The Earth/Moon/Sun

System

Planet Earth

• 40,000 km (25,000 mi.) in circumference.

• Bulges slightly at equator.

• 150 million km (93,000,000 mi.) from Sun.

• Has magnetic field called magnetosphere (generated by our core) that shields us from the solar wind and harmful radiation.

Earth’s Movements

• rotation– spinning on axis– completes one rotation in 23 hrs, 56 minutes

• revolution– orbiting sun– completes one revolution in 365.26 days

• precession– direction of axis moves, like a spinning top– period = 26,000 years

The Right-Hand RuleIf the thumb of your right hand represents the

North Pole, the earth rotates in the direction that your fingers point (west to east).

Apparent Movement

•The rotation of the Earth makes objects (sun, moon, stars) appear to move across our sky from East to West.•Stars between the North Star and the horizon never set, and are called circumpolar.

Sidereal Day vs. Solar Day• Sidereal Day - 23

hrs., 56 minutes – time it takes Earth to make one complete rotation (a distant star returns to the same position in the sky).

• Solar Day – 24 hours, time it takes for the sun to return to the same position in the sky. (4 minutes longer than sidereal day)

Seasons

• Earth’s axis is tilted 23.5°.

• Axis always points to same direction in space.

• Amount of sunlight falling on different regions changes throughout year.

• Northern hemisphere is tilted towards sun in summer, away in winter.

Reasons for Seasons• Summer – Sun passes through less atmosphere at a more direct angle (more concentrated) for a longer portion of the day.•Winter – Sun passes through more atmosphere at a less direct angle (less concentrated) for fewer hours per day.

Solstices – “sun stands still”• Summer Solstice – usually June 21

– First day of summer.– Longest daylight period and shortest night in northern

hemisphere.– Sun directly overhead at noon at Tropic of Cancer (23.5o

N).

• Winter Solstice – usually December 21– First day of winter.– Shortest day daylight period and longest night.– Sun directly overhead at noon at Tropic of Capricorn

(23.5o S).

•The tilt of the axis also determines the Arctic Circle (66.5o N) around the North Pole, in which there is 24 hours of daylight on the Summer Solstice and 24 hours of night on Winter solstice.•Antarctic Circle (66.5o S) around the South Pole has the opposite pattern.

Equinoxes – “equal night”

• Equal day and night length (12 hours each).

• Sun is directly overhead at the Equator.– Vernal Equinox - first day of spring, usually

March 21.– Autumnal Equinox - first day of autumn,

usually September 21.

Orientation of the Earth’s axis relative to the Sun makes the sun appear higher in the sky in summer (when we’re tilted toward it) and lower in winter (when we’re tilted away.

http://www.physicalgeography.net/fundamentals/6h.html

Time Zones

• Local noon is when sun crosses our meridian.

• Local time varies continuously with longitude.

• Standard time zones were invented to avoid confusion.

• International Date Line– gain day going east (subtract 24 hrs., repeat day) – lose day going west (add 1 day to date from

eastern side)

Earth’s Moon• Formed by impact of large body with early

Earth.

• No atmosphere.

• No magnetic field (no core to generate one).

• Many craters visible because the moon has no plate tectonics, weathering, or erosion.

• Completes revolution and rotation in 27.3 days, so same side always faces earth.

• Full moons 29 days apart.

Sidereal Month – 27 1/3 days, time it takes the moon to make one complete revolution around Earth.

Synodic Month – 29 ½ days, time it takes the moon to complete one lunar cycle (new moon to new moon).

Moon Phases

• Half of moon is always lit by sun.

• How much of lit half we see depends upon relative positions of sun, earth, and moon.

• Full moon - moon is on opposite side of earth from sun, rises as sun sets, we see entire lit side.

• New moon - moon is lined up on same side as sun, rises & sets with sun, we can’t see it.

Moon Phases

• Waxing – lit portion of the moon gets bigger each day, lit on the right side.

• Waning – lit portion of the moon gets smaller each day, lit on the left side.

• Crescent – less than half of the moon’s face is lit (excluding new moon).

• Gibbous – more than half of the moon’s face is lit (excluding full moon).

Eclipses• Lunar Eclipse - moon moves into earth’s

shadow. SunEarthMoon

• Partial shadow (penumbra) darkens moon slightly, full shadow (umbra) takes “bite” out of moon & gives shadowed surface a reddish glow.

Eclipses don’t happen every month because plane of moon’s orbit is tilted (moon is usually above or below the Earth’s shadow).

Eclipses• Solar Eclipse - earth moves into moon’s shadow.

SunMoonEarth

• Only a portion of Earth sees partial eclipse (penumbra), tiny portion sees total eclipse (umbra)

Tides

• Caused by gravitational pulls of sun and moon.

• Moon’s influence is greater (less mass, but much closer).

• Spring Tides (extreme) occur when earth, sun, and moon are in line (full & new moons).

• Neap Tides (reduced) occur when sun’s and moon’s pull are at right angles (first and last quarters).

Highest High Tides in the Worldhttp://www.youtube.com/watch?v=qfhNjpu_IU4 -

Bulge of water at high tide is funneled into increasingly narrow bays.

Highest tides are at the far end of the Bay of Fundy.

Incoming tide often appears as a rolling wave moving up the bay, called a tidal bore.

Tidal BoreIn a confined bay, such as the Bay of Fundy, the incoming

tide forms a standing wave that rushes into the bay.

Tidal Bore Turnagain Arm, AlaskaStandup Paddleboard Tidal Bore SurfingKnik Arm Bore TideTidal Bore Nova ScotiaBay of Fundy Tidal Bore rafting

Turnagain Arm of Cook Inlet, Alaska

Atmospheric Astronomy

Auroras

Aurora Borealis – Northern Lights

Aurora Australis – Southern Lights

AurorasCause: Sun emits constant stream of charged particles – solar wind, solar flares, coronal mass ejections (CMEs).

Earth’s magnetic field shields us from most, funnels charged particles into atmosphere around north & south magnetic poles.

Auroras

In upper atmosphere (thermosphere, ionosphere), charged particles interact with atmospheric gases to emit light.Nitrogen – blue or purplish-redLow altitude (100-300 km) oxygen – yellowish-greenHigh altitude (>300 km) oxygen – red (rare)Very low altitude (100 km) nitrogen – reddish fringe on bottom of “curtains”High altitude hydrogen & helium – deep blues & purples

Auroras

Northern Lights

Tourism

Meteors• Solid space debris (meteoroids) penetrate atmosphere & ignite

due to friction (meteors). • Any that survive plunge & land on Earth are meteorites.

Meteorites• Stony – mineral crystal

composition (silica) similar to some earth rocks.

• Iron – mostly iron w 5-30% nickel

• Stony-iron – blend of the two previous

• Lunar• Martian

Fireballs – Large space rocks entering atmosphere at shallow angle,

often disintegrate or explode.

Tunguska Event - 1908