Thursday, 8 May 2025

The Geometry of the Horseshoe Stones at Stonehenge: Design, Alignment, and Cultural Significance

  • May 2025

Abstract

This paper examines the geometric arrangement of the horseshoe stones at Stonehenge, comprising the inner sarsen trilithons and bluestone horseshoe, to understand their design principles, potential astronomical alignments, and cultural implications. Drawing on archaeological surveys, geometric analyses, and comparative studies, I argue that the horseshoe configuration reflects a deliberate application of geometric and astronomical knowledge, rooted in a chord-based system, to encode solstitial alignments and symbolic meanings.

Keywords: Stonehenge, horseshoe stones, sarsen trilithons, bluestone horseshoe, geometry, astronomy, Neolithic, chord-based system, solstice alignment

1. Introduction

The arrangement of Stonehenge’s horseshoe-shaped sarsen trilithons forms a central structure, with the upright stones and lintels creating a focal point, akin to the apse of a cathedral. At the centre is the Altar Stone, surrounded by a semicircular horseshoe arrangement of bluestone monoliths and sarsen trilithons. This configuration directs attention toward the midwinter sunset, suggesting an intentional astronomical orientation.

Yet, this evocative scene is marred by time: fallen trilithons, missing lintels, and displaced bluestones disrupt the monument’s original harmony. How do we reconstruct this ancient design, and what can its geometry reveal about the Neolithic builders’ intentions?

Historical surveys by John Wood (1740) and Flinders Petrie (1880), alongside modern laser scanning, provide critical data, yet 20th-century re-erections and missing stones challenge our understanding of the original layout.

2. Background: The Horseshoe Stones in Context

Sarsen Trilithon Horseshoe

The sarsen trilithon horseshoe comprised of five pairs of sarsen uprights, each originally capped with a lintel, arranged in a horseshoe shape 14 m across, open to the northeast. The trilithons were graded in height, with the tallest, the Central Trilithon (Stones 55–56), originally reaching to 7.6 metres (25 feet) at the southwest, and the smallest at 6 metres (20 feet) in the northeast. The inward-facing surfaces are flat and smoother, suggesting deliberate craftsmanship.

Bluestone Horseshoe

The bluestone horseshoe, set within the trilithon horseshoe, originally formed an oval of up to 24 stones, later modified into a horseshoe by removing stones in the northeast to mirror the trilithons’ alignment. The bluestones’ arrangement likely evolved to prioritize solar alignments, complementing the trilithons’ solstice focus, Clive Ruggles (1997).

Historical Surveys and Reconstruction

Key surveys provide data for reconstructing the horseshoe’s layout. John Wood’s 1747 plan offers a pre-modern baseline, while Flinders Petrie’s 1880 measurements established precise dimensions. William Gowland’s 1901 excavation and re-erection of Stone 56, part of the Central Trilithon, provided detailed socket records, critical for understanding the trilithon’s skew (Gowland, 1902). Richard Atkinson’s 1958 re-erection of Stones 57, 58, and lintel 158 ensured accurate placement of the southwest trilithons (Cleal et al., 1995). Modern laser surveys, such as the 2011 Greenhatch survey, confirm stone positions and reveal unfinished aspects of the outer sarsen circle, suggesting the horseshoe was the intended focal point (Abbott & Anderson-Whymark, 2012). Parchmark discoveries by Banton et al. (2014) indicate the outer sarsen circle was planned to be complete, supporting geometric models reliant on a 60-point circle.

3.Reconstruction Efforts and Archaeological Methodology

The current state of the horseshoe stones—three intact sarsen trilithons, a partially collapsed Central Trilithon (Stones 55–56), and an incomplete bluestone horseshoe—reflects both natural decay and human intervention. Early 20th-century re-erections of stones in the outer circle, such as those in 1919, often relied on speculative placements, introducing errors (Cleal et al., 1995). In contrast, William Gowland’s 1901 excavation and re-erection of Stone 56 (part of the Central Trilithon) prioritized precision. Gowland meticulously documented the stone’s socket and the original position of Stone 55. His records provide a reliable foundation for understanding the Central Trilithon’s placement, particularly its non-perpendicular orientation.

Gowland’s Plan of Stone 56 and 55’s socket (Fig 7 in his excavation report)

Fig.1 Gowland’s Plan of Stone 56 and 55’s socket (Fig 7 in his excavation report)

Richard Atkinson’s 1958 restoration efforts further enhanced reconstruction reliability. Atkinson’s team re-erected Stones 57, 58, and lintel 158, part of the trilithon horseshoe’s southwest side, by excavating stone holes to confirm their original positions. This careful approach, building on Gowland’s methodology, ensured the trilithon horseshoe’s symmetry was accurately restored, complementing historical surveys by John Wood (1747) and Flinders Petrie (1880). However, older plans often assumed perpendicular alignments, overlooking subtle geometric nuances critical to the horseshoe’s design.

The bluestone horseshoe’s reconstruction is more contentious. Socket holes suggest an initial oval configuration, possibly comprising 24–30 stones, before its transformation into a horseshoe mirroring the trilithons’ northeast opening (Cleal et al., 1995; Johnson, 2008). Clive Ruggles (1997) dismisses the oval’s significance, arguing it was a transitional phase with minimal lunar alignment compared to the horseshoe’s clear solar focus. While socket holes confirm the oval’s existence, its brief duration and lack of precise astronomical evidence support Ruggles’ view that the horseshoe, aligned with the solstice axis, was the intended final design.

4.The Twisted Trilithon Hypothesis and The Chord-Based Model

Gordon Freeman (2009 & 2012), a Canadian researcher who first observed the "twist" of the Altar Stone at Stonehenge. He identified that the long axis of the Altar Stone aligns with the Winter Solstice sunrise and Summer Solstice sunset directions—a secondary solstitial alignment distinct from the monument's primary axis.

My (Daw 2015) Twisted Trilithon hypothesis introduced a second similar alignment: the 81 degree skew of the Central Trilithon (Stones 55–56) and the adjacent bluestones, which match the  Altar Stone, aligning with midwinter sunrise and midsummer sunset (calculated at 80° 49' 25" for 2500 BC(Banton pers. Comm.)). This skew, often ignored in older plans (e.g., Wood, 1747) or dismissed as a construction error by Anthony Johnson (2008), is substantiated as original by Gowland’s 1901 excavation records. Gowland’s detailed documentation of Stone 56’s socket and Stone 55’s original position confirms the trilithon’s original non-perpendicular orientation. The skew extends to nearby bluestones and the Altar Stone, suggesting a coordinated geometric design.

Stonehenge plan with overlain “clock face” and chords.

Figure 2: Stonehenge plan with overlain “clock face” and chords.

I (Daw 2024) proposed that the sarsen trilithons might have been positioned using chords across a 60-point sarsen circle, a system resembling Babylonian sexagesimal geometry but likely an independent Neolithic development. The sixty points are the thirty stone positions and the thirty gaps between then in the outer circle.  An 81 degree angle can easily be marked out using a chord from, to use a clock face analogy, the 15 minute mark to the 42nd, where the line of the primary solstitial alignment is being thought of as the 12 across to the 6 hour mark. The trilithons may have been spaced where their gap was on a circle that is half the diameter of the outer circle, marked.

Banton et al.’s (2014) parchmark discoveries further strengthen Daw’s model, revealing stone holes around the outer sarsen circle that suggest it was planned to be complete, with 30 stones and 30 gaps. The trilithons’ flat inward faces, smoother and more precisely worked than their outer surfaces, support this chord model (Cleal et al., 1995; Abbott & Anderson-Whymark, 2012). These faces, particularly on Stones 55 and 56, align with chords, suggesting intentional orientation within a geometric framework.

The Bush Barrow Lozenge, a Bronze Age artifact found near Stonehenge with a similar 81-degree angle, reinforces this as a regional geometric motif, suggesting a continuum of knowledge into the Bronze Age (Daw, 2024; Needham, 2010)

The Bush Barrow Lozenge, Wiltshire Museum Photograph, overlain with angled lines and clockface by Tim Daw

Figure 3. The Bush Barrow Lozenge, Wiltshire Museum Photograph, overlain with angled lines and clockface by Tim Daw

 

The Twisted Trilithon hypothesis has interesting implications for understanding Stonehenge’s geometry. The skew’s presence across the trilithon, bluestones, and Altar Stone indicates intentional planning.  It challenges the assumption of a simple solstice axis, suggesting a more complex astronomical design that integrates dual solar alignments. Furthermore, it underscores the horseshoe’s role as a ritual and observational focal point, with the skewed trilithon framing specific solar events for ceremonial purposes. That the outer stones, 8 and 9, which would have obscured the midwinter sunrise view from the centre, were flattened maybe early in the history of the monument, may not be coincidental.

5. Conclusion

Stonehenge’s horseshoe stones, with their proposed chord-based geometry and 81-degree skew, stand as a testament to the Neolithic builders’ advanced understanding of space, astronomy, and culture. Daw’s model, supported by the trilithons’ flat faces, Gowland’s records, Banton et al.’s parchmarks, and the Bush Barrow Lozenge’s geometric continuity, offers a compelling explanation for the horseshoes’ design, challenging simpler arc-based models and revealing a sophisticated integration of solar alignments. The skew’s alignment with midwinter sunrise and midsummer sunset underscores the horseshoe’s role as a ritual focal point, while the lozenge suggests a tradition of knowledge that persisted into the Bronze Age.

6. References

·         Daw, T. (2015). The Twisted Trilithon – Stone 56 and its Skew. Wiltshire Archaeological and Natural History Magazine, 108, 15–2 https://www.academia.edu/12401342/The_Twisted_Trilithon_of_Stonehenge_Corrected_Version_


 

Figures

Figure 1: Gowland’s Plan of Stone 56 and 55’s socket (Fig 7 in his excavation report)

Fig.1 Gowland’s Plan of Stone 56 and 55’s socket (Fig 7 in his excavation report)

 

Figure 2: Stonehenge plan with overlain “clock face” and chords. 

Figure 2: Stonehenge plan with overlain “clock face” and chords.

 

Figure 3: The Bush Barrow Lozenge, Wiltshire Museum Photograph, overlain with angled lines and clockface by Tim Daw

Figure 3. The Bush Barrow Lozenge, Wiltshire Museum Photograph, overlain with angled lines and clockface by Tim Daw

 

 

 

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