As the Earth spins on its axis, the sky seems to rotate around us. This motion, called diurnal...
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As the Earth spins on its axis, the sky seems to rotate around us. This motion, called diurnal motion , produces the beautiful concentric trails traced by stars during time exposures. Near the middle of the circles is the North Celestial Pole (NCP), easily identified as the point in the sky at the center of all the star trail arcs. The star Polaris, commonly
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Transcript of As the Earth spins on its axis, the sky seems to rotate around us. This motion, called diurnal...
As the Earth spins on its axis, the sky seems to rotate around us. This motion, called diurnal motion, produces the beautiful concentric trails traced by stars during time exposures. Near the middle of the circles is the North Celestial Pole (NCP), easily identified as the point in the sky at the center of all the star trail arcs. The star Polaris, commonly known as the North Star, made the very short bright circle near the NCP. About 12,000 years ago, the bright star Vega was the North Star, and in about 14,000 years, as the Earth's spin axis slowly continues to precess, Vega will become the North Star again.
Homework #1 will be posted later today (check class website)
It will be due Monday, September 14, 8:00 pm
The origins of astronomy and the physical sciences stem from:
observing the sky and nature finding repeatable patterns seeking natural explanations for these patterns
Patterns Observed in the sky:
(observed facts)
Celestial Sphere
Large imaginary spherical surface centered on the Earth.
Stars and other celestial objects “fixed” on its surface.
Conceptual Model, not a physical model
The sun follows the same path around the sky (celestial sphere), repeating this journey once every year.
This path is called the “Ecliptic”
Why does the sun appear to move like this?
Locations of planets in the sky
Mercury: always close to Sun in sky
Venus: always close to Sun in sky
Mars: no restrictions on distance from Sun in sky
Jupiter: no restrictions on distance from Sun in sky
Saturn: no restrictions on distance from Sun in sky
What causes these differences?
On short term (diurnal motion), planets appear to move with the stars, east to west, making a full circuit around the sky (meridian to meridian) in approximately one day
Most of the time, planets move slowly eastward each day relative to the stars: different planets at different rates
Motions of the planets
What causes these motions?
Planets are always close to the “ecliptic”, the apparent annual path of the sun through the sky.
Close grouping of five planets in April 2002.
This is a pattern that was well known to the “ancients”
Why are the planets restricted to these locations?
Some planets occasionally reverse their motion relative to the stars, moving slowly westward relative to the stars, for a few days
apparent retrograde motion
What causes this?
What causes this?
What causes the observed motions of the stars, sun, moon, and planets in the sky?
The Greeks developed a model for the Universe that lasted for nearly 15
centuries.
It did a reasonably good job explaining these motions.
Claudius Ptolemy (100-170 CE)
Developed a model of the universe designed to fit the observational data.
Ptolemy and later scientists were strongly influenced by the belief of
Plato that …
“all natural motion is circular”
Ptolemy’s Geocentric Model● Earth is at center
(Geocentric)
● Sun orbits Earth
●Planets orbit on small circles (epicycles) whose centers orbit the Earth on larger circles (this explains retrograde motion)
Apparent retrograde motion in geocentric model
Geocentric Model Planet orbits lie in
approximately the same plane (this explains why the planets are always near the ecliptic)
Inferior planet epicycles were fixed to the Earth-Sun line (this explained why Mercury & Venus never stray far from the Sun).
Ptolemy’s model fit the data and made accurate predictions, but was horribly contrived!
● Although the geocentric model of Ptolemy gained dominance, Aristarchus of Samos actually proposed that the earth rotated daily and revolved around the sun
Ptolemy’s Geocentric Model
● Relied upon circles upon circles (epicycles & defferents) to explain the motions of planets and the sun.
● Tied to Plato & Aristotle’s belief that “all natural motion is circular”
● With modifications (e.g., additions of epicycles upon epicycles), remained the standard through the middle-ages.
Plato proposed that the orbits of the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles
Plato proposed that the orbits of the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles
The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere
The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere
What is the ecliptic?
when the Moon passes in front of the Sun
the constellations commonly used in astrology to predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial sphere
What is the ecliptic?
when the Moon passes in front of the Sun
the constellations commonly used in astrology to predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial sphere
About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens
About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens
The Revolution Begins!
The Copernican Revolution
● Copernicus, Tycho, Kepler, and Galileo.● Kepler’s three laws of planetary motion
Nicolaus Copernicus (1473-1543)
He thought Polemy’s model was contrived
Yet he believed in circular motion
De Revolutionibus Orbium Coelestium
Copernicus’ Heliocentric Model●Sun is at center of the Universe●Earth orbits the Sun like any other planet●Earth rotates●Circular orbits for all planets●Inferior planet orbits are smaller●Planets move at constant velocities in their
orbits●Retrograde motion occurs when we “lap” Mars
& the other superior planets
Copernicus’ Heliocentric Model●Retrograde motion occurs when we “lap” Mars
& the other superior planets
Simpler, more “elegant”
But, it still required some epicycles in order to make accurate predictions
because
It was still wedded to Aristotle's circular orbit paradigm
Predictions were not much better than those of Ptolemy
Tycho Brahe (1546-1601)● Greatest observer of his
day
• Charted accurate positions of planets(accurate positions of the planets were not fully available)
Tycho Brahe…
was motivated by inadequacy of existing predictions
made very accurate observations of positions (this was prior to the development of the telescope)
advocated a model in which Sun orbits Earth because he could not observe stellar parallax
The parallax problem troubled the Greeks and Tycho. It led both to reject a heliocentric
universe.
The problem was that stars are too distant to produce a parallax large enough to be seen with the technology of those time.
1600 – Tycho brought Johannes Kepler to bear on problem. He assigned him the task of understanding the motions of Mars.
Kepler had great faith in Tycho's measurements; they placed strong constraints on model
