It's the summer solstice! Today the Northern Hemisphere experiences its longest period of daylight of the year, marking the beginning of summer and an extreme point in Earth's seasonality. The photo above, captured from Earth orbit by EUMETSTAT's Meteosat-9, gives us a clue as to why this, but Ron Miller has the full details after the jump.
One of the results of Earth having a tilted axis is, for better or worse, the seasons. On June 21st, we'll be at one extreme point in Earth's seasonality: the summer solstice. Here's what that means.
The axis of the earth is tipped 23.45°. This means that most of the time part of the earth is tipped toward the sun and is getting more sunlight and warmth, and another part is tipped away and is getting less sunlight and warmth. When the northern hemisphere is tilted toward the sun, the days are longer and the sun is high in the sky. The weather is warmer and we have summer. Six months later, the northern hemisphere is tilted away from the sun and the days are shorter and the sun is low in the sky. The temperature grows cold and we have winter.
Every planet with an axial tilt has seasons. Mars has a tilt almost exactly the same as earth's, so its seasons are similar (albeit twice as long). Jupiter, on the other hand, with an axial tilt of just 3.13°, has scarcely any seasons to speak of at all. The weirdest might be Uranus, whose axis is tipped 97.77°.
There are traditional names for the periods when the earth is tipped at its greatest and least angles from the sun. That day in the spring and fall, when the periods of daylight and dark are of equal length, is called the equinox, for fairly obvious reasons. That day in the winter and summer when the difference between daylight and dark is greatest is called the solstice. This is a French word derived from the Latin solstitium, "the point at which the sun seems to stand still," which in turn comes from the Latin word for "sun": sol and sistere: "to come to a stop or make stand still."
It's kind of a bittersweet day, of course, since it's all downhill from here to winter, with every day getting a little shorter.
(Many people think that summer and winter are caused by the changing distance of the earth from the sun—that it is closer in summer and farther away in winter—but they are actually due to the changing tilt of the planet. Actually, the earth is slightly closer to the sun in January and furthest away in July! Seasons in the southern hemisphere are the opposite of those in the northern. When it is winter in the United States it's summer in Australia, and vice versa.)
Art by Ron Miller