The seasons of the Earth are caused by the inclination of our planet's axis. Earth revolves around an axis that has a 23.5 degree tilt.

This means that sometimes the North Pole is angled toward the Sun while the South Pole is slanted away from the Sun. At these times, the Sun appears at a high angle in the sky in the northern hemisphere, and the heat from the Sun is very concentrated.  This causes the northern hemisphere to warm up.

In the southern hemisphere, the Sun is at a low angle, closer to the horizon, and the Sun's energy is more dispersed.

At other times, the situation is reversed, with the South Pole inclined toward the Sun and the North Pole tilted away from the Sun.  At these times, the heat that reaches the southern hemisphere is more concentrated than the heat that reaches the northern hemisphere, and the southern hemisphere warms up while the northern hemisphere cools down.

Solstices

The summer solstice is the day when summer begins. During the summer solstice, the Sun appears higher in the sky than on any other day, of the year and the Sun is in the sky longer than any other day of the year. The summer solstice takes place in June in the northern hemisphere and in December in the southern hemisphere.

The day of the year in which the Sun appears lowest in the sky is known as the Winter Solstice. In the northern hemisphere, the winter solstice occurs in December. It takes place in June in the southern hemisphere. Winter starts at the winter solstice.

The solstices have had a profound effect on culture and religion.

Primitive people who noticed that the days were getting shorter and shorter as the year progressed would have been greatly relieved to discover, at the winter solstice, that the days were getting longer once more, and the Earth was not going to be consumed in darkness.

It is no coincidence that Christmas is celebrated around the time of the winter solstice in the Northern Hemisphere. Religions that preceded Christianity also held important festivals at this time.

The Summer Solstice has been associated with fertility and growth.

Equinoxes

Two equinoxes take place during the year - in March and in September. During an equinox, the Sun shines at a right angle to the Earth's surface; the Earth is not tilted in relation to the Sun.

The sun is in the sky for exactly half of the full day during the equinoxes, which mark the beginning of spring and autumn, or fall. Spring begins in March in the northern hemisphere and in September in the southern hemisphere.  Fall begins in March in the southern hemisphere and in September in the northern hemisphere.

Like the solstices, the equinoxes have important religious and cultural significance. The spring (or vernal) Equinox is associated with rebirth and with religious holidays such as Easter and Passover.

In temperate climates, the autumnal equinox is associated with the harvest. The Jewish High Holy Days often take place around the autumnal equinox.

Polar Regions

The Earth's polar regions include the Arctic, which extends from the North Pole southward to the Arctic Circle ( 66.5 degrees north latitude), and the Antarctic, which extends from the South Pole northward to the Antarctic Circle (66.5 degrees south latitude).

At the North and South Poles, the Sun is always in the sky during the summer and never in the sky in winter.

During the summer solstice, throughout the polar regions, the Sun remains above the horizon day and night. This is why the Arctic is called "The Land of the Midnight Sun." The Sun is never in the sky in the polar regions during the winter solstice.

Every spot in the polar regions experiences at least one full day in which the Sun is always in the sky, and at least one full day when the Sun is always below the horizon

Tropics

The tropics extend from the equator northward to 23.5 degrees north latitude and southward to 23.5 degrees south latitude. The northern boundary of the tropics is known as the Tropic of Cancer, and the southern boundary is known as the Tropic of Capricorn.

This region is not affected by the Earth's tilt.

Therefore, in the tropics, there is no summer, winter, spring or autumn.

The Sun is always overhead, so it is always warm.

The most important influence on weather in the tropics is the amount of rainfall, and places in the tropics have a Wet Season and a Dry Season.