Earth in Space
How Earth Moves
• Earth moves through space in two major
ways: rotation and revolution.
How Earth Moves - Rotation
• The spinning of Earth on its axis is called
rotation. The axis is an imaginary line that passes
through Earth’s center and the North and South
poles.
• Earth’s rotation causes day and night. As the
Earth rotates eastward, the sun appears to move
westward across the sky. It is day on the side of
the Earth facing the sun and night on the side
facing away from the sun.
• It takes Earth about 24 hours to rotate once.
Each 24-hour cycle is called a day
How Earth Moves - Revolution
• Revolution is the movement of one object
around another. The Earth revolves around
the sun while it rotates on its axis.
• One complete revolution around the sun is
called a year.
• The path that Earth follows around the sun is
its orbit. Earth’s orbit is elliptical.
Calendar
• The rotation and revolution of the Earth is the
basis of our calendar. Ancient Egyptians
created one of the first calendars by counting
the days between the first appearance of the
star Sirius in the morning. They found there
were about 365 days in a year.
• Early people used the moon cycle to divide
the year into 12 “moonths”.
Calendar
• In reality the Earth orbits the sun about 365 ¼
days. The Romans adjusted the Egyptian
calendar by adding one day every four years.
This is known as “leap year” and the extra day is
added to February. This helped annual events
like the beginning of summer to occur on the
same date every year.
• Every so often adjustments still need to be made
by adding a “leap second” to adjust for
irregularities in Earth’s rotation.
The Seasons on Earth
• Most places outside the tropics and polar
regions have four distinct seasons: winter,
spring, summer, autumn. But there are great
differences in temperature from place to
place.
How Sunlight Hits Earth
• Sunlight hits Earth most directly near the
equator.
More direct = more intense light and heat.
• Sunlight hits the poles at a steep angle, which
makes it spread over a greater area.
Less direct = less intense light and heat.
How Sunlight Hits Earth
Less direct sunlight.
Spread across a greater area.
Most direct sunlight.
Intense, concentrated in
smaller area.
Less direct sunlight.
Spread across a greater area.
Earth’s Tilted Axis
• Earth has seasons because its axis is tilted as
it moves around the sun. If it wasn’t tilted,
temperatures would remain fairly constant
year round and there would be no seasons.
• Earth’s axis is tilted at an angle of 23.5° from
vertical. As Earth revolves around the sun,
the north end of its axis is tilted away from
the sun for part of the year and toward the
sun for the other part of the year.
Earth’s Tilted Axis
• When the north end of the axis is tilted
toward the sun, the Northern Hemisphere has
summer. The hemisphere tilted toward the
sun has more daylight hours than the
hemisphere tilted away from the sun. The
combination of direct rays and more hours of
sunlight heats the surface more than at any
other time of the year.
Earth’s Tilted Axis
• At the same time, the south end of the axis is
tilted away from the sun and the Southern
Hemisphere has winter. The sun’s energy is
spread over a larger area and the days are
shorter than nights. The combination of less
direct rays and fewer hours of sunlight heats
Earth’s surface less than at any other time of
the year.
Solstices
• Twice a year, the sun reaches its greatest
distance from the equator. This is known as a
solstice.
• The summer solstice happens around June 21st
every year. This is when the sun is farthest north
of the equator. It is the longest day of the year.
• The winter solstice happens around December
21st every year. This is when the sun is farthest
south of the equator. It is the shortest day of the
year.
Equinoxes
• Halfway between the solstices, neither
hemisphere is tilted away or towards the sun.
• Equinox means “equal night”. During an
equinox, day and night are about 12 hours
long all over the world.
• The vernal (spring) equinox is around March
21st and is the beginning of spring.
• The autumnal (fall) equinox is around
September 22nd and is the beginning of fall.