This video is a time-lapse of images taken from a geostationary satellite. It shows a whole year of the Earth’s orbit around the Sun from 2010 to 2011. You can see the difference in illumination between the seasons created by the Earth’s tilt; the angle of the line between light and dark changes as we go around the Sun. This effect is caused by our axis of daily rotation being tilted by 23.5 degrees.
We are usually taught an approximate version of the truth when we learn about the Earth’s orbit. Most people are told that the Earth orbits the Sun like this:
…however the orbit is not really circular, but slightly elliptical. It looks more like this:
Not only is it slightly oval in shape, but it is also slightly off-centre. Our closest approach to the Sun each year is Perihelion (on the right of this image) and our farthest approach is Aphelion (on the left). Crucially though, the Earth is tilted in its own daily rotation by 23.5 degrees. This means that the North and South poles don’t line-up with the top-down view of thus image. So, sideways-on it looks like this:
At around Perihelion (our closest approach to the Sun) the North Pole is pointing away from the Sun – actually it happens a couple of weeks before Perihelion. It is this effect that gives us our seasonal changes in temperature. When the North pole is pointed away from the Sun, as shown here, it means the Northern half of the Earth is receiving less energy from the Sun and so cools down and experiences more darkness. Six months later the Earth has moved around and now the North pole points more toward the Sun and thus it is the Southern Hemisphere that is darker and cools down.
It is not our distance from the Sun that determines the seasons, but our changing exposure to the Sun’s heat and light caused by our axial tilt. Our seasons are the result of the misalignment of our daily, North-South rotation compared with our yearly, Solar rotation. The darkest day is the Winter Solstice in the Northern Hemisphere – usually around December 21st – at the same time the Southern Hemisphere has its longest day. Perihelion occurs in early January (it was Jan 4th in 2014) which means we are closest to the Sun when it is coldest in the Northern Hemisphere and we are farthest from the Sun when it is hottest in the Southern Hemisphere.
The change in our tilt drastically changes how much of the Sun’s energy we receive, as is shown in the following photos of the Earth from Space. You can see that in Europe, for example, our share of daylight changes a great deal over the year. For the same reason, if you go far enough North or South there are places where it is continuously day or night for weeks or months at a time.
The above, amazing images of the seasons come from the same source as the video at the top of this post. They were taken by a EUMETSAT Earth observation satellite. This is a geostationary satellite, meaning that it looks at the same part of the world all the time. It stares at Africa and here you can see the sequence of Winter, Spring, Summer and Autumn in images taken at different times of the year: