The GOES-13 satellite captured this full-disk image of our planet at 7:45 a.m. ET on March 20, just after the 7:02 a.m. ET equinox. The satellite image shows how Earth's two hemispheres receive equal amounts of sunlight during the equinox. In this image, the sun is artificially created to enhance the picture.
Earth's 23.5-degree tilt almost always ensures that the northern and the southern halves of our planet get unequal amounts of solar energy, with longer nights in winter and bigger stretches of sunlight in summer. Twice a year, however, both hemispheres get equal amounts of light, with equal intervals of day and night. That's what's known as the equinox.
Just such an event at 7:02 a.m. ET on Wednesday heralded the official beginning of spring in the Northern Hemisphere, and the start of autumn in the South. This full-disk picture from the GOES-13 weather satellite, captured at 7:45 a.m., shows the equal division between Earth's night and day.
"The visible imagery sensor on GOES requires sunlight to 'see' clouds, and so it provides a useful example of the equinox," the National Oceanic and Atmospheric Administration's Environmental Visualization Laboratory says in Wednesday's advisory. "In this image the GOES imagery extends to each of the poles since the entire hemisphere is equally lit. After the equinox passes today, the Northern Hemisphere will be more lit than the Southern Hemisphere – causing the seasons."
Orbital mechanics may determine the precise moment of the equinox, but scientists say that the effects of the seasonal change can vary widely, due to climatic factors. There's some evidence, for example, that climate change is causing flowers to bloom earlier in the eastern U.S. than they did in the 1850s or the 1930s. Have you noticed changes on shorter time scales? Feel free to spring into action with your comments below.
The sun shines down 42nd Street below the landmark Chrysler Building at sunset on July 12, 2009, during Manhattanhenge.
By Alan Boyle, Science Editor, NBC News
The first night of this year's Manhattanhenge season was a washout, due to cloudy weather, but there's another chance to see the sunset turn the streets of New York aglow tonight.
Manhattanhenge refers to the perfectly placed alignment of the setting sun amid the canyons of midtown Manhattan's east-west streets. The phenomenon, sometimes known as the Manhattan solstice, occurs every year around Memorial Day and major-league baseball's All-Star break.
The Hayden Planetarium's director, Neil DeGrasse Tyson, says future archaeologists might well conclude that these spots on the calendar marked important rites of summer for New Yorkers. (And they wouldn't be far wrong.)
Tyson's the one who came up with the term "Manhattanhenge." Think of it as a modern-day, unintentional version of Stonehenge, with New York skyscrapers standing in for the stones of the 5,000-year-old monument in England.
Stonehenge was constructed to have its stones line up with the rays of the sun on important astronomical dates such as the summer solstice. Manhattan's street grid, however, doesn't line up with the solstice or the equinox. The relevant streets, which reflect the Commissioner's Plan of 1811, are offset 29 degrees from east and west. That would spoil the sunset view on an equinox or a solstice — but on the proper dates, the sun reaches the cleft between skyscrapers just in time to set the streets aglow.
This year's first opportunity for seeing Manhattanhenge's glory came Tuesday night at 8:17 p.m. ET. Under ideal conditions, a pretty half-setting sun could have been seen centered in the gap between the buildings. Unfortunately, conditions were not ideal. In disappointed Twitter tweets, the sight quickly came to be termed "Cloudhenge."
Andrew Dallos via Twitpic
Andrew Dallos' picture of Manhattanhenge, snapped at sunset on Tuesday from 42nd Street, provides a typical view of "Cloudhenge."
"A cloudy and stormy night, so no sun," reported Andrew Dallos, a producer for "The Rachel Maddow Show" on MSNBC who camped out on 42nd Street.
Tonight, on Wednesday night, New Yorkers could get a chance to see the sun's full disk just touching the horizon in the gap at 8:16 p.m. ET. It all depends on the weather: The current forecast calls for partly cloudy skies with a slight chance of thunderstorms — which at least sounds more promising than last night's weather.
Even if tonight's opportunity is clouded out, there'll be a Manhattanhenge replay after the summer solstice, with a full-sun viewing at 8:24 p.m. on July 11 and a half-sun opportunity at 8:25 p.m. July 12.
To enhance your Manhattanhenge viewing experience, Tyson suggests positioning yourself as far east as possible, while still making sure you can see New Jersey when you look west across the avenues. "Clear cross streets include 14th, 23rd, 34th, 42nd, 57th and several streets adjacent to them," he writes in his viewing guide. "The Empire State Building and the Chrysler Building render 34th Street and 42nd Street especially striking vistas."
Thanks in part to Tyson's efforts, Manhattanhenge is the best-known of the modern-day monumental alignments. The clear prospect to the west between New York's towering buildings makes for a nearly unbeatable scene. But other locales have their own "Henge" dates, due to the unintentional effects of a street-grid layout or an architectural feature. Here's a sampling:
Baltimorehenge: The sun lines up with downtown Baltimore's street grid for sunrise on Sept. 18 and March 25, and for sunset on Sept. 29 and March 12. The Baltimore Sun's Frank Roylance explains it all for you.
Phillyhenge: The sunrise moments have come around March 1 and Oct. 11, and sunset alignments are around April 4 and Sept. 5. Precise dates vary from year to year. The Photographer's Ephemeris helps you find the proper lineup.
Torontohenge: The sun lines up with Toronto's street grid for sunrise on April 17-18 and Aug. 23-24, and for sunset on Feb. 15-16 and Oct. 23-24. This entry from Torontopedia helps you figure it out.
Other urban "Henges": If downtown streets line up more precisely with a true east-west axis — as they do in Chicago, Washington and Portland, Maine, for example — the "Henge" moments come around the March 20-21 spring equinox and the Sept. 21-22 autumn equinox.
MIT-Henge in Cambridge, Mass: The rays of the setting sun light up the "Infinite Corridor" at the Massachusetts Institute of Technology in late January and during the second week of November. This video fills in the details.
Watch an 11-second satellite video that tracks Earth's shifting orientation with respect to the sun, through northern autumn, winter, spring, summer and back to autumn. (Credit: NASA / EUMETSAT)
By Alan Boyle, Science Editor, NBC News
In the wake of last week's equinox, the days are shorter than the nights in the Northern Hemisphere, and longer in the south. Every day from now until December will increase the imbalance. How does that happen? It has to do with Earth's changing tilt with respect to the sun, as explained in this tutorial. But sometimes a moving picture can be worth a thousand words.
This 12-second video clip has been assembled from a year's worth of imagery captured by a visible-light and infrared camera on EUMETSAT's Meteosat-9 satellite. Meteosat-9, like other satellites in geosynchronous orbit, has an unchanging view of Earth from a height of about 22,000 miles. Every day, around 6 a.m. local time, the satellite watches the terminator line between night and day move across Africa.
As it spins, Earth is tipped 23.5 degrees on its axis relative to the sun, with the northern point of the axis pointing away from the sun in December and pointing toward the sun in June. That means the Northern Hemisphere is more shadowed in winter and more sunlit in summer. That back-and-forth shift is exactly what you're seeing in the video. At the midpoints between those extremes — for instance, last week's equinox — the terminator line goes straight down the middle of Earth's disk, as seen by Meteosat-9.