The Earth's axis is tilted at an angle of 23.5 degrees. This means that as the Earth orbits the Sun, different parts of the planet are tilted more directly towards the Sun at different times of the year.

In the Northern Hemisphere, the summer solstice occurs around June 21st. During this time, the Northern Hemisphere is tilted more towards the Sun than any other time of the year. This means that the sunlight is more direct, and the days are longer.

The opposite is true in the Southern Hemisphere. The winter solstice occurs around December 21st, and the sunlight is less direct, and the days are shorter.