The obliquity of the Earth

In the solar system, planets have orbits that are all roughly in the same plane. That of the Earth is called the ecliptic. Each planet rotates around its axis of rotation, causing a succession of local days in each planet. The slow change in direction of the axis of rotation of the Earth is called the precession of the equinoxes. The ground tilt is the angle between the axis of rotation of the Earth and its orbital plane, it remains confined between 21.8° and 24.4°.
Currently, she is 23°26.5 'but the axis is recovering about 50 "per year or 1 degree every 71.6 years. Moreover, this axis oscillates around a cone, the full cycle ( 360 °) lasts 25,765 years. This angle (23°27 ') made the changing seasons. Indeed, in summer, the sun is higher in the northern hemisphere than in the southern hemisphere.
The sun is higher in the sky of the northern part of the globe, in the southern part. Sun rays coming to Earth with more intensity. The sun rises early, goes to bed later, and the days are longer. In the south it is winter. The Sun also appears lower on the horizon and the days are shorter, the sun rises later and sets earlier.
At the equator the length of day and night does not vary (although the Sun's position in the sky varies).
At the poles, day and night lasts six months each.
The obliquity characterizes therefore the tilt of the Earth's axis relative to the ecliptic varying between 21.8° and 24.4°. But the Earth is slightly flattened at the poles, gravitational forces exerted by the Sun and the Moon rotate on itself not as a perfectly spherical ball but like a top. This small variation from 21.8° to 24.4° is due to the presence of the moon acts as a stabilizer on the equatorial bulge of the earth. Nevertheless, small variations in the obliquity have broad implications for the sunshine at latitude 65°, which is considered the most reliable criterion of melting ice sheets.

The combination of these two effects produces an oscillation of the Earth's obliquity, very limited, about 1.3° around a mean value close to 23.5°.
The combined period of these oscillations is about 41 000 years. The obliquity has a great importance on high latitudes because it is the cause of the seasons, if the obliquity were zero, there would be no seasons, and thus little variation in temperature. It is a parameter or Milanković Milanković cycles corresponding to three astronomical phenomena affecting the Earth's eccentricity, obliquity and precession.
They are used in the context of the astronomical theory of paleoclimate. They are partly responsible for natural climate changes that have major consequence, the glacial and interglacial periods.

* The inclination of the earth's axis is 23° 26' and the axis recovers about 50" per year or 1 degree every 71.6 years.