Everything you need to know: Autumn equinox 2013

The March equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It marks that special moment when the sun crosses the celestial equator going from south to north. In 2013, this equinox will arrive on March 20 at 11:02 Universal Time, or 6:02 a.m. Central Daylight Time for us in the central U.S.

For the remainder of the 21st century (2001-2100), the March equinox will fall on March 20 or 19. Yet it won’t first occur on March 19 until the year 2044, and it won’t happen on March 21 until the year 2102. The March equinox last took place on March 21 in 2007.

A Chinese perspective on the spring equinox
Equinox 2013 comes before sunrise March 20 for most of U.S.
2013 equinox: Sun rises due east and sets due west

If you’re curious (or confused) about the various dates of this equinox, you’re not alone. In a nutshell, it’s because the tropical year, as measured between successive March equinoxes, doesn’t have an even number of days (365.242 days). Our calendar, on the other hand, has an even 365 days in a common year and 366 days in a leap year.

The centennial year 2000 is a leap year, bringing about earlier dates for the March equinox in the 21st century (2001-2100). However, the suppression of the leap year in the centennial year 2100 pushes the dates upward again in the 22nd century (2101-2200).

Here’s what we all know about this equinox. In the Northern Hemisphere, the sun is rising earlier now, and nightfall comes later. Plants are sprouting. Winds are softening. For us in the Northern Hemisphere, people are enjoying the warmer days of spring. South of the equator, autumn begins.

What is an equinox?

The equinox is an event that happens in Earth’s orbit around the sun. Simultaneously, it happens on the imaginary dome of our sky. The imaginary celestial equator is a great circle dividing the sky into Northern and Southern Hemispheres. The celestial equator wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator, to enter the sky’s northern hemisphere.

Our ancestors didn’t understand the equinoxes and solstices as events that occur in the course of Earth’s yearly orbit around the sun. But they surely marked today as being midway between the sun’s lowest path across the sky in winter and highest path across the sky in summer.

Today, we know each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless motion in orbit.

Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern hemispheres trade places in receiving the sun’s light and warmth most directly. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun.

At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are approximately equal in length. The word equinox comes from the Latin aequus (equal) and nox (night).
But, since Earth never stops moving around the sun, these days of equal sunlight and night will change quickly.

Where should I look to see signs of the equinox in nature?

The knowledge that spring is here – and summer is coming – is everywhere now, on the northern half of Earth’s globe.

If you live in Earth’s Northern Hemisphere, you’ve likely been noticing the earlier dawns and later sunsets for some weeks now.

Also notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the north. Birds and butterflies are migrating back northward, too, along with the path of the sun.

The longer days bring with them warmer weather. People are leaving their winter coats at home. Trees are budding, and plants are beginning a new cycle of growth. In many places, spring flowers are beginning to bloom.

Does the sun rise due east and set due west at the equinox?

Yes, it does. And that’s true no matter where you live on Earth, because we all see the same sky.
No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.

At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration at right shows. This illustration (which is by Tau’olunga) shows the sun’s location on the celestial equator, every hour, on the day of the equinox.

That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west.

This fact makes the day of an equinox a good day for finding due east and due west from your yard or other favorite site for watching the sky. Just go outside around sunset or sunrise and notice the location of the sun on the horizon with respect to familiar landmarks.

If you do this, you’ll be able to use those landmarks to find those cardinal directions in the weeks and months ahead, long after Earth has moved on in its orbit around the sun, carrying the sunrise and sunset points northward.

So enjoy the 2013 spring equinox on March 20 – an event that happens on our sky’s dome – and a seasonal marker in Earth’s orbit around the sun!

Bottom line: In 2013, the vernal equinox comes on March 20 at 11:02 Universal Time, or 6:02 a.m. Central Daylight Time for us in the central U.S. This post explains the significance of an equinox.

How to translate Universal Time to your time zone

- EarthSky