The summer solstice may be one of the most misunderstood phenomenon in all astronomy. Many people mistakenly believe that the summer solstice is the time when Earth is closest to the sun. However, on the first day of summer Earth is actually further from the sun than in December. So what does cause the longest day of the year?
To understand the solstices we must understand the behavior of Earth as a planet. Planet Earth rotates on its axis once every twenty four hours. Yet Earth's rotational axis is not vertical, it is tilted over about 23.5 degrees. This means that the northern hemisphere tilts away from the sun on the winter solstice and toward the sun on the summer solstice. The eqinoxes, when neither hemisphere is tilted toward the sun occur in between.
We observe the effect of this tilt as the sun's altitude in the sky changes. On December 21st, the winter solstice, the sun appears lowest in the sky. It rises in the south east, is very low in the sky at noon and sets in the south west. From December 21st to March 22nd, the Vernal equinox, the sun rises and sets progressively further north, and its noon height is greater. Finally, on June 21st the sun rises & sets furthest north and is at its highest at noon. Then it gradually loses altitude as the year winds down.
Contrary to popular belief, the sun is not directly overhead for most observers on the first day of summer. The only place the sun can be directly overhead on June 21st is at the Tropic of Cancer, which is 23.5 degrees north of the equator. Similarly the sun is only directly overhead at the winters solstice for locations on the Tropic of Capricorn, 23.5 degrees south of the equator. On the euqinoxes, both spring and fall, the sun is only directly overhead at the equator.
Because of the Earth's tilt, observers in the northern hemisphere observe the seasons exactly opposite as an observer in the southern hemisphere. On June 21st, observers in the northern hemisphere observe the sun at its highest altitude in the sky, and mark the start of summer. On the same date, observers in the southern hemisphere observe the sun at its lowest in the sky and mark the start of winter. On December 21st the situation is reversed. On March 22nd and September 22nd, observers in both hemispheres observe the sun rising due east, setting due west and giving us equal hours of daylight and dark.
The term solstice literally means sun still, because our ancient ancestors observed the sun, and noted that it continually changed its altitude in the sky except around December 21st, and June 21st. These were pivotal times for our ancestors who's dependance on the sun drove their agriculture and probably their rituals as well. Our ancestors had no concept of our Earth as a planet, yet their careful observations can be just as easily observed, and understood by observers today.