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EARTH: Basic geography (First page is brief notes)
Size, Shape, Rotation
Latitude, Longitude
Orbit about Sun
Tilt of Earth's rotational axis
Maps and map projections
General shape -- nearly
spherical, radius = 6,371 km
Polar Radius (6357 km) < Equatorial Radius (6378 km)
Rotation (spin) about a N-S axis -- 1 revolution per
day,
or 15 degrees per hour
Actual shape:
Oblate -- slightly higher radius at Equator
Pear-shaped controlled by landmass distribution
Highest Mountain Mount Everest, Himalayas (8840 m)
Deepest Ocean Mariana Trench, Pacific (11,000 m)
Latitudes (parallels):
0° = Equator
Equator is a "great circle" (intersection of surface
with a plane that pass through the center)
90° N or S = Poles
Linear distance between latitudes is constant (1 degree latitude
= 60 nautical miles)
Longitudes (meridians):
0° = Prime Meridian Royal National Observ., Greenwich,
UK
180° = International Date Line
Linear distance between longitudes decrease from Equator to
Poles
Earth's orbit around the Sun
Period = 1 year
Shape of orbit -- almost circular
Tilt of rotational axis
23.5 degree from vertical to orbital plane
Accounts for seasonal variation in daylight and solar radiation
Annual variation in intensity of solar radiation -- greatest in
the temperate, mid-latitude zones!
Maps (projections)
2-D representation of Earth's spherical surface.
Always some distortion.
Maps are constructed by "projection" onto a surface
equatorial cylindrical projection
polar conic projection
polar tangent projection
Bathymetric Maps contour lines connecting points of equal depth
Physiographic Maps highlighted contour intervals
Introduction to the oceans
Pacific Ocean largest and deepest (2X)
Atlantic Ocean - shallowest
Indian Ocean all in southern hemisphere
Antarctic (Southern) Ocean connects all three
Arctic Ocean - smallest and least known - great influence on climate.
This lecture is background so that our study of Oceans is put into context.
Size, Shape, Rotation refer to Fig. 2.3
The shape of our planet closely approximates a sphere, with an average radius of about 6,370 km. Earth spins (rotates) about a N-S (polar) axis at a rate of 1 revolution per day, or 15 degrees per hour.
Because the Earth rotates, the actual shape is slightly oblate. The radius at the Equator about 10 km higher than average, and the radius at the poles about 20 km lower than average. In fact, the Earth is slightly pear-shaped - slightly elevated (higher radius) in dominantly oceanic regions of the Southern Hemisphere and Arcitic Ocean; slightly depressed in continental regions of the mid Northern Hemisphere.
Latitude and Longitude ... is a coordinate (grid) system surface used to express locations on Earth. refer to Figs. 2.4 & 2.6
Latitudes ... are circles parallel to the Equator
Equator: 0° Lat., a "great circle" - Great circles
are the intersection of Earth's surface with planes that pass
through the center of the Earth.
Poles: 90° N or S Lat.
Intermediate latitudes: "small" circles
Latitude of any point on Earth surface is defined as
the angle between
(a) line from that point to the center of Earth and
(b) plane through the Equator
Notice that the distance on Earth's surface between latitudes is constant. One degree of latitude corresponds to 60 nautical miles (69 land miles) or 111 km.
Longitudes (also called "Meridians") are all great circles that pass through the North and South Poles. They are measured in degrees W or E Long. from the Prime Meridian (O° Long) that passes through the Royal Naval Observatory, Greenwich, east of London
Notice that surface distance between longitudes is not constant, but decreases from Equator (where it is same as latitude) to poles (where it approaches zero)
Note that traditionally degrees are further divided into 60 "minutes" (each minute of latitude equals one nautical mile, "a knot"), and then each minute into 60 "seconds" (about 100 feet or 31 meters). In these days of computers it is common just to use what are called decimal degrees where fractional degrees are decimal (powers of ten) as in calculators. For example traditionally the latitude of Champaign would be expressed as: 40°N 07' 30" where as decimal this would be: 40.125 degrees N latitude. C-U Topo map
Earth's orbit around the Sun refer to Figure 2.12 (not really a very clear diagram)
Earth orbits the Sun with a period of one year (365.25 days). The shape of the orbit is nearly circular, but not quite. Earth is slightly closer to the Sun during the Northern Hemisphere winter.
Tilt of rotational axis
Earth's axis of rotation is tilted with respect to its orbital plane ("plane of the ecliptic") by 23.5° from vertical to the orbital plane. The tilt of Earth's axis accounts for why there are seasonal changes in (a) the hours of daylight and (b) the intensity of solar radiation at any location on Earth. The following table is a guide to seasonal changes in the "position" of the Sun (when and where is directly overhead at noon) and in hours of sunlight.
| Date | Sun overhead at | Event | Sunlight |
| Sept. 23 | Equator | Autumnal Equinox | 12 hrs of sunlight everywhere |
| Dec. 23 | 23.5 S (Capricorn) | Winter Solstice | Perpetual sunlight @ > 66.5 (Ant. Circle) |
| March 23 | Equator | Vernal Equinox | 12 hrs of sunlight everywhere |
| June 23 | 23.5 N (Cancer) | Summer Solstice | Perpetual sunlight @ > 66.5 (Arctic Circle) |
How does the intensity of solar radiation vary at different locations throughout the year? Surprisingly, the greatest annual variation is in the temperate, mid-latitude zones. In the tropics (between the Tropics of Cancer and Capricorn) there is little seasonal variation - the Sun is always close to "overhead." In Polar Regions, the intensity of solar radiation throughout the year is always low, and there is not that much variation from season to season.
are two-dimensional representations of Earth's spherical surface. There are a variety of ways to make maps, but all of them involve some distortion of shape, distance, or direction.
Maps of Earth are constructed by projecting light rays from either the rotational center or the center onto screens of various shapes refer to Fig. 2.8
(a) Equatorial cylindrical projection, like the common Mercator projection used in making most maps: In this type of map, low-latitude regions are well represented. But high-latitude regions are distorted because longitudes are parallel (not converging). Used widely because directions are always true, i.e. a vertical line on the map is always true north and thus important in navigating ships.
(b) Polar conic projection - minimizes distortion near the poles
(c) Polar tangent-plane projection - more distortion, but directions better than polar conic.
Homolographic projection is very useful in ocean science refer
to Figure 2.17
You can visualize this type of map as peeling and splitting
the Earth like an orange, then flattening it out. This minimizes
distortion of both shapes and areas.
USGS has a poster and fact sheet describing about 20 kinds on map projections used.
UTM (Universal Transverse Mercator) is a combination grid system, projection system commonly used today that takes advantage of the metric units (The meter was once defined as one ten-millionth of the distance between the pole and the equator).
refer to Figure 2.17
Pacific: Deepest and largest in area and volume (twice that of the Atlantic and Indian)
Atlantic: Shallowest; greatest number of shallow adjacent seas
Indian: Southern Hemisphere ocean, only; but is quite deep.
These three major "gulfs" are interconnected through the "Southern Ocean" that encircles Antarctica.
Arctic Ocean is the 4th commonly accepted ocean, but is smaller and shallower than others - however, it has an uncommonly large effect on the climate!
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