why do we have seasons?
The earth rotates around the sun on an elliptical course at an axis of 23 degrees.  As the northern or southern hemisphere gets closer or further from the sun the amount of light hitting it changes making the seasons.\n\nCombine the axial tilt and the ellipitical path and you create periods of warmth (spring and summer) and periods of cold (fall and winter).\n\nThe primary property is the earth’s 23 degree tilt.  The axial tilt remains fixed as the earth rotates.  When the earth’s axis is turned away from the sun less light falls on that side of the earth and it gets colder.  When the axial tilt is closer to the sun then more light falls upon the earth making it warmer.  This also explains the length of the days.  The earth’s rotational speed doesn’t change, but one hemisphere is tilted closer to the sun so their day is longer.\n\nThen there is the orbital path.  When part of the earth is farther from the sun it gets colder, as it gets closer it gets warmer.  As the earth rotates around the sun it gets closer and further from the sun so the temperature varies.  This is because of the orbital path the earth takes around the sun.  It is not a circular path because to orbit in a true circular path you need to be in a perfectly balanced orbital position with the speed of travel just matching and canceling the attraction of the object.  Satellites in Geo-stationary orbits around the earth are on such a path (since this point matches the speed of the earth’s rotation the satellite is in a fixed position relative to the earth), no planet is in such a position around the sun.  As the earth rotates it gets closer to the sun (and its speed increases, because the sun’s gravity is tugging on it), as the earth goes around the sun it reaches a point where it is closer than at any other point (perihelion), so the earth’s surface gets warmer.  As the earth continues around the sun it slows down, because the sun’s gravity is still tugging on it.  But, since it has accelerated it has a higher velocity so it can travel farther from the sun.  When it reaches its maximum distance (aphelion) the earth is colder.  At this distance it’s speed (velocity) is zero.  The sun is still tugging on the earth so it falls in toward the sun and accelerates, continuing on its orbital path.\n\nSounds real tricky doesn’t it, but counting all the asteriods, planets, and comets over a billion objects rotate around the sun on similar paths and they all have season.