In the previous article (Stonehenge the Calendar) I explained how neither the summer or winter solstice, for which Stonehenge is famous cannot be used to work out the date and how instead a date toward or at the spring or autumn equinox (equal day and night) would be used. I then suggested that some barrows to the east of Stonehenge might be a way to calibrate a calendar to the equinox. (These are recorded as being bronze age, but that could be a possible reuse of earlier features.) Next I suggested that another way to calibrate a date calendar was to use a pyramid like those in Egypt or a suitable mound like Glastonbury.

Tony Marsh beat me to the third possible way of calibrating a calendar date with an excellent article.

However, he left some questions dangling so, this is an attempt to answer them.

But first to explain the idea.

Lintel ring looking SE

Most people are familiar with the view of stonehenge. The site  consists of a series of upright stones with stones laid on the tops like the lintels of a door. It is believed this ring formed a continuous circle and although the site is built on a slight slope it is reasonable to believe it was horizontal.

What is being proposed is that the shadow of this ring fits to a irregular indent on stone T54 shown below.

Location: 51.178844N, 1.826189W


Plan of site

To the right is a plan of the southern edge of Stonehenge reproduced from Tony Marsh's article with a few additions.

The black and white imagery of the stones is from various sites. The pink circle is from a site offering a theoretical depiction of the site - main ring only (after adjustment to fit).

The lilac represent the shadow as it would appear at around noon on the equinox showing that according to Tony, its north side just touches the foot of stone T54.

To justify this he gives a top edge of the ring at 15.92ft (4.85m) and a distance projected to the apparent "arris cut" (the indent) on trilithon T54 of 19.37 ft (5.90m). From this the sun's altitude at equinox would be:

atan (15.92/19.37) = 39.4°

Stonehenge has a latitude of 51.178844N which means the sun will be at 38.82° above the horizon at the equinox. This is 0.6° less than that Tony estimates for the shadow to reach the foot of stone T54.

39.4° degrees which is close to the 38.82° degrees from the latitude of the
site. Checking the figures on the plan shown, I measure 6.0m from the edge of the pink circle to stone T54. According to wikipedia (not my favourite source) the tops of the lintels are 4.9m above ground level - but they don't mention any slope, so this may be out because the remaining lintels are nearly at the opposite side of the circle. Using the revised figures I obtain an angle of

atan (4.9/6.0) = 39.2° (±0.6°)


Note that at the solar equinox, the shadow will move east-west along the line line shown, so if the stone to the south were also aligned east-west, the same measurement could be made any time around noon (Tony used 2pm).

Next Tony says the shadow will move by about 1inch a day. At the solar equinox, the sun is moving by 0.4° per day (northward in spring and southward in autumn). The distance travelled each day by a shadow from an object 4.9m high is:

= length shadow equinox - length of shadow on next day

= 4.9 /tan (38.82°) - 4.9 /(38.82° + 0.4°)

= 0.086m

= 3.4 inch/day.

But as Tony highlights, this does not work unless the west-east to T54 flattening shown in the plan above is at ground level. So here is a picture of the site:

View of foot of T54 showing the flattened area extending from the
grass line up to the writing. As far as can be seen from this image,
the face is aligned west-east (from google earth)

This confirms that the base of T54 has the required cut so that the face is flat along a line west-east as required. This means that at or very close to the equinox, the shadow of any lintel on the ring of uprights at the position suggested by Tony's plan would just touch the base of T54. This position should change by about 8.6cm a day. I had to check that figure several times as it seems quite large.

However what is more intriguing is that when I look closely at the image of T54 there does appear to be a suitable line along this edge:

Base of T54

 Is this an ancient line which if so, it would be very strong evidence this was used to calibrate the date using the equinox. But it could also be a modern line, perhaps added by some enthusiastic antiquarian or pagan. Or it cold be a splash mark whereby rain drops tend to splash up only to a certain level.


Tony's questions:

  1. my 0.7 inch/day figure - I am none too sure! (actual 3.4inch)
  2. Is the arris on T54 at its foot - I have no photos? (yes)
  3. Was ground level in the same position 3000 years
    ago - but with this, wouldn't the keepers maintain a
    a level at the foot which would maintain the precision? (not able to check)
  4. Someone with a good survey could do all this again . .?