Throughout the course of the year, the Earth goes through the four noticeable seasons: summer, autumn (fall), winter and spring, each lasting for about 3 months. The seasons experienced by the northern and southern hemisphere always differ by six months - when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and so on.
Seasons are a direct consequence of the Earth's tilted rotation axis, which makes an angle of about 23.5 degrees to a line drawn perpendicular to the plane of the ecliptic. The direction of the Earth's axis stays nearly fixed throughout one orbit, so that at different parts of the orbit one hemisphere 'leans' towards the Sun (summer), while the other 'leans' away (winter). Six months later, the Earth is leaning in the opposite direction.
For locations north or south of the equator, the main feature accompanying each season is a change in temperature caused by the varying amount of sunlight that falls on each hemisphere of the Earth throughout its annual orbit. The hemisphere tilted towards the Sun will experience longer hours of sunlight, and more direct sunlight.
As the Sun is higher in the sky during summer, the sunlight reaching the surface is more concentrated. In winter, the Sun is lower in the sky, and sunlight is spread out over a larger area. During spring and autumn, both hemispheres receive about the same amount of sunlight.
At the equator, the temperature variation is much smaller throughout the year, and it is common to consider two seasons: dry and wet/monsoon. For observers within the Arctic circle and the Antarctic Circle (latitudes 66.5 degrees north and south), there are only two seasons - a six-month long winter night followed by a six-month long summer day!
The date of the start of the seasons is often chosen to start on the dates of the solstices (summer and winter) and equinoxes (autumn and spring). Alternatively, the start of a new season may be associated with the first day of the month (December, March, June and September) in which a solstice or equinox occurs.
The Earth's changing distance from the Sun due to the Earth's elliptical orbit is sometimes thought to cause the seasons. This is incorrect! The Earth's distance from the Sun varies by about 3% from closest (perihelion distance = 147.09 million km) to furthest approach (aphelion distance = 152.10 million km). This small change in distance cannot account for the temperature differences between summer and winter, and cannot explain how it can be winter in one hemisphere and summer in the other hemisphere.