by Scott Christmas

Created on: June 13, 2008

The summer solstice, which occurs every year on June 21st, is commonly known in the northern hemisphere as "the longest day" and results from the angle of the earth in relation to the sun. Anyone who has glanced at a classroom globe knows that the earth does not rotate straight up and down. If it did, we would have no seasons to speak of and the sun would stay up for exactly 12 hours every day. The angle of the earth in its rotation, in fact, is so important that life as we know it could not have evolved were it not for this convenient angle of our planet to its star.

It is important to note that the summer solstice is the "longest day of the year" only in the northern hemisphere. In the southern hemisphere, the summer solstice is actually the shortest day of the year. (This is, perhaps, a fine example of how our own unique spot in the world tends to overshadow our overall understanding of the planet and the people who live on it - but that is a philosophical discussion for a different topic.)

The earth angles at approximately 23.44 degrees, and as it rotates throughout the year, it also "wobbles." Imagine a top spinning on a table. As it begins to slow its spin, it no longer stays perfectly upright, but tilts and begins to wobble. This is precisely what earth is doing in its orbit around the sun.

This, then, is the scientific basis for the summer (and winter) solstice. From mid-March to mid-September, the earth "wobbles" (that is, it tilts) with its northern end toward the sun. Likewise, the southern hemisphere tilts toward the sun during the winter months. On June 21st, the earth reaches its maximum northern tilt. On this day, a person standing on the Tropic of Cancer (a line that runs through cities such as Havana, Aswan, and Hong Kong) would see the sun passing in an arc directly overhead, splitting the sky in two. Farther north, the sun arcs through the southern sky, providing long days and short nights, thanks to the longer angle. Someone living within the Arctic Circle would see the sun low on the horizon for most of the 24-hour period. The same is true on the winter solstice, but in reverse: the sun is directly overhead at the Tropic of Capricorn, to the north as you go farther south, and visible on the horizon all day and night at the south pole.

The "wobble," then, is the reason why the northern hemisphere generally experiences long days during the summer, and short days during the winter. Throughout the spring, the days get longer leading up to the summer solstice, because the earth is slowly wobbling to the north. After the solstice is reached on June 21st, the earth's rotation brings its tilt slowly around to the south, leading to progressively shorter days until the winter solstice exactly six months later on December 21st.

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