During a discussion last evening, this topic came up. We all agreed the axial tilt of the Earth is the major reason. However, the topic of the Earth's orbit and its possible influence on the seasons was debated as well. IS that orbit an ellipse as is so often pictured in illustrations, or is the orbit more circular? Is the orbit of the Earth a significant factor affecting the seasons in both north and south hemispheres? I would never have known whom to ask for a reliable answer before, but thanks to the Forum, I thought of asking the pros here.
thank you,
geoffrey

The tilt is indeed the major reason.

Earth's orbit is eccentric (as are those of all the planets), but is very close to circular; the difference in distance to the Sun perihelion (closest point) and aphelion (furthest point) is about 3%. Perihelion occurs in January - the Northern Hemisphere's winter - and aphelion in July.

The eccentricity has little effect on the severity, or extremes, of the seasons.

However, perturbations in Earth's orbit do cause changes in the eccentricity over very long periods of time, and those may have effects on the severity of seasons.

Earth's axis itself isn't stationary - it precesses, or rotates, so that in about 12,000 years, perihelion will occur during the Northern Hemisphere's summer. 5,000 years ago, Thuban (Alpha Draconis - the brightest star in the constellation Draco, the Dragon) was the North Star. In a few thousand more years, Vega (Alpha Lyris - brightest in Lyra, the Lyre) will be the "North Star".

If you'd like to get a jump on things for the future, Vega is in the sky now, the brightest star of the three in the "Summer Triangle" - look for it almost overhead (slightly westerly) now after dark.

For more detailed explanations, including other factors that may change the characteristics of the seasons, see here.

The tilt of the Earth's axis will still be the major player, however.

I have always thought it strange that the shortest days (daylight hours) are in December, whereas the coldest weather is January or February and vice versa in summer, May and July / August. Is this due to a gradual warm-up / cool-down of the earth or is there some other reason(s).

Nope, you nailed it - there's a lag between the lessened solar radiation of the period between October 30 and February 10 (roughly the time when the Sun's rays provide the least amount of heat as the days shorten to Winter Solstice - around December 21/22 - then gradually lengthen again) and the time the weather actually catches up. And this lag holds true regardless of the season.

It's similar to a pot of water on the stove - turn the heat up high, and it takes a while to heat up. Remove the heat, and it takes time to cool.

After a month's extensive observation in the Arizona desert, we have decided that sunsets are prettier than sunrises. At least in Winter.

Why is that?


I am not about to get up to check on Summer sunrises. And the times I stayed up late enough to observe the sunrise, I was in no condition to make any intelligent observations. Or remember them.



Also, the (Winter) morning light, just after sunrise is a little more Ektachrome than the light before sunset. At least on the close in ground. The mountains seem to be pretty equal, sunrise or sunset.

How come that is?

Normally nighttimes are cooler and less windy, so the air is clearer and less humid, and that persists until sunrise. The daytime sees more wind and more dust, and more moisture (and clouds); those things cause sunsets to be generally more colorful due to the scattering and absorbtion of light.