What is the Vernal Equinox? - Definition & Facts

Instructor: Dominic Corsini
What causes the first day of spring and how do we know when it occurs? This lesson helps answer those questions through an investigation of the vernal equinox. A lesson summary and brief quiz will follow.

What is the Vernal Equinox?

Winter is tough in many parts of the world. It's tough on the wildlife, and it's tough on us. It's also tough on our wallets because of heating costs! But winter doesn't last forever, and eventually the cold, harsh weather gives way to the warmer, sunnier days of spring. What makes this happen? The answer is twofold: the Earth's tilt on its axis, and its revolution around the sun. These two factors combine to pull us out of winter and into spring, which begins on the vernal equinox. The vernal equinox is a point in time when the sun's rays cross directly over the Earth's equator and provides nearly equal periods of day and night in both hemispheres. The word equinox literally means 'equal night'.

How the Vernal Equinox Works

To help you understand how the vernal equinox works, refer to the illustration below.

Tilt and Revolution of the Earth. Seasons depicted for the northern hemisphere are: fall (front), winter (right), spring (back), summer (left).
Tilt and Revolution of the Earth

This image shows the Earth at various points during its orbit around the sun. These four points represent the beginnings of our four seasons (spring, summer, fall, and winter). Remember how we'd previously mentioned that two things cause seasonal change? Those two things are the Earth's revolution and tilt. Revolution is the Earth's orbit around the sun, and tilt is the Earth's slant on its axis. In this illustration, the Earth is shown on its tilt, while revolving around the sun. Let's explore why both of these items are important to the vernal equinox.

We'll start with tilt. Look at the Earth on the far left side of the image. This shows the northern hemisphere (top half) being tilted toward the sun. What that means is the northern hemisphere will receive more direct solar energy than the southern hemisphere (bottom half). Therefore, the north is experiencing summer, while the south is experiencing winter.

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