The Seasons

The seasons are caused by the tilt of the earth's rotational axis. As the earth orbits the sun, its axis is tilted 23 ½ degrees from perpendicular to its orbit. And it stays tilted the same direction all the time, as shown in the diagram below.

Looking at the above diagram,when the earth is the position labeled June, you can see that the Northern Hemisphere is tilted toward the sun. As a result the sun rises in the northeast and sets in the northwest. It therefore follows a very high path across the sky. At noon, the sun is highest in the sky for the day. In June, in the United States, its noon altitude is around 80 degrees.

When the sun is high in the sky like this, we get a lot of heat from it. Also at this time of year the daylight period is about 15 hours, whereas the nighttime period is only about 9 hours. So we have a long day to soak up heat from the sun, but a short night to lose the heat back into space. As a result, we gain more heat than we lose, so it's hot in June and the other summer months.

On the other hand, when the earth is in the position labeled December in the above diagram, the Northern Hemisphere is tilted away from the sun. Now the sun rises in the southeast and sets in the southwest. It also follows a low path across the sky. At noon, in the United Stares, its noon altitude is around 30 degrees.

When the sun is low in the sky like this, we don't get a lot of heat from it. Also at this time of year the daylight period is only about 9 hours, whereas the nighttime period is about 15 hours. So we have a short day to soak up heat from the sun, but a long night to lose the heat back into space. As a result, we lose more heat than we gain, so it's cold in December and the other winter months.

The date when the sun is highest in the sky for us is called the June Solstice.  This occurs about the 21st of the month.  This is also considered the first day of summer.  The date when the sun is lowest in the sky for the year is called the December Solstice.  This also occurs about the 21st of the month.

Half way between these dates are the March and September Equinoxes.  On these two dates, the sun rises exactly in the east and sets exactly in the west and we have 12 hours of daylight and 12 hours of night.  (Equinox literally means "equal night")

Incidentally, the sun never passes directly overhead any state except Hawaii.

Looking back at the diagram, notice that in June the Southern Hemisphere is tilted away from the sun, so it's winter in June in Australia and South America. In December the Southern Hemisphere is titled toward the sun, so it's summer in December in those places.