CHAPTER 9 Tides Tides http://www.bbso.njit.edu/espr/sci_images/composite_earth1_red.gif

Overview Rhythmic rise and fall of sea level Very long and regular shallow-water waves Caused by gravitational attraction of Sun, Moon, and Earth http://soconnell.web.wesleyan.edu/ees106/lecture_notes/lecture-tides/HTML%20Presentation%20folder/img064.gif

Tide-generating forces http://www.lhup.edu/~dsimanek/scenario/img008.gif Tide-generating forces Fig. 9.1

Gravitational forces Every particle attracts every other particle Fig. 9.2

Centripetal force Center-seeking force – force equal at all points Tethers Earth and Moon to each other Fig. 9.3

Tide-producing forces Resultant forces = differences between centripetal and gravitational forces (blue arrows) Near moon, gravitational forces are greater Away from moon, centripetal forces are greater Tide-generating forces are horizontal components resulting in “bulges” Fig. 9.4

Fig. 9.4

Tidal bulges (lunar) Small horizontal forces push seawater into two bulges on opposite sides of Earth One bulge faces Moon Other bulge opposite side Earth Fig. 9.6

http://www.lhup.edu/~dsimanek/scenario/img008.gif Tidal bulges (lunar) Moon closer to Earth so lunar tide-producing force greater than that of Sun Ideal Earth covered by ocean If the entire earth was covered by ocean and the ocean depth and bottom was uniform, not the case Two tidal bulges Two high tides, 12 hours apart High tide, flood tide, seawater moves on shore Low tide, ebb tide, seawater moves offshore

Lunar Day Moon orbits Earth 24 hours 50 minutes for observer to see subsequent Moons directly overhead High tides are 12 hours and 25 minutes apart Tides are based on lunar cycle Fig. 9.7

Tidal bulges (solar) Sun effects tides, also Similar to lunar bulges but much smaller because sun is farther from earth New/full moon tidal range greatest – spring tide lunar and solar tides in line – constructive interference Time between spring tides about two weeks Quarter moons tidal range least – neap tide lunar and solar tides at 90O – destructive interference

Earth-Moon-Sun positions and spring and neap tides Fig. 9.9

Idealized tide prediction Two high tides/two low tides per lunar day Six lunar hours between high and low tides http://www.aztecsailing.co.uk/theory/Ch2-Fig-2-tidal-cycle.gif

Tidal patterns Semidiurnal Two high tides/two low tides per day Tidal range about same Diurnal One high tide/one low tide per day Rarest Mixed Tidal range different Most common http://www.fhwa.dot.gov/engineering/hydraulics/images/h25_b6.jpg

Global distribution of tides www.nos.noaa.gov/.../ tides/media/

Coastal tidal currents Reversing current Flood current or tide – water rushes into harbor with incoming tide Ebb current or tide – water drains out of harbor or bay with out-going tide Slack tide – current stops at peak of high and low tides High velocity flow in restricted channels Fig. 9.18

Misconceptions Distance from the sun causes the seasons. All tides happen in bodies of water that are very large. Outer space doesn’t really affect Earth. The sun revolves around the Earth.

Seasons

Seasons Solstice Day when the sun reaches its greatest distance north or south of the equator Northern Hemisphere SUMMER – June 21 or 22 WINTER – December 21 or 22

Seasons Equinox Day when the sun is directly over the equator Daylight and nighttime are equal Northern Hemisphere SPRING (Vernal) March 21or 22 FALL (Autumnal) September 21 or 22

Seasons

College of Alameda Physical Geography http://members.aol.com/pakulda/emmod.htm

MOON A cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own - shines by sunlight reflected from its surface.

MOON Rotates on its axis Revolves around the Earth Rotation of the moon takes about as long as its revolution Revolves around the Earth once every 27⅓ days The same side of the moon is facing the Earth

Phases of the MOON WAXING WANING When the moon is between New and Full Portion of moon visible is INCREASING WANING When the moon is between Full and New Portion of the moon visible is DECREASING

PHASES OF THE MOON http://www.harcourtschool.com/activity/moon_phases2/index2.html Good site for clarification and drawings The photographs show what the moon looks like from Earth. The drawings show what the moon would look like from space.

Lunar Eclipse Earth’s shadow falls on the moon can only occur at Full Moon UMBRA – darkest part of the shadow, a region where the Earth blocks all direct sunlight from reaching the Moon. PENUMBRA – cone of shadow cast by deflected light, a zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon

Lunar Eclipse There is a full moon once a month – why not a lunar eclipse? All 3 bodies must be in perfect alignment Must be nighttime when alignment occurs, only half the planet is experiencing night at a given time During Full Moon phase, the moon usually passes above or below Earth's shadows and misses them entirely. No eclipse takes place. Two to four times each year, the Moon passes through some portion of the Earth's penumbral or umbral shadows resulting in an eclipse Lunar Eclipse During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon. Astronauts on the Moon would then see the Earth eclipsing the Sun. (They would see a bright red ring around the Earth as they watched all the sunrises and sunsets happening simultaneousely around the world!) While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon. The total phase of a lunar eclipse is so interesting and beautiful precisely because of the filtering and refracting effect of Earth's atmosphere. If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in Earth's atmosphere. Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earth's atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible. All total eclipses start with a penumbral followed by a partial eclipse, and end with a partial followed by a penumbral eclipse (the total eclipse is sandwiched in the middle). The penumbral phases of the eclipse are quite difficult to see, even with a telescope. However, partial and total eclipses are easy to observe, even with the naked eye.

MOON Earth’s orbit around the sun The moon's orbital plane does not "lie flat" along the ecliptic, but forms an angle of about 5º - affects chances of experiencing an eclipse. (note: the distances in this diagram are not to scale)

MOON Path of moon through Earth’s shadow during total eclipse in 2000. Note the moon does not pass directly through the center of the Umbra Note: the moon does not pass directly through the center of the UMBRA

LUNAR ECLIPSE Partial Lunar Eclipse Total Lunar Eclipse A portion of the Moon passes through Earth's umbral shadow. These events are easy to see, even with the unaided eye. Total Lunar Eclipse The entire Moon passes through Earth's umbral shadow. These events are quite striking for the vibrant range of colors the Moon can take on during the total phase (i.e. - totality).

LUNAR ECLIPSE When an eclipse of the Moon takes place, everyone on the night side of Earth can see it. About 35% of all eclipses are of the penumbral type which are very difficult to detect, even with a telescope. 30% are partial eclipses which are easy to see with the unaided eye. 35% or so are total eclipses, and these are quite extraordinary events to behold.

Indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon. Lunar Eclipse During a total lunar eclipse, the Earth blocks the Sun's light from reaching the Moon. Astronauts on the Moon would then see the Earth eclipsing the Sun. (They would see a bright red ring around the Earth as they watched all the sunrises and sunsets happening simultaneously around the world!) While the Moon remains completely within Earth's umbral shadow, indirect sunlight still manages to reach and illuminate it. However, this sunlight must first pass deep through the Earth's atmosphere which filters out most of the blue colored light. The remaining light is a deep red or orange in color and is much dimmer than pure white sunlight. Earth's atmosphere also bends or refracts some of this light so that a small fraction of it can reach and illuminate the Moon. The total phase of a lunar eclipse is so interesting and beautiful precisely because of the filtering and refracting effect of Earth's atmosphere. If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors from dark brown and red to bright orange and yellow. The exact appearance depends on how much dust and clouds are present in Earth's atmosphere. Total eclipses tend to be very dark after major volcanic eruptions since these events dump large amounts of volcanic ash into Earth's atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible. All total eclipses start with a penumbral followed by a partial eclipse, and end with a partial followed by a penumbral eclipse (the total eclipse is sandwiched in the middle). The penumbral phases of the eclipse are quite difficult to see, even with a telescope. However, partial and total eclipses are easy to observe, even with the naked eye. LUNAR ECLIPSE

If the Earth had no atmosphere, then the Moon would be completely black during a total eclipse. Instead, the Moon can take on a range of colors. The filtering and refracting effect of Earth's atmosphere makes the total phase of a lunar eclipse interesting. The exact appearance depends on how much dust and clouds are present in Earth's atmosphere. Major volcanic eruptions tend to make total eclipses very dark, since these events dump large amounts of ash into Earth's atmosphere. During the total lunar eclipse of December 1992, dust from Mount Pinatubo rendered the Moon nearly invisible. LUNAR ECLIPSE

SOLAR ECLIPSE Moon moves directly between Earth and the sun UMBRA – Dark inner shadow; total eclipses are seen from within this shadow. The track of the Moon's shadow across Earth's surface is called the Path of Totality. It is typically 10,000 miles long but only 100 miles or so wide. PENUMBRA – Faint outer shadow; partial eclipses are seen from within this shadow.

SOLAR ECLIPSE There is a NEW moon once a month – why not a solar eclipse? All 3 bodies must be in perfect alignment During New Moon phase, the moon’s shadow usually passes above or below our planet and misses Earth entirely. No eclipse takes place. At least twice each year, the 3 bodies line up so that some part of the moon’s shadow falls on Earth’s surface An eclipse of the Sun is seen from that region Solar Eclipse Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn't happen because the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region.

2000 Jul 31: Partial Solar Eclipse http://sunearth.gsfc.nasa.gov/eclipse/OH/OH2000.html

QUESTIONS Describe the two types of planetary motion. Why does the Earth have tides? What is the relationship between tides and waves? What are the characteristics of a wave? What causes the Coriolis Effect? What is the Coriolis Effect? Why does the Earth have seasons? What is the difference between a solstice and an equinox?

QUESTIONS What is the difference between a solar eclipse and a lunar eclipse? When can you see a total solar eclipse? A total lunar eclipse?