and the Astronomers of Yore

First You Must Measure...

With a little geometry, students can study alignments. Archaeoastronomers measure four important angles (see figure 3 and figure 4):

 Figure 3 Definition of declination. Declination is the angle between a star and the celestial equator, an imaginary line across the sky that runs parallel to the Earth's equator. To find the celestial equator, hold your arms in an `L' shape and point one arm at the North Star; the other arm will point at the celestial equator. Figure 4 Definition of azimuth and elevation. Azimuth is the compass direction, measured clockwise, between a star and the north. Elevation is the angle between a star and the horizon.

• Geographic latitude, the angle between the horizon and the North Star, Polaris. This indicates your position on the Earth, as measured from the equator. You can look up the latitude on a map or, if you visit the site, measure it with a surveyor's transit.
• Declination, the angle between Earth's equator and a particular star (angle delta in figure 3). Because this angle does not change as the Earth rotates, astronomers use it to describe the positions of stars. You can look up declinations in a catalog of celestial objects.
• Elevation (or altitude), the angle between the observer's horizon and the star (angle E in figure 4). Straight up is 90 degrees, the horizon is 0 degrees. For the North Star, the elevation equals the geographic latitude. For all other stars, the elevation angle changes as the Earth rotates. Elevations can be measured with a protractor on a sketch of the site or with a surveyor's transit.
• Azimuth, the angle between north and the star (angle A in figure 4), measured along the observer's horizon. North is 0 degrees, east is 90 degrees, south is 180 degrees, and west is 270 degrees. As in the case of elevation, azimuth can be measured from a site sketch or with a transit.
Astronomers are always using these angles to determine where to find a particular star at a particular time of year. They look up the star's declination and convert it to elevation and azimuth. The conversion requires a bit of trigonometry; students who feel comfortable with the subject can find the equation in books such as the Observer's Handbook, which the ASP catalog sells, and Practical Astronomy With Your Calculator by Peter Duffett-Smith.

The geometry determines which objects are visible from a given site and how the Earth's rotation affects their motion. Stars fall into different categories:

• Circumpolar stars are always above the horizon, never rising or setting. From North America, the Little Dipper never sets; it just appears to rotate about a point in the sky called the north celestial pole, marked by the location of the North Star. The north celestial pole is the apparent center of the heavens and the place where, according to many cultures, the soul of the deceased went.
• Seasonal stars rise and set, and stay up all night for part of the year. For example, from North America, the constellation Orion is up all night in the winter, and Cygnus in spring. The seasonal stars are the ones that can be used in connection with the Sun to determine approximate times of the year.
• Perpetually obscure stars are always below the horizon. From North America, we never see the constellation Centaurus, for example.

Using their knowledge of these angles, archaeoastronomers have analyzed the great pyramid of the pharaoh Cheops (see figure 5). Two air shafts leading to the king's chamber relate to an afterlife, a strong theme found in Egyptian hieroglyphics. The shaft on the north side leads directly to the north celestial pole, which in Cheops' time corresponded to the star Thuban in the constellation Draco. The shaft on the south side is associated with the constellation Orion, which in Cheops' time passed directly through the line of the shaft every day. Orion was a multipurpose god associated with the hereafter.

 Figure 5 Shafts to King's chamber of Cheops' pyramid. The pyramids, built nearly 5,000 years ago, incorporated the astronomical knowledge of the ancient Egyptians. The sides of the base line up with north, south, east, and west. Two air shafts slope upwards from the main burial chamber. These shafts are aligned with two stars of religious importance: Thuban (the North Star at the time) and Alnilam (the center star in Orion's belt).

...And Then You Can Understand

Geometry also sheds light on the great raised earthworks of the Hopewell. The Hopewell were Native Americans who lived in southern Ohio between the second century B.C. and sixth century A.D. Only a few of their earthworks have survived the farmers' plows and developers' bulldozers. Fortunately, surveys of several dozen monuments by George Squier and E.H. Davis in the mid-1800s preserve the knowledge of the Hopewell (see figure 6). These surveys give the azimuths and the latitude of the sites, so that archaeoastronomers can calculate the declinations of the alignments. Nearly all correspond to noteworthy positions of the Sun or Moon on the horizon.

 Figure 6 Hopewell earthworks. Right, a diagram of the earthworks at Seal in southern Ohio. Left, the circle and octagon mounds at Newark, Ohio, now a municipal golf course. This photograph looks northeast along what some archaeoastronomers think is a moonrise alignment. Other earthworks have been leveled to build shopping malls. Photo courtesy of E.C. Krupp, Griffith Observatory.

Many of the largest earthworks have squares or octagons associated with circles. The squares and octagons have several openings in their perimeter, but the circles do not. This suggests that ceremonial participants entered the earthworks through the square or octagon openings and proceeded to the center of the circle. Which ceremonies were conducted here, we don't know. Yet nearly all the azimuths defined by the direction of the ceremonial procession are associated with a life hereafter. During the ceremonies, spectators could stand on the surrounding earthworks to view the activities.

Geometry also accounts for the well-known alignment at Stonehenge in England. There, the Sun rises over the Heelstone, as viewed from the center of the site, at the summer solstice. Students can easily verify this for themselves, using figure 7, a protractor, and the equation: sin delta = cos phi cos A.
 Figure 7 Layout of Stonehenge in Wiltshire, England. Stonehenge is the most famous of ancient astronomical sites. If you stand in the center of the site and look northeast through the stone arches, you see the Heelstone, which points to the place where the Sun rises in midsummer. Stonehenge was built and rebuilt beginning 5,000 years ago.

With the protractor, measure the azimuth of the Heelstone as seen from the center of the Aubrey Circle. It should be about A = 50 degrees. The direction of geographic north is indicated on the figure. Then, on a map of England, look up the latitude of Stonehenge, near the town of Salisbury. It should be about phi = 51.2 degrees. By substituting A and phi into the equation, find the declination. It should be delta = 23.7 degrees, which is close to 23.9 degrees, the declination of the summer solstice position at the time Stonehenge was built.

The same mathematical expressions demonstrate that this sighting cannot determine the exact time of the solstice. The sighting is of low precision. Because the Sun's path moves slowly at this time of year, there are dozens of days when the positions of the Sun are indistinguishable. From this we conclude that Stonehenge was a great ceremonial center, but not an observatory.

Although naked-eye observations cannot determine the precise time of the solstice, they can determine the precise time of the equinox. At this time of year, the Sun's path changes considerably from day to day, so that the positions on successive days are quite discernible. The length of time it takes the Sun to return to the same equinox is the year. If a long enough baseline in time is used, the year can be determined with precision.

By dividing the year into quarters, ancient peoples predicted the time of the solstices. Whether these predictions agree exactly with our modern calculations does not matter. Generally people don't mind if they get the date of their holidays wrong, as long as everyone agrees to celebrate on the same day. For example, it is unlikely that Christ was born on December 25. If Christians meant to celebrate Christmas on the winter solstice, they are a few days off. But they don't care, since they have agreed to celebrate Christmas on December 25 (in North America and Western Europe).

<< previous page | 1 | 2 | 3 | next page >>

back to Teachers' Newsletter Main Page