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Faculty of Mathematics and Natural Sciences

Horizons lecture

Jacques Laskar - Planetary motions: Order and Chaos

We can predict with the greatest accuracy the date of the next Solar eclipse as well as all eclipses in the past. But what are the limits of these predictions?

Planets colliding, illustration

Planets colliding, illustration
Are planetary colllisions a possibility? Astronomer Jacques Laskar will shed light on the question in this May's Horizons lecture.

The lecture (held in English)

We can predict with the greatest accuracy the date of the next Solar eclipse. We can even find the dates of all the eclipses observed during the past historical periods. But what are the limits of these predictions?

It is now acknowledged that the large climatic variations of the past, such as the ice ages, originate in the variations of the Earth's orbit and of its spin axis resulting from the gravitational pull  of  the other planets.

These variations can be traced over several millions of years in the geological sedimentary records. Is it thus possible to retrace the orbit of the Earth, and therefore the variations of the insolation on its surface up to the time of the dinosaurs, more than 66 million years ago? 

The instabilities that are present in the motion of the planets singularly complicate the answer to this question, also opening up new perspectives such as the possibility of planetary collisions.

Refreshments will be served before the lecture, which starts at 16.15 pm on Thursday 4 of May in "Egget" at the Student Centre. Everybody is welcome!

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About Jacques Laskar:

After a Master in Mathematics at Ecole Normale Supérieure de Cachan, J. Laskar worked as a high school teacher for three years. After completing some degrees in psychology at Censier university, he resigned from his teaching position, and went back to mathematics to pass the very competitive agregation of mathematics. He then followed the graduate program of Paris Observatory where he completed his PhD in Astronomy. He is now Director of Research of highest rank at CNRS and Paris Observatory where he leads the group “Astronomy and Dynamical Systems”. J. Laskar is a member of the French Academy of Sciences and Member of the “Bureau des Longitudes”.

He has been awarded the Silver medal from CNRS and the Brouwer Award from the Division of Dynamical Astronomy from AAS for his work.

J. Laskar's work spans various field of fundamental astronomy, his main interest being the study of motions in planetary systems. He devoted large efforts to obtain accurate solutions for the long-term motion of planets in the Solar System that are used as the world reference for paleoclimate studies.

In pursuing this work, he demonstrated that the orbital motion of the planets of the Solar System is chaotic, with exponential divergence of the orbits of a factor of 10 every 10 million years, making it impossible to predict its motion beyond 60 million years. He showed that planetary perturbations create a large chaotic zone for the spin axis motion of all the terrestrial planets. He demonstrated that without the presence of the Moon, the Earth’s axis would be highly unstable, and could vary from 0 to about 85 degrees. 

He also demonstrated that the spin axis of Mars is chaotic, and can vary between 0 and 60 degrees, inducing high climatic variations on its surface. In order to improve the long-term ephemeris for the Solar System, he initiated the development of the INPOP planetary ephemerides. He is now largely involved in the characterisation of extrasolar planets systems.

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