The mercury's been rising these past few months across the United States as the region heads into summer, which officially starts on June 21 with the summer solstice. The reason for the season, some might think (and not illogically so), is that according to a natural cycle, the Earth has temporarily shifted closer to the sun and is sweltering in the extra heat.
But this explanation is bogus. In fact, in its oval-shaped orbit around the sun, the Earth will be at its annually farthest point from its star, a position called aphelion, on July 5. Perihelion, the closest point, happened back on Jan. 2. Instead of corresponding to the coldest and warmest days of the year for Americans, it's nearly the opposite. So what gives?
Rather than distance from the sun, the true driver of the seasons on our planet is the tilt of the Earth's axis. Essentially, the Earth does not sit up perfectly straight as it spins; the planet has a slight "lean" as it rotates relative to the plane of its orbit. (Infographic: What Causes Earth's Seasons?)
Over the course of a year, this axial tilt causes the planet's northern and southern halves, or hemispheres, to bask in different levels of sunlight, which in turn sends temperatures spiking or plummeting.
"As we orbit around the sun, the tilt of the Earth relative to that orbit ends up leading to dramatic seasonal changes in the amount of energy we receive from the sun," said Dan Barrie, program manager at the National Oceanic and Atmospheric Administration's Climate Program Office. "As a result, the temperatures we end up experiencing are radically different."
Off-kilter on its axis
Scientists think the Earth got an initial axial tilt from collisions with myriad objects pinballing around during the early solar system's formation 4.5 billion years ago.
Many factors, such as gravitational interactions with the moon, sun and planets, continue to alter the Earth's tilt by a few degrees on long-term scales of tens of thousands of years. The planet's current axial tilt — that is, the amount it deviates from a perpendicular, zero degrees relative to the plane of its orbit — is 23.44 degrees, and on the decline.
To visualize the Earth's axial tilt, think of the globe as a toy top that wobbles as it whirls on a surface. "As the Earth rotates around the sun, it's not an upright top — it spins around a little bit," said Barrie.
A sunrise on a hot summer day.Cjwhitewine | Dreamstime.com
Equilibrium, for a moment
As the Earth spins around, there are two dates every year on which each hemisphere receives the same amount of solar illumination: the vernal equinox in spring and the autumnal equinox in fall. Days and nights are nearly equally long — about 12 hours — on those dates, depending on one's latitude.
After the vernal equinox, the Northern Hemisphere starts tilting toward the sun, giving sun rays both a more direct path to the ground and a longer time to shine as the days grow longer. The ground, surface waters and atmosphere, along with everything in between, all heat up thanks to this bonus solar energy.
Come summer-solstice time, the Northern Hemisphere actually starts tipping away from the sun. But enough hemispheric sunbathing and warming-up of the oceans has already occurred and will continue on through the season to keep things toasty. (Album: Stunning Photos of the Summer Solstice)
Hot "here," cold "there"
Given the Earth's axial tilt, summer — and all the seasons for that matter —obviously don't grace the whole planet at the same time. Right now, as the Northern Hemisphere increasingly feels summer's warmth, the Southern Hemisphere is heading deeper into its winter.
"Summer is a season. It's really just nomenclature assigned to a particular part of the year, and it's also relative to which side of the equator you live on," said Barrie. "The Northern and Southern Hemispheres both experience summer, but at opposite ends of the year."
Because the Southern Hemisphere's summer season coincides with the planet's closest swing by the sun, in January, the hemisphere does receive more solar energy than the Northern Hemisphere during its respective summer. However, about 80 percent of the Southern Hemisphere is ocean instead of land. Water takes much longer to heat up than continental landmass, thus tempering the hemispheric solar energy imbalance, Barrie noted.
This same water-land hemispheric dichotomy also probably explains a bit of global bias when it comes to personal ideas about seasonality. About 90 percent of the world's population lives north of the equator, making it easy to forget that for 10 percent of humanity, summertime is six months away. "It's sort of our bias, a Northern Hemisphere-centric perspective," said Barrie.
Regardless of where one lives, fortunately, there are any number of great ways to beat the heat, from air conditioning to ice cream to hitting the beach — all future subjects of the Science of Summer series.
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