Image: Flickr Creative Commons

Image: Flickr Creative Commons

Spring officially began in the northern hemisphere Friday on  22:45 GMT, around 30 seconds later than last year. As it so happens, every year spring gets shorter by 30 seconds to a minute, losing the time to summer which gets longer by the same amount. Whose to blame for the later bloom? Why, that wretched tilted axis of course.

Our planet traces the sun in an elliptical orbit, which means that at a specific moment it’s closest (perihelion) or farthest (aphelion) away from the sun. But don’t get fooled by the Earth’s motion around the sun – seasons are in fact determined completely by the fact that the Earth is tilted on its axis by 23.5°. This is why when it’s summer in the Northern Hemisphere, it’s winter in the Southern Hemisphere, and vice-verse. Without this tilt, there would be no seasons and the weather from day to day across the globe would be relatively uniform. For instance, spring precisely began during the vernal equinox when the Earth’s tilt reaches a halfway mark, pointing neither toward the sun (as it does on the summer solstice) nor away from the sun (as it does on the winter solstice).

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This event isn’t constant in time, however, since the Earth’s axis cyclically wobbles – this movement is called a precession. So each year, because of this wobble the halfway mark slightly shifts. This is why spring gets shorter and summer longer, and conversely autumn loses time to winter. FYI, this year summer is the longest season, with 93.65 days, followed by spring with 92.76 days, autumn with 89.84 days and winter with 88.99 days. Spring will be shortest in about the year 8680, measuring about 88.5 days, or about four days shorter than this year’s spring.

earth tilt

Earth’s Tilt At Present Time (Now)

The weirdness doesn’t stop here. Some 13,000 years from now, the axis will have shifted so that summer in the northern hemisphere will occur on the right side of the Sun where Earth is closer, so summers will be hotter, and winters will be colder (and longer). According to Harvard climate scientist, Peter Huybers, this is an important factor that triggers ice ages.

Earth's Tilt 13,000 years from Now

Earth’s Tilt 13,000 years from Now