ABOUT TIME / EQUINOX, SOLSTICE EXPLAINED

Equinox and Solstice are such cool words but when I was asked to explain the difference to a grandson recently I had to evoke my usual change-the-subject ruse by saying “how about those Padres?  You think they’ll win the pennant this year?”

Checking for more expert definitions, I discovered an outstanding website called www.timeanddate.com, which kindly explained most of the following.

First lesson learned is realizing the Vernal Equinox (first day of spring) doesn’t always occur on the same day.  For example, this year’s Spring Equinox occurred on last Saturday, March 19 at 9:30 pm Pacific Daylight Time.

But let’s return to the gnawing question what’s an equinox?  Timeanddate.com explains the March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens on March 19, 20 or 21 every year.

Equator passes through downtown Nyahururu, Kenya

Quito, Ecuador looking West

Equinoxes and solstices are opposite on either side of the equator, and the March equinox is also known as the "spring (vernal) equinox" in the Northern Hemisphere and as the "autumnal (fall) equinox" in the Southern Hemisphere.

Why is it called “Equinox”?

On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an “equinox”, derived from Latin, meaning "equal night". However, in reality equinoxes don't have exactly 12 hours of daylight.

Why is it called a “Solstice”?

The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; that is, the seasonal movement of the Sun's path (as seen from Earth) comes to a figurative stop before reversing direction.

What Happens on the Equinox?

The Earth's axis is always tilted at an angle of about 23.4° in relation to the ecliptic, the imaginary plane created by the Earth's path around the Sun. On any other day of the year, either the Southern Hemisphere or the Northern Hemisphere tilts a little towards the Sun. But on the two equinoxes, the tilt of the Earth's axis is perpendicular to the Sun's rays, like the illustration shows at the beginning of this blog.

More on the Vernal Equinox:

--In San Diego we’re in the aura of the Spring Equinox but in Perth, Australia, the downunders are celebrating their autumnal Equinox.

--March is the first Equinox of the year.  The second equinox, the September Equinox, takes place on or around September 22 every year. It's the Southern Hemisphere's Spring Equinox and is called the Autumnal (Fall Equinox) in the Northern Hemisphere.

--Equinoxes are just bookmarks. In the Northern Hemisphere, astronomers and scientists use the March Equinox as the start of spring, which ends on the June Solstice, when astronomical summer begins. For meteorologists, on the other hand, spring in the Northern Hemisphere begins three weeks before the March Equinox on March 1 and ends on May 31.

--An Equinox or a Soltice are only a moment in time. While cultures around the world celebrate the whole day as the March Equinox, the equinox in reality occurs at a specific moment in time. It is the exact moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s Equator – from south to north. At this moment, the Earth's axis is neither tilted away from nor towards the Sun.

--Don’t hold your breath for the next Spring Equinox on March 21.  Because as we learned today, dates and times of the Equinox can vary.  The last time the Equinox occurred on March 21 was in 2007.  The next time it will happen is in 2101!

Contrary to popular wisdom, the March Equinox can take place on March 19, 20 or 21. In the 21st century, the March Equinox has only occurred twice on March 21 – 2003 and 2007. A March 19 equinox will be more frequent during the last decades of the century.

Note: These dates are based on the time of the equinox in UTC. Due to time zone differences, locations ahead of UTC may celebrate the March Equinox a day later and locations behind UTC may celebrate it a day earlier.  Note: Greenwich Mean Time (GMT) is often interchanged or confused with Coordinated Universal Time (UTC). But GMT is a time zone and UTC is a time standard.

--Conventional wisdom suggests that on the equinox everybody on Earth gets to experience a day and night of equal lengths – 12 hours of daylight and 12 hours of night time. In fact, the name equinox is derived from the Latin words aequus, meaning equal and nox, meaning night. But not quite. In reality though, most places on Earth get to see more daylight than night time on the equinoxes. This is because of two reasons: how sunrise and sunset is defined and atmospheric refraction of sunlight.

--What about that myth that the Spring Equinox is the only day you can balance an egg on its end? The truth however is that there is nothing magical about the equinox or the time it occurs – you can balance an egg perfectly on its end on all days of the year.  The real trick is to balance a checkbook, but that’s a topic for another blog.