Earth Science
Chapter 19
Section 1
A. Days and Years:
The study of the moon, stars and other objects in space is
called Astronomy.
Motions of the Earth:
Earth - Sun Relationships:
- The Earth intercepts only a minute percentage of the suns
energy - less than 1/2000000000 ( one two-billionth )
- Solar radiation represents 99.9% of the energy that heats
the Earth
- Solar energy is not distributed equally over the Earth's land
and sea surfaces
- This unequal heating drives the ocean's currents and creates
winds that transport heat from the tropics to the poles in an
attempt to reach an energy balance
Motions of the Earth:
The Earth travels around the sun in a path called an orbit.
The Earth has two principle motions in it’s orbit:
1. Rotation = The spinning of the earth about its axis,
(an imaginary line running through the poles) Our
planet rotates once every 24 hours producing the daily
cycle of daylight and darkness. (one day)
2. Revolution = The movement of the earth in its orbit
around the sun once every 365 1/4 days. (one year)
The earth travels at a speed of 113,000 Km/hour
in it’s orbit.
- The distance between the sun and earth is about 150
million Km (93 million miles)
- Because the earth's orbit is not perfectly circular, the
distance between the earth and the sun varies during the
course of the year.
- Each year on about January 3rd, our planet is about 147
million Km from the sun, closer than any other time of the
year. This position is called Perihelion.
- 6 months later, on July 4th, our planet is about 152
million Km from the sun, farther away than any other
time of the year. This position is called Aphelion.
- Calendars based on a 365 ¼ day Earth orbit with a leap
year of 366 days every 4th year or orbit.
B. The Seasons on Earth:
-We have seasons because the sun strikes the earth's surface
at different angles and distances at different times of the year.
The earth is tilted at 23 1/2 degrees on its axis from the
perpendicular elliptical plane of the axis. This is termed
"Inclination of the Axis"
- On June 21 or 22 the earth is in a position where the axis in
the northern hemisphere is tilted at 23 1/2 degrees toward the
sun. At this time the vertical rays of the sun are striking at
23 1/2 degrees North latitude at a point known as the Tropic
of Cancer.
For people living in the northern hemisphere June 21 or 22 is
known as the Summer Solstice.
- On December 21 or 22 the earth is in an opposite position,
where the sun's vertical ray's are striking at 23 1/2 degrees
South latitude at a point known as the Tropic of Capricorn.
For people living in the northern hemisphere December 21 or
22 is known as the Winter Solstice.
- The Equinoxes occur midway between the solstices.
September 22 or 23 is the date of the Autumnal (Fall)
Equinox in the northern hemisphere and March 21 or 22
is the date of the
Vernal (spring) Equinox. On these dates the vertical rays
of the sun are striking the equator ( 0 degrees latitude ) ,
because the earth is tilted neither toward or away from the
sun.