Chapter 3 Section 1 The Phases of the Moon

The Motion of the Moon 2 things about motion of the moon: We see it moving eastward against the background of stars It moves about 13˚ eastward in the sky each night We notice that the markings on its face don’t change Moon orbits around Earth at an average distance of 384,400 km (30 times Earth’s diameter) Its orbit is slightly elliptical, so its distance from Earth can vary up to 6%

The Motion of the Moon Sidereal Period is the period of rotation or revolution of an astronomical body referred to the stars It takes the Moon days for the moon to circle the sky once and return to the same place among the stars We see the same side of the moon because it rotates on its axis to keep one side facing Earth If it didn’t rotate, then we would see a different sides of the moon in different parts of its orbit

The Phase Cycle As the moon moves around the sky, the sun illuminates different amounts of the side of the moon facing Earth, and so the moon passes through a sequence of phases When we see no moon in the sky on a clear night it is called a new moon, which is the start of the lunar phases As we continue to watch the progression the light we see reflected waxes (grows) until it is fully illuminated, which is called a full moon The 2 nd half of the lunar cycle reverses the first half because the reflected light of the moon wanes (shrinks)

The Phase Cycle Synodic Period – period of rotation or revolution of a celestial body with respect to the sun Cycle of lunar phases takes days (little over 4 weeks) Each phase takes about a week to complete

integration.pbworks.com/w/page/ /M oon%20Phase%20Images 3 rd quarter or Last Quarter Starting at New Moon the phases move around the diagram in a counter-clockwise direction. Similar to Figure 3-3 in your book

Chapter 3 Section 2 The Tides

The Causes of the Tides Tides are produced by a difference between the gravitational force acting on different parts of an object The side of the Earth facing the moon is about 4000 miles closer to the moon that is Earth’s center Moon’s gravity pulls more strongly on the oceans on the near side than on Earth’s center This difference is small, only about 3% of the moon’s total gravitational force on Earth, but it is enough to make the ocean waters flow into a bulge on the side of Earth facing the moon

The Causes of the Tides A bulge also forms on the side of Earth facing away from the moon Earth’s far side is about 4000 miles farther from the moon than Earth’s center, and the moon’s gravity pulls on it less strongly than it does on Earth’s center This causes a small force that makes the ocean waters on Earth’s far side flow away from the moon

The Causes of the Tides Even though Earth rotates on its axis, the tidal bulge remains fixed along the Earth-moon line As Earth turns and carries us into the tidal bulge, the ocean water deepens, and the tide crawls up the beach Since there are two bulges on opposites sides of Earth, the tides rise and fall twice a day, and the times of high tide and low tide depend on the phase of the moon

Both of these picture were taken in a harbor on the Bay of Fundy. The picture on the right was taken at high tide and picture on right was taken at low tide. The Bay of Fundy has the largest tidal range (over 12 meters) in the world.

The Causes of the Tides At new moon and full moon, the moon and sun produce tidal bulges that join together to cause extreme tidal changes Spring tides – occur at new moon and full moon phases High tide is very high Neap tides – occur at 1 st quarter and 3 rd quarter moon phases Low tide is very low

Tidal Effects Tidal forces can have surprising effects on both rotation and orbital motions Friction of Earth’s ocean waters against the seabeds slows Earth’s rotation – days a getting second longer per century Tides are slowing Earth’s rotation by about second per day – adds almost a second longer per year Earth’s gravitational force has slowed the moon’s rotation Moon’s orbit is growing about 3.8 cm per year, so it is getting further away from us

Chapter 3 Section 3 Lunar Eclipses

Earth’s Shadow A lunar eclipse occurs at full moon when the moon moves through Earth’s shadow Earth’s shadow consists of 2 parts: Umbra – region of total shadow Penumbra – region where sunlight is dimmed, but not totally extinguished This diagram was copied from Nick Strobel's Astronomy Notes. Go to his site at for the updated and corrected version.Nick Strobel's Astronomy Noteswww.astronomynotes.com

Total Lunar Eclipses A lunar eclipse occurs when the moon passes through Earth’s shadow and grows dark If the moon passes through the umbra and no part of the moon remains outside the umbra in the partial sunlight of the penumbra, the eclipse is called a total lunar eclipse Total Lunar Eclipse Progression

Total Lunar Eclipses Even when the moon is totally eclipsed, it doesn’t disappear completely The bending of sunlight by our atmosphere leaks into the umbra and bathes the moon in a faint glow Since blue light from the sun is scattered by Earth’s atmosphere more easily than red light, it is the red light that penetrates to illuminate the moon in a coopery glow

Partial and Penumbral Lunar Eclipses Not all eclipses are total If the moon’s orbit carries the full moon too far north or south of the umbra, the moon may only partially enter the umbra This results in a partial lunar eclipse Only part of the moon remains outside the umbra, it receives some sunlight and looks brighter in contrast with the dark part of the moon inside the umbra If the orbit of the moon carries the moon far enough north or south of the umbra, the moon may only pass through the penumbra and never reach the umbra This results in a penumbral eclipse Moon is in the partial shadow of the penumbra which causes the moon to only partially dim Most people glancing at this type of eclipse won’t notice any difference from a full moon

Chapter 3 Section 4 Solar Eclipses

A solar eclipse occurs when the moon moves between Earth and the sun Total Solar Eclipse – takes place when the moon covers the disk of the sun completely Partial Solar Eclipse – takes place when the moon covers only part of the sun SEgallery1/SEgallery1.html

The Moon’s Shadow The moon’s total (umbral) shadow produces a spot of darkness roughly 167 miles in diameter on Earth’s surface If we are in this spot of total shadow, we will see a total Solar Eclipse If we are just outside the total shadow but in the penumbra, we see part of the sun peeking around the moon, and the eclipse is partial If we are outside of the penumbra, we will see no shadow at all Found on google images

Total Solar Eclipses A total solar eclipse begins when we first see the edge of the moon encroaching on the sun. This is the moment when the edge of the penumbra sweeps over our location During this partial phase, part of the sun remains visible, and it is hazardous to look at the eclipse without protection Dangerous invisible heat radiation (infrared) can burn the retina of our eyes Best to observe the partial phases with a pinhole projection Next total solar eclipse visible from the United States will occur August 21, 2017