The word equinox is generally taken to refer to the days when, at every point on the earth, day and night are of equal length. But this definition of the equinox is a bit misleading. Since it gets light before the sun rises and remains light after the sun sets, the actual period of darkness at the equinox will be substantially less than twelve hours, the exact amount depending on latitude and how one defines the boundary between twilight and night. It is even misleading to say that the equinoxes are the days when the time between sunrise and sunset is the same as that between sunset and sunrise, because this definition assumes a flat horizon and the absence of atmospheric effects, particularly refraction. In practice, one cannot determine the equinox by measuring the length of time between sunrise and sunset.
From an astronomical perspective, the definition of the equinoxes is much clearer. They represent the positions in the earth’s orbit where the axis joining the two poles leans neither toward nor away from the sun. These points are roughly a quarter of the way around from the two solstices, although not exactly, because the earth’s orbit is elliptical rather than circular. It is also possible to express this technical definition from the perspective of an earth-based observer, using the concepts of the celestial sphere and declination. The equinoxes are the times when the sun, in its progress along its annual circuit through the stars known as the ecliptic, crosses the celestial equator, in other words, when its declination is zero. This occurs twice in each year, at the vernal, or spring, equinox, which generally falls on March 20 or 21 in the modern (Gregorian) calendar, and the autumnal equinox on September 22 or 23. (Of course, spring and autumn are transposed in the southern hemisphere.)
This concept of the equinox, in the sense in which astronomers use the term today, was a natural one for the astronomers of Hellenistic Greece, because they were attempting to develop geometric models that fit the available observational data. For example, Hipparchus in the second century b.c.e. developed a model of the motion of the sun around the earth in which the sun orbited in a circle at uniform speed but the earth was displaced from the center of the circle. If we were in Hipparchus’s shoes, approaching the problem geometrically, then we might naturally progress as follows. First, draw a circle to represent the sun’s orbit around the earth. Then divide it into quarter-points (the solstices and equinoxes). Then consider how these could best be determined through observation. Then measure the time intervals between them (leading to the discovery that they are not uniform). Finally, try to find a way of modifying the model to fit the observations. The point of this exercise is to show that, while the equinoxes do enter this particular thought process quite naturally, this is only because we are approaching the problem in a geometrical way. What if, in common with many other human societies in the past, especially the prehistoric past, as well as in the indigenous present, we were not inclined to try to understand or explain the cosmos using geometrical models? In other words, how relevant is the concept of the equinoxes likely to be among other human cultures in general?
To add a further complication, when people use the term equinox in a cultural context, they commonly mean not the true equinox (as just defined) but the halfway point between the two solstices. Unfortunately, this, like the “night and day” definition, is problematic in practice. For a start, one might mean the halfway point in space or the halfway point in time. The first of these might be identified, say, by observing the rising points of the sun at the two solstices and then marking out the day when it rises halfway between them. The second would involve counting the number of days between the adjacent summer and winter solstices, then counting half that number of days. In the case of the spatial halfway point, unless one is close to the equator, the sun does not rise vertically but at an angle. This means that, unless the horizon is smooth and flat, the resulting “equinoctial sunrise” could be as much as several degrees (and the resulting “equinox” many days) away from the true one. The temporal halfway point is less problematic, in that it is independent of place, thus making it easier for us to spot if things were consistently aligned upon sunrise (or sunset) on that date. The engineer Alexander Thom did believe that this event was extensively known in prehistoric Britain and marked by monumental alignments; for this reason the day halfway in time between the solstices is sometimes referred to as the Thom equinox. Because the earth does not orbit the sun at a constant speed, it actually occurs one or two days later in March, and earlier in September, than the true equinox, at a time when the sun’s declination is about +0.5 degrees.
When monumental alignments are found facing close to east and west, such as the two passages at the passage tomb of Knowth in the Boyne Valley of Ireland, it is often assumed that they are equinoctial in the “halfway between the solstices” sense. It is certainly easier to argue on pragmatic grounds that the “halfway” rather than true equinox was the one marked in prehistoric times, in that the halfway point (whether in space or time) could have been identified using direct observations from the place concerned. However, this explanation leaves open the question of how this might have been achieved in practice, and to what precision. Identifying the Thom equinox, for example, requires an efficient system of recording or memorizing numbers of days up to at least 180 and presupposes that the solstices can themselves be defined to pinpoint precision—which is not self-evident, as is seen at the Bronze Age site of Brainport Bay in Argyll, Scotland. This is quite different from suggesting, for example, that past communities in temperate latitudes may have divided the seasonal year into two main portions, the start of summer and winter halves being recognized and celebrated at times that happen to approximate the equinoxes. The point is whether the equinox existed as a meaningful concept for them, and hence a meaningful concept for us in trying to explain, for example, monumental alignments to the east or west.
This question leads to another, fundamental question. Even if certain human societies in the past had the technical means to accurately determine the halfway point between the solstices, either in space or time, are they likely to have been motivated to do so? Historical and anthropological evidence suggests that the answer in most cases is no. To judge from historical and ethnographic examples, the norm is to identify dates that are significant in a local context, such as in a ceremonial calendar related to seasonal agricultural activities. Ceremonial significance might also be attached, say, to the day on which the sun rises or sets in line with a significant feature in the visible landscape, such as a sacred mountain: an example of this occurs at Ca-caxtla in Mexico. The point is that space and time are generally not conceived in the abstract, as in the modern scientific tradition, but in relation to physically perceived objects and events. Now, the solstices are physically discernible (to whatever precision) as the days when the lengths of day and night are at their longest or shortest. In addition, the solstitial rising and setting points of the sun have a concrete significance in any landscape as the limits of the sun’s motion, and the boundaries of those parts of the horizon where the sun can rise or set. The equinoxes, and the positions of equinoctial sunrise and sunset, on the other hand, have no inherent significance. Division into two equal parts only tends to strike the modern investigator as an obvious way of subdividing up the arc of horizon where the sun rises, or the time between the solstices, because he or she perceives space and time in an abstract way in the first place.
The fact that the equinox has, nonetheless, acquired crucial liturgical importance within the Christian world in connection with the timing of Easter is attributable to the roots of that tradition in the Classical world. The difficulties of recognizing and marking the equinox in medieval times were considerable, and this is reflected in the process and practice of orienting churches.
But outside this context the evidence from around the world that people in the past were directly interested in the equinox (however defined) and intentional equinoctial alignments is rather thin. For instance, if we look impartially at the patterns of orientation of local groups of European prehistoric temples and tombs, rather than just picking out individual examples, then the evidence suggests a clear interest in the motions of the sun, manifested in many different ways in different places and times, but no clear preference for equinoctial alignments. In Mesoamerica, to take another example, the easterly direction was itself of fundamental importance, and convincing evidence exists for horizon calendars marking sunrise at different dates in the year against topographic landmarks. However, there is little or no direct evidence of precise equinoctial alignments. Despite this, some researchers continue to argue that topographic landmarks could have been used to mark off equal divisions of the year into quarters or eighths.
When trying to interpret the orientations of ancient structures, it has become commonplace to apply a “toolkit” of potential astronomical targets in which sunrise and sunset on the solstices and equinoxes invariably appear at the top of the list. One problem is that the term “equinoctial” is often poorly defined and used all too often simply as a convenient label for east-west alignments. More seriously, it is usually considered self-evident (or else implicitly assumed) that the equinox was a meaningful concept, whatever the human society being investigated, and hence that sunrise or sunset on that date was a possible target for orientations. In fact, outside the framework of the modern Western scientific tradition (which has its roots in the Greek geometrical tradition), it is far from self-evident in most cases that the equinox was of any significance at all.
The equinoxes, perhaps more than any other astronomical concept, demonstrate the dangers of applying Western concepts too readily and uncritically in interpreting the material remains of human cultures in the past. It is essential to deconstruct concepts that are particular to our own mindset if we want to understand actions that arise as a result of the mindsets of others.
