conditions on the surface and in the atmosphere of planets.





List of words

stellar                of the stars
period of the star        time it takes for a full cycle from maximum brightness to minimum                 and back to maximum again
cosmology            scientific study of the formation and evolution of the universe
habitable zone        the region of space around a star where the surface conditions of any                 planets could be favourable for life to evolve
signature of life        any evidence from observations and measurements that life has                 existed int he place that’s being studied