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UC Merced Journal of California and Great Basin Anthropology

Title A Simple Method of Determining Archaeoastronomical Alignments in the Field

Permalink https://escholarship.org/uc/item/7vd4h0xw

Journal Journal of California and Great Basin Anthropology, 1(1)

ISSN 0191-3557

Authors Seymour, Timothy P Edberg, Stephen J

Publication Date 1979-12-01

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California A Simple Method of Determining Archaeoastronomical Alignments in the Field

TIMOTHY P. SEYMOUR STEPHEN J. EDBERG

As an aid in achieving this goal, we have EDITOR'S NOTE: While it is not nor­ developed the following simplified algebraic mally our policy to publish papers of a purely expressions, derived from spherical trigo­ methodological nature, the following paper is nometry, which can be used to determine useful to archaeologists with an interest in whether or not a celestial object (such as the archaeoastronotny and can be employed in , , or particular ) of possible sig­ making observations of the type described in nificance will rise or set at a point on the the preceding paper. horizon indicated by an apparent . The only field equipment required consists of ECAUSE of the recent interest on the a surveyor's transit, a book of trigonometric B part of archaeologists in the possible tables or hand calculator with trigonometric astronomical significance of various archaeo­ functions, and an inexpensive star . In logical features, field archaeologists are addition, a great deal of preliminary analysis beginning to look for possible archaeoastro­ can be accomplished prior to going into the nomical alignments with ever-increasing field with nothing more complex than a topo­ vigilance. This awareness has resulted in a graphic map and a protractor. number of notable discoveries in the past Two equations are used in the calculations; several and promises many more as one is a simplified version of the other. Which research continues. However, archaeologists equation is more appropriate will depend upon in the field face a number of problems in their the topographic conditions prevailing at the efforts to identify and describe such sites, not site. Equation (1) can be used for sites located the least of which has been their general in very flat terrain where there are no hills, unfamiliarity with mathematical astronomy. "obstructions," or depressions. These features Clearly, a means whereby the archaeologist should not exceed about 7° from the level himself could quickly calculate such align­ mathematical or astronomical horizon which ments is sorely needed. Such calculations the observer would see if the were a would in no way supercede field observations, smooth sphere. Equation (2) should be used but would provide a method of making those when more uneven topography must be taken observations vastly more efficient. into account.

[64] DETERMINING ARCHAEOASTRONOMICAL ALIGNMENTS 65

(1) D = sin"'(cos L cos A) clockwise from true north (see Fig. la). (2) D = sin''(cos H cos A cos L + sin H sin L) H represents either the apparent height of The various letters used in the preceding any horizon obstruction such as a hill equations can be interpreted as follows: or mountain, in positive degrees, or any D is the (celestial latitude) of depression of the horizon below the the hypothetical heavenly object (sun, astronomical horizon, in negative moon, star) whose rising or setting degrees (see Fig. lb). point is indicated by the alignment. L is the latitude of the site. Procedures for employing the foregoing A is the azimuth direction, in degrees, equations are explained in the following the alignment is pointing, measured examples.

SOLSTICE SUN?.

TRUE HORIZON NORTH OBSTRUCTION (MOUNTAIN I

HORIZON OBSTRUCTION

ASTRONOMICAL HORIZON

Fig. 1 (a & b). Hypothetical alignment showing the proper determination of the azimuth and H angles. 66 JOURNAL OF CALIFORNIA AND GREAT BASIN ANTHROPOLOGY

Example 1. An archaeologist suspects that of error, we may therefore assume that a sig­ a rock feature at a site is significantly aligned nificant alignment does exist in the situation with a distant mountain peak, and hypothe­ described above. Since the sun's declination sizes that the sun may rise behind the peak on varies systematically from +23° 27' at the the morning of the summer solstice. His hypo­ summer solstice, to 0° at the equinoxes, to thesis can be quickly checked as follows. -23° 27' at the winter solstice (as the earth First, the azimuth (A) of the alignment is moves through its yearly ), other align­ measured with a compass or transit and found ments at other azimuth headings might be to be 63° 54'. Second, the latitude (L) of the looked for. If there were alignments to the site, taken from a USGS quad sheet, is found equinoxes or to the winter solstice, the to be 37°30'. Third, the apparent height (H) declination values given by the equation would or elevation of the mountain behind which the be D = 0° and D = -23° 27'. (Note that plus or sun is suspected to rise is measured by transit minus values indicate whether an object is and determined to be +5°. Since the horizon is above or below—i.e., north or south—of the not flat and high accuracy is desired, equation celestial .) (2) is selected. Substituting appropriate values into the equation gives the following results: It should be recognized that alignments that point to objects rising on the eastern D = sin-i [(cos 5°)(cos 63°540(cos 37°30') horizon may also be used to point to objects + (sin 5°)(sin 37°30')] setting in the west. In this case the azimuth of = sin-i [(0.9962)(0.4400)(0.7934) the western alignment is simply the eastern + (0.0872)(0.6088)] azimuth plus 180°. The declination of the = sini [(0.3478) + (0.0531)] setting object, in this two-way situation, would = sin' (0.4009) be the same except for a change in sign. In = +23° 38' the example just given, the declination of an The sun's declination at the time of the summer object setting in the west in line with the rock solstice is +23°27'. With a reasonable margin feature would be -23° 38' (rather than +23° 38');

/ OBSTRUCTED HORIZON ACTUAL SUNRISE T H

i ASTRONOMICAL THEORETICAL HORIZON SUNRISE

Eastern Horizon As Viewed From The Site Fig. 2. Schematic of the rising sun showing the relationship between H and azimuth. DETERMINING ARCHAEOASTRONOMICAL ALIGNMENTS 67 this would correspond to the sun's setting posi­ would be to examine the possible association tion at the winter solstice. However, it should of nearby mountain peaks with already cata­ be noted that each declination value for the loged sites. This can be done by following the rising and setting horizon points may have to azimuth heading east or west from the site to be calculated individually if each horizon has ascertain if any Hkely topographic features lie a different elevation or H value (see Fig. 2). along this line of sight. It should be emphasized that the major All such calculations of azimuths have to source of error in these calculations will arise take into account elevation differences from an inability to determine the azimuth between the site and mountain peak. This heading of an alignment precisely. We caution value (H) can be easily determined by using the that uncertainties in the direction of a possible quad map and employing a little high school alignment will result in a range of appropriate trigonometry. declination in the sky rather than a single The difference in elevation between the mtn. value. The archaeologist must take this tan H = peak and the site (in meters). uncertainty factor (which will vary widely from The distance (in meters) between the mtn. site to site) into account when assessing the peak and the site. significance of an alignment. This simple can also be used to calculate Since alignments with the rising or setting reasonably accurate values for H when the solstice or equinox sun are usually the most researcher does not have access to a transit. common, it is often desirable to know in Let us now examine a situation where a advance what azimuth headings to look for in site is located in flat terrain and there are no alignments in a particular area or at a parti­ elevated features on the horizon. cular site to be examined. This can be done Example 2. The alignment of two rock before going into the field by solving equation at an is suspected to (1) or equation (2) for azimuth instead of for have astronomical (particularly solar) signifi­ declination. cance. This hypothesis can be checked as follows. The azimuth (A) is measured as (la) A = cos '/sin D\ or 52°03', while the latitude (L) is the same as in \cos L/ example 1 (37° 30'). Substituting appropriate (2a) A = COS" sin D (sin H sin L)[ values into the equation gives the following [ cos H cos L J results:

This will provide a quickly calculated estitnate D = sin-' (cos 37°30')(cos 52°03') of the pointing direction of any possible align­ = sin-i (0.7934X0.5299) ments. Once in the field, any such orientations = sin' (0.4879) can be precisely measured and calculated using = +29° 12' the formal method given. Such prior calculations of likely azimuth Since the sun is never farther north than headings have a number of possible applica­ +23°27', this alignment clearly has no solar tions. They might conceivably be used to significance. However, the moon varies cycli­ locate solstice sites associated with prominent cally in declination between about +29° and mountain peaks by following the predicted -29°. Since many cultures were probably azimuth on a USGS quad map east or west aware of this cycle, we might postulate a lunar from the peak to a point of likely observation. alignment in the example above. This would be A complementary type investigation (and difficult to verify unless there was an associ­ perhaps a more immediately practical one) ation with lunar symbols in . 68 JOURNAL OF CALIFORNIA AND GREAT BASIN ANTHROPOLOGY

Similar outside corroboration will be TABLE 1 necessary in situations where there is more OF SELECTED BRIGHT * than one possible celestial object associated with a particular declination. In the above DecL Star Name +89°02' Alpha Ursa Minoris Polaris example, a declination of +29° is also close +62°or Alpha Ursa Majoris enough to that of the star Beta Tau, given the +57° 19' Delta Ursa Majoris uncertainties of azimuth determination, to +56°39' Beta Ursa Majoris +56°14' Epsilon Ursa Majoris make it another candidate. +55° 11' Zeta Ursa Majoris Such alignments with bright stars are in +53°58' Gamma Ursa Majoris some respects easier to ascertain than solar or +49°4r Alpha Persei lunar alignments, since the declinations of +49° 34' Eta Ursa Majoris +45°57' Alpha Aurigae stars vary only slightly with time (i.e., over +45°06' Alpha Cygni Deneb centuries) compared to those of the sun (over +44° 57' months) and moon (over days). Here, star +38°44' Alpha Lyrae +32°00' Alpha Geminorum Castor charts such as Norton's Star Atlas (Norton +28°34' 1973) or the A'^^'M' Popular Star Atlas (Inglis +28°09' Beta Geminorum Pollax 1974) may be of more help (when used in con­ +27°5r Beta Cygni junction with a table of stellar declinations) +23°58' Pleides +20° 06' Gamma Leonis than a table alone (see Table 1). Once a pos­ + 19°27' Alpha Bootes sible declination has been calculated, the + 16° 27' Delta Geminorum indicated parallel of declination line may be + 16°25' Alpha Tauri Aldeberan + 12°13' Alpha Leonis followed across the face of the star chart and all +08°44' Alpha Aquilae Altair likely bright stars near it noted. This also facili­ +07° 24' Alpha Orionis tates the visualization of any other possible +06° 18' Gamma Orionis alignments with rising or setting +05°2r Alpha Canis Minoris -00°20' Delta Orionis in the area. Such a procedure has been used by -01° 13' Epsilon Orionis the authors to successfully identify alignments -01°58' Zeta Orionis with the first and last rising "belt stars" in -05°09' Beta Eridani -08° 15' Beta Orionis Orion at a site near Barstow's Ceremonial -09°41' Chi Orionis Point. -10° 54' Alpha Virginis Spica -16°39' Alpha Canis Majoris Like the sun and moon, the stars have a -17° 56' Beta Canis Majoris yearly motion that is known to have been -26° 19' Alpha Scorpii employed for calendrical reckoning by ancient -28°54' Epsilon Canis Majoris -29°53' Alpha Piscis Austr. cultures around the world. Once a (or for -47° ir Gamma Velorum a few days once a year), each star rises in the -52°40' Alpha Carinae morning, only to be quickly drowned in the -56°50' Gamma Crucis glare of the rising sun. These "heliacal risings" -57°29' Alpha Eridani Achernar -58°28' Delta Crucis of certain bright stars were often used to mark -59°21' Epsilon Carinae important yearly ceremonies and events. Since -59° 25' Beta Crucis Beta Centauri the verification of such observations offers -60°08' -60° 38' great potential for determining seasonal -62°49' Alpha Crucis activities at a site, we have included a simple -69°48' Large Magellanic Cloud Small Megallanic Cloud method of determining such risings as a com­ -73°06' plement to the techniques already described. (Roth 1975:518) The calculation of the formal date ofheli- • 1950 DETERMINING ARCHAEOASTRONOMICAL ALIGNMENTS 69 acal rising of a star (i.e., the date when the star each month. This is the date of the formal and the sun's centerpoint rise at the same . The actual heliacal rising will moment) can be simply accomplished as be one one or two weeks later because the sun follows. Lay a protractor on the star chart in must move away from the region of the star. Fig. 3, with its center on the celestial equator For example, to calculate the approximate and its flat edge (which represents the heliacal rising of Deneb (Alpha Cygni) at a observer's horizon) perpendicular to this site located at 35° north latitude, rotate the same equatorial line. The round edge of the protractor counterclockwise to 90° - 35° = 55°. protractor should be to the left. Rotate the Then slide the protractor along the celestial protractor to 90° minus the site latitude (L), equator until its straight edge intersects Deneb turning it counterclockwise for northern hemi­ (the brightest star in Cygnus) and read the sphere sites and clockwise for southern hemi­ approximate date where it also intersects the sphere sites. Slide the protractor from right to . The estimated heliacal rising is about left until the flat side intersects the star of November 23rd. Note, however, that only the interest. Read the date where the flat side stars with names in Table 1 are likely candi­ intersects the ecliptic, which is marked for the dates for use by prehistoric observers since sun's position on the first and fifteenth day of only such very bright stars can be seen under

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RIGHT ASCENSION Fig. 3. Starchart of prominent constellations for determination of appro.ximate heltacal rising dates. 70 JOURNAL OF CALIFORNIA AND GREAT BASIN ANTHROPOLOGY heliacal rising conditions. ACKNOWLEDGEMENTS The accuracy of this method decreases the This report has benefited from the com­ farther a star is from the celestial equator, ments and advice of many individuals. Thanks since the protractor's flat side only approxi­ go to Thomas Blackburn, Travis Hudsoq, mates the curvature of the horizon and the and Ken Hedges for their encouragement and modified mercator projection used on the reviews of the early text. A similar debt is chart distorts positions near the poles. Pre- owed to Ed Krupp for his advice on astro­ cessional effects have also changed the dates nomical problems, to Valerie Kramer, and to of heliacal risings over the centuries. James Cleary and Gary Hartzell for their fine When using this method for determining illustrations. Finally, thanks go to Robert the approximate date of heliacal rising, the Brown and Mike Morgan, who helped test observer should remember that stars with out our theories in the field. positive declinations of 90° minus the site latitude (L) or greater are always above the California State Polytechnic University, horizon (i.e., circumpolar), while those with Pomona negative declinations of 90° minus the site latitude (L) or lower are always below the San Fernando Observatory, horizon. In the southern hemisphere the situ­ California State University, Northridge ation is reversed: negative declinations are REFERENCES circumpolar and positive declinations never rise. Inglis, R. M. G. 1974 New Popular Star Atlas. Sky Publishing This paper has, of necessity, been brieL Corp. Further data, lists of new stars or other objects, or more complete tabulations may become Norton, Arthur P. necessary as the current state of knowledge 1973 Norton's Star Atlas and Reference Hand­ advances. The method we have outlined does, book. Sky Publishing Corp. however, provide an immediate and long- Roth, G. D. overdue tool for archaeologists dealing with 1975 Astronomy: A Handbook. Sky Pub­ ancient astronomies. lishing Corp.