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National Weather Service
We experience seasons because Earth rotates on an axis that's tilted in its orbit. The 23.5-degree tilt causes different hemispheres to be at different angles to the sun at different times of year. During winter, the energy from the sun must travel through more atmosphere to reach the poles. Also, a given amount of the sun's energy is spread over a larger area.The seasonal turning points are listed for the Northern Hemisphere.
The seasons are a powerful force in our lives. They affect the activities we do, the foods we crave, the clothes we wear — and quite often, the moods we are in. The seasons officially changed once again on Wednesday, with spring beginning in the Northern Hemisphere and autumn starting in the south.
What is it that causes the change?
The ability to predict the seasons — by tracking the rising and setting points of the sun throughout the year — was key to human survival in ancient times. The Babylonians, the Maya and other cultures developed complex systems for monitoring seasonal shifts. But it took centuries more to unravel the science behind the seasons.
Nicolaus Copernicus (1473-1543) radically changed our understanding of astronomy when he proposed that the sun, not Earth, was the center of the solar system. This led to our modern understanding of the relationship between the sun and Earth.
We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of spring and summer, fall and winter.
Solstices and equinoxes
The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt.
The solstices mark the points at which the poles are tilted at their maximum toward or away from the sun. This is when the difference between the daylight hours and the nighttime hours is most acute. The solstices occur each year on June 20 or 21 and Dec. 21 or 22, and represent the official start of the summer and winter seasons.
NASA / EUMETSAT
These views of Earth from the Spinning Enhanced Visible and Infrared Imager on EUMETSAT's Meteosat-9 satellite shows the terminator line on Dec. 21, 2010 (upper left), March 20, 2011 (upper right), June 21, 2011 (lower left) and Sept. 20, 2010 (lower right). The sequence illustrates how Earth's northern half receives more sunlight in June, while the southern half gets more light in December.
The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun is directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet.
On March 20 or 21 of each year, the Northern Hemisphere reaches the vernal equinox and enjoys the signs of spring. At the same time, the winds turn colder in the Southern Hemisphere as the autumnal equinox sets in. The year's other equinox occurs on Sept. 22 or 23, when summer fades to fall in the north, and winter’s chill starts giving way to spring in the south.
From year to year, there is always some variability in the equinoxes and solstices because of the way Earth's changing tilt matches up with its orbit around the sun. This year, the precise moment of the March equinox came at 7:02 a.m. ET Wednesday. That's the moment when the sun is exactly overhead, as seen from the point on Earth's equator directly facing the sun at that time.
Wednesday is thus the day that comes the closest to offering equal amounts of sunlight and darkness all over the globe — at least until the September equinox.
Effect on climate
Here’s how the seasonal change affects the weather: Around the time of the June solstice, the North Pole is tilted toward the sun and the Northern Hemisphere is starting to enjoy summer. The density of the solar radiation is higher because it's coming from directly overhead — in other words, the sun's rays are concentrated over a smaller surface area. The days are longer, too, meaning that more radiation is absorbed in northern climes during the 24-hour cycle. Another factor is that radiation takes a shorter path through the energy-absorbing atmosphere before striking the earth.
At the same time that the Northern Hemisphere is entering summer, the South Pole is tilted away from the sun, and the Southern Hemisphere is starting to feel the cold of winter. The sun’s glancing rays are spread over a greater surface area and must travel through more of the atmosphere before reaching the earth. There are also fewer hours of daylight in a 24-hour period.
The situations are reversed in December, when it’s the Southern Hemisphere that basks in the most direct rays of the sun, while the Northern Hemisphere receives less dense solar radiation for shorter periods of time.
Although the solstices represent the pinnacles of summer and winter with respect to the intensity of the sun’s rays, they do not usually represent the year's warmest or coldest days. This is because temperature depends not only on the amount of heat the atmosphere receives from the sun, but also on the amount of heat it loses due to the absorption of this heat by the ground and ocean.
It is not until the ground and oceans absorb enough heat to reach equilibrium with the temperature of the atmosphere that we feel the coldest days of winter or hottest days of summer.
More about spring:
- Gallery: 10 spring flings with science
- Spring begins a day earlier ... kind of
- Even for sperm, there is a season
This is an updated version of an article originally published in March 1999. An earlier, not-quite-completely updated version included a reference to December rather than March.
This story was originally published on Tue Mar 19, 2013 10:27 PM EDT
Leave a Comment:
Actually, the coldest quarter of the year ended about 9-10 days ago. A sine-regression of mid-latitude temperatures shows pretty consistent sunlight - temperature phase shift. Imagining a precise pi/4 phase shift is conflating the precision of astronomy with the reality of thermodynamics. The Old English word for winter solstice was midwinter, not the first day of winter.
Thanks for adding this. Articles often leave this important logical step out of the explanation. Earth just happens to have a sunlight-temperature phase shift that is fairly close to pi/4, and that is why it is even possible to use our vernal equinox as a reasonable definition for the start of spring.
"This year, the precise moment of the March equinox comes at 7:02 a.m. ET Dec. 21."
That is truly amazing!
What's amazing is that all of this can be found in the Holy Q'uran.
This year, the precise moment of the March equinox comes at 7:02 a.m. ET Dec. 21.
Doesn't anyone there proofread anymore?
They forgot to convert from the metric calendar.
Definitely a big "blooper!" According to heavens-above.com (a very useful website for this kind of thing), the spring equinox happens at 7:01 a.m. EDT on March 20 (this year). You can login, set your location, and get data for any time zone you want.
Yup, I blew it with that update. I was so focused on the 7:02 part that I missed changing December over to March. As the italic note at the end suggests, this story is just a basic explainer on the seasonal change that is updated for each turn of the season. We do the same with the time change. If there's anything new to report, we usually handle that as a separate story. I just hustled this version up and missed that Dec. 21 reference. Sorry about that, folks.
What's amazing to me is that so few people understand what a solstice or equinox is, much less how these happen (physically).
I know educated people who think the Earth's rotational axis precesses with a period equal to that of the orbit, and that this is what causes the seasons. They remain convinced.
True. The Earth's axis does precess, but with a much, much longer period.
It's going to be a long winter if the March equinox comes in December. In which month does the September equinox fall this year?
@yourliver -
Thats a good question. I broke out my old slide ruler to determine that this years September equinox will fall in April. I know, its a little unusual, but if you integrate the sinusoidal frequency of the Earth's spheroid maxima at the equator during the height of March Madness, one would easily observe that due to the innate spiciness of the US Standard for Buffalo wings divided by the cubed root of the Earth's magnetic poles in a hypothetical fifth dimension, you would have to conclude that this years September equinox must fall in April. Now, a question that has been vexing me for years, and hopefully someone could help me out here, is this: What was the color of George Washington's favorite brown horse?
rock jock
Excellent! Too bad the author does not check for accuracy.
Uuuhhhh, A horse of a different color. (from original Wizard of Oz)
Good one -- jack from Jax!
I had forgotten about that one....
and after crossing that large field of poppies, I'm sure that the horse was many colors.....
.
Since Venus is behind the Sun, Jupiter is Dominate pressing Saturn, the Earth is outside the Triangle free from the vortex between these two giants. Venus hits three minutes behind schedule she being behind the Sun bouncing her light off Jupiter at 9:30 am CST rather than 9:27 am CST.
Venus echos in the hour of Jupiter at 8:32 pm CST
Morel mushrooms always burst on Wed. the day of Mercury, in the second hour of the Sun. Happy hunting.
But they are most likely to burst around 9:00 am rather than later because Mercury is retrograde and Venus dominate. But then Venus is behind the Sun giving power to Jupiter in the Hour of Venus around 9:00 am CST.
@igby - Is that really true? About the morel mushrooms? Or is it an ole wives tale? I've heard that before, but I can't find much about it on the net.... Or did you just prank gullible ole' me?
Pootly written article; with articles like this, it is no wonder astronomy is so misunderstood. For example, "The ability to predict the seasons... was KEY to survival in ancient times..."; I wonder how all those other creatures that roamed the earth survived without being able to predict the seasons? At the equinoxes "the sun APPEARS to be directly overhead"; it seems to me that the sun IS directly overhead at a PRECISE moment. And, "Another factor that MAY come into play is that radiation takes a SOMEWHAT shorter path through the atmosphere..."; it does take a shorter path and the less atmosphere, the less energy absorption. I know I am being picky, but for good reason. For years I carried the misconception that the earth rotated and wobbled "like a top"... CAUSING the seasons. You know, like it passed through one wobble PER YEAR causing the seasons. The teacher forgot to tell me a complete wobble was every 25,000, or so, years; or I didn't hear it or understand it.
These are good points ... I'll make these amendations and carry them over to September, when I hope everything will be perfect at last.
Here again, mainstream media is giving credit to scientists for 'deciding when spring is', when the earth actually makes up the rules and does all the changing. Scientists only notice it and quantify it mathematically to make themselves appear more intelligent and in charge. It is going to happen whether scientists decide when it happens or not.
I think the point here is how the start of each season is defined. It has more to do with the human definition of the start of the season, in astronomical terms, rather than with the natural rhythm.
The seasons appear to lag the sun by a considerable extent.
Uh....our ancestors had NO problem knowing when Spring or any OTHER season had come. Seasons aren't just tied to "science", they're also tied to traditions, memory, and the heart.
And those traditions, memory and "the heart" as you call it were all formed by science, whether they knew to call it that or not.
FORMED by science? Meaning what? We look at natural phenomena today through the lens of science. Our ancestors did not. You can rightly say that the same scientific principles were at work then, as they are now, that what I called "traditions, memory, and heart" were shaped by scientifically provable natural occurrences, but you can't say that "science" formed their understandings of the world around them, since they didn't see or understand the scientific basis for what they experienced. Seasonal changes were, for our ancestors, tied to village celebrations, planting, harvest, myths, songs, and customs. They knew when the seasons changed. I swear that some science-minded individuals can be as joyless and rigid as some religious fundamentalists are. The vast range of things that make us fully human don't matter to either group if they don't square with scientific or Biblical "truth".
Thanks, Hans. You said what I was trying to say much more articulately than I did. Instead of "formed" I should have said "shaped" or "influenced."
I was about to write a very similar rant. Getting the dates for the Equinox and Solstice mixed up was just sloppy writing with obviously no editors or fact checkers to catch that glaring blooper.
Scientists are able to now determine the exact point in time when the earth/sun positions are right at the tipping point, but the Solstices and Equinoxes have been predicted accurately to the day for thousands of years which was more than close enough for their religious and agricultural needs.
The sun directly overhead? The position of the sun is pretty basic to the change in the seasons. Being directly overhead is referring to the middle of the arc though and not the center of the sky as some might infer from that. The minimum and maximum angles occur at the Solstices with the midpoint at Equinoxes. I guess they also forgot about it being Daylight Saving Time, though, which technically puts the sun directly overhead off an hour too early by the DST clock.
Temperature always lags with seasonal changes as we have a huge regulator of heat called the oceans along with other atmospheric conditions that don't react instantaneously to the changes. The longer days of sunlight do eventually win out in more heat being absorbed with the opposite effect in winter. The tilt does cause areas to get more direct sunlight and the Earth's orbit is elliptical (but just barely), but the main factor driving the seasons is that more of the northern (or southern) hemisphere is getting more sunlight for more hours per day.
I think the "directly overhead" refers to the position with relation to a point on Earth's equator. Wouldn't that be truly in the center of the sky? As always, feel free to correct me if I'm wrong. You're right that the maximum angle would apply to the solstices. As for daylight saving time, I'm not sure whether one can really say whether the sun is "too early." The time is just the time. But in any case, thanks for shedding more light on the subject, so to speak.
"Directly overhead" (or at the zenith) at the equinox does indeed happen only at the equator, at the longitude where it's local noon when the sun's declination is zero degrees crossing from south to north (or the opposite in the fall). For other locations at or near the equator, the sun will still be very close to overhead at a different time. Outside of the tropics, the sun is never overhead. Instead, the maximum angle above the horizon (at the solstice) would be 90 degrees - Latitude + 23.5 deg (axis tilt).
You tell a lot about a species by whether they use science or ground hogs to determine the start of spring.
When the groundhogs start coming out again so's I can eat 'em, it's spring!
Thanks to our Creator, everything so perfectly aligned.
How could anyone believe that this resulted from anything other?
Not knocking the Creator, but... perfectly aligned? With what? The Earth travels in a slightly elliptical orbit, it's tilted at an angle, its tilt wobbles over time, the solstices and equinoxes are not always on the same day, the week (which it took God to create everything, supposedly) doesn't match either the lunar month or the calendar year, the year isn't exactly an even number of days, nor does the lunar month equal an exact fraction of the calendar year. It sure isn't perfect, and it isn't "aligned" with anything that I can see!
Too bad the creator doesn't come down here and straighten out the thousands of whackos that do harm in his name.
Only it's not. I'm sure that early man took the fact that there were 4 different, repeating changes in weather and created the seasons around that, not the other way around. That's like saying God gave us ten fingers so that we can use the base ten number system, without bothering to think that maybe we developed the base ten number system because we have ten fingers.