Astronomy

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Previous Lessons
Open Chapter Ch. 1: A Modern View of the Universe
Lesson #1 The Scale of the Universe
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Lesson #2 The History of the Universe
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Lesson #3 Spaceship Earth
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Open Chapter Ch. 2: Discovering the Universe for Yourself
Lesson #4 Patterns in the Night Sky
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Lesson #5 The Reason for Seasons
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Lesson #6 The Moon, our Constant Companion
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Lesson #7 Ancient Mystery of the Planets
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Open Chapter Ch. 3: The Science of Astronomy
Lesson #8 The Ancient Roots of Science
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Lesson #9 Ancient Greek Science
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Lesson #10 The Copernican Revolution
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Lesson #11 The Nature of Science
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Open Chapter Ch. 4: Understanding Motion, Energy, and Gravity
Lesson #12 Describing Motion
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Lesson #13 Newton's Laws of Motion
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Lesson #14 Conservation Laws in Astronomy
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Lesson #15 The Force of Gravity
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Open Chapter Ch. 5: Light: The Cosmic Messenger
Lesson #16 Basic Properties of Light and Matter
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Lesson #17 Learning from Light
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Lesson #18 Collecting Light with Telescopes
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Exam Exam 1
Open Chapter Ch. 6: Formation of the Solar System
Lesson #19 A Brief Tour of the Solar System
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Lesson #20 The Nebular Theory of Solar System Formation
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Lesson #21 Explaining the Major Features of the Solar System
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Lesson #22 The Age of the Solar System
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Open Chapter Ch. 7: Earth and the Terrestrial Worlds
Lesson #23 Earth as a Planet
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Lesson #24 The Moon and Mercury: Geologically Dead
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Lesson #25 Mars, a Victim of Planetary Freeze Drying
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Lesson #26 Venus, a Hothouse World
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Lesson #27 Earth as a living planet
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Open Chapter Ch. 8: Jovian Planet Systems
Lesson #28 A Different Kind of Planet
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Lesson #29 A Wealth of Worlds: Satellites of Ice and Rock
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Open Chapter Ch. 9: Asteroids, Comets, and Dwarf Planets
Lesson #30 Classifying Small Bodies
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Lesson #31 Asteroids
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Lesson #32 Comets
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Lesson #33 Pluto and the Kuiper Belt
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Lesson #34 Cosmic Collisions - Small Bodies vs Planets
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Open Chapter Ch. 10: Other Planetary Systems
Lesson #35 Detecting Planets Around Other Stars
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Lesson #36 The Nature of Planets Around Other Stars
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Lesson #37 The Formation of Other Planetary Systems
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Exam Midterm Exam
Open Chapter Ch. 11: Our Star
Lesson #38 The Sun, Our Star
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Lesson #39 Nuclear Fusion in the Sun
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Lesson #40 Sun-Earth Connection
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Open Chapter Ch. 12: Surveying the Stars
Lesson #41 Properties of Stars
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Lesson #42 Patterns in the Stars
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Lesson #43 Star Clusters
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Open Chapter Ch. 13: Star Stuff
Lesson #44 Star Birth
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Lesson #45 Life as a Low Mass Star
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Lesson #46 Life as a High Mass Star
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Open Chapter Ch. 14: The Bizarre Stellar Graveyard
Lesson #47 White Dwarfs
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Lesson #48 Neutron Stars
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Lesson #49 Black Holes: Gravity’s Ultimate Victory
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Exam Exam 3
Open Chapter Ch. 15: Our Galaxy
Lesson #50 The Milky Way Revealed
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Lesson #51 Galactic Recycling
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Lesson #52 The History of the Milky Way
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Open Chapter Ch. 16: A Universe of Galaxies
Lesson #53 Islands of Stars
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Lesson #54 Distances of Galaxies
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Lesson #55 Galaxy Evolution
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Lesson #56 The Role of Supermassive Black Holes
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Open Chapter Ch. 17: The Birth of the Universe
Lesson #57 The Big Bang Theory
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Lesson #58 Evidence for the Big Bang
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Lesson #59 The Big Bang and Inflation
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Open Chapter Ch. 18: Dark Matter, Dark Energy, and the Fate of the Universe
Lesson #60 Unseen Influences in the Cosmos
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Lesson #61 Structure Formation
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Open Chapter Ch. 19: Life in the Universe
Lesson #62 Life on Earth
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Lesson #63 Life in the Solar System
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Lesson #64 The Search for Extraterrestrial Intelligence
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Lesson #65 Interstellar Travel and Implications for Civilizations
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Exam Final Exam

Assignments:

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Lesson Objectives:

- Seasons
- Solstices and equinoxes
- Precession



What causes seasons? Seasons are affected by the position of the Earth in relationship to the Sun. The Earth is pointed towards Polaris (the North Star) throughout the year.

The orientation of the Earth's axis in relation to the Sun changes as it orbits the Sun.

For example, the Northern Hemisphere is tipped towards the Sun in June and away from the Sun in December. The opposite is true for the Southern Hemisphere. That is why these hemispheres have opposite seasons. Changing amounts of sunlight gives rise to seasons.

When the angle of the Earth to the Sun is steeper, more sunlight occurs, leading to warmer temperatures. When the angle is more shallow, less sunlight occurs, leading to cooler temperatures.



To help us mark changing seasons, we define four specific times of the year. Equinoxes and solstices help mark the progression of seasons.

June solstice, or summer solstice, occurs when the Northern Hemisphere is tipped *towards* the Sun and gets the most sunlight. This happens on June 21st.

December solstice, or winter solstice, occurs when the Northern Hemisphere is tipped *away* from the Sun and gets the least sunlight. This happens on December 21st.

March equinox, also known as spring or vernal equinox, occurs when the Northern Hemisphere goes from being tipped away from the Sun to slightly *towards* the sun. This happens on March 21st.

September equinox, or fall or autumnal equinox, occurs when the Northern Hemisphere first tips *away* from the Sun. This happens on Sept 22nd.

The Sun rises precisely due east and sets precisely due west only on the days of the March and September equinoxes.

Seasons also vary by latitude. For example, Vermont has a much longer summer day and a much longer winter night than Florida does. At high latitudes, the summer Sun remains above the horizon all day long.



How does the orientation of the Earth's axis change over time? While we maintain our solstice and equinoxes on the same dates, the constellations appear differently over time. This is because of a change in orientation in the Earth's axis known as precession.

Precession is the gradual wobble altering the orientation of the Earth's axis in space. Precession occurs with rotating objects. For example, if you spin a top, you will notice that while the top spins rapidly, the axis sweeps out a circle at a slower rate. Gravity pulls the object downward but does not succeed in pulling it over, just slowing it down. The same thing happens with the Earth except at a much slower rate. Each cycle of an Earth's precession is estimated to last 26,000 years, which gradually changes the direction in which the axis points in space.

The tilt of the Earth's axis stays at 23 1/2 degrees, however, and therefore seasons are not affected. This change in the orientation of the Earth's axis changes how we see constellations.