Unit 1 - Sky Observation & Measurement
1. History
a. Plato/Aristotle (384-322 BCE, ancient Greece) - Greek philosophers
i. Geocentrism - Earth was center of universe, planets and stars
revolved around in circles.
ii. Uniform circular motion - Circles are “perfect,” and the heavens must
be perfect, therefore they must move in circles.
b. Hipparchus (190-120 BC, ancient Greece) - “Father of astronomy” &
inventor of trigonometry.
i. Almagest - One of the oldest existing records of actual
measurements/explanations of movements of celestial bodies.
ii. First mathematical models of Sun/Earth/Moon system - Worked out
motions & distances of these allowing prediction/explanation of eclipses, solstices/equinoxes, etc.
1. Sextant/Astrolabe/Armillary sphere - Used (possibly invented?) these devices to record
positions & motions of the heavens.
2. Parallax - Observations from different locations can be used in combination with trig to
determine distances. Founder of trigonometry.
c. Claudius Ptolemy (100-170, Greco-Roman Egypt) -
i. Retrograde motion - The planets (wanderers) appear to change
direction during certain times of the year and begin to move
backward. Difficult to explain using geocentric model.
ii. Epicycles - “Circles on circles.” In order to maintain uniform circular
motion, he uses multiple circles on circles to approximate motion of
planets.
iii. Ptolemaic Model -
d. Nicolaus Copernicus (1473-1543, Prussia)
i. Heliocentrism - Borrowing ideas from those like Aristarchus &
Al-Tusi, postulates that apparent motion of the heavens is
actually due to Earth’s rotation & revolution around Sun
ii. Retrograde motion revisited - “Backward” motion of planets is
only apparent. It is due to an inferior planet passing a superior
planet in orbit.
e. Tycho Brahe (1546-1601, Denmark) -
i. Built a giant observatory (Uraniborg) where he corrected/refined
much of the earlier celestial data (often predicted events were
off by days.)
ii. Tychonic model & Epicycles on epicycles - All planets were in orbit around the sun, but the sun still
orbited around earth. Animated tychonic model.
f. Johannes Kepler (1571-1630, Germany) - Tycho Brahe’s student in his later years, inherited his work.
i. Laws of planetary motion - More to come on these in unit 2
1. Law of Ellipses - Planets orbited not in perfect circles, but in elliptical shapes.
2. Law of Areas - Describes the relative velocity of planets during different parts of orbit.
3. Law of Harmonies - Describes orbital periods of planets at different distances
g. Galileo Galilei (1565-1642, Italy) -
i. Telescope - Improved upon telescope design and pointed it skyward, making several discoveries
1. Jupiter’s moons - Four moons of Jupiter (Galilean) were visible, looking very much like a mini
solar system. Showed that there could be more than one center of motion in universe.
, 2. Phases of venus - Venus goes through phases very similarly to those of the moon. Geocentric
model struggled to explain the observed phases. Geocentric vs heliocentric venusian phases.
ii. Heliocentrism returns - These two discoveries convinced him that the sun-centered model was
correct. Published a book supporting the Copernican view (somewhat censured), was later tried
for heresy by the inquisition and put on quasi house arrest.
h. Isaac Newton (1642-1726, England) - Previous explanations of celestial motion focus on how objects
move, not why they move in this way.
i. Universal law of gravity - Any two objects with mass experience an attractive force as expressed
by the following equation: FG=Gm1m2/d2
2. Astronomy tools/technology
a. Behavior of waves and light
i. Structure of EM waves - Consist of an electric
wave and magnetic wave propagating at right
angles to one another.
ii. Light variable relationships
1. Wavelength/frequency relationship c = λ * 𝜈
○ c = 3.00 x 108 m/s (speed of light)
○ λ = wavelength (meters)
○ 𝜈 = frequency (1/s or Hertz)
○ Example:
2. Energy/frequency relationship E = h * 𝜈
○ E = energy of a photon (Joules)
○ h = Planck’s constant (6.626 x 10-34 J*s)
○ 𝜈 = frequency ( 1/s or Hertz)
○ Example:
iii. EM spectrum - Various types of
electromagnetic waves of different
wavelengths fit into the categories
pictured at right.
iv. Behavior of waves
1. Transmission - Light passes
straight through an object (glass)
2. Reflection - Light bounces off an
object at the same angle it came in
(mirror)
3. Refraction - Light bends as it
changes mediums. Short
wavelengths bend more than long
wavelengths as in a prism
4. Diffraction - Light bends around
corners & through small slits
5. Absorption - Light strikes atoms in
a substance and causes them to
vibrate, becomes thermal energy
6. Scattering - Light bounces off objects in random directions. This is why the sky appears blue.
1. History
a. Plato/Aristotle (384-322 BCE, ancient Greece) - Greek philosophers
i. Geocentrism - Earth was center of universe, planets and stars
revolved around in circles.
ii. Uniform circular motion - Circles are “perfect,” and the heavens must
be perfect, therefore they must move in circles.
b. Hipparchus (190-120 BC, ancient Greece) - “Father of astronomy” &
inventor of trigonometry.
i. Almagest - One of the oldest existing records of actual
measurements/explanations of movements of celestial bodies.
ii. First mathematical models of Sun/Earth/Moon system - Worked out
motions & distances of these allowing prediction/explanation of eclipses, solstices/equinoxes, etc.
1. Sextant/Astrolabe/Armillary sphere - Used (possibly invented?) these devices to record
positions & motions of the heavens.
2. Parallax - Observations from different locations can be used in combination with trig to
determine distances. Founder of trigonometry.
c. Claudius Ptolemy (100-170, Greco-Roman Egypt) -
i. Retrograde motion - The planets (wanderers) appear to change
direction during certain times of the year and begin to move
backward. Difficult to explain using geocentric model.
ii. Epicycles - “Circles on circles.” In order to maintain uniform circular
motion, he uses multiple circles on circles to approximate motion of
planets.
iii. Ptolemaic Model -
d. Nicolaus Copernicus (1473-1543, Prussia)
i. Heliocentrism - Borrowing ideas from those like Aristarchus &
Al-Tusi, postulates that apparent motion of the heavens is
actually due to Earth’s rotation & revolution around Sun
ii. Retrograde motion revisited - “Backward” motion of planets is
only apparent. It is due to an inferior planet passing a superior
planet in orbit.
e. Tycho Brahe (1546-1601, Denmark) -
i. Built a giant observatory (Uraniborg) where he corrected/refined
much of the earlier celestial data (often predicted events were
off by days.)
ii. Tychonic model & Epicycles on epicycles - All planets were in orbit around the sun, but the sun still
orbited around earth. Animated tychonic model.
f. Johannes Kepler (1571-1630, Germany) - Tycho Brahe’s student in his later years, inherited his work.
i. Laws of planetary motion - More to come on these in unit 2
1. Law of Ellipses - Planets orbited not in perfect circles, but in elliptical shapes.
2. Law of Areas - Describes the relative velocity of planets during different parts of orbit.
3. Law of Harmonies - Describes orbital periods of planets at different distances
g. Galileo Galilei (1565-1642, Italy) -
i. Telescope - Improved upon telescope design and pointed it skyward, making several discoveries
1. Jupiter’s moons - Four moons of Jupiter (Galilean) were visible, looking very much like a mini
solar system. Showed that there could be more than one center of motion in universe.
, 2. Phases of venus - Venus goes through phases very similarly to those of the moon. Geocentric
model struggled to explain the observed phases. Geocentric vs heliocentric venusian phases.
ii. Heliocentrism returns - These two discoveries convinced him that the sun-centered model was
correct. Published a book supporting the Copernican view (somewhat censured), was later tried
for heresy by the inquisition and put on quasi house arrest.
h. Isaac Newton (1642-1726, England) - Previous explanations of celestial motion focus on how objects
move, not why they move in this way.
i. Universal law of gravity - Any two objects with mass experience an attractive force as expressed
by the following equation: FG=Gm1m2/d2
2. Astronomy tools/technology
a. Behavior of waves and light
i. Structure of EM waves - Consist of an electric
wave and magnetic wave propagating at right
angles to one another.
ii. Light variable relationships
1. Wavelength/frequency relationship c = λ * 𝜈
○ c = 3.00 x 108 m/s (speed of light)
○ λ = wavelength (meters)
○ 𝜈 = frequency (1/s or Hertz)
○ Example:
2. Energy/frequency relationship E = h * 𝜈
○ E = energy of a photon (Joules)
○ h = Planck’s constant (6.626 x 10-34 J*s)
○ 𝜈 = frequency ( 1/s or Hertz)
○ Example:
iii. EM spectrum - Various types of
electromagnetic waves of different
wavelengths fit into the categories
pictured at right.
iv. Behavior of waves
1. Transmission - Light passes
straight through an object (glass)
2. Reflection - Light bounces off an
object at the same angle it came in
(mirror)
3. Refraction - Light bends as it
changes mediums. Short
wavelengths bend more than long
wavelengths as in a prism
4. Diffraction - Light bends around
corners & through small slits
5. Absorption - Light strikes atoms in
a substance and causes them to
vibrate, becomes thermal energy
6. Scattering - Light bounces off objects in random directions. This is why the sky appears blue.
