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 2014, this equinox arrives on March 20 at 16:57
UTC, or 11:57 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.
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. Follow the links below to learn more about the
vernal equinox.
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 2014 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 2014, the vernal equinox comes on March 20 at 16:57
UTC, or 11:57 a.m. Central Daylight Time for us in the central U.S.
Everything you need to know about the equinox, here.
How to translate Universal Time to your time zone
2014 equinox: Sun rises due east and sets due west
A Chinese perspective on the spring equinox
- EarthSky
