AST 111 Lecture 6 Seasons, Phases of the Moon and Planets.
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AST 111 Lecture 6 Seasons, Phases of the Moon and Planets
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Transcript of AST 111 Lecture 6 Seasons, Phases of the Moon and Planets.
AST 111 Lecture 6
Seasons, Phases of the Moon and Planets
Earth’s Seasons
• Light is a form of energy that can heat things up
• Consider light coming from the sun hitting different sized areas– Which heats up faster?
Earth’s Seasons
• Now consider light impinging upon a large sphere.– Which parts get hottest? Coolest?– Consider how “sections” of light spread out
Earth’s Seasons
Equal amounts of sunlight hit unequalamounts of area.
Earth’s Seasons
• What if Earth’s axis were perpendicular to the plane of the solar system?
• Temperature would vary with latitude, but not time of the year.
Earth’s Seasons
• In this picture, say you’re in Arizona. – Hotter in July or January?
So what causes the temperature to change,over the year, at a FIXED latitude?
Earth’s Seasons
• Earth’s axis is tilted.– This causes the seasons.– Tilted 23.5o to Solar System plane
Earth’s Seasons
Sunlight hits different parts of the Earth more directly or less directly during different
parts of the year.
Earth’s Seasons
Northern Hemisphere:
Winter
Northern Hemisphere:
Summer
Earth’s Seasons
Southern Hemisphere:
Summer
Southern Hemisphere:
Winter
Tropics
• Summer in Northern Hemisphere:– Sun at zenith in Tropic of Cancer
• Summer in Southern Hemisphere:– Sun at zenith in Tropic of Capricorn
• What are the latitudes of each tropic?
Earth’s Seasons
• The vertical motion in the picture goes along with the seasons.
• The Sun is:– Closest to zenith in
summer– Closest to horizon in
winter
Terminology(applies to Northern hemisphere)
• Summer Solstice (June 21): – Most direct sunlight– Sun highest in the sky
• Winter Solstice (December 21):– Least direct sunlight– Sun lowest in the sky
• Spring (Vernal) Equinox (March 21):– Axis: tipped away to tipped toward
• Fall Equinox (September 21): – Axis: tipped toward to tipped away
Earth’s Seasons
• Common misconception: ellipticity of Earth’s orbit causes the seasons
– Distances do not even correspond to the seasons.
– North and South hemispheres would not have opposite seasons.
– Earth’s orbit only slightly eccentric.
Elliptical Orbit
Eccentricity greatly exaggerated.
SummerWinter
Seasons
• Jupiter has an axial tilt of 5 degrees.
• Saturn has an axial tilt of 27 degrees.– Does Jupiter have mild
seasons?– Saturn?
Phases of the Moon
Waxing Crescent First QuarterNew Moon Waxing Gibbous
Full Moon Waning Gibbous Third Quarter Waning Crescent
Phases of the Moon
• How much of the Moon is illuminated by the Sun at any given time?
Phases of the Moon
• Moon orbits Earth every 27 1/3 days
• How does its orbit around Earth affect how much of the Moon we see?
Phases of the Moon
• The Moon shows phases because the half that is lit faces Earth differently.
Synchronous Rotation
• The Moon spins on its axis in the same time it completes an orbit around Earth.
• What does this imply about how we see the Moon?
Inner Planets Have Phases
Full Venus or Mercury?
• Do we ever see a “full Venus”? Or a “full Mercury”?
Inner Planets Have Phases
And the outer planets?
• From Earth:– The inner planets have phases– The outer planets do not
• Why?
Outer Planets Do NOT Have (significant) Phases
• Consider the geometry of the solar system• Earth is always looking at the lit-up face of the
outer planets
Outer Planets do NOT Have (significant) Phases
• Earth is much closer to the source of light than the outer planets, and so we see them more “from the Sun’s point of view”
