Those who spend much time studying Stonehenge hear a lot about solstice alignments built into the ancient megalithic structure (particularly summer solstice alignments), but very little about equinoctial alignments.

In his amazing work Hidden Stonehenge, Professor Gordon R. Freeman explains that there are in fact subtle equinoctial alignments built into Stonehenge, in addition to subtle lunar alignments and beautiful (and more widely-recognized) solstice alignments as well.

More precisely, Professor Freeman found that Stonehenge contains alignments to the rising and setting of the sun on "Equalday/night" (slightly different from the equinox).  Professor Freeman discovered the distinction by careful observation of the Sun Temple Ring Ómahkiyáahkóhtóohp in Alberta, CanadaHis observations found that the sacred circle site there contains clear alignments for days near the days we call the equinoxes, but slightly before that date in March and slightly after it in September. 

In page 65 of his book, Professor Freeman explains:
[. . .] the time of an Equinox is selected "theoretically" as the time when the centre of the Sun is directly above the Equator, and the "theoretical" Sun rise is when the centre of the Sun is physically horizontal from the observer.  "Theory" treats the Sun as if it were a tiny dot, instead of its actual broad disk.  The radius of the Sun is one-quarter of a degree, and the near-horizontal light of the first flash from the Sun's tip is bent downward more than one-half of a degree as it penetrates the Earth's atmosphere, so the first flash of sunlight appears when the centre of the Sun is more than three-quarters of a degree below the horizon.  So the observed rise time is a few minutes before the "theoretical" sun rise.
Similarly, the last flash occurs a few minutes after the "theoretical" Sun set.  At the latitude of Ómahkiyáahkóhtóohp, five minutes are added to each end of the day and taken from each end of the night.  The so-called Equinox days are 12.2 hours long and the nights are 11.8 hours.  So the 12.0-hour-day/12.0-hour-night, or the Equalday/night, occurs to to three days, an average of 2.8 days, before the Equinox as the days lengthen in March, and two to three days after the Equinox as the days shorten in September.
Professor Freeman goes into a great deal more detail about this important concept in his book, which is an absolutely essential reference and full of gorgeous photography and clear and detailed explanation.  

Regarding the alignments at Stonehenge which encode Equalday/night, Professor Freeman deserves credit for being the first in modern history to rediscover these. He notes on page 116, "Strange as it may seem, during a century of speculation about a possible calendar in Stonehenge, nothing was published about an attempt to observe an Equinox Sun rise or set there."  

Professor Freeman explains that the observation lines for the winter solstice sunset and the summer solstice sunrise were well known, and aligned to some of the most massive stonework in the complex, but that the summer solstice sunset and the winter solstice sunrise both used a subtle alignment through the notches carved in Sarsen 58 of the "West Trilithon" (the trilithon composed of Sarsen 58 and Sarsen 57, and topped by the mighty lintel stone 158.  This trilithon can be seen in the image at top, on the right-hand side of the picture -- the notches in Sarsen 58 are clearly visible and form little "windows" with the edge of Sarsen 57 in the photo.

Below is a photograph showing Sarsen 58 and some of the other nearby stones, with their numbers indicated for ease of reference.

Amazingly, Professor Freeman found that the dramatic "windows" on Sarsen 58 also figured in the alignments for the Equalday/night sunrise and sunset, which makes the design and construction of this ancient site all the more mind-boggling in its sophistication.

A sample chapter from his book which is available online explains the Equalday/night sunrise and sunset alignments in detail, with beautiful photographs showing the "sight windows" created by the ancient builders to frame the sunrise and sunset on these important days (the resolution of the online version is not great, but the photographs in the book itself are wonderful and well worth the price of the book all by themselves).  

The discussion of Stonehenge's Equalday/night alignment begins on page 116 in that online chapter (same pagination as the book itself).  The photograph marked "Figure 4-45" on page 126 of that file (and the actual book) is perhaps the most dramatic, clearly showing the window formed by the lower notch on Sarsen 58 framing the setting sun of Equalday/night (taken on September 24, 2002).  That photograph shows that Sarsen 3 (on the other side of the circle from the image above) forms the left edge that creates the window with the notches in Sarsen 58.  

The photograph above is looking towards 58 from almost due west of the center of Stonehenge, with a view towards the northeast, and so the beam of light from the setting sun can be imagined coming in from the left side of the above picture and piercing through the notch in Sarsen 58 on its way towards an observer on the other side of the stones, on the other side of the circle. 

In order to visualize this phenomenon more clearly, the numbered diagram below is provided.  It is from Wikimedia commons here, and comes from a 2008 book by Anthony Johnson called Solving Stonehenge, which Professor Freeman praises highly and at one point declares, "I just want everyone to know that Johnson's work is more important than even he imagined" (317).  

In the online sample chapter linked above, Professor Freeman explains that the sight-line for the Equalday/night sunrise ran along the northern edge of Sarsen 20 (no longer present), which was on the circle just below Sarsen 21 (still standing and visible in the left side of both the photographs above and in the map diagram) and just to the east and a bit north of the fallen lintel stone marked 120 on the map above (you can see 120 lying embedded in the earth in the photos).  From the north edge of 20, the Equalday/night sight-line ran across the circle to the southern edge of a Sarsen on the far side of the circle, Sarsen 2 (still standing and holding up a lintel designated 102, which is also supported by circle Sarsen 1).

The Equalday/night sunset line ran in the opposite direction (of course), and an observer on the eastern side of the Sarsen circle looking west would use the northern edge of Sarsen 3 as the "near sight" and the notched southern edge of Sarsen 58 as the "far sight" to frame the setting sun.  The image on page 126, referenced earlier, shows this important sunset taking place between the edges of Sarsens 3 and 58.

Even after so many thousands of years, the precision of these alignments is breathtaking.  The fact that they are executed using such enormous stones makes the achievement even more so, and the fact that these same stones incorporate solstice and Equalday/night alignments, as well as the more complex lunar rise and set patterns, is almost incomprehensible.

But that's by no means all.  Professor Freeman also discovered that Stonehenge, like  Ómahkiyáahkóhtóohp in Canada, incorporates mechanisms to track a four-year "leap year" pattern (created by the fact that the earth itself does not turn an equal number of times during its circuit from one March or September Equalday/night to the next March or September  Equalday/night each year).  Because the earth turns an additional one-quarter of a rotation (almost -- for more detail see here) during its annual circuit each year, this has the effect of "adding a day" every four years.

Professor Freeman found that the very precise window created by Sarsen 58  sees the setting sun cross from left to right (heading from the south to the north, as one looks to the west) during March (a few days after the spring equinox) and sees the setting sun cross again from right to left (heading from the north back towards the south on its way to the December solstice) in September.  The alignments are constructed such that the sun rises can be seen looking east through Sarsens 2 and 3 (as described above) against even further mounds on the horizon which indicate the first two years of this four-year cycle, and such that sun sets are seen in the Sarsen 58 window during the last two years of this four-year cycle!

This kind of precision beggars belief.  Even more intriguing is the fact that Professor Freeman was alert to such a mechanism at Stonehenge because he had already found a similar "four-year" or "leap-year" mechanism encoded in the alignments at the sacred circle in Canada! 

As he explains in his book, Equalday/night varies greatly by latitude on the globe.  Ómahkiyáahkóhtóohp in Canada is in 51 north latitude, as is Stonehenge over in England!

One final observation is in order, and it is an insightful one which only Professor Freeman could make (because he is the first in modern history to discover these Equalday/night alignments at Stonehenge).  He points out (page 180) that the designers of these incredible sites had to have their alignments already planned out before they began to place stones on the ground.  That means that the widths of the stones at Stonehenge (such as Sarsen 58 and Sarsens 2 and 3 and 20 discussed above), as well as the widths of the trilithons and all the other stones were dictated by these very precise alignments that the architects wanted to establish!  

In other words, the builders of Stonehenge didn't just haul up a bunch of huge stones to the site and see what kinds of alignments they could make with them -- it is not in any way a haphazard arrangement.  They knew what they were doing before they did it, which means they knew what sized stones they would need before they obtained and transported them.  The sophistication of this site, executed in such a ponderous medium, speaks to the genius of the ancients.

As equinox approaches (and the ecliptic plane passes back below the celestial equator during the day for those in the northern hemisphere, as discussed in detail here and here), take some time to consider the vital distinction between equinox and Equalday/night, and then to appreciate the ancient people on either side of the Atlantic who constructed incredible monuments that still encode the subtle aspects of this important concept.