Solar System OverviewSolar System Overview

Early IdeasEarly Ideas It was assumed that the Sun, planets, and It was assumed that the Sun, planets, and

stars orbited a stationary universestars orbited a stationary universe This is known as a “geocentric” model, This is known as a “geocentric” model,

which means Earth centeredwhich means Earth centered Problem with this model though: doesn’t Problem with this model though: doesn’t

explain all aspects of planetary motionexplain all aspects of planetary motion EX: normal direction for motion of planets EX: normal direction for motion of planets

is toward the East as observed from Earth; is toward the East as observed from Earth; sometimes though a planet will appear to sometimes though a planet will appear to move the opposite direction across the sky move the opposite direction across the sky in what is called in what is called retrograde motionretrograde motion

Example of retrograde motionExample of retrograde motion

Mars retrograde motionMars retrograde motion The search for an explanation for this The search for an explanation for this

retrograde motion motivated early retrograde motion motivated early astronomers to keep searching for a astronomers to keep searching for a better model of our solar systembetter model of our solar system

Heliocentric ModelHeliocentric Model

1543 Polish scientist Nicolaus Copernicus 1543 Polish scientist Nicolaus Copernicus suggested Sun was center of solar systemsuggested Sun was center of solar system

First time a sun-centered or “heliocentric” First time a sun-centered or “heliocentric” model was proposedmodel was proposed

In a heliocentric model, the inner planets In a heliocentric model, the inner planets move faster in their orbits than the outer move faster in their orbits than the outer planets do; as Earth bypasses a slower planets do; as Earth bypasses a slower moving outer planet it appears the outer moving outer planet it appears the outer planet temporarily moves backward in the planet temporarily moves backward in the skysky

Retrograde Motion in Heliocentric Retrograde Motion in Heliocentric ModelModel

Heliocentric retrograde motionHeliocentric retrograde motion

Kepler’s First LawKepler’s First Law

Ideas of Copernicus not originally accepted Ideas of Copernicus not originally accepted by scientific community, but within a by scientific community, but within a century other astronomers found evidence century other astronomers found evidence to support the heliocentric modelto support the heliocentric model

Using accurate data of planetary Using accurate data of planetary observations, Kepler demonstrated each observations, Kepler demonstrated each planet orbits the Sun in a shape called an planet orbits the Sun in a shape called an ellipse instead of a circleellipse instead of a circle

This is Kepler’s First LawThis is Kepler’s First Law

KeplerKepler

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

KeplerKepler Ellipse is oval shape centered Ellipse is oval shape centered

on two points instead of a on two points instead of a single point, like a circlesingle point, like a circle

The two points are called the The two points are called the foci (singular focus)foci (singular focus)

Each planet’s ellipse is a Each planet’s ellipse is a different shape and size, and different shape and size, and the Sun is always at one focusthe Sun is always at one focus

The major axis is the line that The major axis is the line that runs through both foci and is runs through both foci and is the maximum diameter of the the maximum diameter of the ellipseellipse

Half the length of a major axis Half the length of a major axis is called the semi-major axisis called the semi-major axis

KeplerKepler

The semi-major axis is the average The semi-major axis is the average distance between the Sun and the distance between the Sun and the planetplanet

For the Sun and the Earth, it is 1.496 For the Sun and the Earth, it is 1.496 x 10x 1088 km, or 1 astronomical unit (AU) km, or 1 astronomical unit (AU)

Average distance between Sun and Average distance between Sun and each planet are measured in each planet are measured in astronomical unitsastronomical units

Kepler’s LawsKepler’s Laws

A planet in a elliptical orbit is not at a A planet in a elliptical orbit is not at a constant distance from the Sunconstant distance from the Sun

When planet is at closest distance to the When planet is at closest distance to the Sun in its orbit, it is at perihelionSun in its orbit, it is at perihelion

When a planet is at farthest distance away When a planet is at farthest distance away from the Sun in its orbit, it is at aphelionfrom the Sun in its orbit, it is at aphelion

The shape of a planet’s elliptical orbit is The shape of a planet’s elliptical orbit is defined by eccentricity (ratio of distance defined by eccentricity (ratio of distance between foci to the length of the major between foci to the length of the major axis)axis)

Kepler’s LawsKepler’s Laws

Eccentricity values range from 0-1Eccentricity values range from 0-1 0 is a perfect circle0 is a perfect circle Nearly 1 is a very elongated ovalNearly 1 is a very elongated oval 1 is equal to a parabola1 is equal to a parabola Most planets have orbits not very Most planets have orbits not very

eccentric and are close to being circleseccentric and are close to being circles Orbital periodOrbital period – time required for a planet – time required for a planet

or body to travel a complete elliptical orbit or body to travel a complete elliptical orbit around the Sunaround the Sun

Kepler’s LawsKepler’s Laws

Planetary MotionPlanetary Motion

Gravity and OrbitsGravity and Orbits

Newton realized any two bodies attract each other with Newton realized any two bodies attract each other with a force that depends on their masses and the distance a force that depends on their masses and the distance between the two bodiesbetween the two bodies

This relationship is called the This relationship is called the law of universal law of universal gravitationgravitation

The greater the distance between two bodies, the less The greater the distance between two bodies, the less the force between them isthe force between them is

The smaller the distance between two bodies, the The smaller the distance between two bodies, the greater the force between them isgreater the force between them is

BarycenterBarycenter Newton also determined each planet Newton also determined each planet

orbits a point between it and the Sun orbits a point between it and the Sun called the called the barycenterbarycenter

The barycenter is the balance point The barycenter is the balance point between two orbiting bodies (where all between two orbiting bodies (where all the mass of an object is concentrated)the mass of an object is concentrated)

This is similar to the pivot point on a see-This is similar to the pivot point on a see-sawsaw

If one of two bodies orbiting each other is If one of two bodies orbiting each other is more massive than the other, the center more massive than the other, the center of mass is closer to the more massive of mass is closer to the more massive bodybody

If two bodies are similar in mass, their If two bodies are similar in mass, their center of mass is near the middle position center of mass is near the middle position between thembetween them

For any planet and the Sun, the center of For any planet and the Sun, the center of mass is just above the surface of the Sun mass is just above the surface of the Sun (or within the Sun) because the Sun is (or within the Sun) because the Sun is more massive than any planetmore massive than any planet

BarycenterBarycenter Barycenter is closer to the end of the broomBarycenter is closer to the end of the broom

The Earth does not revolve around the Sun, but rather the barycenter!