What causes the seasons? Not only is this asked on page 32 of Chapter 2, but it is often asked by many individuals residing on Earth. However, a surprisingly few number of people actually know. Seasons occur because the title of Earth’s axis causes sunlight to fall differently on Earth at different times of year. Notice that the seasons on Earth are caused by axis tilt not by its distance from the Sun. While Earth’s distance to the Sun changes over a year, Earth is only about 3% farther at its farthest point than it is at its closest point. Interestingly, Earth is farthest away from the Sun in July and closest to the Sun in January! The difference in strength of sunlight is completely overwhelmed by the difference in the Earth’s tilt. We use Solstices and Equinoxes to help us mark the change of seasons, and there are four special positions in Earth’s orbit that mark the change in seasons. The June solstice, known as the summer solstice for those who reside in the Northern Hemisphere, occurs arround June 21 and is the moment when the Northern Hemisphere is tipped most directly toward the Sun and receives the most direct sunlight. The longest day and the highest solar noon almost always occur during the June solstice, too. The December solstice, called the winter solstice by people in the Northern Hemisphere, occurs on December 21 and is the moment when the North Hemisphere receives the least direct sunlight. Coincidentally, the shortest day and lowest solar noon almost always occur on this day. The March equinox, called the spring equinox by people in the Northern Hemisphere, occurs around March 21 and is the moment when the Northern Hemisphere transitions from being tipped slightly away from the Sun to being tipped slightly toward the Sun. Lastly, the September equinox, called the fall equinox in the Northern Hemisphere, occurs around September 22 and is the moment when the Northern Hemisphere first starts to be tipped away from the sun.
The exact dates and times of the equinoxes and solstices can vary up to a couple days from the given dates above. Also, the Solstices and Equinoxes happen in reverse for the Southern Hemisphere. For example, the June Solstice results in the Southern Hemisphere being tipped most away from the Sun and receives the least direct sunlight. All in all, many people believe that seasons are caused by variations in the Earth’s distance from the sun. If this was true, the Northern and Southern Hemispheres would have the same seasons at the same time, which we know is not true. The real cause of seasons is the Earth’s axis tilt, which causes the two hemispheres to rotate being tipped toward the sun over the course of a year.