Earth as a Rotating Planet - Study Notes

The Earth’s shape is close to spherical.
Actually an oblate ellipsoid (bulges at the equator and flattens at the poles).
A more accurate representation is the geoid, which illustrates the pull of gravity on Earth.

Rotation is counterclockwise when viewed from above the north pole.
It is from west to east when viewed with the north pole up.
Rhythms of the Sun cause:
- Day & night
- Daily air temperature cycle
- Motions of the atmosphere and oceans
- Weather systems and ocean currents
The Earth’s rotation and the Moon’s gravitational pull create the rise and fall of tides.
Tidal currents – life-giving pulse for plants & animals, clock for human coastal activities.

Meridian – north/south lines.
Parallel – east/west lines.
Every point on the Earth has a combination of one parallel and one meridian, defined by their intersection.

Latitude (Parallels):
- $1^ ext{o} ext{ latitude} = 111 ext{ km}$ per degree.
Longitude (Meridians):
- $1^ ext{o} ext{ of longitude} = 111 ext{ km} ext{ at the equator and } 0 ext{ km at the poles}$
Latitude is the angle between a point on a parallel and the center of the Earth and a point on the equator.
Longitude is the angle between a point on a meridian at the Equator (P) and a point on the prime meridian at the Equator (Q) as measured at the Earth’s center.
Prime Meridian is at the Royal Observatory in Greenwich, England.

Small Circles: Created when a plane passes through the Earth but does not intersect the center.
Great Circles: Created when a plane passes through the Earth and intersects the center point.

Map projection – how to display the Earth’s surface.
Polar projection centered on the North or the South Pole:
- Parallels centered on the pole.
- Meridians radiate outward from the pole.
- Shows one hemisphere, the equator at the outer edge of the map.
- Intersections of the parallels & meridians form right angles; projection shows the true shapes.
Mercator Projection:
- Mercator projection shows a line of constant compass bearing as a straight line.
- Used to display directional features such as wind direction.
Winkel Tripel Projection:
- Minimizes distortion in area, distance and direction.
- Shows countries and continents of the globe with minimal distortion in shape and area.
GIS – Geographic Information Systems:
- Computer-based mapping and analytical ability provided by complex software.
- Maps, diagrams, satellite images and aerial photographs can be stored and manipulated.
- Geographic spatially referenced data.
- Spatially referenced data used to solve complex problems.

Standard Time:
- Standard time system – global time kept according to adjacent standard meridians; normally differ by one hour.
Based on the east-west position of the Sun; solar day defined by one sun circuit.
Time is determined by longitude, not latitude.
Time zone boundaries follow preexisting natural or political boundaries.
International Date Line:
- Crossing the International Date Line in an eastward direction, travelers set their calendars back one day.
- Dashed lines represent $15^ ext{o}$ meridians and bold lines represent $7.5^ ext{o}$ meridians.
- Alternate zones appear in color.
- 12 hours from Prime Meridian.
- Opposite side of globe is the 180° meridian (180th meridian).
- Earth rotates $15^ ext{o}$ per hour; time zones differ by 1 hour ( $\frac{360^ ext{o}}{15^ ext{o}} = 24 ext{ hours}$ ).
- Date changes on either side of the line.

DST: Clocks are set ahead by one hour (or more) for part of the year, matching modern societal rhythms.
United States: DST begins the second Sunday in March and ends the first Sunday in November.
Not all of the US observes DST.
European Union daylight saving: summer time begins on the last Sunday in March and ends on the last Sunday in October.

Precise timekeeping – most accurate form using atomic clocks.
Based on the frequency of microwave energy emission from atoms of the element cesium cooled to near absolute zero.
Keeps time to better than one part in 1 trillion.

Revolution: the circle around the Sun (approximately $365 ext{ days}$ ).
Orbits counterclockwise when viewed from the north pole.
Elliptical path; orbits in the plane of the ecliptic.
The Earth is nearest to the Sun at perihelion, which occurs on or near $ext{January 3}.$
The Earth is farthest from the Sun at aphelion, on or near $ext{July 4}.$
The distance between the Sun and the Earth varies by about $3 ext{ percent}$ during one revolution.

The Moon rotates on its axis and revolves around the Earth in the same direction that the Earth rotates and revolves around the Sun.
The Moon’s rate of rotation is synchronized with the Earth’s rotation (one side of Moon permanently directed toward the Earth; the other side hidden).
Phases of the Moon are determined by its position in its orbit around the Earth.
A full cycle from one full Moon to the next takes $29.5 ext{ days}$ .

The Earth’s orbit around the Sun lies in the plane of the ecliptic.
The rotational axis remains pointed toward Polaris (the North Star).
The axis makes an angle of $66\frac{1}{2}^ ext{o}$ with the ecliptic plane.
The axis of the Earth is tilted at $23\frac{1}{2}^ ext{o}$ away from a right angle to the plane of the ecliptic.

Four seasons occur because the Earth maintains a constant orientation (tilted $23\frac{1}{2}^ ext{o}$ ) with respect to the perpendicular to the plane of the ecliptic as it revolves around the Sun.

Solstice (sun stands still): December or winter solstice – December 22.
The north polar end of the Earth’s axis leans at the maximum angle away from the Sun, $23\frac{1}{2}^ ext{o}$ .
The Southern Hemisphere is tilted toward the Sun and receives stronger heating.
The opposite occurs for the June or summer solstice.

Solstice (June 22): subsolar point is $23\frac{1}{2}^ ext{o} ext{N}$ (Tropic of Cancer).
Solstice (December 22): subsolar point is $23\frac{1}{2}^ ext{o} ext{S}$ (Tropic of Capricorn).