Summer is caused by the tilt of the Earth's axis. The Earth's axis is tilted at an angle of about 23.5 degrees relative to its orbit around the sun. As the Earth orbits the sun, different parts of the Earth receive more or less direct sunlight depending on the tilt of the axis. When the northern hemisphere is tilted towards the sun, it is summer in the northern hemisphere and winter in the southern hemisphere. This is the opposite when the northern hemisphere is tilted away from the sun, it is winter in the northern hemisphere and summer in the southern hemisphere.