Starry Night
The seasons are caused by Earth's tilt. In the Northern Hemisphere it is summer when the Northern Hemisphere is tilted toward the sun.
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updated 6/20/2011 6:24:19 PM ET 2011-06-20T22:24:19

The summer solstice shouldn't come as a surprise. It arrives at pretty much the same time every year. But some of the little-known facts behind and surrounding the solstice are fascinating. First, the basics:

Summer in the Northern Hemisphere will officially arrive on Tuesday at 1:16 p.m. EDT: the June solstice.  At the same time, winter officially begins for the Southern Hemisphere.

At that moment, the sun will reach the point where it is farthest north of the celestial equator. To be more precise, when the summer solstice occurs, the sun will appear to be shining directly overhead at a point on the Tropic of Cancer (latitude 23.5 degrees north) in the Great Bahama Bank, roughly halfway between Andros Island and central Cuba.

Extreme daylight
From no point in the contiguous 48 United States can the sun appear directly overhead.  From New York, for instance, at 12:57 p.m. Eastern Time, the sun will attain its highest point in the sky for this entire year, standing 73 degrees above the southern horizon or about four-fifths of the way up from the horizon to the point directly overhead.

And since the sun will appear to describe such a high arc across the sky, the duration of daylight is now at its most extreme.  In fact, north of the Arctic Circle, which encompasses northern Alaska, far-northern Canada and much of Greenland as well as the northernmost parts of Norway, Sweden and Finland, the sun now remains above the horizon for an entire 24-hour day, leading to the effect known as the "midnight sun."

However, contrary to popular belief, the earliest sunrise and latest sunset do not coincide with the summer solstice.  For mid-northern latitudes, the earliest sunrise actually occurred on June 14, while the latest sunset is not due until June 27.

Hotter weather
If the insolation — the total energy received from the sun — alone governed the temperature, we should be experiencing the year’s hottest weather right now. 

But the atmosphere in temperate regions continues to receive more heat than it gives up to space, a situation that lasts a month or more, depending on the latitude. Though it depends on the local climate, most locations see the hottest part of the year occurring in late July.  A reverse process occurs after the winter solstice in December; most places see their coldest weather in late January. 

The solar heating depends directly on the sun's altitude in the sky, which also controls its daily path and the number of hours the sun is above the horizon. As an example, although on April 12 the insolation is the same as on Aug. 31, thanks to the seasonal temperature lag, the northern and central United States can still experience a freeze at the former date, or endure a 90-degree heat wave at the latter. 

Armistice
Just as the word "armistice" is defined as a staying of the action of arms, "solstice" is a staying of the sun's apparent motion over the latitudes of the Earth.

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At the summer solstice, the sun stops its northward motion and begins heading south.  At the winter solstice, it turns north.

So technically on Tuesday, even at 17:17 UT, the sun will have turned around and started on its six-month journey south.  It will cross the equator at the autumnal equinox, passing into the Southern Hemisphere on Sept. 23, at 9:05 UT.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y.

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Explainer: 10 slices of summer science

  • Scott Barbour  /  Getty Images

    At sunrise on the summer solstice, druids, pagans and partygoers converge on a circle of stones in the English countryside to welcome the longest day of the year. The ritual at Stonehenge likely dates back thousands of years: Archaeologists estimate wooden posts and timbers were in place as early as 3100 B.C., although the monument's original purpose remains a mystery. One theory holds that it was a cemetery; another that it was a place of healing. Most archaeologists believe the large sarsen stones in the post-and-lintel formations seen in the background of this image served to mark the seasons, making Stonehenge an astronomical observatory as well as a religious center of one sort or another. Click the "Next" label for nine more slices of summer science.

    — By John Roach, msnbc.com contributor

  • Catch a 'shooting star'

    Image: meteor shower
    Ethan Miller  /  Getty Images stock
    Meteor above Lake Mead National Recreation Area in Arizona.

    On some summer nights, the sky sizzles with shooting stars — which are actually space particles, most commonly shed by comets, that vaporize when they hit Earth's atmosphere. The season's most famous meteor shower is the Perseids. The show peaks in the second week of August, with dozens of meteors per hour streaking across the sky.

    Other notable summer showers include the southern Delta Aquarids in late July, followed a few days later by the Alpha Capricornids.

  • The science of salmon and dams

    Image: Salmon
    Don Ryan  /  AP file

    Every spring and summer, salmon attempt to navigate human-enhanced obstacle courses on their journeys from the oceans to the streams where they were born. If they make it back, most fish will spawn there and die.

    Every year, swarms of scientists, conservationists and engineers document every move the fish make to understand why most salmon runs are in decline. Many people point their fingers at hydroelectric and irrigation dams, which can restrict fish passage. But fish ladders, such as the one shown here at the Bonneville Dam on the Columbia River, help the fish navigate the obstacles.

    A recent study indicated that such measures to assist the fish appear to be working. Other factors to consider as the cause for the salmon decline include agricultural pesticides and natural weather cycles such as El Nino and La Nina.

  • The season of shark attacks

    Image: Shark swimming near beach
    Jason C. Miller  /  AP
    A bull shark swims off a Florida beach

    Shark attacks tick up in the summer. Scientists suspect the main reason for that is simply that more people swim when the weather is warm. Experts say you are much more likely to die in an auto accident on the way to the beach than in a shark attack after you arrive.

    Nevertheless, here is their advice to reduce the risk of a shark encounter:

    • Swim in groups.
    • Shed reflective clothing.
    • Refrain from dips when bleeding or menstruating.
    • Avoid areas where sharks have recently been seen.

    If bitten, you should fight back with swats to the shark's head, eyes and snout, scientists say. Sharks will likely spit you out once they realize you're not a very nutritious meal, but they may not do so until you've been dragged underwater and are bleeding profusely.

  • A dead zone grows in the Gulf of Mexico

    Image: effluents deposited at the Mississippi River delta
    NASA
    Soil and fertilizer runoff in waters around the Mississippi River Delta.

    Each spring, more than 200 million pounds of nitrogen-rich fertilizer from cropland in the Mississippi River Basin wash into the Gulf of Mexico. There, microscopic plants gorge on the excess nutrients, creating enormous blooms of algae. The algae die and sink to the ocean bottom, robbing the surrounding waters of oxygen. Devoid of oxygen, most life can no longer survive.

    The affected region is known as a dead zone. The one in the Gulf of Mexico is about the size of New Jersey, and scientists fear that it could grow even larger as farmers plant more corn for ethanol fuel processing. Corn responds well to nitrogen fertilizer, so farmers apply a lot of it, according to researchers.

  • Hurricane forecasting: An improving science

    NOAA/AP
    This infrared satellite image from 2004 shows Hurricane Francis over Florida as Hurricane Ivan lurks in the lower right.

    When a hurricane forms over the Atlantic, eyes quickly turn to the forecasters at the National Hurricane Center to learn where the storm will go and how strong the winds will blow.

    Although the forecasts are almost always laden with uncertainty, state-of-the-art computer models filled with real-time data on wind, precipitation, pressure and temperature — combined with more observational experience — have enabled forecasters to narrow the cone of uncertainty in hurricane tracks over the past 50 years. Improvements in intensity forecasts, however, are lagging.

  • The science of summer droughts

    Laura Rauch  /  AP file

    While some people welcome long, sunny summer days, too many gorgeous days in a row can dry out the land enough to trigger a drought — a prolonged shortage of the rainfall that humans need to grow crops and brush their teeth, among other activities.

    Natural records such as tree rings and lakebed sediments indicate that drought is part of the natural climate cycle, affecting parts of North America a few times per century. But what exactly governs drought? Some research indicates a link between the El Nino and La Nina weather phenomena, which periodically warm and cool the tropical Pacific Ocean, affecting weather patterns around the world.

    Other research suggests that drought could become more common in response to global climate change. That might parch people dependent on water from the already-shrinking Lake Mead in Nevada, shown here.

  • It's wildfire season, too

    Image: Firefighter
    Mark Reis  /  AFP-Getty Images file

    A fourfold jump in the average annual number of wildfires since the mid-1980s is linked to a trend toward earlier springs and drier summers, scientists concluded in a study. The earlier springs mean snow melts off the mountains sooner, leaving them high and dry. As the dog days of summer linger, all it takes is a spark from a passing lightning storm to send the forest up in flames.

    Another potential factor behind the recent flare-up is a decades-long policy of fighting fires as soon as they start, leading to increasingly fuel-loaded forests.

  • Scientists eye the Arctic ice

    Peter West  /  AFP - Getty Images

    Much of the concern about global climate change stems from what scientists see occurring in the Arctic: a meltdown of historic proportions.

    To gauge just how much ice is melting, scientists keep close tabs on the extent and thickness of the Arctic ice cap, a huge but shrinking expanse of ice that covers the Arctic Ocean. Each summer, some of the ice melts and the surrounding waters heat up, melting more ice. In the winter, new ice forms, but most years the extent and thickness of the ice cover is less than the year before.

    At the rate the meltdown is occurring, some scientists predict the Arctic will be ice-free in the summer as early as 2040.

  • Museums offer lasting slice of science

    Mike Derer  /  AP
    Kids touch a large plastic replica of lips and a nose at the Infection Connection exhibit at the Liberty Science Center in Jersey City, N.J.

    For parents, summer brings the annual question: What shall we do with the kids? One option espoused by science educators is to take them to a science museum, aquarium or zoo where outside-the-classroom, hands-on learning might take hold and nurture a new generation of scientists and engineers. At the very least, the experience might help stave off what education experts refer to as summer brain drain while kids laze away the hazy days of summer.

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