What Causes the Seasons?

The seasons have nothing to do with how far the Earth is from the Sun.  If this were the case, it would be hotter in the northern hemisphere during January as opposed to July.  Instead, the seasons are caused by the Earth being tilted on its axis by an average of 23.5 degrees (Earth's tilt on its axis actually varies from near 22 degrees to 24.5 degrees).  Here's how it works:

The Earth has an elliptical orbit around our Sun.  This being said, the Earth is at its closest point distance wise to the Sun in January (called the Perihelion) and the furthest in July (the Aphelion).  But this distance change is not great enough to cause any substantial difference in our climate.  This is why the Earth's 23.5 degree tilt is all important in changing our seasons.  Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude.  This situates the northern hemisphere in a more direct path of the Sun's energy.  What this means is less sunlight gets scattered before reaching the ground because it has less distance to travel through the atmosphere.  In addition, the high sun angle produces long days.  The opposite is true in the southern hemisphere, where the low sun angle produces short days.  Furthermore, a large amount of the Sun's energy is scattered before reaching the ground because the energy has to travel through more of the atmosphere.  Therefore near June 21st, the southern hemisphere is having its winter solstice because it "leans" away from the Sun.

Advancing 90 days, the Earth is at the autumnal equinox on or about September 21st.  As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator.   Basically, the Sun's energy is in balance between the northern and southern hemispheres.  The same holds true on the spring equinox near March 21st, as the Sun is once again directly over the equator. 

Lastly, on the winter solstice near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude.  The southern hemisphere is therefore receiving the direct sunlight, with little scattering of the sun's rays and a high sun angle producing long days.  The northern hemisphere is tipped away from the Sun, producing short days and a low sun angle.

What kind of effect does the earth's tilt and subsequent seasons have on our length of daylight (defined as sunrise to sunset).  Over the equator, the answer is not much.  If you live on or very close to the equator, your daylight would be basically within a few minutes of 12 hours the year around.  Using the northern hemisphere as a reference, the daylight would  lengthen/shorten during the summer/winter moving northward from the equator.  The daylight difference is subtle in the tropics, but becomes extremely large in the northern latitudes.  Where we live in the mid latitudes, daylight ranges from about 15 hours around the summer solstice to near nine hours close to the winter solstice.   Moving to the arctic circle at 66.5 degrees north latitude, the Sun never sets from early June to early July.  But around the winter solstice, the daylight only lasts slightly more than two hours.  There becomes a profound difference in the length of daylight heading north of the arctic circle.  Barrow, Alaska at slightly more than 71 degrees north latitude, lies just less than 300 nautical miles north of the arctic circle.  Barrow sees two months of total darkness, as the Sun never rises for about a month on each side of the winter solstice.  On the other hand, Barrow also has total light from mid May to early August.  And what about the north pole, or 90 degrees north latitude?  The Sun rises in the early evening near the spring equinox and never sets again until just after the autumnal equinox, or six months of light.  Conversely, after the Sun sets in the mid morning just after the autumnal equinox, it will not be seen again until the following spring equinox, equating to six months of darkness.