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Exhibition Guide

Planetary Motion and Gravity

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Purpose of Exhibition

This exhibit shows you how the Earth and other planets move around the Sun. The hole located at the center represents the Sun and the balls launched from the perimeter are planets. You can observe that the ball moves faster as it approaches the center.
The laws governing planetary motion were first discovered by a German astronomer, Johannes Kepler, in the early 17th century.
This exhibit was transferred from the old Astronomy Building and has been renovated. The curved surface and the mechanism inside were overhauled and are now in use without any changes made.


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Additional Knowledge


[Kepler's Laws]
Kepler has discovered the laws that could explain planetary motion in general. The laws he discovered consists of three laws.
1. First law: The orbit of the planets are ellipses, where the Sun is placed at one of their focul points.
2. Second law: A ray comming from the Sun to any planet sweeps out equal areas during equal time intervals.
3. Third law: The square of the orbital period of a planet is proportional to the cube of the semi major axis of its orbit.

[About the Surface in which the Ball is Rolling in the exhibition]
The surface on which the balls roll is made into a trumpet-like form. The gradient increases toward the center, and it is more flat at its perimeter. Due to this form, the balls move faster when they near the center and slower around the perimeter, just like a planet on its orbit.

[About the Discovery of Kepler's Laws]
Johannes Kepler (1571-1630) discovered the laws of planetary motion by precisely examining the data of motion of Mars in the sky as observed by his teacher Tycho Brahe.
He considered that the force acting on the planets and causing that orbit was some sort of magnetic interaction. This was found out to be wrong by the discovery of Newton's law of universal gravitation. Kepler's discovery, however, made a significant contribution to the development of astronomy later on.




【 References 】

Article by Yoshitaka Yamada, curator and Astronomy Section

 

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