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The Earth and Its Moon

Part 1: The Earth in Space

Day and NightEarth’s axis: an

imaginary line from the North Pole to the South Pole through the center of the Earth

Earth rotates on its axis in a counterclockwise direction, from west to east.

1 rotation = 1 day = about 24 hours

Attribution: Dennis Nilsson2

Day and NightThe Earth’s axis is tilted

23.5° from perpendicular with the line of orbit.

This tilt causes the length of daylight and darkness to vary from place to place and season to season.

If the axis were perpendicular, every place would have 12 hours of daylight and 12 hours of darkness every day.

Attribution: Dennis Nilsson3

A Year on Earth

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A year is the length of time it takes to orbit the sun, or one revolution.

One year = 365.26 daysEvery four years we add one day

to the calendar (leap day), to make up for the extra 0.26 days.

What would happen is we didn’t have Leap Years?

Seasons on Earth

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Also caused by the tilt of the Earth’s axisThe tilt of the axis is constantIn the summer the North Pole is tilted

toward the sun and in the winter it is tilted away from the sun.

Note that during our summer the Earth is actually at its furthest distance from the sun.

Seasons on Earth

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The hemisphere of the Earth that is tilted toward the sun receives more direct rays of sunlight and also has longer days than the hemisphere tilted away from the sun.

This results in more heat received and causes the summer season.

Note that at the equator about ½ the Earth is illuminated at all times.

Solstices

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Summer Solstice: June 20th or 21st The longest day

in the Northern Hemisphere

When the North Pole is tilted closest to the sun (23 ½°)

Winter Solstice:December 21st or

22nd Shortest day in

the Northern Hemisphere

When the North Pole is tilted farthest from the sun (23 ½°)

Equinoxes

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An equinox is when the tilt of the Earth’s axis is neither toward the sun or away from the sun.

Equinox (Latin) means equal night and on those two days each year day and night are about the same length

Also the time when the position of the Sun in the sky is directly overhead at the equator.

Vernal EquinoxMarch 20th or 21st

Autumnal EquinoxSeptember 22 or

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Solstices and Equinoxes

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The Earth at the start of the 4 (astronomical) seasons as seen from the north and ignoring the atmosphere (no clouds, no twilight).

A Magnet in Space

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Magnetic Field: invisible lines of force that connect the two ends (poles) of a magnet

A magnetic field points away from the north pole and toward the south pole.

Source: Newton Henry Black, Harvey N. Davis (1913) Practical Physics, The MacMillan Co., USA, p. 242, fig. 200 This image (or other media file) is in the public domain because its copyright has expired.

A Magnet in Space

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The Earth is much like a bar magnet.

The magnetism arises from electrical currents produced by the spinning liquid metallic outer core of iron and nickel

Supercomputer model of Earth's magnetic

field.

A schematic diagram of Earth's interior. The outer core is the source of the geomagnetic field.

North and South Poles

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The Magnetic North Pole and the Magnetic South Pole are not in the same place as the geographic North Pole and the geographic South Pole.

The magnetic poles are constantly shifting position and periodically trade places.

Magnetic north pole positions of the Earth.

Magnetosphere

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Solar wind (the plasma of charged particles coming out of the Sun in all directions at very high speeds) has its own magnetic field

The magnetosphere is the region of space around a planet where the planet’s magnetic field is dominant

Our magnetosphere deflects the solar wind around the Earth and protects us from harmful radiation.

Magnetosphere

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The strong solar wind compresses the magnetosphere on the sun side and elongates it out in a tail called the magnetotail on the other side.

Pictures from NASA

Van Allen Radiation Belts

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Regions of high-energy particles, mainly protons and electrons, held captive by the magnetic influence of the Earth.

Inner belt (identified in 1958 by Dr. James Van Allen) is mostly protons; 4,000 mi above Earth’s surface.

These belts can pose a hazard to orbiting satellites.

Aurora

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Radiation from the Van Allen Belts can travel along the magnetic field lines of force to the Earth’s poles.

When they collide with particle’s in the upper atmosphere they give off light creating the auroras.Aurora borealis =

Northern LightsAurora australis =

Southern LightsNorthern Lights in SwedenPhotos: Jerry MagnuM Porsbjer