1. Earth’s Season
MOTIONS OF THE EARTH

2. DEFINITION OF Terms Season
A traditional division of the year based on distinctive weather conditions.
Climate
The average weather or the regular variations in weather in a region over a period of years.
Weather
The state of the atmosphere with regards to temperature, cloudiness, rainfall, wind and other meteorological conditions.

3. MOTIONS OF THE EARTH (EARTH’S SEASON)
MILANKOVITCH CYCLES
Milankovitch cycles are three periodic variations in Earth’s orientation toward the Sun.
Scientist believe the three variations combine to produce cyclical changes in climate about once every 100, 000 years.

4. MOTIONS OF THE EARTH (Earth’s Season)
1. PRECESSION
Conical sweep of axis of rotation.
Earth wobbles in a way similar to that in which a top wobbles when spun.
The wobble called precession.
Earth completes one full circle of precession about once every 25, 000 years.

5. MOTIONS OF THE EARTH (EARTH’S SEASON)
1. PRECESSION (continued)
Precession causes the time of year that Earth is closest to the Sun to cycle through the seasons.
Currently the Earth is closest to the Sun during winter in the northern hemisphere (summer in the southern hemisphere).

6. MOTIONS OF THE EARTH (EARTH SEASON’S)
2. ORBITAL ELLIPTICITY
Earth’s orbit around the Sun flexes from a near circle to a more elliptical or oval shape over a period of about 100, 000 years.
When the orbit is most elliptical, the distance from the Earth to the Sun and hence the intensity of sunlight falling on the planet varies significantly over the course of the year.

7. MOTIONS OF THE EARTH (EARTH’S SEASONS)
3. NUTATIONS
Wobbly rotation.
Earth’s axis is tilted relative to the plane in which the planet orbits the Sun.
This tilt causes the seasons as first one hemisphere and then the other are tilted toward and away from the Sun during a year.
The axis is currently tilted 23.5 degrees, but the amount of tilt varies from 21.5 to 24.5 degrees over a period of about 40, 000 years.

8. MOTIONS OF THE EARTH (EARTH’S SEASONS)
3. NUTATIONS (continued)
This variation in axial tilt is called nutation.
Nutation causes the seasons to be slightly more or less severe - the greater the tilt, the harsher the seasons.

9. MOTIONS OF THE EARTH (EARTH’S SEASON)
CLIMATE VARIATION
Precession, Orbital Ellipticity and Nutation combine every 100, 000 years or so to produce significant changes in climate.
When the three cycles all unite to produce cooler summers in the Northern hemisphere, winter ice tends not to melt and enormous glaciers form, precipitating an ice age.
Sea levels falls as ocean water is converted into vast sheets of ice.

10. MOTIONS OF THE EARTH (EARTH’S SEASON)
CLIMATE VARIATION (continued)
Conversely, when the cycles combine to produce hot summers in the Northern Hemisphere, glaciers melt and sea level rises.
At the same time, of course, the Southern Hemisphere experiences cooler summers. Ice ages are not precipitated by cool summers in the South, however, because the Southern Hemisphere contains much less land on which glaciers can grow. Scientist believe that the Earth is currently coming out of an ice age.