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?.
2 – A saga of mankind
What do you know about the motion of the planets?
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?
2-1 The birth of astronomy
phases of the Moon carved on a bone
Origin: Abri Blanchard,
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
The natural units for calendars are:
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
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
~ 183 days = half a year
Reconstruction of the temple/observatory
Temples were built with specific places aligned to the heavens and based on the geometry of the land
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
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
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?
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