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Astronomy Chapter1


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Early observers of the sky noticed how many planets?
How did they distinguish planets and stars?
To them the planets "wandered" in direct motion and retrograde motion
What is considered the "direct motion" early planetary observers saw?
Normal eastward movement across the sky.
What is considered the "retrograde motion" planets were observed to make?
westward movement. Making the planet appear to move in loops.
Early models of planetary movement had earth where? these models were termed?
at the center; egocentric
The first geocentric models were based on who's work?
In the first geocentric models, Earth was at the center while the other planet's and the sun circled in what shape?
How did the theory term one period around earth? wat was an epicycle?
Deferent ; an epicycle was a smaller circular orbit the planet made.
Who was known to have constructed one of the best geocentric models?
What did Ptolemy's theory explain correctly and what did it fail to explain?

-fairly good approximation of planetary orbits (circles)

failed to explain:
-retrograde motion
-planetary brightness
What was the name of the early greek philosipher who was nearly entirely correct about planetary motion?
Nicolas Copernicus was given credit for several IMPORTANT discoveries (5). What were they?
-First heliocentric model of planetary motion.
- All planets CIRCLE the sun
- Only the moon orbits the earth
-Earth spins on an axis.
-Retrograde motion is just an "optical illusion"
Why is Copernicus' concept of retrograde motion as being an "optical illusion" correct?
Because as Earth is about to overtake a planet in it's orbit around the sun, the other planet (f.e Mars) appears to be moving backwards.

In reality both planets are moving in the same direction, but Earth is travelling faster.
Galileo Galilei is often called the father of _______.
Experimental science.
Galileo was credited with being the first person to do what?
Use a telescope to observe objects in the sky.
Galileo is credited with several discoveries (4). What are they?
- Moon's craters
- Jupiter's moons
-Sun spots
- phases of Venus
Galileo is termed the father of experimental science because he first used what?
the scientific method.

He used it as a full circle.
Observations are used to explain which are used to predict observations....etc.
Tyco Brahe is considered one of the best what? What did he do to recieve this title

he made extremely accurate observations of the planets and kept accurate records.
When tyco brahe died, who inherited his data?
Johanes Kepler.
Who was considered a pure theorist?
Johannes Kepler
Kepler is credited with what two astronomical conclusions?
1.) He used Brahe's data to propose a mathematical theory of planetary speeds and motions

2.) proposed that planets were on eliptical orbits not circular and that there were NO EPICYCLES.
What is Kepler's FIRST law?
That planetary motion is in the shape of an elipse with the Sun at one focus.
For what astronomical bodies does Kepler's First law apply?
All orbiting bodies (planets, moons, asteroids)
What is the equation to determine a planets eccentricity?

major axis - minor axis / major axis
What is a planet's eccentricity?
the amount the planet's orbital DEVIATES FROM A PERFECT CIRCLE. (not very much in most cases)
What is Kepler's second law?
Planets sweep out equal areas of their orbit in equal times.
In order for Kepler's second law to be true, what must occur?
the planet must move faster where there is more area to cover and slower where there is less to cover.
What accounts for the changes in the speed a planetary orbits the sun?
The distance from the sun.

Remember eliptical orbits with sun at foci not center. Thus as planets get closer to the sun they travel faster because of its gravitational pull. Likewise as they move further from the sun the orbit is slower.
What is the point in the planet's orbit where the planet is moving the fastest (closest to the sun)?

Moving the slowest (furthest from the sun)?
perihelion ; aphelion.

peri- around sun
a- away from sun
What is Kepler's Third law?
The square of a planet's orbital period is equal to the cube of its semi-major axis.

P^2 = a^3
For Kepler's Third law calculations, what must be the units of period? semi-major axis?
Period: Earth years
Semi-major axis: AU
what relationship exists as outlined by Kepler's third law?
Linear. Planet's have p^2/a^3 very close to 1.
Kepler's laws are used to determine 2 very important properties of other planets. What are they?
1.) The shape of each planet's orbital.

2.) its distance to the sun (relative to Earth)
How are actual distances in the sky measured?
RADAR- RAdio Detection And Ranging. Radio waves are aimed at a planet and the time it takes to travel and return indicates how far the object is.
Can we measure the distance from earth to the sun by RADAR? why?
No, because the sun absorbs radio waves. No signal is transmitted back.
Newton among other achievements is credited with what 3 achievements?
1.) Three laws of movement
2.) Law of Gravity
3.) Calculus
What is Newton's first law of motion?
An object in motion or at rest will stay that way until an external force acts on it.
What is newton's second law of motion?
the acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass.
What is the equation of Newton's second law?
F= ma or a= F/m
What is Newton's third law of motion?
For every action there is an equal and opposite reaction.

ex. when a body exerts a force on a second body, the second body exerts an equal and opposite force.
Law of Gravity. What can of force is Gravity.
Gravity is an attractive force.
Why is gravity considered an inverse squared force?
because it is inversely propotional to the square of the two objects' distances.
What is the equation for gravity?
F= G m1m2/ r^2
Why does mass not equal weight?
An object's mass is irrespective of gravity. A person;s weight is the amount of gravitational force the Earth exerts on their body. Thus weight can change where there are different gravitational pulls.
What is responsible for the Earth's orbit?
The earth's orbit is a result of two forces. The Earth's velocity which is a tangential acceleration and the gravitational pull from the sun. the resultant vector puts it in an eliptical orbit around the sun.
why doesn't the sun orbit the earth?
While object's orbit eachother, they technically orbit the center of mass between the two bodies. the center of mass between earth and the sun is in the sun. Thus the sun does move minimally. its movement is called a wobble.
What is the recvision to Kepler's Third law:
P^2 = a^3 / total mass
why is this revision almost pointless?
because total mass is that of the sun and its orbiting body. Yet the sun's mass is so enormous in comparison to anything else. total mass is virtually = to the sun's mass.

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