GENERAL INTRODUCTION

SESSION 1

HISTORY• Humankind has long been fascinated by the

heavens• The Sun and Moon played dominant parts in

many early religions• We probably began to study the heavens

about 10,000 years ago with the introduction of agriculture

• Shamans became a privileged class whose needs were looked after by the local community enabling them to study the night sky amongst many other things

SUMERIAN OBSERVATIONS

• The oldest written observations were made by the Sumerians about 5,000 years ago

• They believe the Earth was a flat motionless platform covered by a dome where the Gods played around with the sun, moon and stars

• However, some stars did not appear to follow the rules

THE PLANETS• Nearly all stars kept constant relationships to

one another• However, five that could be observed with

the naked eye appeared to walk about the sky independently. They are the ones that were called “wanderers” or “planets”

• They are now known as Mercury, Venus, Mars, Jupiter and Saturn after major figures in Roman mythology. Mars attracted special attention because of its reddish colour and became associated with war and conflict

BIRTH OF ASTRONOMY (1)• The Greeks made major contributions

to our understanding of the solar system

• Aristotle concluded the earth was probably round because its shadow on the moon during a lunar eclipse was curved

• Aristarchus found that the Sun, not the Earth, was the centre of the planetary system and that the Earth was simply another planet orbiting around the sun. He also concluded that the Earth rotated around its axis

BIRTH OF ASTRONOMY (2)

• The Greek astronomer Hipparchus, who lived on the island of Rhodes at about 150 BC worked out a system for predicting the positions of the Sun and Moon at any time of the year

• He even calculated the distance between the Earth and the Moon (384,400 km)

PTOLEMY

• He lived in the second century AD and believed in a stationary Earth located in the centre of the Universe with all the other heavenly bodies moving around it

• He argued that the Earth could not possibly be spinning around its axis because, if it did, the birds would fly off their perches

PTOLEMY (continued)

• At this time it was known that some planets appeared to reverse direction – a phenomenon that was particularly obvious in the case of Mars

• He attempted to explain this by suggesting that the planets were moving in small circles (epicircles), at the same time as they revolved around the Earth

• During the Middle Ages his ideas unfortunately became part of the dogma of the Roman Catholic Church

COPERNICUS

• He published a manuscript in 1543 in which he placed the Sun in the centre with the Earth and the other planets moving around it but he also believed that they were travelling at constant velocity in circular orbits

• Aristarchus had suggested this model 1,800 years earlier. Copernicus was aware of this and credited him with the original discovery

JOHANNES KEPLER

• In the early 1,600’s he used high quality astronomical observations by Danish scientist Tycho Brahe to hypothesize that the planets traveled around the Sun in elliptical orbits with the Sun located in one of the foci

• The planets move fastest at perihelion and slowest when they are at aphelion

Mars travels much further per unit time when in perihelion than in aphelion

With the Sun at the centre of the solar system theapparent reversal of the course of Mars is easilyexplained

OTHER ACHIEVEMENTS

• Kepler correctly calculated the relative distances of the planets from the Sun

• The distance from the Earth to the Sun was unknown so Kepler called it one astronomical unit, now known to be 149.51 million km measured by radar in the 20th Century

• He showed that Mars was an average distance of 1.523 astronomical units from the Sun or 227.72 million km

GALILEO

• He was an Italian astronomer at the University of Padua who adopted Kepler’s ideas after reading his book “Astronomia Nova”

• In the same year heard about the invention of a primitive telescope by the Dutch astronomer Huygens.

• He started to build a succession of telescopes the best of which had 32X magnification

MARS THROUGH THE TELESCOPE

• Galileo became the first human being to examine Mars through a telescope but could not see any details probably due to unfavourable conditions

• The distance from Earth to Mars varies from 78 to 399 million km depending on whether they are on the same or on opposite sides of the solar system – approximately a two year cycle

• Mars features can also be obscured by occasional dust storms

VENUS THROUGH THE TELESCOPE

• Galileo also studied Venus and found that like the Moon it went through a full range of phases from a narrow crescent to practically full

• This told him that Ptolemy’s Earth centered model of the solar system had to be incorrect

At left is Ptolemy’s model of the solar systemwith the Earth at the centre. At right is thesolar centred model of Galileo based on hisstudy of the phases of Venus

NEW MOONS

• In 1610, while examining Jupiter through one of his telescopes, Galileo discovered four moons rotating around the planet. The Earth was not the only planet with a moon

• He also observed Saturn and discovered its rings but could not figure out what they were

HUYGENS• Huygens was the first astronomer to

identify a surface feature on the planet Mars. He discovered a dark v-shaped marking now known as Syrtus Major

• From its movement he was able to calculate that Mars turned around its axis at almost the same time as Earth (24 hours and 38 minutes)

• He also discovered the south polar ice cap. The north polar icecap was discovered a few years later

GALILEO IN RETREAT• The Roman Catholic Church took a dim

view of his sun centered model of the solar system and in 1616 he was told to stop teaching heresies

• In 1632 he published a book explaining his hypothesis. He narrowly escaped being burnt at the stake and was forced to recant his views. He died in 1642 while still under house arrest

• It took the church another 100 years to accept his views

GIORDANO BRUNO

• He was a scholarly Catholic cleric living in Italy in about 1,600 AD who dared to suggest that the Universe might contain other inhabited worlds, a view that is quite popular today

• His views so enraged the Church that he was burnt alive at the stake

NEWTON• In 1687 Newton formulated his laws of

gravity together with their mathematical proof

• He found that all objects attract one another in direct proportion to their mass and in inverse proportion to the distance between them

• It made it possible to predict the movement of bodies in the solar system with uncanny accuracy

• The same laws are still used today in the navigation of space probes

IMPROVED TELESCOPES

• Newton also designed a new reflecting telescope with a concave lens and a flat mirror which produced much sharper images than earlier instruments

• This type of telescope was used by William Herschel in 1781 to discover the planet Uranus, the first discovery of a new planet since ancient times