2 – A saga of mankind
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2 – A saga of mankind. What do you know about the motion of the planets?. 1 – What is the shape of the Earth’s orbit around the Sun?. 2 – Do the planets orbit the Sun at constant speed?. 3 – Which planet has the largest orbital speed: Mercury or Jupiter?.

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2 – A saga of mankind

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2 – A saga of mankind

What do you know about the motion of the planets?

1 – What is the shape of the Earth’s orbit around the Sun?

2 – Do the planets orbit the Sun at constant speed?

3 – Which planet has the largest orbital speed: Mercury or Jupiter?

4 – The Voyager spacecraft has now left the solar system. What force does it take to keep it moving in a straight line at constant speed?

5 – You bring your bathroom scale with you for a trip to the Moon. What does it display when you weigh yourself on the Moon?

6 – You drop amarble and a hammer from the top of the CN tower. Which reaches the ground first?

2-1 The birth of astronomy

First astronomical record

30,000 BC:

phases of the Moon carved on a bone

Origin: Abri Blanchard,

Perigord, France

Picture yourself as an ancient farmer

What is the evening show?

What does your survival depend on?

Weather and seasons

One of the most important question farmers had to answer was: ‘What is today’s date?’ (When should I sow?)

Religious ceremonies where often linked to astronomical events

Winter solstice (shortest day of

the year): people would gather

and pray for the return of the warm season

(Christmas Eve: 24th)

Priests had to answer: “what is today’s date?” . They were often astronomers as well


  • Day (between two culminations of the Sun)

The natural units for calendars are:

  • Month (from one full Moon to the next one)

  • Year (from one winter solstice to the next one)

Challenges of the calendar:

between 2 full moons: 29.5 days ≠ 30

between to winter solstice: 365.24 days ≠ 365



(3000 – 100 BC)

Where writing was invented

Artist view of Babylon

Next to Baghdad

Ishtar gate

The hanging garden of Babylon

Babylonians believed that the planets and their motions influenced the fortune of kings and nations

Very precise observations but never tried to explain what they saw

Could forecast conjunctions and oppositions of planets

Babylonian astronomers knew the approximate length of the year

How could you measure the length of the year?

Every day at noon, record the length of the shadow of a post

June 21

March 21

~ 183 days = half a year

Dec 21

Archeological site: Ziggurat, Ur

Reconstruction of the temple/observatory



Temples were built with specific places aligned to the heavens and based on the geometry of the land

Astronomical records

Letter reporting

an eclipse

Tablet library

British Isles: Stonehenge

Built from 2800 to 1500 BC

People used stones to keep track of the motions of the Sun and Moon








3000 years ago, Egyptians adopted a calendar based on a 365-day year

Senenmut calendar

Egyptian map of the sky

The ceilings of many royal tombs depict the night sky as groups of star-gods

Egyptians knew that five bright objects, known today as “planets”, move from night to night against the background of the stars

They portrayed the planets as deities sailing across the heavens in barques, and they were known as the "stars that know no rest".

Sirius : brightest star of the night sky

Each year, Sirius rose above horizon at dawn for ten days

Egyptians kept careful track of the rising of Sirius to forecast the flooding of the Nile river

Sirius in Canis Major

Central America


Ruins of the Caracol , Chichen Itza, Yutacan, Mexico

1000 years ago

“Morning star”, “Evening star” = planet Venus

Maya: Sophisticated calendar based on the motions of Venus

You are lost on an isolated island in the Pacific ocean. How can you tell when is the shortest day of the year?

Sept 21

March 21

June 21

Dec 21


Aztec calendars


Intihuatana, Machu Pichu

Also used as a calendar

Solar eclipses happen only once in a lifetime


Polynesians learned to navigate by the stars over hundred of kilometers

It enabled them to colonize new islands

The odds came from the sky (rain, storms, haze) so people associated motions in the sky to their fate

Planets were symbols of gods that controlled human lives

Weather, diseases, accidents, were thought to be whims of the gods

Priests tried to understand what gods had in mind

Many civilizations developed their own astrology. Position of the Sun, Moon and 5 visible planets determined the fate of humans

Calvin’s opinion

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