STANDARD S6E1 d. Explain the motion of objects in the day/night sky in terms of relative position. S6E2 b. Explain the alignment of the earth, moon, and sun during solar and lunar eclipses. c. Relate the tilt of the earth to the distribution of sunlight throughout the year and its effect on climate.
The Earth in Space The earth is a round three dimensional shape called a sphere. The Earth revolves around the sun. In one year it travels 940,000,000 km. The Earth rotates on its axis, an imaginary vertical line. These two combined movements affect both day and night and the seasons on Earth.
Day and Night The Earth s rotation causes day and night every 24 hours. The Earth rotates in a counterclockwise pattern ( west to east) causing the sun to appear to rise in the east and set in the west. Since the Earth s axis is tilted when the North Pole is leaning toward the sun, the South Pole is leaning away. The hemisphere that leans toward the sun has long days and short nights.
The Earth takes 365.25 days to complete the revolution around the sun. An extra day is added to the calendar every four years in February. This is called a leap year. A Year on Earth Tilt Since the earth is spherical and tilted, different areas of the earth receive different amount of sunlight.
Direct and Indirect Radiation Areas near or at the equator receive direct solar radiation. As you move away from the equator, north or south, you receive less direct (indirect) radiation. The equator receives direct (90º) radiation year round
Indirect and Direct Radiation Direct radiation covers a smaller area and is more concentrated. The more indirect the light, the larger the area it covers and the less concentrated each area.
Seasons on Earth The Earth has four seasons: winter, spring, summer and autumn. Five of the other planets, Mars, Saturn, Uranus, Neptune and possibly Pluto. The other planets, Mercury, Venus and Jupiter, do not have seasons since they are not tilted on their axis.
Earth s Orbit The earth s orbit is an ellipse. The earth is not in the center of the ellipse. It is closest to the sun in January. It is the tilt that causes the seasons.
Tilt The Earth s tilt also causes the Sun s radiation to strike the hemispheres at different angles. The part tilted toward receives more direct (90) angles of radiation.
Seasons When the Northern Hemisphere is tilted toward the sun, that part of the Earth has summer since the sun s rays are more direct and the days are longer. At the same time, the Southern Hemisphere is tilted away and has winter. Summer and winter are not affected by the distance from the sun.
Solstices Summer begins in the Northern Hemisphere on June 20 or 21. The summer solstice is the longest day of the year. It is the time when the sun reaches its greatest distance North or South of the equator. The winter solstice is the shortest day of the year. This occurs when the Sun is directly above the Earth s equator. When the Northern Hemisphere has the longest day, the Southern Hemisphere has the shortest day.
Equinox Twice a year neither hemisphere is tilted toward the sun. These times are known as equinoxes. Spring begins on March 20 or 21, the vernal equinox. Autumn begins on September 22 or 23, the autumnal equinox.
Solstice and Equinox
The Earth, the Moon and the Sun As the Earth moves in its yearly revolution around the sun, the moon moves in a monthly revolution around the Earth.At the same time both rotate. The motion of these three result in the changing appearance of the moon as seen from the Earth and the blocking of light.
A solar eclipse occurs when the new moon comes directly between the sun and the Earth. Solar Eclipse
Penumbra and Umbra The complete or perfect shadow o f an opaque body, as a planet, where the direct light from the so urce of illumination is completely cut off. The people in the umbra of the shadow see a total solar eclipse. The partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off. The people in the penumbra, the larger outer shadow, see a partial solar eclipse.
A lunar eclipse occurs when the moon passes through the Earth s shadow. Lunar Eclipse
Lunar Eclipses When the moon moves through the umbra, a total lunar eclipse occurs. When the moon moves through the penumbra, a partial lunar eclipse occurs.