Earth-Sun Relationships and Illumination of the Globe
I. Earth-Sun Relationships:
Why we have to understand? Because by understanding earth-sun relationships one will be able to determine the apparent path of the sun in the sky, the angles at which sun's ray (light) strike, the lengths of day and night, and the occurence of seasons.
The earth is turning on its axis at the same time that it is moving in a path about the sun, and because the earth's axis is tilted with respect to the plane of its orbit, therefore, it will be much easier for us to view these relationships in 3-dimension, or to view the earth from space.
II. Motion of the Earth: Rotation and Revolution
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B. Revolution of the Earth:
Revolution: The motion of the earth in its orbit around the sun
Direction of Revolution: Looking down upon the North Pole of the Earth, the Earth is traveling counterclockwise around the sun, and clockwise revolution when you look at the South Pole.
Time of Revolution: Period of revolution (year) is the time required for the earth to complete one circuit around the sun.
Sidereal year: Time required for the earth to return to a given point in its orbit with
reference to the fixed stars.
Tropical year: The period of time from one vernal equinox to the next. Usually it takes 365
days 5 hours 48 minutes 45.68 seconds (365 1/4 days)
Calendar year: 365 days.
Leap year: 366 days. Feb. 29, every 4 years.
Velocity of Revolution: Earth is revolving around the sun on earth's orbit. and the distance is approximately 583 million miles.
Earth Orbit: It is an ellipse rather than a circle. The ellipticity or degree of flattening of the ellipse is very slight. The mean distance between earth and sun is about 93 million miles (150 million km) but because of the ellipicity of the orbit the distance may be 1.5 million miles greater or less than this figure.
Perihelion (around or near, helios: Sun): the least distance from earth to sun is about 91.5 million miles (147 million km) happen on about January 3.
Aphelion (Ap: away from; helios: sun): on July 4, the earth is at its fartherest point from the sun at a distance of 94.5 million miles (152 million km).
The mean velocity of the earth in its orbit is about (583,000,000 miles / 365 days) 66,600 miles (107,000 km) per hour or 18.5 miles/second. The velocity is greatest at perihelion, least at aphelion.
III. Inclination of the Earth's Axis
This is the most important fact of Earth-sun Relations.
A. Plane of the Ecliptic: For tilted globes the plane in which the earth's orbit and the sun lie is imagined to be horizontal and to pass through the center of the globe. The plane of the equator is inclined 23.5 degrees with the plane of the ecliptic. 23 degrees 27 minutes more exactly. The earth's axis makes an angle of 66.5 degree with the plane of the ecliptic, and is tilted 23.5 degrees from a line perpendicular to that plane.
(1) The earth's axis keeps a fixed angle with the ecliptic plane.
(2) The axis always points to the same place among the stars.
The seasons result because the tilted earth's axis keeps a constant orientation in space as the earth revolves about the sun.
B. Solstice and Equinox:
Summer Solstice: On June 21 or 22, the North Pole end of its axis leans at the maximum angle 23.5 degrees toward the sun.
Winter Solstice: On December 21 or 22, the southern hemisphere tipped toward the sun. Circle of illumination tangent to the arctic and antarctic circles.
Vernal Equinox: March 20 or 21.
Autumnal Equinox: September 22 or 23.
C. Circle of Illumination: The great circle that marks the boundary...