Spring officially arrives for everyone, including astronomers on March 20. The word “Equinox” literally means “equal night”. It’s all about the balance of light – not the myth of balancing eggs. On Thursday, both the day and night are the same length. But what’s so special about it? It’s a date that most of us recognize as symbolic of changing seasons. North of Earth’s equator we welcome Spring, while people south of the equator are gearing up for the cooler temperatures of Autumn.
These all too brief, but monumental moments in Earth-time, owe their significance to the slightly more than 23 degree tilt of the Earth’s axis. Because of our planetary angle, we receive the Sun’s rays most directly during the Summer. In the Winter, when we are tilted away from the Sun, the rays pass through the atmosphere at a greater slant, bringing lower temperatures. If the Earth rotated on an axis perpendicular to the plane of the Earth’s orbit around the Sun, there would be no variation in day lengths or temperatures throughout the year, and we would not have seasons. At Equinox, the midway between these two times in Spring and Autumn, the spin axis of the Earth points 90 degrees away from the Sun.
If your head is spinning from all of this, sit and ponder for a moment. Now is a great time to choose a marker and observe what’s happening for yourself. Trying a real science experiment for equinox is much better than the myth of balancing eggs. Just place a stake of some type into the ground (or use a fencepost or signpost) and periodically over the next few weeks measure the length of the shadow when the Sun is at its highest and write down your measurements. It won’t take long before your marker’s shadow length changes and you notice how the Sun’s position changes in the sky, and with it the ecliptic plane.
In the language of astronomy, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The Vernal Equinox is also known as “the first point of Aries” – a the point at which the Sun appears to cross the celestial equator from south to north. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This is what’s known as the precession of the equinoxes – a motion first noted by Hipparchus roughly in 120 B.C. But what causes it?
The precession is caused the gravitational attraction of both the Moon and Sun on the equatorial bulge of the Earth. Imagine the Earth’s axis patterning itself in a cone as it moves, like a spinning top. As a result, the celestial equator, which lies in the plane of the Earth’s equator, moves on the celestial sphere, while the ecliptic, which lies in the plane of the Earth’s orbit around the Sun, is not affected by this motion. The equinoxes, which lie at the intersections of the celestial equator and the ecliptic, now move on the celestial sphere. Much the same, the celestial poles move in circles on the celestial sphere, so that there is a continual change in the star at or near one of these poles.
After a period of about 26,000 years the equinoxes and poles lie once again at nearly the same points on the celestial sphere. Because the gravitational effects of the Sun and Moon aren’t always the same, there is some wobble in the motion of the Earth’s axis called nutation. This wobble causes the celestial poles to move, not in perfect circles, but in a series of S-shaped curves with a period of 18.6 years that was first explained by Isaac Newton in 1687.
Go ahead and balance eggs for fun… But believe in science!
P.S. The Bad Astronomer Phil Plait has a tutorial video on his website, teaching you how to stand an egg on end, any time of the year. Click here to watch it.
Lately I’ve been trying out what it would be like if the equinoxes and solstices were the seasonal mid-points rather than the beginnings. Feb 4 would have been spring, March 20 mid-spring, May 6 summer, June 21 mid-summer, Aug 6 autumn, Sept 21 mid-autumn, Nov 5 winter, Dec 21 mid-winter.
Over the past year it seems to work really well especially for the Canadian winter: it started while it was still nice and ended early. Mind you it takes a while longer for the spring melt to begin but the days are truly starting to get longer in early February and the promise of spring is in the air. Summer really feels like it’s underway in May and is reaching its end in August. The leaves here in Saskatchewan are gone by mid September so Autumn does start early. By the beginning November if the snow hasn’t fallen then something is wrong! By Dec 21 with the long nights it’s a good feeling to know that spring is only 6 weeks away!
Try celebrating the first day of summer on May 5, seven weeks early, and see how it feels.
We’re lucky the moon nutates us so that the Northern hemi faces the sun during apogee & tilts away during perigee, somewhat buffering the temp extremes. We won’t be so lucky in 26K since we lack the heatsink capacity to ameliorate these climate extremes. Southern hemi’s mostly buffering water.
As for spring, fohghetabowdit till at least May here in 40th parallel. West Virgina’s 75% forest, but the canopy does poorer job of buffering winter, than cooling summers. Driving through forest can be 10-20 cooler than interstate or city(heat island) b/c foilage absorbs insolation(lowers albedo) & evapotranspirates.
The earth absorbs & loses heat to depths of soil strata, producing at least a month lag in temps behind light. Therefore, coldest weather occurs in late January & warmest in July-August.
Think about tropics, the sun passes directly overhead now & Sept 21. Luckily the canopy & moisture keeps temps in 80s. Did you know Florida never gets to 100F. But humidity makes the heat index(can’t evaporate sweat) higher than 120F Southwest. Luckily shores produce daily convection cells. Thermals rise during day heading seaward, whence off-shore cooler breeze sweeps horizontally landward. Cell reverses a noches. Here in the Ohio Valley, the warmed air flows up the basin by day & the cooler night air flows downwards w current, just above the river. I live smack in the middle of the Ohio, largest inhabited river island in U.S., forever on flood watch.
The west coast gets pacific moisture, which can form high-albedo snow in Canada. Also, mountains behind you may trap orographic moisture. Remember this, thermocline’s strongest in winter & disperse in summer. It can be hotter in Chicago or Fargo than deep South. Typically we’re hotter than Southern Calif in summer. The cool Aleutian current sees to that. They have wetter, warmer Mediterranean climate winters.
Global warming affects different areas, some will cool. Hotter tropical air evaporates seawater, which should rise & migrate poleward, dropping it’s spent cargo along the way. This increases albedo & permafrost acting as counter to ‘warming’. For now, let’s wait for the movie! Les
[…] La primavera llega oficialmente para todos los habitantes del Hemisferio Norte, incluyendo a los astrónomos, el 20 de marzo [a las 05:48 (TU), tres horas menos en Buenos Aires]. […]
This balancing eggs myth seems to be a North American thing-it’s never mentioned in Australia. Until I started reading US-based urban legends websites I’d never heard of it.
BTW, here we officially commence our seasons at the beginning of the month. Hence, Autumn begins on March 1, Winter on June 1, Spring on September 1 and Summer on December 1.
I agree with Tina. I’d never heard of balancing eggs until reading this article!
I’m glad it sparked some interest! No one is quite sure how the myth got started, but to me it seems like an age-old game of “post office”. Tell a group of people a certain point in time like vernal equinox is associated with eggs and fertility, the balance of day and night, and many more assoicated beliefs, and a hundred years later of passing on the story and you’ll have a garbled fable that only retains two words – balance and eggs.
Does it work? Well, yes! I have a whole dozen of them balanced in my refrigerator right now. Does it work without the box? Don’t tell anyone you were watching, but I tried it when making breakfast this morning. The first three happily rolled over on the countertop, where my dog watched with great glee hoping they’d drop to the floor. The fourth stood up proudly… For about as long as it took for me to find the frying pan.
While I don’t believe the arrival of Vernal Equinox caused the egg to stand (more likely a sticky spot from where I made toast and jam), I do believe that when I apply heat and a bit of melted butter they will turn from a rigid encrusted viscous oval into tasty, warm semi-solid mass!
Enjoy the day…
~Tammy
I don’t think my last comment posted. I apologize if this turns out to be a double post.
Where I live, sunrise and sunset times were identical about two days ago. How could these times be equal on any day other than the equinox?
The times, if I remember correctly, were 722 a.m. and 7:22 pm.