Distance Learning Module: Seasons and Equinoxes
Read It!
Start here for information about what causes Earth’s seasons, and what it means to experience an equinox
Earth is in constant motion. Approximately every 365 days, Earth completes one revolution around the Sun (a year), and approximately every 24 hours, Earth completes one rotation around its axis (a day). Because Earth's axis is not straight up-and-down (it is tilted about 23.5 degrees), the way in which the Sun's light hits Earth's surface varies throughout the Earth’s orbit. Seasons are the result of a combination of the tilt of Earth's axis and its place in its orbit.
When the north pole of Earth's axis is pointed toward the Sun, it is summer in the Northern Hemisphere and winter in the Southern Hemisphere. How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth? There are two effects we need to consider. When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth’s surface. (You can get a similar effect by shining a flashlight onto a wall. If you shine the flashlight straight on, you get an intense spot of light on the wall, then the spot of light is more spread out and less intense.) The second effect has to do with the length of time the Sun spends above the horizon. In June, the Sun rises high in the sky over the Northern Hemisphere, and stays above the horizon in the United States for as long as 15 hours. Thus, the Sun not only heats us with more direct rays, but it also has more time to do it each day.
Two days out of the year, we experience an equinox, when the Earth is tilted neither toward nor away from the Sun. This happens because the Earth’s axis (the imaginary line running from the North Pole to the South Pole) falls exactly parallel to the plane of the Earth’s orbit around the Sun, leaving the Sun directly over the equator at mid-day. During an equinox, the hours of daylight and the hours of darkness are about equal almost everywhere on Earth (hence, the name).
The vernal equinox marks the beginning of spring. After the vernal equinox, the days become longer and the nights become shorter in the Northern Hemisphere. Autumn, or fall, begins with the autumnal equinox. After the autumnal equinox, the days become shorter and the nights become longer.
In 2020, the vernal equinox occurred on the night of Thursday, March 19—making Friday, March 20, the first day of spring.
Try It! Complete these hands-on activities to see today’s lesson in action
Activity: Modeling the Seasons (adapted from an activity from the Exploratorium)
Materials
flashlight
a room that can be made dark
a Styrofoam ball (in a real pinch, a fresh orange or a rounded sphere of Play-Doh will do, but it won’t be completely spherical!)
a large drinking straw
a rubber band
a flexible plastic cup
Scotch tape
scissors
Make a Model of the Earth
Push the straw through the center of the Styrofoam ball. This represents the axis about which the earth rotates. One end of the ball is north and the other end is south. Place a rubber band around the center of the ball to represent the equator.
Place the tack or sticky dot into the ball at an appropriate location on the ball to represent your town or city. (For instance, the latitude of Concord, New Hampshire is about 43.2˚ north, so you’d place the tack not quite halfway between the equator and the North Pole.)
With an adult’s help, use the scissors to make a hole in the bottom of the plastic cup, near the side, as shown below. It should be just large enough to accommodate the diameter of the straw.
Cut a long (approx. 8 cm) piece of tape. Cut a second, shorter piece (approx. 2 cm) and place it in the center of the long piece, with the sticky sides together. Place the straw into the hole in the cup. The smooth part of the small tape should allow the straw to rotate in the hole, while the sticky parts of the long tape will hold it in place against the side of the cup. Your model should look something like the picture on the left. Use a protractor to check the angle of the earth’s tilt. It should be 23.5 degrees.
Set up the “Solar System”
Enter the room that is designated to represent outer space. Choose a small object and put it in a stationary location to represent Polaris, the North Star. Your straw should point to Polaris throughout the activity.Set up the flashlight (Sun) in the center of the room, and darken the rest of the room.
Model a Day on Earth
Turn the straw so that the earth spins counterclockwise (when viewed from the north) for one rotation. Notice that the dot is in light (day) for about half of the rotation and is in shadow (night) for about half of the rotation.
Model the Seasons
Station yourself at four different locations around the Sun, to represent the changing of the seasons on December 21, March 21, June 21, and September 21. Your movement should look like this if seen from above:
From each location (date), spin the Earth on its axis (rotate the straw in its hole in the drinking cup) to simulate one day. Make observations about the following:
For what fraction of the day is the dot in the light? More than half? Less than half? About half?
For what fraction of the day is the North Pole in the light?
How is the light from the sun striking the dot? Is it direct or at an angle?
What is happening in the Southern Hemisphere, versus in the Northern Hemisphere?
When you have been to each of the four dates, have modeled one year on Earth (one revolution around the Sun)!
What’s Going On?
Many people think the seasons are caused by variations in our distance from the Sun. While the Earth’s orbit is slightly elliptical, it’s very close to circular, and the variation in distance between the Earth and Sun is not enough to account for our seasons. The seasons are caused by the tilt of the earth. The earth holds its rotation axis (tilt) fixed in space as it moves around the Sun. In the summer, the Northern Hemisphere tilts toward the Sun. It’s warmer because the midday sun shines more directly head-on. In the winter, when the Northern Hemisphere tilts away from the Sun, the Sun’s rays strike the Earth at a lower angle, and the energy from the sunlight is spread out over a larger area, which reduces its effectiveness at heating the ground. The way the Sun hits the Earth is opposite between the Northern and Southern Hemispheres—so the seasons are reversed between the two halves of the Earth.
