Earth orbits the sun every year, but it does so on a tilt, which means sunlight hits the planet at an angle. Depending on the time of year, the angle of the light hitting the Northern Hemisphere changes, which affects our temperature. In the winter, we are tilted away and so are colder. The opposite is true in the summer.

Astronomical seasons are defined by how the sun is positioned, and change each time there is a solstice or equinox. These can vary each year, depending on celestial mechanics.

Meteorological seasons fit closely to our calendar months and temperature cycles. These occur ever so slightly before the astronomical change and are at the same time each year.