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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
• Rigorous testing in controlled experiments
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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
• Rigorous testing in controlled experiments
• Communicating results to peers
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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
• Rigorous testing in controlled experiments
• Communicating results to peers
• More testing by independent researchers/teams
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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
•
•
•
•
Rigorous testing in controlled experiments
Communicating results to peers
More testing by independent researchers/teams
An accepted theory will be able to make accurate
predictions about physical phenomena
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The Scientific Method
• Scientific method
– Hypothesis evolves into a theory after
•
•
•
•
Rigorous testing in controlled experiments
Communicating results to peers
More testing by independent researchers/teams
An accepted theory will be able to make accurate
predictions about physical phenomena
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General Relativity Observation
Deflection of Stellar Light Path
Control Observation: measure star positions outside the Sun’s influence.
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General Relativity Observation
Deflection of Stellar Light Path
Prediction: mass of sun warps space, and therefore light
paths; stars’ positions will appear to shift
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General Relativity Observation
Deflection of Stellar Light Path
Observation that confirms prediction: stars appear to shift position 10
General Relativity Observation
Deflection of Stellar Light Path
Physical picture
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General Relativity Observation
Deflection of Stellar Light Path
Another Control Observation: reduce measurement uncertainties.
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General Relativity Observation
Deflection of GPS Signals
• Clock corrections in GPS prevent errors of
up to 5 miles/day from accumulating.
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What is Astronomy?
• A branch of science.
• Studies the motions,
origins, and properties
of celestial objects
using the scientific
method.
• Has its origins in
ancient history.
Crescent Jupiter and Io from Cassini Spacecraft
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What is Astronomy?
• Almost all knowledge
is gained through
study of light.
• Almost always
impossible to perform
experiments (too far)
• Astronomy has made
great advances with
new technologies.
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What is Astronomy?
• Scientific method
– Astronomy is a mostly observational science
– Can do only a few controlled experiments in
astronomy
• Stellar evolution   nuclear reactors
• Cosmology   particle accelerators
• Solar System
– Send satellites & rovers to planets
– Crash probe into comet: Deep Impact Mission
– Collect comet matter and return to Earth: Stardust Mission
– Can run computer models in place of experiments
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Origins of Astronomy - Time
• The Sun, Moon and stars served humanity
for thousands of years as a clock and a
calendar. Their motions were predictable.
• Today we have other means of measuring
time but long ago an accurate understanding
of the skies could mean life or death.
• Studying the skies told people when to plant
and harvest, when to go out to sea, when to
expect good or bad weather, etc.
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Time - Day & Night
• How do you know
what time it is?
• How high is the sun in
the sky?
• Morning - sunrise
• Evening - sunset
• Does the Sun really
rise and set?
Sunrise behind the Space Shuttle
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Time Day & Night- Earth’s Spin
• In reality because the Earth rotates on its axis from
West to East the Sun, Moon and stars all appear to
move from East to West.
• The Sun appears to rise in the East and set in the
West because the Earth spins.
• The Moon however also orbits the Earth traveling
from West to East but it takes much longer than 24
hours to orbit the Earth. However from night to
night you can see the Moon change position.
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Time - The Moon and Months
• It takes the Moon about one
month to orbit the Earth
• Our calendar of months was
originally based on the Moon
• The word “Month” is derived
from the word “Moon”
• Jewish and Islamic calendars are
still based on the Moon. The
New Moon starts a new month.
Photograph of crescent Moon and crescent Venus
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Time - Year and Seasons
• Cultures around the world
marked the Sun’s yearly motion
through the stars.
• Summer - Sun is farthest North.
• Winter - Sun is farthest South.
• Knowing when these dates
occurred was very important for
agricultural societies
First day of Summer at Stonehenge, England
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Earth’s Orbital Motion
• As the Earth
orbits the Sun
it appears that
the Sun is
actually
moving.
• The Sun
appears to
move through a
set of 12
constellations
over one year.
• Some
constellations
can only be
seen during
certain times of
year.
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Earth’s Orbital Motion
The path
through the
sky along
which the Sun
appears to
move is called
the ecliptic
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Earth’s Orbital Motion
The other planets appear to follow similar paths near the ecliptic.
The band through which the Sun and the planets appear to move
is called the Zodiac
But the planets really do move!
As seen from Earth their motions
appear more complicated than they
actually are. In fact they sometimes
appear to move backwards! This is
called retrograde motion.
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