The Earth in its orbit at the solstices and equinoxes.
Click on image for full size

The Seasonal Merry-Go-Round

The tilt of Earth's rotational axis and the Earth's orbit around the Sun work together to create the seasons. As the Earth travels around the Sun, it remains tipped over in the same direction, with its north pole pointed towards the star Polaris. This 23.5 degree tilt of the Earth's rotational axis to the Earth's orbital plane about the Sun (the ecliptic plane) causes Earth's two hemispheres to be exposed to different intensities of sunlight for different amounts of time throughout the year. The changing intensity and changing amount of sunlight to the different hemispheres has given rise to the seasons of summer, fall, winter and spring.

In the Northern Hemisphere, the first day of summer, called the summer solstice, is around June 21st. The summer solstice marks the point at which the north pole of the Earth is tilted at its maximum towards the Sun. For any location in the northern hemisphere, the day of the summer solstice is the longest day of the year with the Sun reaching its greatest angular distance north (or its highest point in the sky for the year for that given location). During the time surrounding the summer solstice, the northern hemisphere is getting more direct sunlight for a longer amount of time, which heats that hemisphere efficiently causing warmer temperatures on average.

Notice that when the northern hemisphere is tilted towards the Sun, the southern hemisphere is tilted away. This is why people in the North America, Europe, Asia, and other places north of the equator have the opposite season of people in South America, Australia, and other places south of the equator. So, while the summer solstice marks the beginning of summer for the northern hemisphere, it marks the beginning of winter for the southern hemisphere.

The first day of winter for the northern hemisphere is called the winter solstice. This day, around December 21st each year, is when the north pole of the Earth is tilted at its maximum away from the Sun. The Sun’s rays are less intense at this time of year because they are spread over a greater surface area and must travel through more energy-absorbing atmosphere to reach the Earth. Also, the winter solstice is the shortest day of the year for those who live in the northern hemisphere. The decreased intensity of the sunlight received along with less daylight hours leads to the cooler temperatures often felt by those living in the northern hemisphere during winter months. Again, things are reversed for the southern hemisphere, where summer is being ushered in at the time of the winter solstice.

Of course, between the season of summer and winter in the northern hemisphere comes the season of fall. The beginning of fall in the northern hemisphere is marked by the autumnal equinox (around September 23rd each year). And between the seasons of winter and summer in the northern hemisphere is spring. The beginning of spring in the northern hemisphere is marked by the vernal equinox (around March 21st each year). At the two equinoxes, neither the north pole nor the south pole is inclined toward the Sun. Equinox literally means "equal night". On the vernal (spring) and autumnal (fall) equinoxes, day and night are about the same length all over the world.

Last modified October 31, 2006 by Travis Metcalfe.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

North and South: Opposite Seasons

NASA's Earth Observatory has recently started making images of the entire surface of the Earth every month. There are no clouds in the images because they combine many pictures taken at different times...more

President of Peru Visits Ancient Incan Sites

Peru's new president, Alejandro Toledo, was sworn in on July 28, 2001. The next day he took part in traditional ceremonies asking for the blessing of the Incan gods. Toledo is himself one of 16 children...more

What is the distance from the Earth to the Sun when the Earth is at perihelion and when it is at aphelion? At what month is the Earth closest to the Sun? What is the circumference of the Earth? How fast is the Earth moving about its axis; how about around the Sun?

Ok. Let's get some terminology down first. If a body (like the Earth) is orbiting around the Sun, we say it is closest to the Sun at perihelion and farthest from the Sun at aphelion. If a body (like the...more

Cuzco

This is an aerial view of Cuzco. In the center of the picture, the cathedral of Cuzco can be seen. This cathedral was built in the 17th century. Cuzco is located in Southern Peru. It is the ancient capital...more

What Causes the Seasons?

Let's get rid of some common misconceptions about the seasons. The Earth's orbit is in the shape of an ellipse, so that sometimes the Earth is a little bit closer to the Sun than at other times. Is this...more

Universal Time

When it is noon where you live, it is midnight on the opposite side of the world. Usually when we think of time, we mean "the time of day where I live". If we say something happened at 9 AM, we mean it...more

Native American Astronomy

By 30,000 B.C,. Asian hunter-gatherers had crossed the Bering Strait into North America. These people were the first to inhabit this new land and so they are known as the Native Americans of North America....more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA