Earth tilt angle is 23.5 degrees
Earth is tilted at an angle of 23.5 degree across its axis of rotation. This tilt is responsible for seasonal changes, without this there will be no seasonal changes but instead single season per area. The most direct sun rays hit areas would always be warmer. The increase in tilt of the Earth severe the the seasons. Winter becomes even more colder and summer becomes more warm. Nowadays tilt is 23.5 degree but it changes with a cycle of about 40,000 years the tilt varies between 22.1 and 24.5. Cooling of summers are thought to allow snow and ice formation on the high altitude areas which can stay for more time and becomes solid making massive ice sheets. Earth covered with more snow will reflect more sun light and Earth will becomes more and more cold. This is responsible for the ice age where Earth is covered with snow.
The tilt as implies to the seasonal changes other factors that gives seasonal changes are the change in the rotation of Earth's orbit. The other is the precession, changes in axial precession increase seasonal contrast between northern and southern hemisphere. When northern hemisphere have summer southern will be away from sun which will be the winter season.
Eccentricity is the change in Earth's orbital which can be away from sun or near the sun. When it is at the furthest distance from the sun, temperature will be cooler. The near sun orbital, heating process will be at peak warming up the Earth. Ice sheets will melt and transgression will occur, water level will rise. Changes in eccentricity affect the Earth sun distance, The shape of Earth becomes nearly circular from elliptical in a cycle that takes between 90,000 to 100,000 years. This cycle is called Milakovitch cycle which was after Milutin Milankovitch (1879-1958).