ASTR 001 Lecture Notes - Lecture 23: Lunar Phase, Moe Williams, Tidal Locking
15 Jul 2020
School
Department
Course
Professor
Elementary Astronomy
Moe Williams
msowae@go.pasadena.edu
Astr 1
Seasons:
● Sun’s path in the local sky of Charlottesville:
○ Summer solstice: highest path, 75.5 degree above horizon
○ Winter solstice: lowest path 28.4 degree above horizon
● What causes seasons: Tilt of Earth’s axis
● How does the orientation o the earth’s axis change with time:
The Moon:
● Want to know:
○ Why do we see phases of the moon?
○ What us the so-called “dark side of the moon”?
● The crescent moon is caused by the Earth’s shadow False
● Why do we see phases of the moon?
○ Lunar phases are a consequence of the Moon’s orbit around the Earth
○ 3 motions, same direction
■ Orbit of the moon around the Earth is counter-clockwise
■ 3 different time scales
■ Important, used later
○ Phases of the moon
■ Half of moon is illuminated by the sun and half is dark
■ We see a changing combination of the bright and dark faces as the
moon orbits
• New moon illuminated part faces the sun. Moon is in between the
• First Quarter moon half of the moon is visible, Rise noon, Set
midnight, sun leads the first quarter moon by 90 degrees
• Waxing Gibbous Rise 3 pm, set 3 am
• Full moon Earth is in between moon and Sun, Rise 6 pm, Set 6 am,
highest Midnight
• Waning Gibbous Rise 9 pm, Set 9 am
• Third Quarter Rise midnight, Set noon, leads sun by 90 degrees
• Waning Quarter Rise 3 am, set 3 pm
○ 29.5 day cycle
■ Waxing= gets “fuller”, trailing the sun in the sky
■ Waning= gets “less”, leading sun in the sky
● The “Dark” Side of the Moon
○ “Dark side of the moon refers to the side of the moon not visible to Earth
○ We see only one side of the moon
■ Moon rotates exactly once with each orbit, called “synchronous
rotation” (orbits Earth once a month and rotates on its axis exactly
once a month)
Document Summary
Sun"s path in the local sky of charlottesville: Summer solstice: highest path, 75. 5 degree above horizon. Winter solstice: lowest path 28. 4 degree above horizon. What causes seasons: tilt of earth"s axis. How does the orientation o the earth"s axis change with time: The crescent moon is caused by the earth"s shadow false. Lunar phases are a consequence of the moon"s orbit around the earth. Orbit of the moon around the earth is counter-clockwise. Half of moon is illuminated by the sun and half is dark. We see a changing combination of the bright and dark faces as the moon orbits: new moon illuminated part faces the sun. Waxing= gets fuller , trailing the sun in the sky. Waning= gets less , leading sun in the sky. Dark side of the moon refers to the side of the moon not visible to earth. We see only one side of the moon.
