One of the first science facts we learn is that the Earth travels in a circle around the Sun.
Unfortunately, that is not strictly true. Yes, the Earth orbits the Sun but its orbit only approximates a circle. It is actually an ellipse as illustrated by Kepler's First Law. The Earth's orbit has an eccentricity of 1.7 per cent or it varies by 1.7 per cent during the course of a year.
Today at 8 p.m., we will reach our maximum distance from the Sun at 152.6 million kilometres. Next Jan. 4, we will be at our point of closest approach at 147.5 million kilometres. These two points are called "aphelion" and "perihelion", respectively.
But if the Sun is at its furthest in July and at its closest in January, why is July much hotter than January?
The answer is that the Sun's distance to the Earth does not drive our seasonal climate. That is a consequence of the tilt of the Earth relative to its orbital plane. Essentially, the Earth orbits the Sun in a flat plane, called the "ecliptic", but the axis of rotation of the planet is tilted relative to this plane by 23.5 degrees.
This means that the north celestial pole is, for now, Polaris as the Earth maintains its tilt throughout its annual orbit. In January, in the northern hemisphere, this mean that the Earth is tilted away from the Sun. In July, it is tilted towards the Sun.
Of course, in the southern hemisphere, exactly the opposite is observed with the consequences that the seasons are reversed. Christmas is a summer holiday in Australia and South Africa.
In the equatorial zone, seasonal variations are minimal as the Sun's position does not vary significantly from overhead. However, in Prince George, changes in the Sun's position have a tremendous impact on the amount of ambient solar radiation that we receive.
This is seen both in the duration and intensity of light. In the winter, with the Sun low in the sky, our days are much shorter and what sunlight we get strikes the ground at an oblique angle. The amount of heat per square metre is lower. We can think of this as a tilted board observed at a distance. The area of the board hasn't changed but it appears smaller.
In the summer, the reverse is true. Our days get incredibly long and the angle of incidence for incoming solar radiation is much closer to perpendicular. The amount of energy per unit area goes up as does the temperature. The tilt of the Earth drives the seasonal variations in temperature.
However, there is another interesting aspect associated with aphelion.
Despite the fact that the Earth is at its farthest from the Sun, the average mean surface temperature of the Earth is some 2.3 C warmer than at perihelion. That is, the further we are from the Sun, the warmer the Earth is on average. Strange but true.
Of course, this doesn't really have anything to do with our distance from the Sun. Rather it is a consequence of the distribution of land and ocean on the surface of the planet.
If you hold a globe up and look at it from the South Pole, you see mostly water. Antarctica and Australia are the only continents totally in the southern hemisphere. Africa and South America straddle the equator but most of Africa is actually in the northern hemisphere.
On the other hand, if you looking at a globe from above the North Pole, you see mostly land - the large land masses of North America and the European/Asia continent.
This preponderance of land in the northern hemisphere results in the northern and southern hemispheres heating up differently. Land heats up much more rapidly than water and it returns that heat to the atmosphere much more quickly.
Consider the desert around Osoyoos versus the coastal climate of Vancouver. Both are at the same latitude, yet Osoyoos undergoes huge temperature extremes during a summer's day - from boiling hot to freezing cold. Vancouver, on the other hand, has a much more moderate climate that avoids both temperature extremes. The oceans moderate the coastal temperatures.
Even though Earth reaches its furthest point from the Sun in July, the land in the north makes our summers a little warmer and the Earth a little hotter. In addition, the oceans keep their heat, moderating the year round temperature, so that the southern winter is not as cold as ours.
Just ask the World Cup players in Brazil.
Those of us living in the north do have one advantage. Kepler's 2nd Law of Planetary motion says planets travel slower at aphelion than at perihelion. As a result, the northern summer is 2 to 3 days longer than down south.
So get out and enjoy a warm summer's day while they last!
