The Earth's changing climate is caused by the tilt of the Earth's axis and its elliptical orbit around the sun. (See Reference 1) Sunlight hits the surface of the planet at different angles during the planet's year-long journey, causing parts to experience different seasons as the amount of heat and light change.(see Reference 2) There is no noticeable seasonal change at the equator, where sunlight amount and strength is relatively constant throughout the year.(see reference 3)
The earth's axis of rotation is 23.4 degrees in relationship to its orbital plane. The axis is fixed, constantly pointing in the same direction regardless of movement of the planet. The tilt causes different amounts of sunlight exposure to the southern or northern hemisphere dependent on where earth is in its orbit around the sun.
The earth travels an elliptical path around the sun, completed within 365 days and five hours. During this journey the earth's tilt presents different sides of its surface toward the sun. The tilt produces the most dramatic change in light at the summer and winter solstice and the least at the vernal and autumnal equinoxes. The change in the amount and strength of the sunlight throughout the year is what produces the seasonal changes.
At the winter solstice the hemisphere tilted as far away from the sun as possible experiences the longest night and shortest day of the year. The sunlight that reaches this hemisphere hits the surface at a smaller angle. This causes the same amount of light to be spread over a greater area, causing there to be less heat. There are fewer hours of daylight each day as the earth approaches the solstice. At the pole there will be 24 hours of darkness.
Spring and Autumnal Equinox
As the earth moves along the elliptical path, it points neither towards or away from the sun. Both hemispheres receive equal amounts of light during the spring and autumnal equinox day. The hemisphere moving from winter will have increasingly longer days with shorter nights as the planet's axis moves to expose that hemisphere to the sun more directly. This will also increase the heat experienced by that hemisphere. The other hemisphere will begin to move towards autumn, with cooler days and longer nights after the equinox.
The hemisphere experiencing summer and moving towards the summer solstice, is receiving more sunlight directly than it has at any other point in the cycle. This increase in direct light also increases the heat for that hemisphere during this time. The summer solstice is the longest period of daylight, and the pole of that hemisphere will have 24 hours of daylight. The opposite hemisphere will be experiencing the winter solstice on this day.