USING OBSERVATIONS OF THE SUN
Another method of determining the date of the solstices is to observe the variations in the position of the sun in the sky as it progresses through the year.
The earth makes one rotation around the sun every year. Further, the earth's axis is tilted 23 degrees and 27 minutes from the plane of its orbit around the sun. The result is that the height of the sun at mid-day, and the direction of sunrises and sunsets, varies throughout the year - reaching maximums and minimums at the solstices. Thus, measurement - or simple observance - of the related angles can be used to determine the solstices.
NOTE: All of the information relative to observations of the sun assume that you are in the northern hemisphere. If in the southern hemisphere, simply substitute north for south and vice versa.
Click here to view a more extensive article on the solar system and solstices.
There are two different approaches to solar observations of the solstice.
The first uses the fact that you already know from other sources when the solstice will occur. You then set up an observation method that can be fixed by a single observation on the day of the solstice, and that can be used in subsequent years to again observe the event. Thus, you would not have to travel to Stonehenge or any of the other many similar sites to enjoy the event - and perhaps make it a social occasion with your friends.
The second is much more challenging and involves determining when the solstice will occur by making your own observations or measurements. That is the main thrust of this section.
There are two TYPES of observation that can be made, and two different METHODS of observation that can be used for either type.
As the earth passes around the sun the height of the sun at mid-day goes from a high point at the summer solstice to a low point at the winter solstice. The size of the angle formed by this height above the horizon is called Elevation. The winter solstice occurs when this angle is a minimum and the summer solstice occurs when it is a maximum.
Additionally, the sun rises and sets in a more southerly direction at the winter solstice and a less southerly direction at the summer solstice. The size of the angle of the sun on the horizon at sun rise, or sun set, from true North is called Azimuth. Similarly, the winter solstice occurs when the sun rises and sets with the most southerly angle and the summer solstice occurs when the sun rises and sets with the most northerly angle.
DEFINITION OF TERMS
The measurement of the sun's altitude and azimuth are illustrated in the following graphic.
Direct viewing involves directly seeing the sun and aligning its position on a horizon, distant object, or compass with respect to the eye.
Since it is difficult to view the sun directly at mid-day without a filter, direct viewing with the naked eye is limited to observing the azimuth of the sun just as it rises (sets) or when its passage is nearly shadowed by an intermediate object such as a large stone, the wall of the entrance to a cave, or a mountain top. In these cases the amount of light being viewed may be reduced by either or both of the following phenomena. First, as part of the sun is shadowed or below the horizon so that only a portion of the sun's light is visible. Second, the atmospheric bending of the sun's rays causes the brightness to be less for the few moments of the apparent sunrise and sunset.
Use of a sextant to measure elevation is also a direct viewing method since the eye views the sun directly, although such views are through light filters to protect the viewers eye when viewing a bright object such as the mid-day sun. However, such use is not considered to be an amateur method and is not considered in this discussion.
Indirect viewing involves allowing the sun to cast either a shadow on a surface, or a reflection from a fixed position surface, such that the position of the shadow or the reflection provides information about the elevation or azimuth of the sun.
Indirect viewing is more frequently used for mid-day elevation measurements, since more light is available to cast a distinct shadow. However, indirect viewing using a poor, and possibly filtered, reflecting surface may also be used for either azimuth or elevation measurements since adequate light is available for good readings in both cases.
MEASUREMENT ACCURACY REQUIREMENTS
While actual elevation and azimuth data is provided later for illustration purposes, it is noted that the azimuth or elevation angles at the solstices will vary depending upon the latitude of your observation location. Further, it is not important to know the specific size of the angles. It is only important to observe when the angle stops changing and reverses its direction of change.
The effect of the earth's orbit around the sun makes the rate of change of both the azimuth and elevation decrease proportional to a trigonometric sine wave as the time for the solstice nears, to reach a value of zero at the instant of the solstice, and then to increase in the same fashion after the solstice has passed. Conversely, the rate of change is near the maximum at the in-between equinoxes.
Thus, it is impossible to determine the instant of the solstice without very sophisticated scientific equipment.
However, although data is available on the date and time of this instant, it is customary to merely determine and report the day on which the solstice occurs. Further, it is likely that the earliest peoples to think in terms of the solstice were not concerned about the specific day - and were more interested in determining the general period in which the solstice took place. In fact, many of the related celebrations took place over a several day period.
An after the fact determination of the approximate date of the solstice may be made using relatively inaccurate observation methods, plotting the results and then, after the data has been plotted, observing when the change of direction probably occurred.
However, if you want to determine the exact day on which it occurred, then more precise observations are required.
Further, if you want to demonstrate your ability to verify its occurrence on the publicized date, then you must have previously determined the solstice and have precisely set markers based on that previous date that you can now observe with precise observations.
Quantitatively, the change in the sun's elevation or azimuth is only on the order of 1/2 arc minute per day on the two days surrounding the exact instant of the solstice. (An angle of one degree = 60 arc minutes.)
Further, while the elevation of the sun does not change significantly during the minute or so that you might need to make a mid-day elevation observation, the azimuth of the sun does change rapidly during sunrise and sunset at all latitudes except the equator. At the San Francisco latitude the sun rises in an about 45 degree angle rather than the 90 degree vertical rise that one might have thought.
Since the diameter of the sun is on the order of 30 arc minutes, the accuracy of precise observations must be on the order of 1/60th of the sun's diameter.
While this is possible using careful direct vision of the edge of the rising or setting sun shadowed by an object whose shadowing face is at an angle related to its latitude, it is not reasonable using simpler shadow marking (for either elevation or azimuth observations) since the very small elevation change per day would cause shadow marks made on subsequent days to be so closely spaced that they would over-write each other. However, it is possible by using a simple home-made device having closely spaced grooves where the position of the shadow is determined by counting the number of grooves from the leading edge of the shadow to a fixed point.
Finally, do not expect to determine the day of the solstice by an initial observation on the day of the solstice. By definition, the solstice occurs at the exact moment when the change in declination or elevation completely stops. Since you can not tell when it completely stops until you make another observation showing that it has reversed course, by definition, you can only back calculate when it did occur.
Of course this limitation only occurs for your first series of observations. Once you have determined the date of the solstice, and have fixed the position of your observation by appropriate "stones", pictures, shadow marks, etc., then you can determine the solstice in subsequent years by merely referring back to the prior year's findings. Even better, maintain a record over several years, and average the results, to give a more exact result. (After all, the ancients had centuries of observations to average to come to their results.)