The Earth has two main motions that affect its seasons: rotation, which causes day and night, and revolution around the Sun. The tilt of the Earth's axis of rotation combined with its revolution cause the seasons by changing which hemisphere receives more direct sunlight throughout the year. The solstices mark the summer and winter beginning in each hemisphere when the Sun is at its maximum northern and southern declinations, while the equinoxes occur when day and night are equal in length as the Sun crosses the celestial equator.

1. Earth
Motions and Seasons
By Uday VI-E

2. MOTIONS of the EARTH
• REVOLUTION
The movement of the earth in its orbit.
• ROTATION
The spinning of an object on its axis.

3. • Precession –
The spin axis of the Earth undergoes a motion
• This changing of direction of the spin axis is called
precession.

5. Seasons of the Earth

6. Earth at Winter Solstice

7. Earth at Summer Solstice

8. Why do we have seasons?

9. The Winter & Summer Sun

11. Seasons in the Northern &
Southern Hemisphere

12. Solstices
• The solstices are days when
the Sun reaches its farthest
northern and southern
declinations.
• The winter solstice occurs on
December 21 or 22 and marks
the beginning of winter (this is
the shortest day of the year).
• The summer solstice occurs
on June 21 and marks the
beginning of summer (this is
the longest day of the year).

13. Equinoxes
• Equinoxes are days in which day
and night are of equal duration.
The two yearly equinoxes occur
when the Sun crosses the
celestial equator.
• The vernal equinox occurs in late
March (this is the beginning of
spring in the Northern
Hemisphere and the beginning of
fall in the Southern Hemisphere);
the autumnal equinox occurs in
late September (this is the
beginning of fall in the Northern
Hemisphere and the beginning of
spring in the Southern
Hemisphere).

14. Summary