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.
More about the changing seasons:
- How we know that spring has sprung
- Spring begins a day earlier, kind of
- Gallery: 10 spring flings with science
Tip o' the Log to LiveScience's Douglas Main.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log and the rest of NBCNews.com's science and space coverage, sign up for the Tech & Science newsletter, delivered to your email in-box. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.