ANW summary
Chapter 10
Ancient Astronomy
We know from ancient times that the Earth isn’t flat because the Earth’s shadows were circular during eclipse and
they noticed North Pole star was higher when you went north also when ships sailed out the boat disappeared first
and then the masts and ships. Constellations move circular across the sky while planets move along fairly straight
line but sometimes moves backwards (retrograde motion). 500 BCE followers of Pythagoras suggested the earth was
the center of the universe.
Copernican revolution
Aristotle (384-322 BCE) made model with 55 spheres not good enough. Apollonius thought each planet moves in
small circle while the centre of this moves round a larger circle centered on the earth Ptolemy (110-170AD) made
adjustmentsgeocentric model. Around 1260 AD Copernicus suggested it would be simper if the sun was the
centre heliocentric model. The retrograde motion can then be explained by the difference in distance to the sun.
He wrote book about his findings published after death in 1543he was afraid of what others would say and the
Church, so he checked and improved a lotcomplicated model. Book wasn’t immediately noticed but Church did
put it on list of forbidden bookssuppress unorthodox views.
Argument for geocentric model
Response Copernican
Earth feels like it doesn’t fly
We’re moving along, so it’s the same as vehicles
Why don’t we feel wind from fast rotations
Earth’s atmosphere moves along
We don’t observe parallax (moving stars)
It’s too far away
Convincing others
Tycho Branch was interested in astronomy especially in prediction were stars etc. would be. It started when the
conjunction of Jupiter and Saturn was days off. He set of best equipped observatory of EU and was able to record
accurate measurements over 20 years. He believed in geocentric model but seeing a supernova dim over months
and noticing that the earth’s path crossed through the orbits f other planets, made him question this model. So he
compromised the model and said that the moon and the sun were going round the earth and the other planets went
round the Sun. Galileo Galilei seemed to have been convince of the heliocentric model and he expressed his views
confidentially. 1609, he got first telescope and saw heavenly bodies were not perfect spheres. He also observed
Jupiter and his moons and saw that objects could orbit around a centre other than earth. He also wrote a letter
saying that the heliocentric model is a ‘fact’. Church didn’t agree so he was censured. The Heliocentric model could
only be used for calculating. Until it proven that Earth moved. He wrote another book in dialogue style about the
mode, for this he was trialed and had to renounce Copernican views. This argument was based on whether the
decision we make should it be based on evidence or tradition, rather than science vs. religion. Johannes Kepler was
born in 1517 and was an astronomer who believed that the heliocentric model explained the movement rather than
just predicted it. He worked as Tycho’s assistant and inherited his data when h passed away. He tried to work out
orbit of Mars, but Tycho’s data didn’t exactly fit Copernican model. He came to conclusion that orbits of planets
were not circles but ellipses. Keplers laws:
- The orbits of the planets are ellipses
- Each planet moves round its own orbit so the line joining it to the Sun sweeps out equal areas in equal times
- Time that a planet takes to orbit Sun (T) is related to its average distance from
the Sun (R) by a simple rule: R3:T2 is same for every planet
An explanation for the planetary motions
Isaac Newton (1662) wondered why the moon orbits around the Earth at a constant
distance from it unlike objects near the earth that fall towards the Earth. He argued that the Earth’s gravitational
force kept the Moon from going in a straight line and pulled it towards the Earth. His inverse square law led to
Kepler’s three laws: the ‘natural’ path of an object orbiting a larger one, due to an attractive force between them
that obeyed an inverse-square law, is an ellipse.
Discovering new planets
1781, Herschel discovered Saturn with a home-made telescope. It had been observed before but had been mistaken
for a star. Uranus was far beyond Saturn. Predictions didn’t fit with Uranus, so perhaps Newton’s law of gravitation
doesn’t apply at longer distances. 1843, Adams thought that movement of
Uranus might be caused by another unknown planet. His calculations were 2®
off, but no one looked at it. Verrier could too predict the position. Both send
paper to Airy and when he received them he asked them to search for the
unknown planet. Challis (Adams director at Cambridge Observatory) observed
the planet twice, before he thought that the star was a planet. But he decided
to come back next night. In the mean time a letter from Verrier arrived at the
observatory in Berlin and immediately a search was set up. They checked for
stars that weren’t on the map and found the new planet, Neptune. Later both
Verrier and Adams were recognized as discoverers. People were more
convinced of the heliocentric model and Newton’s idea of universal gravitation.
An understanding that works
1970-1980Voyager missions were planned by NASA because four planets would come in line on the same side of the
Sun, which would make it possible to travel by these planets and collect data and photos. Voyager 1 passed close to
Jupiter and Saturn, but Voyager 2 passed close to Jupiter, Saturn, Uranus and Neptune and some of its moons. The
gravity fields of these planets were used as ‘slingshots. It took 12 years and they travelled 7128 km to reach Neptune
within 5000km from surface.
How far are the stars?
1640-1650 heliocentric models got accepted because of Kepler’s simple model based on elliptical orbits and accurate
predictions. If earth is moving then stellar parallax (change in positions of the stars) should appear. So they started
observing. To observe them you have to know which one to observe. The closest stars are the brightest and
astronomers used this and other ‘hunches’ to select stars. 1838, Wilhelm Bessel succeeded in measuring parallax of
star 61 Cygni form which he calculated distance . Closest stars are 1000 times further from Sun than Neptune.
New Techniques
One of the new techniques was invention of photography which enabled us to use long exposure times to record
objects that are too vague to be observed with a telescope. The other invention was spectroscopy. Newton showed
with a prism that sunlight is made out of continuous band of colours from red to violet. 1800, Young showed light
beams produce interference and thus light travels in wave. We now know that the color of light is related to wave
length: red long wavelengths, violet shorter wavelengths. Fraunhofer noticed that spectrum of Sun crosses dark
lines. Bunsen and Kirchhoff found that spectrum of glowing glass consisted of bright lines of different colours, with a
pattern that was characteristic of the element present. Kirchhoff concluded Fraunhofer had seen line in Suns
spectrum due to light from Sun’s surface passing through slight cooled vapor used to discover composition of
SunMainly hydrogen and helium.
Beyond the closer stars
Astronomers could only measure positions in the sky, brightness and composition of light of stars. Harvard College
Observatory set up group for women to compile a catalogue of the spectra of stars. Cannon also classified stars
according to spectra13 classes (blue to red) Russell plotted brightness of star against its spectral class. Using
apparent brightness would be useless but base it for stars whose distances are known so you can base it on intrinsic
brightness pattern emerges break through  could star could estimate intrinsic brightness.
Cepheid variables
Leavitt a women in Harvard, studied Cepheid variables (stars which pulse in brightness)  choose 1 of 2 Magellanic
Clouds stars are about same distance away can compare apparent brightness noticed that the brighter a
variable star, the longer its period calculate intrinsic brightness when they know period and know distance from
apparent brightness. Shapley used a number of additional assumptions to estimate the distance to one
Cepheidallowed distance to others.
The structure of the universe
Nebulae are fuzzy, cloudy objects; Milky Way, Magellanic Clouds and Andromeda. They are clouds of gas and dust or
clusters of stars far away. Kant speculated (correctly) that the Milky Way is really our edge-on view of our own galaxy
and nebulae’s are galaxies beyond our own. Herschel (who discovered Uranus) looked at nebulae and classified them
in five types not all nebulae are the same spectroscopy: some nebulae have spectra like that of a star, whilst
others have spectra characteristic of a glowing gas. Astronomers also believed it were stars at different stages in the
process of formation.
Globular clusters and the Milky Way
Shapley observed nebulae too and he found that the clusters are the centre of our galaxy and not the Sun. 1920, he
made a wrong estimation of the Milky Way, because he thought it included the whole universe and everything was
within it. Curtis argued that some nebulae were galaxies far beyond the Milky Way. Hubble observed difference in
apparent brightness of stars of Andromeda nebulaeCepheid variablecalculated 900.000 light years
awayAndromeda Galaxy was galaxy beyond our ownended debate about structure of our universe. Slipher
noticed nebulae travelling away from us (600 miles per second)measured with Doppler effect and spectra.
An expanding universe
Einstein’s theory of general relativity said gravity was because of curvature of space by all objects that have
masslight should bend by a certain amount when it passes a massive bodycorrect. Einstein also made equations
which showed that our universe is expanding. Hubble provided evidence for this by first working out intrinsic
brightness of brightest stars in galaxy. Then observed brightest stars in remote galaxies assuming they’d the same
intrinsic brightness. He compared this with apparent brightness to find out how far they were away. Then he
calculated speed of moving away with red shift (pg. 177).
A big bang
An expanding universe suggests a big bang. This idea was resisted because it implies a creation’ at one moment in
past and no normal physical laws can explain it. They argued ‘ready state’ theory matter is produced all the time
as the universe expands. Calculated backwards that the universe was 4 billion years old (Earth 3.4 bill)  now 15
billion years is estimated.
The ‘echo’ of the big bang
Believers of the big bang showed Einstein’s theory of general relativity implied Earth must have started as an
infinitely small pinpoint, so they looked for ‘echo’ to be heard not easy to detect Wilson and Penzias heard radio
noise coming from every direction first thought it were pigeon droppings, but this was not the case this was the
best antenna in worldfinally published findingsNobel Prize for Physics, 1978.
Unresolved questions
Every matter in Universe attracts every other bit gravitational force between any pair of masses. High density
means lower speed of expansion which then stops and then galaxies move closer. But if this happens below this
density it stays expanding forever. Astronomers estimate that the average density of universe and amount of mass it
contains, in some cases it showed that some masses of matter were 10 times bigger than they needed to be to
account for emitted radiationsuggests 90% of matter is invisible, this dark matter. Dark matter also means dark
energy. Two astronomers were trying to measure the rate at which the universe is slowing downexpansion was
speedingsuggested that was caused by sort of gravitationally repulsive material on dark energyalso driving force
behind pushing the universe apart.
Chapter 11
Search for life beyond Earth
Other beings might communicate through waves of electromagnetic radiation and also might be looking for other
beings. Radio telescopes are used to detect radio waves and we’ve already send out those waves 80 light years out
to other possible beings. Not all life forms communicate and lots of people are working together to predict the
environment these creatures could live in so they can design tools, to detect this on habitable planets.
Search for habitable planets
UK scientists work internationally and look e.g. for life in habitats with extreme conditions or collect data from our
own atmosphere, because this might show chemical signs to look for on their planets.
Briefing the science minister
2007, seven scientist, chief scientific adviser and minister of science held a meeting to discuss challenge of
identifying Earth-like planets, recognize successful research areas and understand how much we can learn about
planets remarkable event.
Is life common elsewhere, how will we find it?
Astronomers believe in life forms are most likely to be found on planets with conditions like on Earth (temp., rock,
pressure etc.) and evolution must have had an opportunity to take place. Also if other planets orbit further out from
the star it’ll have protected as shield against comets or asteroids.
Searching for planets round distant stars
Aigrain said that we need powerful telescopes to detect the slightest wobble of a star caused by the gravitational
pull of a large planet in orbit. Also slight shift can be seen in the lines in the spectrum of light from stars which can
indicate movement away from us. If a planet is to be detected like that it needs to have a large gravitational effect
on heir star. Udry detected Gliese 581c, a planet which orbits a star and is in the habitable zone. Earth’s telescopes
are not powerful enough, s Aigrain is also part of the EU team in COROT projectsatellite with telescope (can detect
1 in 10.000 changes in brightness) which collects data from 60.000 stars.
Detecting life on Earth-like planets
Life forms leave traces because of biological activity. Spectroscopy is used to detect chemical composition of planets
and stars. Reflected light is used to detect life (life when O2, ozone, H2O or methane is found). Light from planet
compared to its bright shining star is hard to detectsolution is focusing telescopes so that they cancel out light
from central star. ESA (European space agency) will start mission in 2018 to search for Earth-like planets in which
they’ll launch 3 space telescopes and one communication hub (Darwin mission). They’ll scan 500 stars within 60 light
years and study light of 50 alien planets. Most likely is that alien’s will be microbes.
Next steps
Longmore said we can be certain of habitable planets around stars other than the Sun and we can be surer of
prediction because of good observational techniques. He also made outline of the next key steps:
- Establish numbers and variety habitable planets, probably within 7 years with facilities that already exist
- Directly image nearest examples of habitable planets to estimate size and surface properties
- Start search for life itself which requires measuring of spectrum of planets to detect molecules
Most of this would have to be done international because of the costs. UK needs access to data via special
contribution or helping engineer technology.