beginners astronomy

BEGINNERS ASTRONOMY

The Solar System

Martin Crow Crayford Manor House Astronomical Society


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.

The Solar System


The Solar SystemHow 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.


The Solar System

With the advent of agriculture the observing the sky became evenmore important for timing the planting of crops.

In ancient Egypt the heliacal rising of Sirius occurred close to the timeof 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.


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The Solar SystemThe Greeks view and their influence

Eudoxus (410?BC – 350?BC)

Eudoxus developed a two sphere model. It divided the Cosmosinto two regions: a spherical Earth central and motionless and a sphericalheavenly realm centred on the Earth containing multiple rotating crystallinespheres.

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 retrogrademotions and it couldn’t account for the planets change in brightness.


The Solar System


The Solar System

Aristotle’s world view

384 BC – 322 BC

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


The Solar System

Apollonius of Perga (262 BC – 190 BC)

In response to some of the problems with the Eudoxan system heintroduced two mechanisms that allowed a planet to vary its distance and speed.

Eccentric

Deferent and Epicycle


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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.


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Claudius Ptolemy (90 AD – 168 AD) a Roman citizen of Egypt

Ptolemy wrote the Almagest around 150 AD and is a critical source ofinformation 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.


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 useof eccentrics, deferents and epicycles.


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The Ptolemaic system

Note that the order of the planetshas changed slightly from the Eudoxan system


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The Antikthera mechanism


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The Geocentric system lasted untilwell into the 17th century

Links with the Arab world brought Ptolemy back to Europe in the Middle ages.

Ptolemaic science was lost in Europe but kept alive and advanced upon in the middle East.


The Solar SystemThe beginnings of our modern understanding

Nicolaus Copernicus (1473 – 1543)

Although generally thought of as the author of the Heliocentric systemCopernicus 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.


The beginnings of our modern understanding

The Heliocentric system


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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 humanachievement 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.

His book ‘De revolutionibus orbium coelestium’ was not published until after his death in 1543.


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 ofHven.


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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 celestialspheres 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.


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Despite all of this Tycho still held that the Earth did not move but producedhis own Tychonic system.


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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.


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But none of these things actually proved that the Earth moved!!!


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Johannes Kepler (1571 – 1630)

Kepler was Tycho’s assistant for a short period in Prague prior to Tycho’sunexpected 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.


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1st LawThe orbit of every planet is a conic section or ellipse with the Sun at one of the two foci.

Kepler’s three laws of planetary motion


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2nd Law A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.


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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 ittakes 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.


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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.

It did not prove that the Earth moved.

Newtonian gravitational theory would hold sway for the next 250 yearsand is still used today.


The Solar System

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.

So how do you prove that the Earth moves?


The Solar SystemHow to look for stellar parallax and things to consider.

Assuming that all stars are the same brightness then the brighterthe star the nearer it is and vice versa.

The stars are at different distances.

The Ariel telescope is not suitable.

Atmospheric refraction must be taken into account.

The way forward is to use a Zenith telescope.


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Both used a Zenith Telescope to look for parallax.

Robert Hooke (1635 – 1703)

John Flamsteed (1646 – 1719)Astronomer Royal

The star they decided to study was Gamma Draconis.This star passes directly overhead and is relatively bight at mag 2.2.


The Solar SystemGamma Draconis 2011 May 06 2:54ut


The Solar System

One of the telescopes used by Hookestill exists today and you may have been up it without realising.Completed in 1667 the Monumentwas designed as a scientific instrument. Sadly it was not up to the job asit expanded at different rates andswayed in the wind!!


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’.


The Solar SystemThe 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 Thamesand noticed the effect of the apparent wind when the boat changed direction.

This was the discovery of the ‘Aberration of Light’ and was conclusive evidencethat 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 wasmeasured.


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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 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 gravityand Newton’s laws of motion.

The seasons are caused by the tilt of the Earth’s axis.


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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?


The Solar SystemEnter 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 accuratelypredict where it would be seen from i.e. Europe.

Transits of Venus are very rare and occur in pairs with 8 year gaps separatedby long gaps of 121.5 years and 105.5 years.


The Solar SystemSo 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.


The Solar SystemJeremiah 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 friendWilliam Crabtree from Salford on 4th December 1639.

He estimated an Earth – Sun distance of 59.4 million miles which is approx2/3 the actual distance but a more accurate figure than any suggested up to that time.


The Solar SystemThe 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, Newfoundlandand Madagascar.

Captain Cooks voyage to the South seas was specifically to observe the transitof Venus although he managed to discover Australia while he was down there.

Using the combined results from both events the French astronomerJerone 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.

Guillaume Le Gentil spent 8 years away from home and saw neither.


The Solar SystemThe black drop effect


The Solar SystemTransit 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 of433 Eros during its close opposition.

The accuracy of this figure was not superceded until 1968 when radar techniqueswere used.


The Solar SystemWe 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.


The Solar SystemThings had really changed but ‘our’ solar system still only contained the ‘classical’ planets.


The Solar SystemWilliam 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 becometary 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.


The Solar SystemFollowing on from Herschel's discovery a great effort went into looking for a possible planet between Mars and Jupiter also predicted by Bodeslaw.

In 1801 Giuseppe Piazzi discovered Ceres the first of the asteroids.


The Solar SystemObservations of Uranus’s orbit showed slight changes which it was deducedwere 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.


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In 1930 Clyde Tombaugh using photographic plates discovered Pluto the tenthplanet.

Perturbations in Neptune's orbit led to a search for Planet X as it was dubbed.


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) comprisetrue 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.


The Solar System


The Solar SystemSo the solar system as we know it today looks like this:


The Solar System

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