Spring is almost here! You can feel it in the air, smell it in the earth, and see it in the gradual lengthening of the hours of daylight.
While we’re reveling in the warmer (we hope!) weather, younger children are also bubbling with questions about how and why the seasons change. So, in honor of the first day of spring—the spring equinox—we’re taking the opportunity to help parents and Learning Coaches answer some of those questions, and hopefully spark students’ interest in the science behind the season.
Let’s take it step by step and start with …
What causes day and night?
The earth rotates on the imaginary axis running through its center from top to bottom or north to south. This axis or pole also permanently tilts the earth at a 23.5-degree angle. (More on that later.) The earth makes one complete rotation approximately once every 24 hours, or one “solar day.” As one side of the earth rotates toward the sun, that side experiences daylight. The side facing away from the sun experiences darkness or night.
To help your kids visualize this, grab a ball or globe and a flashlight. If you’re using a ball, stick a Post-it Note marked “You are here!” on one side. Turn off the room lights and, holding the ball or globe in one hand, shine the flashlight onto one side of the ball. Now ask your student to slowly turn it counterclockwise while looking down from its “north pole.” See how the “earth’s” rotation makes the difference between day and night where you live?
Good! Now let’s move on!
Why do the seasons change?
So we know that the earth is spinning on its axis one full turn every 24 hours. At the same time, the earth is also moving around the sun in an elliptical or egg-shaped orbit that takes one full year or 365 days to complete. Contrary to what you might think, this elliptical orbit actually moves the earth closer to the sun in December and farther from the sun in June. So the earth’s orbit isn’t the answer!
The answer lies instead in the earth’s tilt. Again, the earth tilts at a constant 23.5 degrees along an imaginary axis running north to south. As the earth moves around the sun over the course of the year, the northern or “top” half tilts toward the sun in summer and then away from the sun in winter. So it’s the earth’s tilt toward or away from the sun, not its distance from the sun, that explains our change of seasons.
If all this rotating and orbiting has your head spinning …
Check out this fun video from Bill Nye the Science Guy, where he demonstrates why “the earth’s tilt is the reason for the seasons!”
Take an astronomical stroll through the seasons in this 90-second National Geographic video.
When is the first day of spring? Why is it called the vernal equinox?
In the Northern Hemisphere, the first day of spring occurs on March 20 or 21 of each year. This year it happens on March 20!
On March 20, the sun shines directly over the earth’s celestial equator, the imaginary line above the earth that divides it into northern (top) and southern (bottom) hemispheres or halves. Because of this perpendicular alignment, we get approximately 12 hours of daylight and 12 hours of night.
On September 22 or 23, when the earth has traveled halfway around the sun, its tilt will align this way again—causing another 12 hours of day and 12 hours of night. We refer to this day as the autumnal equinox.
In the Northern Hemisphere, the vernal equinox signals the beginning of spring and the autumnal equinox signals the beginning of autumn. In the Southern Hemisphere, the seasons are reversed, so when it’s the spring equinox here in the United States, it’s the autumnal equinox in countries that lie below the equator like Argentina and Peru.
When are the first days of summer and winter?
In the Northern Hemisphere, the first day of summer, or summer solstice, occurs between June 20 and June 22. At that time, the earth’s axis tilts the Northern Hemisphere closest toward the sun. The first day of winter, the winter solstice, occurs sometime between December 21 and December 22 when the earth’s axis tilts the Northern Hemisphere away from the sun.
What’s the difference between an equinox and a solstice?
To sum up, an equinox is a day that has an equal number of hours of day and night. A solstice is either the longest night of the year (winter) or the longest day of the year (summer). Put another way, the winter solstice has the fewest hours of daylight in the year, and the summer solstice has the most hours of daylight.
An equinox occurs when the earth’s tilt aligns the equator perpendicularly to the sun. A solstice occurs when the earth’s tilt, as it moves through its yearlong orbit, either points farthest from the sun (winter) or closest to the sun (summer).
Now that you know the reasons for the seasons, we hope you’ll appreciate the first day of spring even more!