Explore the science behind the first day of spring.

If you live in the Northern Hemisphere, you're probably looking forward to buds on the trees, longer days, and warmer weather. The vernal equinox is almost here — that's the first day of spring.

What Is an Equinox?

1. To understand what an equinox is, you will have to imagine you are the sun. Need help getting into character? Read these answers to kids' questions from an astronomer at the Space Telescope Science Institute. What's one interesting fact you learned about the sun?

2. Now that you're in character, visit the Plymouth State Weather Center to see the spring equinox from the point of view of the sun.

For most of the year, the Earth is titled on its axis, which allows different amounts of sunshine to reach different parts of the globe creating seasons.

But on the two equinoxes — one in spring and one in fall — the Earth is not tilted so the sun passes directly over the equator. On the equinoxes, day and night are equal in length, 12 hours each. Now take a look at the Earth's rotation from another view.

When Is the Equinox?

3. One year is 365 days long... but not exactly. The length of an astronomical year changes over time because of a number of scientific factors including gravity. This means the equinoxes occur at different times — or even different days — from year to year.

Print out an equinox worksheet (PDF) and use this chart to fill in the dates and times for the year. Include the summer and winter solstice dates, too. Now you know when all four seasons will begin.

Once you've labeled your worksheet, you can check your answers here.

Do Eggs Really Balance on End?

4. Have you ever heard that on the equinox you can balance an egg upright? Many people believe that it's true. Some scientific-minded students in Ecuador decided to test this theory. See their results. After reading about their experiences, do you think the equinox-egg connection is fact or myth?

What About the Southern Hemisphere?

5. The Earth is divided into northern and southern hemispheres by the equator: an imaginary line, like a belt, all the way around the middle. The southern hemisphere is the half of the Earth south, or below, the equator.

On the first day of spring in the northern hemisphere, it's the first day of fall in the southern hemisphere. Can you imagine why this is? Explain what you think in one or two sentences.

Now, watch this short video about the Earth's rotation around the sun and complete the three interactive challenges at the end. (This will take you about five minutes.)

Now that you've completed this Web Hunt, you should have a better understanding of the equinox. You'll enjoy spring even more by understanding the science behind it!