At a time when the geocentric model of the universe was dominant, the work of one scientist and his mentor largely changed the orientation of humans among the stars.
Johannes Kepler, drawing on the research of mentor Tycho Brahe, theorized three universal concepts to explain the motion of the planets around the sun in our solar system. This sun-centered, or heliocentric, model conflicted with the popular geocentric system. While Kepler was not the first to suggest heliocentrism, he was the first to describe the orbits of the planets as less than a perfect circle. Kepler was a pioneer, the first to break from the traditional model of heavenly perfection. More information on this shift can be found here: http://www.scientificamerican.com/article/galileo-kepler-iya/
Kepler’s first law of planetary motion was an entirely novel concept, and understandably must have been disorienting at the time. It states that the orbit of each planet around the sun is an ellipse, with the sun located at one of the ellipse’s foci. As a result, the planet’s distance from the sun fluctuates at different points in its orbit, though this difference is very slight. When the planet is closest to the sun, it is at a point called the perihelion, and when it is farthest it is at the aphelion. The average of the distances of these two points from the sun is the planet’s average distance from the sun.
Earth’s orbit looks more like this:
And not this:
Kepler’s second law states that a planet moves faster when it is closer to the sun and slower when it is farther away. Therefore, a line drawn from the planet to the sun sweeps out equal areas in equal times, despite varying distances from the sun, due to this correlation between distance and speed. This is attributed to conservation of angular momentum: if earth does not transfer angular momentum to another object, its angular momentum is conserved and thus its velocity must increase as its radial distance decreases. The cause of this phenomenon was not understood until Newton observed gravity and angular momentum later.
Kepler’s third law explains distance as a function of time in terms of planetary orbits. He observed that more distant planets orbit the sun at slower average speeds. This applies the concept of gravity, because gravitational pull is weaker between objects that are farther apart. However, gravity was not understood as the reason for this phenomenon until Newton articulated it years later. Mathematically, Kepler's concept states orbital period of a planet in years squared is equal to the planet’s average distance from the sun in astronomical units cubed:
p2=a3
This law set the stage for Newton, who later devised his own version to be able to calculate the mass of objects orbiting the sun.
These three laws are significant because they accurately apply to a heliocentric system, and match the observed behavior of the Solar System. These laws can even apply to distant solar systems beyond our own, broadening our understanding of the universe. The context of Kepler’s achievements is also important. This new and relatively shocking information was released in a hostile environment, on which the majority of people were not receptive to this change. It took time for these laws to become accepted scientific theory, and used as a basis for Newton to further explore the universe.
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