The summer solstice is the day each year when the Earth tilts directly towards the Sun. In the Northern Hemisphere, this happens in mid-June, when the northern half of the Earth leans towards the sun; in the Southern Hemisphere the summer solstice happens in mid-December, when the southern half of the Earth inclines sunward. Whenever it occurs, the summer solstice is the longest day of the year. In fact, near the Earth's poles daylight lasts for 24 hours during the summer solstice.
Ancient astronomers from cultures around the world were aware of the solstice's importance, and celebrated its arrival with temples and monuments. Unfortunately for us, the tendency of the Earth to shift in its orbit over time means that many temples built to showcase the solstice are no longer perfectly aligned with the Sun. Nonetheless, these ancient observatories are some of the most remarkable constructions in human history.
Stonehenge The most famous ancient observatory, Stonehenge deserves a place at the top of the list for its sheer magnitude alone. The monument was once much larger, but time and vandalism resulted in a gradual decay of the stone circles. During the summer solstice, the Sun rises directly over the Heel Stone, and casts its shadow into the central circle of stones; however, many experts believe that at the time Stonehenge was assembled several thousand years ago, the Heel Stone was one of a pair of stones that framed the rising sun during the summer solstice.
The Sun Dagger Created by the Anasazi tribe from the southwestern US, the Sun Dagger is found at the top of Fajada Butte in Colorado. It is a spiral-shaped carving on the bluff's surface, placed so that during the summer solstice a dagger of sunlight pierces the exact center of the spiral. In recent years erosion around the site has caused the rock slabs that support the Sun Dagger to shift, moving the spiral gradually out of alignment and prompting local officials to limit public access to the site for fear of further damage.
Abu Simbel The summer solstice was a vitally important time in the lives of the ancient Egyptians. The solstice typically coincided with the annual flooding of the Nile, an event which literally brought life to Egypt. One of the most extraordinary Egyptian solstice temples is Abu Simbel, in Nubia. There the temple of Ra, the Egyptian god of the sun, is aligned so that at dawn on the summer solstice the light illuminates the entire length of the temple, brightening the statues of Ra, Amun, and Ramses II (the pharaoh responsible for building the temple). The statue of Ptah, the Egyptian god of night, is the only part of the building not illuminated by the solstice dawn.
Chichen Itza Another New World observatory built by the Mayans. Interestingly enough, unlike most ancient solstice temples the buildings at Chichen Itza are aligned with sunset on the summer solstice, rather than sunrise. The Temple of the Warriors, the Temple of the Jaguars and the Caracol are all designed to produce light effects during sunset of the summer solstice.