A little science this week. Specifically, the impact of Kepler’s First Law of Planetary Motion.
Most diagrams in textbooks have the planets going around the sun in circular orbits. While this is almost true, the planets actually orbit in ellipses.
I learned to draw an ellipse by tying a loop of string, sticking two pins in a paper, looping the loop around the pins, and then drawing while stretching the loop with a pencil. The resulting figure is an ellipse which has two focal points or foci.
In the case of our orbit around the sun, the two foci are about 5 million kilometres apart, with the centre of the sun occupying one of them.
From a practical perspective, this means our distance from the sun varies on an annual basis. On July 4, we were as far from the Sun as we can get and on Jan. 4, we will be as close as we get – give or take a few hours in both cases.
I know that doesn’t sound right. How can we be furthest from the sun in July when it is so hot and closest to the sun in January when it is so cold? The seasonal temperatures are a consequence of the tilt of the Earth’s axis and not the distance to the sun.
The two points are respectively referred to as “aphelion” and “perihelion,” meaning “away from” and “near” in Greek. At aphelion – a couple days ago – we were 152,093,250 kilometres from the centre of the sun. Over the next six months, we will fall towards the sun, reaching perihelion at 147,098,924 kilometres only to climb back up to aphelion.
The difference is three percent and doesn’t really affect Earth’s surface temperature all that much. Indeed, counter-intuitively, it is when we are furthest from the Sun that Earth’s average surface temperature is highest.
This is not a result of the distance to the sun but the northern summer where much of the land mass of the planet is facing the sun. Land has very low heat capacity, so it warms quicker and heats the atmosphere more than the southern oceans because water has a much higher heat capacity.
The good news? Summer in the northern hemisphere is also longer.
Todd Whitcombe is a chemistry professor at UNBC.
