Dr. Sten Odenwald
"The sun casts a shadow." No one knows when this revelation occurred to ancient humans, but it is clear from carvings on animal bones from 30,000 ago that some awareness of the sun and moon were thought to be important enough to encode for convenient reference by Shamans.
The fact that the shadow of an object changes its position on the ground in a regular way is also one of those key astronomical facts that also seems older than the written historical record. The oldest 'shadow stick' called a gnomen was built as part of a sundial by ancient Egyptians about 1000 BC. Temple and monument architecture also shows that many civilizations knew how to use the changing solar shadow as a local clock to measure the daily passage of time.
Once you have a sense that the position of the sun's shadow can be measured in a practical way to mark the passage of time, it is not such a major leap to recognize that from day to day, the length of this shadow, and the sun's rising and setting each day, also change but more slowly. The passage of the seasons is easily seen in the changing circumstances of the rising and setting location of the sun along the southern horizon, and in its maximum noon-time height in the southern sky. Astronomers call this the Meridian Transit.
Almost universally, civilizations have eventually discovered the importance of the Spring Equinox as the start of the planting season. In very arid locations, such as the Southwest Desert of the United States, the growing season is short due to rainfall patterns, so it was important to know within days when to plant. Spring also had important religious connotations as the 'rebirth' of the world from the grip of winter - a time for both joyous festivities and solemn ceremony. Astronomically, this happens when the sun rises on the eastern horizon, half-way between its extreme winter position and its extreme summer position. By carefully noting these three locations on the horizon, you can anticipate when winter has ended and the seasons are moving towards spring. It was common for this equinoctial position to be encoded into important buildings through sightlines in the monument architecture, such as windows that let the light from the sun reach an interior wall only at the appointed day during the year (Hovenweep, Abu Simbel, Newgrange). For other civilizations, marking the time of the summer and winter solstices was equally important (Machu Pichu). Although the equinox position of the sun is identical for spring and fall, the solstice positions are easily distinguished by their extreme southern (winter: Newgrange) and northern (summer: Angkor Wat) locations along the horizon.
Other civilizations also may have paid attention to the location of certain stars such as Sirius, which was used by the Egyptians to predict the all-important flooding of the Nile around July, which began their planting season.
Once a building or temple is aligned with the rising and setting of the sun for either religious or practical reasons, it automatically will have architectural elements, such as hallways, colonnades, etc, that fall along a north-south and east-west axis. An example is the Hypostile Hall of the Temple at Karnak. The role of the North Star as a unique geographic direction did not play nearly as much of a role in ancient alignments as the intentional alignments with the rising and setting sun. Since buildings are often rectangular in shape, once one axis is set along the East-West orientation, the other axis will automatically follow the north-south axis as a 'freebie'.
Abu Simbel: It was built by Ramses II between 1279 and 1213 B.C As you walk to the rear of the temple you come to the Holiest of Holies located at the back wall, where you will find four statues of: Ra-Harakhte, Ptah, Amun-Ra and King Ramses II. The sun shines directly on the Holiest of Holies two days a year: February 21, the king's birthday, and October 22, the date of his coronation.
Great Pyramids at Gizeh: The Great Pyramid of Pharaoh Khufu is accurately aligned with the cardinal points of the earth, and some people have suggested that one of the shafts in the pyramid is aligned with the star Sirius.
Newgrange: The Megalithic Passage Tomb at Newgrange was built about 3200 BC. A shaft of sunlight shines through the roof box over the entrance and penetrates the passage to light up the chamber. The dramatic event lasts for 17 minutes at dawn from the 19th to the 23rd of December.
Angkor Wat: Standing near the south-western corner in Angkor Thom the rising sun at summer equinox will visible through or over the eastern gate. 6 months later the alignment has shifted till its northern point of sunrise at winter solstice. The sun itself was so important to the builders of the temple that solar movement regulates the position of the bas-reliefs.
Medicine Wheel: The wheel was built between 1200 and 1700 AD. A line drawn between the central cairn and an outlying cairn at the Bighorn Medicine Wheel pointed to within 1/3 of a degree of the rising point of the sun at the summer solstice. The actual astronomical purpose of the design of these wheels remains controversial.
Chichen Itza: This is a square-based, stepped pyramid approximately 75 feet tall, constructed by the Mayans ca 1000-1200 AD, directly upon the multiple foundations of previous temples. The axes that run through the northwest and southwest corners of the pyramid are oriented toward the rising point of the sun at the summer solstice and its setting point at the winter solstice.
Easter Island: Over 880 statues called moai can be found on this isolated island, located 2,300 miles from the coast of Chile. The seven moai at Ahu Akivi were built around 1460 C.E. and face the point at which the sun sets during the equinox.
Gaocheng Observatory: Established in 1279 AD by the famous astronomer Guo Shoujing, it is the oldest of 27 ancient observatories in China. Linking the entry and exit to the platform are stairs and pathways. Between the two pathways is the 93 foot long stone Chinese sundial, which was paved by 36 slates. It was designed originally for use in predicting the time of the solstice each year.
Gotland Groves: There are about 3600 known grooves on stones scattered throughout the island of Gotland. The most important feature of the grooves appears to be in their grand alignment when looked at over the entire island. A recent study of 1256 grooves showed that they are aligned with certain positions of the celestial bodies, apparently the sun or the moon.
Hovenweep: Tree-ring dating of timbers used in the construction of the 'Sun Room' suggest that it was added in 1277 AD about 100 years after the main structure, called the Castle, was completed. Two ports, or windows, in the large tower admit the rays from the sun into the interior room, and it has been proposed that this arrangement was used as a solar calendar. The equinox port points to the sunrise azimuth four days after the vernal equinox.
Jantar Mantar: The great Indian astronomer-king Maharaja Jai Singh II of Jaipur built these five astronomical observatories between AD 1724 and 1730 during the period generally known as the dark age of Indian history. The Jantar Mantar consists of a number of masonry instruments for predicting time, measuring the position of a celestial body and determining the latitude. The two pillars on the southwest of Mishra Yantra were designed to determine the shortest and longest days of the year. In December one pillar completely covers the other with its shadow while in June it does not cast any such shadow at all.
Karnak: Built by the Ancient Egyptians in several episodes of construction and enlargement from 2055 B.C to 395 A.D. The earliest axis included the famous Great Hypostyle Hall built by Ramses II on an east to west alignment. Sir Norman Lockyer (1836-1920) proposed a midsummer sunset alignment of the Main Axis of the Great Temple of Amon-Re Karnak was By some accounts, the temple at Luxor may have no less than four well-defined alignment changes involving stars. Unlike solar alignments which can generally last for thousands of years intact, stellar alignments are much more critical because of the precession of the equinoxes, and last only a few hundred years. Lockyer's measurements showed several Karnak temples had been altered over the centuries to match the precessional changes in their aligned stars.
Machu Pichu: A number of features distributed throughout the site are aligned with the June solstice azimuth of 65-245 degrees. The Temple of the Three Windows forming the easterly side of the plaza, opens to the plaza and faces the solstice sunset. The solstice alignment, and the importance of solstice ritual to the Inca, suggest that this was a primary ceremonial consideration of this shrine. The Torreon is popularly called the Temple of the Sun. A stone enclosed within the Torreon is reported to receive a ray of sun light through the east facing window during the June solstice.
Casa Rinconada: This is one of five great kivas in Chaco Canyon - 60 feet in diameter and 15 feet deep. Casa Rinconada was probab;y built between 1070 and 1110 AD. The symmetry axis defined by the two T-shaped doors is aligned with the North-South line to within 20 arcminutes. Shortly after sunrise on the summer solstice, as the Sun rises a beam of light shines through a lone window on the N-NE side of the kiva and moves downward and northward until it illuminates, on the interior West wall, one of the five larger, irregularly spaced niches in the kiva.
Basilica San Petronio: Gian Domenico Cassini constructed the meridiana del tiempo to find a better value for the length of the year by counting the days and hours between the sun's successive returns to the same solstice or equinox. An exact value of the year is needed to calculate the date of Easter. Surprisingly, many catholic churches have now been re-discovered' to have such features dating from the 17th century. The 'heliometer' consists of two separate pieces. One piece lies on the floor; it is a perfectly horizontal rod running due north for some 191 feet from a spot under one of the side chapels to the front door of the church. The other part is a small hole one-inch in diameter set in a horizontal metal plate fixed in the roof of a chapel. The hole is permanently open so as to give free access to the sun's rays around noon throughout the year.
Sun Dagger: At the top of Fajada Butte, along a narrrow ledge is a sacred Native American site given the name Sun Dagger. On the day of the summer solstice, a slender beam of sunlight passes between two rock monoliths, and bisect the center of a spiral-shaped petroglyph. Two parallel daggers bracket the larger spiral at the spring and fall equinoxes. The Sun Dagger formation is estimated to be a thousand years old, and was constructed by the Anasazi. Chaco Canyon was abandoned during the late 13th Century for unknown reasons, and the sun dagger remained hidden until 1977 when it was rediscovered by archeologists studying the pertoglyphs.
Great Zimbabwe: The construction includes a 'Great Enclosure' consisting of a ring of stone walls and platforms about 250 meters in circumference, last used about 800 years ago. Several of the stone monoliths line up with certain bright stars in the constellation Orion as they rise on the morning of the shortest day of the year, the winter solstice.
Thanks to the advent of satellite imagery and the interactive capabilities of GOOGLE Earth, we can fly over the monuments we have just described and explore their shape and orientation in space. Here's what to do!
The following are 4-page problem sets that introduce you to the very simple geometry that goes along with designing monuments to favor certain orientations and alignments. It really isn't 'rocket science' but it did take thousands of years for humans to get it right! The problems require a basic understanding of angular measure, the use of compasses and protractors, and creating and using scaled images and models. The problems have been developed by Space Math @ NASA. For additional math problems with a space science and astronomy flavor, visit http://spacemath.gsfc.nasa.gov.
The Sun-Earth Day 2005 'Ancient Observatories - Timeless Knowledge' website contains dozens of detailed essays about ancient observatories. These essays, written by NASA Astronomer, Dr. Sten Odenwald, can be found in the Technology Through Time series Issue 1 to 30 located at http://sunearthday.gsfc.nasa.gov/2005/locations/trace.htm
A transit of Venus occurs when Venus passes directly between the Sun and the Earth.