phases, eclipses, & tides chapter 1, section 3, p20-27
TRANSCRIPT
Phases, Eclipses, & Tides
Chapter 1, Section 3, p20-27
Motions of the Moon
• Moon moves through space in two ways– Revolves around Earth• Relative positions of sun, Earth, and moon change
– The changing relative positions of the moon, Earth, and sun cause the phases of the moon, eclipses, and tides
– Rotates on its axis• 1 moon “day” = 1 moon “year”
– Time to rotate once is same as time to revolve around Earth– Same side of moon always faces Earth
Phases of the Moon
• Moonlight is reflected sunlight– Moon doesn’t give off own light• Able to see things in dark room when light bounces off
them to your eyes
• Phases: different shapes of the moon you see from Earth– One phase cycle = one revolution of moon around
Earth
Phases of the Moon
• Moon phases caused by changes in relative positions of sun, Earth, and moon– Half of the moon always lit– Moon revolves around Earth– We see the moon from different angles– The side of the moon we see, isn’t always the “lit” side– The phase of the moon you see depends on how much of
the sunlit side of the moon faces Earth• New moon – side of moon facing Earth isn’t lit (sun behind moon)• Full moon – side of moon facing Earth is lit (sun in front of moon)• Cycle ≈ 29.5 days
Eclipses
• Eclipses– Eclipse: when a space object moves between the
sun and a third object casting a shadow on that third object• When the moon’s shadow hits Earth, or the Earth’s
shadow hits the moon, an eclipse occurs– Moon’s orbit is slightly tilted with respect to Earth’s orbit
» Moon revolves around Earth without moving into Earth’s shadow or casting a shadow on Earth
Eclipses
• When Do Solar Eclipses Occur– A solar eclipse occurs when the moon passes
directly between Earth and the sun, blocking sunlight from Earth• Solar eclipse: new moon blocks your view of the sun
– Solar – Latin word for “sun”
Eclipses• Total Solar Eclipses– Umbra: darkest part of shadow; (cone-shaped)
• From any part in umbra light is completely blocked– Moon’s shadow just long enough to reach Earth– Only few people on Earth in umbra experience total solar eclipse
» Sky grows dark as night» Air becomes cool» Able to see stars and solar corona (outer atmosphere of sun)
• Partial Solar Eclipses– Penumbra: part of shadow less dark than the umbra;
(larger part of shadow)• From any part in penumbra light is partial blocked
– Part of sun is still visible – don’t look at sun
Eclipses
• When Do Lunar Eclipses Occur– During a lunar eclipse, Earth blocks sunlight from
reaching the moon• Lunar eclipse: full moon is in Earth’s shadow and looks
dim
Eclipses
• Total Lunar Eclipses– Umbra: darkest part of shadow• Total lunar eclipse occurs when moon is in Earth’s
umbra– Able to see Earth’s shadow on moon before and after eclipse
• Able to be seen anywhere on Earth that moon is visible
• Partial Lunar Eclipses• Partial lunar eclipse occurs when moon passes partly
into umbra of Earth’s shadow
Tides• Tides– Tides: the rise and fall of ocean water that occurs
every 12.5 hours or so• Cycle:
– Water rises ≈ 6 hours– Water falls ≈ 6 hours
– Tides are caused by differences in how much the moon’s gravity pulls on different parts of Earth
Tides• Tide Cycle– Figure 16• Moon’s gravitational pull exerts a force
– Strongest on water closest to the moon» Water flows towards this point = high tide
– Pulls on Earth more than water on far side of Earth– Weakest on water farthest from the moon
» Water flows towards this point = high tide– In between these two points are low tides
» Water flows away from these areas
Tides• Spring Tides– Spring tide: tide with the greatest difference between
consecutive low and high tides• Occur twice a month when sun and moon are in a straight
line– Full moon– New moon
• Neap Tides– Neap tide: tide with the least difference between
consecutive high and low tides• Occur twice a month when sun and moon pull at right angles
– 1st Quarter moon– 3rd quarter moon
Homework
• Read Section 3: p20-27• Answer ?s 1-3 p27