The Equinox: A Unit Study

The equinox is the time of year when the sun is in the same plane as the earth’s equator. Named for the Latin aequus (equal) and nox (night), during an equinox day and night are roughly the same length — although they actually never are exactly the same length. There are two times when this occurs in a year — the spring equinox on or around the 20th of March, and the fall equinox on or around the 22nd of September.

When Does an Equinox Occur?

An equinox occurs when the earth is such that the sun covers both the northern and southern hemispheres evenly, causing spring in the one and fall in the other. Why does the sun cover both hemispheres evenly only twice a year? The earth is tilted 23.26 degrees on its axis. This tilt is always in the same place regardless of where the earth is relative to the sun. As an example of what this means, take an orange, tilt it, and orbit it around a lamp while keeping the tilt always pointing in the same direction regardless of where the orange is in relation to the lamp. When the earth orbits the sun, the quantity of both the northern and southern hemispheres that is being illuminated changes. The equinox will occur when the sun has reached the same plane as the earth’s equator, as the equator divides the earth in two, causing both hemispheres to be illuminated evenly.

The Dates of the Equinoxes

The date for the spring equinox was originally set at March 25th by Julius Caesar when creating his calendar. However, Julius Caesar’s calendar drifted, as it was too long relative to the true length of the year. The result was a drift so large that in the 1500s (according to the Julian calendar) the true equinox was occurring around March 11. In the 1500s Pope Gregory the XIII, created the Gregorian calendar to remedy the problem. To accomplish the elimination of the drift, he removed three days every four centuries by decreeing that leap years on any year ending with two zeros would be canceled unless the first two digits alone were divisible by four. In this new calendar, Pope Gregory the XIII set the date of the spring equinox to March 21st.

The true spring equinox doesn’t necessarily arrive on the “first day of spring,” however. The date of the first day of spring varies from calendar to calendar in any given year — some using March 21st, the day Pope Gregory the XIII used, others using March 20th, or even March 19th depending on the day when the true equinox falls. This shift in dates is due to the fact that the equinox only happens when the sun crosses the celestial equator, the imaginary line above the earth’s equator dividing the sky in two. Because the calendar is not exact, and has corrective leap years every four years except on century years, the equinox falls earlier and earlier. While every leap year helps the equinox to catch up with March 21st, it takes a non-leap-year century year to correct the dates.

Suggestions

Do the orange and lamp experiment explained above.
Have your child tell you why we have seasons.
Have your child tell you everything he can about equinoxes.

Further Investigation

The Seasons, the Equinox, and the Solstice
Brief explanations from NOAA.

The Sun in the sky during the Spring and Fall Equinox
Brief explanation with helpful illustration.

The Equinox
An explanation in more story-like form from PBS.

More About Rotation
More complicated explanations with diagrams. Follow the “Show Me More” link at the bottom to follow the discussion through equinoxes and to the end of More About Rotation IV.

What is an Equinox
Video explanation. (You may want to install an ad blocker before viewing.)

Activities

Where Is the Sun?
Activity that helps you plot the sun’s movements. “On March 21, the Vernal (spring) Equinox, and September 21, the Autumnal (fall) Equinox, you will find the Sun in exactly the same position in the sky.”

Why Do We Have Seasons?
Interactive from PBS.

Making a Shadow Plot
Scroll down for activity.

Unit Studies & Lesson Plans

A Time for All Seasons
Core Knowledge lesson plan that covers the four seasons by exploring the rotation of the earth and its revolution around the sun.

The Reasons for the Seasons
Lesson plan from Miami University in Ohio exploring shadows, sunrise and sunset, and the earth’s tilt and revolution.

As the Calendar Turns
Cute lesson plan from Crayola where students create a changeable calendar to keep track of the seasons.

Kinestetic Astronomy
Lesson plan from NASA.