# Solar System Overview. Early Ideas  It was assumed that the Sun, planets, and stars orbited a stationary universe  This is known as a “geocentric” model,

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Solar System Overview

Early Ideas  It was assumed that the Sun, planets, and stars orbited a stationary universe  This is known as a “geocentric” model, which means Earth centered  Problem with this model though: doesn’t explain all aspects of planetary motion  EX: normal direction for motion of planets is toward the East as observed from Earth; sometimes though a planet will appear to move the opposite direction across the sky in what is called retrograde motion

Example of retrograde motion  Mars retrograde motion Mars retrograde motion Mars retrograde motion  The search for an explanation for this retrograde motion motivated early astronomers to keep searching for a better model of our solar system

Heliocentric Model  1543 Polish scientist Nicolaus Copernicus suggested Sun was center of solar system  First time a sun-centered or “heliocentric” model was proposed  In a heliocentric model, the inner planets move faster in their orbits than the outer planets do; as Earth bypasses a slower moving outer planet it appears the outer planet temporarily moves backward in the sky

Kepler’s First Law  Ideas of Copernicus not originally accepted by scientific community, but within a century other astronomers found evidence to support the heliocentric model  Using accurate data of planetary observations, Kepler demonstrated each planet orbits the Sun in a shape called an ellipse instead of a circle  This is Kepler’s First Law

Kepler  The orbits of planets are not circles but oval-shaped curves called ellipses!

Kepler  Ellipse is oval shape centered on two points instead of a single point, like a circle  The two points are called the foci (singular focus)  Each planet’s ellipse is a different shape and size, and the Sun is always at one focus  The major axis is the line that runs through both foci and is the maximum diameter of the ellipse  Half the length of a major axis is called the semi-major axis

Kepler  The semi-major axis is the average distance between the Sun and the planet  For the Sun and the Earth, it is 1.496 x 10 8 km, or 1 astronomical unit (AU)  Average distance between Sun and each planet are measured in astronomical units

Kepler’s Laws  A planet in a elliptical orbit is not at a constant distance from the Sun  When planet is at closest distance to the Sun in its orbit, it is at perihelion  When a planet is at farthest distance away from the Sun in its orbit, it is at aphelion  The shape of a planet’s elliptical orbit is defined by eccentricity (ratio of distance between foci to the length of the major axis)

Kepler’s Laws  Eccentricity values range from 0-1  0 is a perfect circle  Nearly 1 is a very elongated oval  1 is equal to a parabola  Most planets have orbits not very eccentric and are close to being circles  Orbital period – time required for a planet or body to travel a complete elliptical orbit around the Sun

Kepler’s Laws  Planetary Motion Planetary Motion Planetary Motion

Gravity and Orbits  Newton realized any two bodies attract each other with a force that depends on their masses and the distance between the two bodies  This relationship is called the law of universal gravitation  The greater the distance between two bodies, the less the force between them is  The smaller the distance between two bodies, the greater the force between them is

Barycenter  Newton also determined each planet orbits a point between it and the Sun called the barycenter barycenter  The barycenter is the balance point between two orbiting bodies (where all the mass of an object is concentrated)  This is similar to the pivot point on a see-saw  If one of two bodies orbiting each other is more massive than the other, the center of mass is closer to the more massive body  If two bodies are similar in mass, their center of mass is near the middle position between them  For any planet and the Sun, the center of mass is just above the surface of the Sun (or within the Sun) because the Sun is more massive than any planet

Barycenter  Barycenter is closer to the end of the broom The Earth does not revolve around the Sun, but rather the barycenter!

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