Planetary Motion Planetary Motion & &

Historical People in Historical People in AstronomyAstronomy

Earth ScienceEarth Science

Overview of our Solar SystemOverview of our Solar System Earth is one of the 8 planets revolving Earth is one of the 8 planets revolving

around, or orbiting, the sun.around, or orbiting, the sun.

All the planets, as well as most of their All the planets, as well as most of their moon (satellites), orbit the Sun in the moon (satellites), orbit the Sun in the same direction, and all their orbits, lie same direction, and all their orbits, lie near the same plane.near the same plane.

Humans have been watching the skies for Humans have been watching the skies for 1000s of years, and early ideas were 1000s of years, and early ideas were developed solely on the basis of Earth-developed solely on the basis of Earth-based observations.based observations.

We have gathered much information We have gathered much information through the use on modern technology. through the use on modern technology. Spacecraft, telescopes (ground based & Spacecraft, telescopes (ground based & space based).space based).

Influential People in Influential People in AstronomyAstronomy

1. 1. PtolemyPtolemy (85 – (85 – 165 AD) – 165 AD) – proposed that proposed that objects ORBIT objects ORBIT around an around an unmoving Earth unmoving Earth ((GEOCENTRISMGEOCENTRISM).).

PtolemyPtolemy

2. 2. TychoTycho BraheBrahe (1546 – (1546 – 1601)1601)

Made important Made important contributions by contributions by devising the devising the most most preciseprecise instrumentsinstruments available before available before the invention of the invention of the the telescopetelescope for for observing the observing the heavens.heavens.

The astronomical instruments of Tycho Brahe

Tycho Brahe

Initially Tycho Initially Tycho designed a zodiacal designed a zodiacal armillary sphere, armillary sphere, which could be used to which could be used to obtain the ecliptic co-obtain the ecliptic co-ordinates of celestial ordinates of celestial bodies (celestial bodies (celestial longitude and latitude) longitude and latitude) directly, without any directly, without any calculation. calculation.

Using Trigonometry Using Trigonometry

Brahe cont…Brahe cont… His observations of planetary motion, His observations of planetary motion,

particularly that of Mars, provided particularly that of Mars, provided crucial data for later astronomers like crucial data for later astronomers like KeplerKepler to construct our to construct our presentpresent model of the model of the solarsolar systemsystem..

Brahe cont…Brahe cont…

BraheBrahe proposed a model of the solar proposed a model of the solar system that was intermediate system that was intermediate between the between the PtolemaicPtolemaic and and CopernicanCopernican models (it had the models (it had the EarthEarth at the at the centercenter).).

It proved to be It proved to be incorrectincorrect, but it was , but it was the most widely accepted model of the most widely accepted model of the Solar System for the time.the Solar System for the time.

3. 3. CopernicusCopernicus (1473 – 1543)(1473 – 1543)

Copernicus…Copernicus… 11stst to propose that planets ORBIT to propose that planets ORBIT

aroundaround the Sun the Sun ((HeliocentrismHeliocentrism).). CONTROVERSIAL at the time!!!CONTROVERSIAL at the time!!!

4. 4. GalileoGalileo (1564 – 1642) (1564 – 1642)

GalileoGalileo…… Adhered to Adhered to CopernicusCopernicus

theories and as a result he theories and as a result he was brought forward in was brought forward in 16331633, , and, there, in front of his and, there, in front of his “betters,” he was, under the “betters,” he was, under the threat of torture and death, threat of torture and death, forced to his knees to forced to his knees to renounce all belief in renounce all belief in Copernican theories, and was Copernican theories, and was thereafter sentenced to thereafter sentenced to imprisonment for the imprisonment for the remainder of his daysremainder of his days..

5. 5. KeplerKepler (1571 – 1630) (1571 – 1630)

Developed the 3 Developed the 3 laws on planetary laws on planetary motion.motion.

Described how Described how planets moved planets moved around the sun.around the sun.

11stst Law (Eccentricity): Law (Eccentricity): The The orbitorbit of each planet is the shape of of each planet is the shape of

an an ellipseellipse (oval –shaped)(oval –shaped) with the with the sunsun located at one located at one focusfocus. (There are 2 foci . (There are 2 foci in an ellipse).in an ellipse).

Ellipses can have different shapes, usually characterized by their "eccentricity." A circle is a special case of an ellipse. It has zero eccentricity.

When planet is at its When planet is at its farthestfarthest point point from the from the sunsun. 93 million miles!!!. 93 million miles!!!

AphelionAphelion

PerihelionPerihelion When a planet is at its When a planet is at its closestclosest point point

from the from the sunsun. 91 million miles!!!. 91 million miles!!!

22ndnd Law (Law of Equal Law (Law of Equal Areas):Areas):

In any time interval, a line from a planet to In any time interval, a line from a planet to the sun will sweep out the sun will sweep out equalequal areas.areas.

NOTE: As the planet goes around the sun, NOTE: As the planet goes around the sun, the further away it is, the slower it the further away it is, the slower it ORBITSORBITS. . This is due to the gravitational attraction This is due to the gravitational attraction between the Sun and Earth.between the Sun and Earth.

A planet sweeps out at equal amounts of A planet sweeps out at equal amounts of area in equal amounts of time.area in equal amounts of time.

Planet will “sweep” faster here.

Planet will “sweep” slower here.

Gravity gets stronger as the planets come “near” the sun.

Time: AB = CD

Universal Gravitational Law:Universal Gravitational Law:

6.67300 × 10-11 m3 kg-1 s-2

Universal Gravitational LawUniversal Gravitational Law Example:Example: There are 2 objects in space. There are 2 objects in space.

One is 50 kg and another is 65 kg. They One is 50 kg and another is 65 kg. They are 12 m apart. What is the gravitational are 12 m apart. What is the gravitational force between them?force between them?

F = F = 6.67300 × 10-11 [(50 kg)(65 kg)][(50 kg)(65 kg)] 12 m12 m F = 1.505 x 10F = 1.505 x 10-9-9 N N

Basically, a) the bigger the object, the Basically, a) the bigger the object, the stronger the gravity and b) the closer the stronger the gravity and b) the closer the object, the stronger the gravity.object, the stronger the gravity.

33rdrd Law (Law of Harmonies) Law (Law of Harmonies) The ratio of the squares of the The ratio of the squares of the

periodsperiods of any of any twotwo planetsplanets is is equalequal to to the ratio of the cubes of their average the ratio of the cubes of their average distances from the sun. Pdistances from the sun. P22 = a = a33

P = timeP = time22 it takes to go around the it takes to go around the sun.sun.

A = distanceA = distance33 from the sun. from the sun.

Using the 3Using the 3rdrd Law (write Law (write out):out):

We can use the 3We can use the 3rdrd law to law to find:find:

a)a) Distance from sun.Distance from sun.

b)b) Orbital period of a planet.Orbital period of a planet.

Example:Example:

Jupiter’s average distance from the sun Jupiter’s average distance from the sun is 5.20 AU. What is its orbital period?is 5.20 AU. What is its orbital period?

If If aa = 5.20, then = 5.20, then aa3 3 = 140.6.= 140.6. Use formula:Use formula: PP22 = a = a33

The orbital period must be the square The orbital period must be the square root of 140.6, which equals about 11.8.root of 140.6, which equals about 11.8.

Let’s try… Determine the orbital Let’s try… Determine the orbital period:period:

PlanetPlanet Distance from Distance from Sun (AU)Sun (AU)

Orbital PeriodOrbital Period

PP22 = a = a33

MercuryMercury 0.3870.387

VenusVenus 0.7230.723

EarthEarth 11

MarsMars 1.5241.524

JupiterJupiter 5.2035.203

SaturnSaturn 9.5399.539

UranusUranus 19.19119.191

NeptuneNeptune 30.07130.071

PlutoPluto 39.45739.457

0.2410.2410.6160.616111.881.8811.911.929.529.584.084.0165.0165.0248.0248.0

6. 6. NewtonNewton (1643 – 1727) (1643 – 1727)

Newton’s view on planetary Newton’s view on planetary motion…motion…

NewtonNewton proposed that the proposed that the planet’s motion is due to planet’s motion is due to 22 forces: forces: InertiaInertia and and gravitygravity. . When combined, the When combined, the planets move in an elliptical planets move in an elliptical orbit.orbit.

2. Sun’s gravity pulls Earth. 1. Earth’s

tendency of movement is in a straight line (Inertia).

Combined, it causes the planets to orbit around the sun.

RotationRotation

The time it takes for a planet spin The time it takes for a planet spin or rotate on its axis once.or rotate on its axis once.

Examples:Examples:

a)a) Earth = 24 hoursEarth = 24 hours

b)b) Mercury = 59 daysMercury = 59 days

RevolutionRevolution The time it takes for a planet to go The time it takes for a planet to go

around the sun once.around the sun once.

Examples:Examples:

a)a) Earth = 1 yearEarth = 1 year

b)b) Pluto (dwarf planet) = 248 yearsPluto (dwarf planet) = 248 years