The Solar System
BEGINNERS ASTRONOMY
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
This week:
The Solar System from a historical point of view
How the Ancients viewed the Solar System \ Universe
The Greeks view and their influence
Copernicus, Tycho, Galileo, Kepler and our modern view of the
Solar System.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
How the Ancients viewed the Solar System \ Universe
For ancient people there was no Solar System it was the sky.
The sky was their clock / calendar
The appearance of certain constellations would herald the change
of the seasons and the appearance \ disappearance of certain animals.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
With the advent of agriculture the observing the sky became even
more important for timing the planting of crops.
In ancient Egypt the heliacal rising of Sirius occurred close to the time
of the Nile’s inundation and so seed needed planting. This also marked
the beginning of their New Year.
Heliacal rising means: when a star, that has not been visible for a while,
is visible just before the Sun rises. This is weather dependent.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The Greeks view and their influence
Eudoxus (410?BC – 350?BC)
Eudoxus developed a two sphere model. It divided the Cosmos
into two regions: a spherical Earth central and motionless and a spherical
heavenly realm centred on the Earth containing multiple rotating crystalline
spheres.
Each sphere carried a planet in the order:
Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, Fixed Stars
This Eudoxan system had a number of critical flaws:
It predicted motions poorly. It could not account for the planets retrograde
motions and it couldn’t account for the planets change in brightness.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
384 BC – 322 BC
Aristotle’s world view
The five elements:
Fire, which is hot and dry
Earth, which is cold and dry
Air, which is hot and wet
Water, which is cold and wet
Aether, which is the divine substance that makes up the heavenly
spheres and heavenly bodies
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Apollonius of Perga (262 BC – 190 BC)
In response to some of the problems with the Eudoxan system he
introduced two mechanisms that allowed a planet to vary its distance
and speed.
Eccentric
Deferent and Epicycle
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Aristarchus of Samos (310 BC – 230 BC)
Aristarchus proposed a system where the Earth revolved around the Sun
This was not well received as it meant that the Earth moved !!!!
The Greeks knew that the Earth was a sphere and how big that it was.
It was obvious that we would all be thrown off if this was the case.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Claudius Ptolemy (90 AD – 168 AD) a Roman citizen of Egypt
Ptolemy wrote the Almagest around 150 AD and is a critical source of
information on Greek astronomy
Ptolemy claimed to have derived his geometrical models from selected
astronomical observations by his predecessors spanning more than
800 years.
Ptolemy presented his astronomical models in convenient tables which
could be used to compute the future or past position of the planets.
The Almagest also contains a star catalogue which is an appropriated
version of a catalogue created by Hipparchus.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The Ptolemaic system said:
The Earth was chaotic.
The heavens were unchanging.
The heavenly bodies moved only in perfect circles.
The retrograde motions of the planets were accounted for by the use
of eccentrics, deferents and epicycles.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The Ptolemaic system
Note that the order of the planets
has changed slightly from the
Eudoxan system
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The Antikthera mechanism
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Ptolemaic science was lost in
Europe but kept alive and advanced
upon in the middle East.
Links with the Arab world brought
Ptolemy back to Europe in the
Middle ages.
The Geocentric system lasted until
well into the 17th century
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The beginnings of our modern understanding
Nicolaus Copernicus (1473 – 1543)
Although generally thought of as the author of the Heliocentric system
Copernicus was to a large extent reworking earlier Greek ideas.
Although he put the Sun at the centre of the Universe the planets were all
still required to orbit in perfect circles as in the Greek tradition.
Because of this the predictive abilities of this model were no better than the
old Ptolemaic system.
It did account for the daily motion of the Sun, Moon and star plus the
retrograde motion of the Planets and the seasons.
Martin Crow
Crayford Manor House Astronomical Society
The Heliocentric system
The beginnings of our modern understanding
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
His book ‘De revolutionibus orbium coelestium’ was not published until after
his death in 1543.
Copernicus wasn’t accepted immediately or by all.
Predictions were no better and how do you prove that the Earth moves?
However, there was a growing appetite for change.
In the medieval mind the ancients were seen as the pinnacle of human
achievement and were infallible. The fact that dates predicted for Planetary
apparition could be out by a couple of weeks indicated to some that the very
mechanism of the universe was wearing out and the world was coming to
an end.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Tycho Brahe (1546 – 1601)
Tycho was probably the greatest pre-telescopic observer there had ever been.
Tycho was a Danish nobleman and had been granted an estate on the island of
Hven.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
In 1572 Tycho observed a new star. His observations showed that it lacked
parallax and therefore could not be a sub-lunar phenomenon.
In ‘De nova Stella (on the new star) 1573 he refuted the theory of the celestial
spheres in showing that the heavens were not unchanging.
Tycho went on to demonstrate with comets seen in 1577, 1580, 1582, 1585,
1590, 1593 & 1594 that they also were at a great distance and not atmospheric
phenomena.
This also proved that the crystalline spheres did not exist.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Despite all of this Tycho still held that the Earth did not move but produced
his own Tychonic system.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Galileo Galilei (1564 – 1642)
With the invention of the telescope by Hans Lipperhey in 1608 Galileo made
his own and pointed it towards the sky in 1609.
With the aid of the telescope Galileo discovered:
The moons of Jupiter - showing that it was a centre of rotation.
The phases of Venus - demonstrating that it revolved around the Sun.
That the wandering stars were other worlds.
That there were stars that we could not see unaided.
That the Sun’s surface sometimes had spots on it.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
But none of these things actually proved that the Earth moved!!!
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Johannes Kepler (1571 – 1630)
Kepler was Tycho’s assistant for a short period in Prague prior to Tycho’s
unexpected death in October 1601. Kepler was appointed imperial
mathematician.
Using Tycho’s observations of Mar’s motion he produced his three laws of
Planetary motion.
The first two laws were published in ‘Astronomia Nova’ 1609.
The third law was not published until 1619.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Kepler’s three laws of planetary motion
1st Law
The orbit of every planet is a conic section or ellipse with the Sun at one
of the two foci.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
2nd Law
A line joining a planet and the Sun sweeps out equal areas during
equal intervals of time.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
3rd Law
The square of the orbital period of a planet is directly proportional to the
cube of the semi-major axis of its orbit.
What this means is that if you know an objects orbital period (i.e. how long it
takes to go round once) you can work out the ratio of distances between
the planets. Therefore, if you know the orbital radius of one planet
you can work out the rest.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Isaac Newton (1643 – 1727)
Newton published Philosophiæ Naturalis Principia Mathematica
published in 1687.
Normally referred to as ‘Principia’ it describes the three laws of motion,
universal gravitation and demonstrated the consistency of Kepler’s three
laws of planetary motion and his theory of gravitation.
Newton showed why the planets moved as Kepler had discovered and
helped to remove doubts about heliocentrism.
Newtonian gravitational theory would hold sway for the next 250 years
and is still used today.
It did not prove that the Earth moved.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
So how do you prove that the Earth moves?
One way of proving that the Earth moved
was to look for parallax in the stars.
This method would also give a distance to
the star.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
How to look for stellar parallax and things to consider.
The stars are at different distances.
Assuming that all stars are the same brightness then the brighter
the star the nearer it is and vice versa.
The Ariel telescope is not suitable.
Atmospheric refraction must be taken into account.
The way forward is to use a Zenith telescope.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Robert Hooke (1635 – 1703)
John Flamsteed (1646 – 1719)
Astronomer Royal
Both used a Zenith Telescope to look for parallax.
The star they decided to study was Gamma Draconis.
This star passes directly overhead and is relatively
bight at mag 2.2.
Martin Crow
Crayford Manor House Astronomical Society
Gamma Draconis
2011 May 06 2:54ut
The Solar System
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
One of the telescopes used by Hooke
still exists today and you may have
been up it without realising.
Completed in 1667 the Monument
was designed as a scientific
instrument.
Sadly it was not up to the job as
it expanded at different rates and
swayed in the wind!!
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Other zenith telescopes
The well telescope Greenwich 1679
It would appear that it was not a success, probably
because the tube and lenses were too unstable, the
object glass was of poor quality and, as was said in 1737,
‘because of the damp of the place’.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The beginnings of our modern understanding
James Bradley (1693 – 1762)
Working with Samuel Molyneux trying to measure the parallax of Gamma
Draconis they measured an apparent motion, but it was not as they expected
it to be.
What Bradley found was an apparent motion of 20” that reached its most
southerly point in March, and its most northerly point in September and that
could not be accounted for by parallax. This was at first a mystery.
Allegedly, Bradley realised what was going on when sailing on the Thames
and noticed the effect of the apparent wind when the boat changed direction.
This was the discovery of the ‘Aberration of Light’ and was conclusive evidence
that the Earth moved through space orbiting the Sun
James Bradley published his results in the Philosophical Transactions in
January 1729. It was to be another hundred years before stellar parallax was
measured.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
At last the we had left Ptolemy behind.
So now we knew that:
The Earth does move through space rotating as it does so.
The seasons are caused by the tilt of the Earth’s axis.
The Earth along with all of the known planets orbit the Sun.
The orbits of planets and comets are all conic sections - elliptical.
We now knew that this was all caused through the action of gravity
and Newton’s laws of motion.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
But how large is the solar system?
From Kepler’s third law we knew the ratio of the distances.
What was required was one accurately known distance.
So how was this to be done?
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Enter the transit of Venus.
First predicted by Kepler in 1627
He said a transit of Venus would occur in 1631 and 1761 but could not accurately
predict where it would be seen from i.e. Europe.
Transits of Venus are very rare and occur in pairs with 8 year gaps separated
by long gaps of 121.5 years and 105.5 years.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
So why is the transit of Venus so important?
If viewed from two widely spaced locations parallax can be used to
determine the Earth - Sun distance. This method was first described
by James Gregory (1638 – 1675) in Optica Promota in 1663.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Jeremiah Horrocks (1618 – 1641) corrected Kepler’s prediction of 1631
Showing that another would happen in 1639 at around 3:00 pm
This was observed by Horrocks from Much Hoole near Preston and his friend
William Crabtree from Salford on 4th December 1639.
He estimated an Earth – Sun distance of 59.4 million miles which is approx
2/3 the actual distance but a more accurate figure than any suggested up to
that time.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The transits of 1761 and 1769.
International collaboration was organised so that observations could be
made around the world.
Scientists and explorers from Britain, Austria and France travelled to
destinations around the world, including Siberia, Norway, Newfoundland
and Madagascar.
Captain Cooks voyage to the South seas was specifically to observe the transit
of Venus although he managed to discover Australia while he was down there.
Guillaume Le Gentil spent 8 years away from home and saw neither.
Using the combined results from both events the French astronomer
Jerone Lalande calculated a value of 153 million kilometres ± 1 million km.
The precision was less than expected due to the ‘black drop effect’. However,
we now had a very good idea of the size of the solar system.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
The black drop effect
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Transit observations in 1874 and 1882 allowed this value to be refined further.
The American astronomer Simon Newcomb combined the data from the last four
transits and derived a value of 149.59 million kilometers (±0.31 million km).
In 1931 an even more accurate figure was obtained by measuring the parallax of
433 Eros during its close opposition.
The accuracy of this figure was not superceded until 1968 when radar techniques
were used.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
We observed the 2004 transit from the manor house in Crayford.
Your last chance to see a transit will be 6th June 2012 just as the Sun rises.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Things had really changed but ‘our’ solar system still only contained
the ‘classical’ planets.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
William Herschel (1738 – 1822)
In March 1781 while searching for double stars Herschel discovered an
object which was not stellar in appearance and at first he thought it to be
cometary in nature.
Further observations showed that was a planet and orbited beyond Saturn.
He had discovered Uranus.
Interestingly, it was in approx. the position predicted
by Bodes law.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Following on from Herschel's discovery a great effort went into looking
for a possible planet between Mars and Jupiter also predicted by Bodes
law.
In 1801 Giuseppe Piazzi discovered Ceres the first of the asteroids.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Observations of Uranus’s orbit showed slight changes which it was deduced
were due to gravitational perturbation of an unknown planet.
On 23 September 1846 Johann Galle observed the planet Neptune which was
within a degree of the position predicted by Urbain Le Verrier.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Perturbations in Neptune's orbit led to a search for Planet X as it was dubbed.
In 1930 Clyde Tombaugh using photographic plates discovered Pluto the tenth
planet.
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
In 2006 Pluto was reclassified as a Dwarf planet and a member of the
Edgeworth-Kuiper belt objects.
The Edgeworth – Kuiper belt or Trans Neptunian Objects (TNOs) comprise
true Kuiper belt objects with stable long lived orbits and Scattered Disk
objects who’s orbits are affected by Neptune and, it is thought, give rise to
short period comets. 30 – 100 a.u.
Long period comets are thought to originate from the Oort cloud which
surrounds the solar system at a distance of around 50,000 a.u.
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
So the solar system as we know it today looks like this:
Martin Crow
Crayford Manor House Astronomical Society
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Martin Crow
Crayford Manor House Astronomical Society
The Solar System
Martin Crow
Crayford Manor House Astronomical Society