DO YOUR OWN TEST - ELEVATION

While determination of the solstices by observation of the minimum or maximum elevation of the sun is possible by the use of an expensive sextant, this section only discusses the use of an astrolabe and the use of the measurement of the movement of the shadow as the sun changes its elevation.

An astrolabe is essentially a poor man's sextant, as discussed in the Solar Observations/Details section. While useful as a demonstration of the principle, it's accuracy will be far less than desired unless the light tube is quite long - and unwieldy, and it's compass rose is large enough to read elevations to the 1/2 minute level. It is not recommended for serious amateur use in determining the solstice.

To use the moving shadow method follow these steps.


First - Find the direction of South and the time of Mid-Day

While it is not important that you make your observations with a shadow cast by a straight edge facing true south, nor that you make your observations at true mid-day, it will be easier to make accurate observations if you are close to these conditions.

The simplest method to find the direction of south, if you are willing to use a modern clock or watch, is to find a vertical shadow at 12:00 noon (not daylight saving time). Then mark this shadow or by other means keep this direction for later use.

This vertical shadow mark can also be your basis for determining mid-day, for the vertical shadow will return to this position at the same time each day.

Of course, this determination of the direction of south and of the time of mid-day will be in error if you are not in the middle of your time zone. This will cause very minor errors in determining the solstice. However, if you want to determine it more accurately, and as the ancients had to do it without a compass or modern time piece, then Select FIND TRUE SOUTH and FIND MID-DAY from the menu.

Second - Find a suitable mid-day shadow and shadow marking surface

You will need two components for this shadow, an object that casts the shadow and a surface on which the shadow is cast allowing you to make daily markings of the position of the shadow.

Ideally this object should cast a horizontal shadow which will move up and down with the sun's daily change in elevation, and will cast a vertical shadow which will move horizontally to act as a sun dial to allow you to make your observations at exactly the same time of day each day.

The horizontal shadow which moves up and down may be cast by a straight roof line which is oriented to the south.

The sun dial shadow which moves horizontally with the passage of time should be a fine, but distinguishable, line. This can easily be achieved by attaching a nail or some other narrow or pointed object onto the bottom edge of the roof line. Call this the sun-dial marker.

The objects which cast the shadows should be at least several feet away from the selected surface in order for the shadow to move an adequate distance along the marking surface as the sun changes its mid-day elevation in the sky and to act as a sun-dial to allow the conduct of observations at the same time of day.

The size of the marking surface will be related to how far away the shadowing object is, and the amount of elevation and timing options you wish to be able to observe. The further away the greater distance the shadow will travel each day - but also will give a wider shadow width.

For relatively simple observations up to a week or so prior to the solstice, with a shadow coming from a one story roof overhang, a single strip of masking tape mounted vertically on the wall should suffice. However, a more complex system may be appropriate for the very closely spaced marks on days close to the solstice. Select MARKING SURFACE from the menu for more instructions on the size and shape of the marking surface.

A set of practice runs done in advance of the real test dates will help you select an appropriate marking panel and its location.

THIRD - Mark the mid-day location of the sun dial point

In order to make your marks at the same time every day, on the first day make a vertical mark where the sun dial marker shadow exists. Then draw a line straight up and down on the masking tape. While the horizontal roof line shadow will move up and down, the sun dial marker will always cross this vertical line to tell you when it is mid-day.

FOURTH - Mark the elevation of the edge of the main shadow

Mark the edge of the shadow at mid-day each day.

Select TIME AVAILABLE under Elevation to see how much time you can take in making your observations before the sun moves enough to introduce a significant error.

As discussed in the menu section on Details, the shadow will have a width, making it difficult to determine the exact place to put your daily mark.

It is not important to be precise with your mark until the solstice is approaching. At that time you will note with experience that the width of the shadow is greater than the daily movement of the shadow. Therefore, for a few days surrounding the solstice, it is advisable to use a more precise method for determining the position of either the leading or trailing edge of the shadow and use it for making your mark.

Select PRECISION METHODS to see a discussion on more precise marking systems.


FIFTH - Compare your markings

You will note that as you approach the Winter Solstice, with the sun going lower each day, that your daily marks will be higher each day until the Winter Solstice is reached. However, as the solstice is approached, the spacing of your marks will come closer and closer together and will eventually over-write each other, making a determination of the solstice by the mark with the highest elevation impossible.

Select MARK SPACING under Elevation to see a discussion of mark spacing near the solstice.

After the solstice has occurred the marks will start going back down as the elevation of the sun starts to be higher each day. When they become separated enough they will start having the same spacing as the previous set made when approaching the solstice. You can then estimate the date that the solstice occurred by counting the days back to where a mark was made at the same elevation, dividing this number in two, and subtracting that number from the current date. See example below.

If you have used a more precise method where you separately record the position of the shadow, then a plot of that data would look similar to the chart above.

LAST - Schedule your winter celebration

Having done the above you can predict the winter solstice and can schedule your next winter's celebration. If you wait about three days to make sure you read your marks correctly, and that no one will be able to criticize you for calling it too early, you will find that you can safely schedule Christmas.

FINAL NOTE

While the change in elevation and declination of the sun at the solstices is stopped, it is at the maximum rate of change at the equinoxes. Thus a shadow movement will be the fastest and more easily marked for future use if you want to schedule a spring holiday. Select ELEVATION AT EQUINOX to see data on the rate of change at the vernal equinox.