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