Earth’s Cycles

SPI 6.3 Distinguish among a day, lunar cycle, and year based on the movements of the earth, sun, and moon.

Earth’s Cycles There are several cycles that we go through: Day Lunar Year

What causes these cycles? The answer to this question has to do with the motion of Earth as it revolves around the sun. Learning more about how Earth moves will help you understand why we have daylight and night and why we have the seasons of the year.

Revolve vs. Rotate Revolve: to move in a circle around a fixed point (you can use orbit in its place)

Revolve vs. Rotate Rotate: to turn on or around an axis Like a top spinning on its axis

Day Cycle It takes Earth 24 hours to rotate once on its axis. Earth's axis is an imaginary line that runs through Earth's center from the North Pole to the South Pole. A day cycle is one complete rotation on the Earth’s axis. This rotation gives us night and day.

Day Cycle In the picture, City C is experiencing sunrise. Which city has had sunlight for about one hour? Which cities are still dark? Which city will be the next to experience sunrise?

Lunar Cycle A complete lunar cycle is 29.5 days. We know this as the phases of the moon

Year Cycle It takes Earth 365 ¼ days to revolve around the Sun. A year cycle is one complete revolution around the Sun. Every four years we add an extra day to the calendar: February 29

Seasons

SPI 6.6 Use a diagram that shows the positions of the earth and sun to explain the four seasons.

What are seasons? Seasons are periods in a year marked by specific weather conditions, temperatures, and length of the days. Most modern day calendars around the world divide the year into 4 seasons: spring, summer, fall (autumn), and winter.

Why do we have seasons? Seasons are caused mainly by the 23.5° tilt of Earth's axis. This tilt causes the number of daylight hours to change as the time of year changes.

Why do we have seasons? The tilt of the Earth’s axis never changes. Earth's tilt and shape affect the amount of sunlight that an area receives.

Solstices A solstice happens when the sun is farthest north or south of the equator. In the month of June, the Northern Hemisphere tilts toward the sun, so the Northern Hemisphere is warmer than the Southern Hemisphere. In December, the opposite is true.

Equinoxes In March and September, neither end of Earth's axis is tilted toward the sun. So, both hemispheres receive the same amount of solar energy because the sun is directly above the equator. The time when the sun is directly above the equator is called equinox

Equinoxes & Solstices March 21 (Spring) September 23 (Fall or Autumn) June 22 (Summer) December 22 (Winter)

Cycles & Seasons