THE EARTH IN SPACE (the Final Frontier)
Shape of the earth
During the Dark Ages, most Europeans believed that the earth was flat
To this day, there is a "Flat Earth Society" http://www.flat-earth.org
Today, we've gone back to the ancient Greek and mediaeval Arab idea
that the earth is round
It's not perfectly round, though
The earth's rotation creates a bulge along the equator and a
flattening along the poles: The shape, then, is a
flattened oval
The shape is called an "oblate ellipsoid" = "flattened oval"
It's not quite a perfect oblate ellipsoid, either
There's a slight constriction in the Northern Hemisphere
This may reflect the concentration of the land masses there
The effect is a slight tendency to a pear-like shape
This pear-like oblate ellipsoid is called a "geoid"
Size of the earth
Circumference: ~40,000 km (39,840 km) or ~25,000 mi. (24,900 mi.)
Diameters
~13,000 km or ~8,000 mi.
Very slightly different diameters depending on the direction of
measure because of the oblateness of the earth):
Equatorial: 12,757 km (7,927 mi.)
Polar: 12,715 km (7,900 mi.)
Earth motions
Rotation: Earth's spinning movement around its own axis each day
24 hours
Angular speed of motion: 15 degrees an hour
Linear speed, therefore, is about 1,600 km/hr or 1,000 mph at the
equator (only)
Major effects:
Night and day
Apparent motion of the heavenly bodies from east to west, as
the earth rotates from west to east
Revolution: Earth's movement around the sun each year
365.24219 days
Rotation and revolution are not in synchrony, therefore
Calendar fixes are needed to keep them aligned
See: http://www.ast.cam.ac.uk/RGO/leaflets/leapyear/
leapyear.html
Leap year -- every year evenly divisible by 4 gets a Feb 29
Except for the turns of the century
Unless the century can be divided by 400
So millenial changes are leap years
Orbital distance from the sun
Orbit is elliptical, with variations in speed to conserve
angular momentum
Planets, including Earth, sweep out equal areas in
equal amounts of time
So, when the earth is closer to the sun, it has to
travel faster, and, when farther from the sun, it
has to slow down
See http://www.astro.virginia.edu/~eww6n/physics/
Kepler'sLaws.html
Average distance from the earth to the sun
~150,000,000 km (~93,000,000 mi.)
This distance is called an astronomical unit (AU) for
measuring the orbits of other planets in our solar
system (Saturn, for example, is about 10 AU from
the sun) or other objects in the neighborhood of
the solar system
Minimum distance
~147,250,000 km (~91,500,000 mi.)
This happens at perihelion (~3 January)
Maximum distance
~152,100,000 km (~94,500,000 mi.)
This happens at aphelion (~4 July)
The major effect of revolution is the seasons, but revolution
cannot produce seasonality all by itself: it has to work
with a second factor to produce this effect
This second factor is the tilt of the earth's axis
The plane of reference for our little dirtball spinning
through space is the plane of ecliptic
This is an imaginary flat surface, or plane, resting on
the earth's orbital path
The earth's axis is tilted 23 1/2 degrees from the
vertical of the plane of ecliptic
This axial tilt is constant through time
The axis points the same direction at the same angle
all year round (that is, the North Pole always
points to Polaris, the North Star)
There is a very long scale (e.g., ~24,000 years) wobble
in the earth's axis, which makes the North Pole
point at different "north" stars over the
millenia, but we can disregard this at the scale
of human lifespans
The constancy of the axial tilt throughout the earth's
revolution means that, at one part of the year (around
June), the North Pole is pointed toward the sun, while,
at the opposite time of year (around December), the
South Pole points toward the sun
So, in June, the Northern Hemisphere gets most of the
insolation (INcoming SOLar radiATION) and warmth,
while the Southern Hemisphere gets little; in
December, the situations are reversed
Looking at the geometry of the Earth-Sun relationship in
more detail at particular points in the earth's orbit
June 21 or so
Solstice (Latin for the sun stands still)
Summer solstice in the Northern Hemisphere
Winter solstice in the Southern Hemisphere
Direct ray strikes well north of the equator
23 1/2 degrees north
The Tropic of Cancer
This is the latitude experiencing the noon
overhead sun
In other words, the declination is 23 1/2
degrees north
Northernmost tangent ray (the one that just barely
brushes the planet and continues on into
space)
Strikes beyond the North Pole
66 1/2 degrees north
The Arctic Circle
Southernmost tangent ray
Doesn't quite make it to the South Pole
66 1/2 degrees south
The Antarctic Circle
The Circle of Illumination
If you drew a line connecting all places
touched by the tangent rays of the sun
Defines the line between night and day
The Circle of Illumination is a great circle
dividing Earth into a night half and a
day half
How this geometry affects the seasonal temperature
The wedge of insolation between the Arctic
Circle and the Equator is larger than
that between the Antarctic Circle and
the Equator
The sun rays striking the Northern Hemisphere
are more direct and concentrated than
those hitting the Southern Hemisphere
Together, these factors account for the
heating of the Northern Hemisphere in
its summer and the chilling of the
Southern Hemisphere in its winter
How this geometry affects the length of day
Follow any place on the Equator over the
course of the day's rotation: half of
the rotation faces the sun and half
faces away from the sun
Follow any place from 1 degree north to 66
degrees north, and you'll find that it
spends more time during the rotation in
the sunny part of the planet and less
time facing the night sky
The days become relatively longer and the
nights shorter the farther you are from
the Equator
At 66 1/2 degrees north, night disappears:
someone along the Arctic Circle spends
24 hours in technical daytime, and you
have reached the "Land of the Midnight
Sun"
As you travel north from the Arctic Circle,
the length of time without sunset gets
longer and longer until, at the North
Pole, you experience six months of
daylight: the June solstice is the
"noon" of the North Pole's six month
day!
December 21 or so
Solstice (Latin for the sun stands still)
Winter solstice in the Northern Hemisphere
Summer solstice in the Southern Hemisphere
Direct ray (or declination) strikes well south of
the equator
23 1/2 degrees south
The Tropic of Capricorn
Northernmost tangent ray
Strikes short of the North Pole
66 1/2 degrees north
The Arctic Circle
Southernmost tangent ray
Overshoots the South Pole
66 1/2 degrees south
The Antarctic Circle
The Circle of Illumination and the geometry of
Earth's rotation at this point in the
revolutionary round
The wedge of insolation between the Antarctic
Circle and the Equator is now larger
than that between the Arctic Circle and
the Equator
The sun rays striking the Southern Hemisphere
are now the ones that are more direct
and concentrated than those hitting the
Northern Hemisphere
So, now, the Northern Hemisphere is chilled
and the Southern Hemisphere is toasty
warm: it's winter in the north and
summer in the south
How this geometry affects the length of day
The days become relatively longer and the
nights shorter the farther south you are
from the Equator
At 66 1/2 degrees south, night disappears
As you travel south from the Antarctic
Circle, the length of time without
sunset gets longer and longer until, at
the South Pole now, you experience six
months of daylight:
The December solstice is the "noon" of
the South Pole's six month day
It's also the "midnight" of the North
Pole's six month night)!
March 21 and September 21 (or thereabouts)
These are the equinoces (from equal nights)
The March equinox is the spring or vernal
equinox in the Northern Hemisphere and
the fall or autumnal equinox in the
Southern Hemisphere
The September equinox is the fall or autumnal
equinox in the Northern Hemisphere and
the spring or vernal equinox in the
Southern Hemisphere
At this point in the earth's orbit, both poles are
just tangentially visible from the sun: the
tangent rays hit the poles
The Circle of Illumination passes through the
poles
This puts the declination squarely on the Equator
Insolation is now evenly split between the two
hemispheres
Day and night are everywhere the same length
Obviously, as usual, at the Equator
Also for all the places from 1 degrees north
or south to 66 degrees north or south
Ditto for the Arctic and Antarctic circles
This is true even at the poles!
The first twelve hours of 21 March are
the last twelve hours of the six
month night in the Northern
Hemisphere and the last twelve
hours are the first twelve hours of
the new six month day there
The first twelve hours of 21 March are
the last twelve hours of the six
month day in the Southern
Hemisphere and the last twelve
hours are the first twelve hours of
the new six month night there
The opposite is true on or about 21
September
Those cartographic coordinates, again
The Arctic and Antarctic circles are the outer limits of the
"midnight sun" phenomenon
The tropics of Cancer and Capricorn are the outer limits of
the noon overhead sun (or direct ray) phenomenon
These two tropics experience the noon overhead sun on
one day each year:
Cancer experiences it around 21 June
Capricorn around 21 December
All places between the two tropics experience the noon
overhead sun two days each year (for the Equator,
those two dates are the equinoces)
last revised: 06/08/98