Retrograde motion
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73 Terms
In what direction do stars appear to move across the sky during one night?
East to west.
In what direction do the Sun, Moon, planets, and stars rise and set?
They rise in the east and set in the west.
What is the North Star called?
Polaris.
What is the circumpolar region?
The region of the sky where stars appear to circle around Polaris without rising or setting.
How far is Polaris from the center of the circumpolar region?
About 0.65 degrees.
Why do circumpolar stars never rise or set?
Their apparent paths remain above the horizon as they circle near Polaris.
What does geocentric mean?
Earth-centered.
What is a geocentric model?
A model in which Earth is at the center and the Sun, Moon, stars, and planets orbit Earth.
Who is associated with the early Greek geocentric model?
Plato and Aristotle.
What did the early Greek model assume about celestial motion?
Earth was at the center, celestial objects moved on transparent spheres, and their motions were perfectly circular.
What major observation did the Plato/Aristotle model fail to explain adequately?
Retrograde motion.
What is prograde motion?
A planet's normal movement west to east relative to the background stars.
What is retrograde motion?
The apparent temporary movement of a planet east to west relative to the background stars.
What is Mars's usual motion relative to the background stars?
West to east, or prograde motion.
What direction does Mars move during apparent retrograde motion?
East to west relative to the background stars.
Does Mars actually reverse its orbital direction during retrograde motion?
No. It only appears to move backward from Earth's point of view.
What causes Mars's apparent retrograde motion in the heliocentric model?
Earth moves faster in its smaller orbit and overtakes Mars. As Earth passes Mars, our changing line of sight makes Mars appear to move backward.
What everyday analogy did the lecture use for retrograde motion?
A faster motorcycle passing a slower motorcycle: from the faster rider's perspective, the slower motorcycle can appear to move backward even though both are moving forward.
Can planets other than Mars undergo apparent retrograde motion as viewed from Earth?
Yes. All seven other planets can exhibit retrograde motion as seen from Earth.
About how long can Mars's retrograde motion last?
About 60-80 days.
About how long does Mars take to complete one cycle around the sky?
About 26 months.
If planets rise in the east and set in the west, how can prograde motion be west to east?
These describe two different timescales. During a single night, planets travel across the sky east to west. But when you compare a planet's position against the background stars from night to night, its usual/prograde drift is west to east.
During one night, which way does Mars move across the sky?
East to west.
Over many nights during normal/prograde motion, which way does Mars shift relative to the background stars?
West to east.
Was Ptolemy's model geocentric or heliocentric?
Geocentric.
What was at the center of Ptolemy's model?
Earth.
What was an epicycle in Ptolemy's model?
The small circle on which a planet moved.
What was a deferent?
The larger circle around Earth on which the epicycle traveled.
How did Ptolemy explain retrograde motion?
Planets moved around small circles called epicycles, while those circles traveled around larger circles called deferents. This produced backward motion in the model.
What is the easiest way to remember epicycle vs. deferent?
Epicycle = small circle. Deferent = big circle.
How did Ptolemy's model differ from Aristotle's?
Ptolemy added smaller circles (epicycles) attached to larger circles (deferents) to explain planetary motion.
Why did Ptolemy's model remain in use for so long?
It was sufficiently accurate for predicting planetary positions and remained in use for about 1,500 years.
What does heliocentric mean?
Sun-centered.
Who proposed the Copernican heliocentric model?
Nicolaus Copernicus.
What did Copernicus's model propose?
A Sun-centered system in which the planets orbit the Sun.
What old Greek idea did Copernicus retain?
He still assumed celestial orbits were perfect circles.
How did Copernicus's model explain retrograde motion?
Retrograde motion occurs naturally because Earth and the other planets orbit the Sun at different speeds.
What do Copernicus's and Kepler's models have in common?
Both are heliocentric and place the Sun at the center of the planetary system.
What major difference exists between Copernicus's and Kepler's orbital models?
Copernicus used circular orbits; Kepler used elliptical orbits.
What important observations did Galileo make with his telescope?
He observed mountains and craters on the Moon, moons orbiting Jupiter, and the phases of Venus.
What did Galileo's observation of Jupiter's moons demonstrate?
Not everything revolves around Earth.
What did Galileo observe about the Moon?
It had mountains and craters, challenging the idea of perfectly smooth or perfect celestial bodies.
What does 'Ori.' mean in Galileo's observations?
Oriens = east.
What does 'Occ.' mean?
Occidens = west.
What did Galileo observe about Venus?
Venus goes through a full set of phases, including crescent through gibbous/near-full phases.
Why were the phases of Venus important?
They showed that Venus orbits the Sun, not Earth.
Why were Venus's phases a problem for the Ptolemaic model?
The Ptolemaic/geocentric arrangement could not produce a near-full Venus as Galileo observed.
Which model did Galileo's observations of Venus support?
The Copernican/heliocentric model.
What was Tycho Brahe's major contribution to astronomy?
He compiled extremely precise naked-eye measurements of planetary positions.
Did Brahe use a telescope for these measurements?
No. They were naked-eye measurements, using instruments such as quadrants and sextants.
Why was Brahe's data important?
His observations did not fully agree with older models and provided the precise data Kepler needed to develop a better model.
Who worked as Tycho Brahe's assistant?
Johannes Kepler.
What did Kepler discover about planetary orbits?
Planets move in elliptical, rather than perfectly circular, orbits.
Where is the Sun located in a planet's elliptical orbit?
At one focus of the ellipse.
What is a focus?
One of the two special points inside an ellipse; in a planetary orbit, the Sun is located at one focus.
What did Kepler's model successfully account for?
Retrograde motion of Mars, the phases of Venus, and Brahe's measurements.
What is Kepler's First Law?
Planets orbit the Sun in ellipses, with the Sun at one of the two foci.
What is Kepler's Second Law?
A line connecting the Sun and a planet sweeps out equal areas in equal amounts of time.
What does Kepler's Second Law tell us about a planet's speed?
A planet does not move at a constant speed throughout its orbit.
What is perihelion?
The point in a planet's orbit where it is closest to the Sun.
What happens to a planet's speed at perihelion?
It moves fastest.
What is aphelion?
The point in a planet's orbit where it is farthest from the Sun.
What happens to a planet's speed at aphelion?
It moves slowest.
What is Kepler's Third Law?
The square of a planet's orbital period is proportional to the cube of its semi-major axis.
What is the equation/relationship for Kepler's Third Law?
T^2 is proportional to r^3.
What does T represent in Kepler's Third Law?
The planet's orbital period.
What does r represent in your lecture's version of Kepler's Third Law?
The planet's semi-major axis.
Plato & Aristotle
Geocentric model.
Ptolemy
Geocentric model with epicycles and deferents.
Copernicus
Heliocentric model with circular orbits.
Galileo
Telescopic evidence supporting heliocentrism.
Tycho Brahe
Extremely precise observations and planetary data.
Johannes Kepler
Heliocentric model with elliptical orbits and three laws.