Earth Motions



Earth Motions

Terms

• Zenith- sun is directly overhead (90 degrees)

• Horizon- 0 degrees

• Zenith angle- difference in latitude between the latitude where the sun is at zenith and a given latitude. If the two latitudes are in the same hemisphere the latitudes are subtracted, if the two latitudes are in different hemispheres then latitudes are added.

• Solar angle- highest altitude of the sun that is reached at a given location. It is calculated by subtracting the zenith angle from 90.

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The Earth has two principal motions- rotation and revolution

Revolution-

• the movement of the Earth around the sun

• it takes 365.25 days which accounts for leap year.

Rotation-

• as the earth moves around the sun it is spinning on its axis.

• it takes 24 hours to complete a rotation

Effects of Rotation

• day and night

• movement of the sun across the sky (the sun rotates west to east, counterclockwise)

Evidence for rotation

• Coriolis effect- the apparent deflection of the winds, if the earth did not rotate, winds would blow from the poles ( high pressure) to the equator (low pressure)

• Winds are deflected to the right in the Northern hemisphere.

Rate of rotation

• The earth completes one complete circle (360 degrees every 24 hours) so every location on earth moves at a rate of 15 degrees per hour 360/24 = 15 degrees.

• the speed of rotation differs, it is based on latitude (remember that latitude runs North to South)

• at the equator - fastest

• at the poles - slowest ~ 0 km/hr

Revolution

• slightly elliptical

• we are not always the same distance from the sun

• Perihelion- closest to the sun occurs ~ Jan. 3rd( 147,000,000 km)

• Aphelion- furthest from the sun ~ July 4th

• ( 152,000,000 km)

Tilt

• the earth is tilted from perpendicular (90 degrees) 23.5 degrees

• Parallelism of axis - means the direction of the axis does not change. It always points to the Polaris

• effect = sometimes the North pole points toward the sun, sometimes it points away from the sun

• when it is tilted toward the sun- longer periods of daylight in the Northern Hemisphere

• when it is tilted away from the sun- shorter periods of daylight in the Northern Hemisphere

Altitude of the sun

• When the sun is high in the sky, solar rays are most concentrated.

• The lower the angle, the more spread out and less intense is the solar radiation.

Summer and Winter Solstice

On June 21- the North Pole tilts toward the sun, on this day the sun is striking the earth at a 90 degree angle along the Tropic of Cancer. ( 23.5oN)

This is called the summer solstice, it marks the beginning of SUMMER in the Northern Hemisphere.

Characteristics

• most hours of daylight, longest day of the year

• the farther North you are, the more hours of daylight

• Arctic Circle has 24 hours of daylight

• From this day the hours of daylight begins to decrease

By December- Sun ½ through its orbit

• North Pole points away from the sun

• occurs on ~Dec. 21

• Sun strikes the Tropic of Capricorn at 90 degrees angle (23.5oS)

• Winter solstice

• marks the beginning of winter in Northern Hemisphere

• sun follows lowest path across the sky

• Places North of Arctic Circle have 24 hours of darkness

• From this day forward the hours of daylight begins to increase

• shortest hours of daylight

September 22

• sun’s rays at 90 degrees at the equator (0o)

• called the Autumnal Equinox

• marks beginning of fall in Northern Hemisphere

• Equal amounts of daylight and darkness

March 21

• 90 degree angle at the equator (0o)

• Vernal equinox

• marks the beginning of Spring

• hours of daylight and darkness the same

Time Zones

Using the sun as the basis for measuring time, 12:00 noon is defined as the time when the sun is highest in the sky.

This is called solar noon.

• the sun appears highest over different locations at different times

• ex assume sun is over head at noon in NY

Philadelphia, a short distance away- the sun would appear highest a few minutes later

Baltimore, a short distance away - the sun would appear highest a few minutes later

Each community would have a different time even though they are not far apart.

To avoid this, the earth is divided into 24 standard times zones.

• In each zone, noon is at a set time when the sun is at the highest

• each time zone covers 15 degrees called time meridian.

• The starting point is called the prime meridian.

• It passes through Greenwich, England

• the time in each zone is 1 hour earlier than the time in the zone to its east

Excluding Alaska and Hawaii there are 4 time zones

• Eastern = 75oW longitude

• Central = 90o W longitude

• Mountain = 105 o W longitude

• Pacific 120 o W longitude

• Alaska = Alaska time zone 150 o W longitude

• Hawaii = Aleutian time zone 165 o W longitude

International Date line- a line running north and south through the pacific ocean.

• It does not cut through islands or continents.

• It is the place where the date changes.

• When the International date line is crossed, the time does not change but the date does.

• ex. If it is 8am Thursday on the east side of the date line then it is 8 am Friday to the west of the International date line

Daylight savings time

• the time is advanced by one hour

• adds an hour of daylight to the part of the day when most people are awake

• What time of year does this occur?

• marck

• Standard time- is resumed in November by moving the clocks back one hour.

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