How accurate is the sundial?
Sometimes we can meet passersby comparing the time on the sundial with their own watch, and in most cases you can see the differences between the indications of both timers. Today we set our watches according to very precise atomic clocks, so are sundials not working properly? On the contrary, there are two corrections to make this possible. The first is related to the longitude of the place where our clock is located, and the second results from the so-called the equation of time that eliminates the variable speed with which the Earth orbits the sun. But one by one. The sundial shows the local time, which is the real time determined by the apparent movements of the sun. As 12 o'clock, that is, noon, we assume the moment when the Sun towers over a given place, that is, it reaches the highest point of its daily path from east to west. This means that in the places north and south of this place exactly the same solar hour. For example (looking from the coast to the mountains) in Gdańsk, Grudziądz, Toruń, Wieluń, Rybnik and Cieszyn, solar noon falls more or less at the same time. However, differences in time will be noticeable in towns located east and west of each other. In the case of our country, this difference can reach even approx. 50 minutes (e.g. in Chełm the sun reaches its peak 48 minutes earlier than in Zgorzelec). In order to overcome these differences, the zone time was introduced. Almost all of Europe is in Central European Time, which is determined on the basis of the 15 ° meridian running at our western border.In other words, 12 o'clock in Poland, Germany, France and many other European countries is when the Sun is exactly above the meridian of this longitude.
The longitude correction for our sundial will calculate the difference in degrees between the longitude where the clock stands and the zonal meridian (15 °) and multiply the result by 4, as 1 degree equals 4 minutes. Let's calculate the correction for Krakow, the city lies at the length of 20 °.
20 ° - 15 °=5 °, the result is multiplied by 4, and the obtained 20 minutes are subtracted from the reading from the sundial (these differences we subtract, not add, because most Polish cities lie east of the 15th meridian).
And one more example, this time for Warsaw, which lies at the length of 21 °:
21 ° - 15 °=6 °, the result is multiplied by 4. The correction is 24 minutes, which we subtract from the reading sundial.
This way we are one step away from establishing the official time.Now it is enough to take into account the correction resulting from the equation of time. We can read the correction for each day of the year from the table (photo 2).
For example, if on May 11 the sundial shows 9.46 (after taking into account the longitude correction), then after adding 3 minutes and about 38 seconds we get rounded to 9.50. In the period from March to October daylight saving time applies, so you should add one more hour to the obtained result. So in our example it will be 10.50 am DST. Four times a year, the correction of the time equation is 0, so sundials show the same time as mechanical watches. This takes place on April 15, June 14, September 2 and December 25.
Finding the local meridian
In order to set the sundial correctly in the garden, it is necessary to set a local meridian (photo 3). This makes it possible to determine in which plane the Sun is towering over the garden and the site of future time measurements. Note: do not use the compass for this, because the magnetic north differs quite significantly from the geographic north.
1.Choose a flat, sunny place. Let it be empty and free of inequalities. It should be the place where we plan to put the clock.
2.Stick a vertical rod or a straight stick into the ground. We plumb it very carefully. It will be a measuring gnomon, the same one used by the ancients. The optimal length is 1-1.5 m (too short will give unreliable readings, and too long will make the end of the shadow blur on the ground).
3.2-3 hours before solar noon we make a circle around the gnome with a radius equal to the length of its shadow (during summer time solar noon takes place around 1 p.m.) .
4.Mark the exact spot where the end of the gnomon's shadow touches the circle.
5.We wait for the end the shadow of the gnomon touches the circle again. This will happen 2-3 hours in the afternoon. We mark this place.
6.From the two points marked on the circle we draw arcs of the same radius, greater than the distance between the points (the size of this arc does not matter, but we suggest that to be 1.5-2 times greater than the distance between points).We mark exactly the places of their intersection - there should be two on each side of the gnomon.
7.We connect the points of intersection of the arcs with a straight line - it should pass through the place where we set it gnomon. Congratulations, this is our local meridian!
To increase the accuracy of such a measurement, you can make several circles around the gnomon with a different radius and mark the places where you touch them with a shadow on each one. The following procedure should be repeated for each of the circles, and the drawn meridian line should be the same for all of them. Regardless of the construction of the sundial, we can consider fixing the meridian and setting up a gnomon that will always show the moment of true noon. This is a unique attraction that no one else has at home.
The shadows are very short in summer and long in winter. Therefore, the most precise measurements are made around the equinox. This takes place twice a year on March 20/21 and September 22/23.
Drawing a horizontal clockThe dial can be drawn with the use of any of the free computer programs available on the Internet (eg Shadows Pro) or by drawing tools. Here is a step-by-step instruction (Figure 4).
1.Draw two perpendicular lines AB and CO intersecting at point O. The segments created in this way will be used later to mark the hours: CO will be the 12 o'clock line, AO - 6 am, and OB - 18. They will also mark the geographic directions of the world.
2.From point O we draw a line FROM so that the angle COD has the value equal to the latitude the place for which the clock is designed. In our example it is 50 °, which corresponds to the southern districts of Krakow.
3.From any point E on OD we draw a line EF perpendicular to OD and intersecting CO at point F.
4.From point F we draw a line GH parallel to AB.
5.From point F we measure the length of the segment FE with a caliper, and then mark point C on CO so that FC equals FE.
6.From point O we measure the length of the segment OC with a caliper, and then on the line AB we mark points A and B so that OA and OB equal OC.
7.From points A, B and C plot arcs of any equal radius.
8.We divide arcs into angles at 15 °.
9.From point C, extend the arms of the angles so that they intersect the GH line.
10.From the resulting points J and L we draw lines JK and LM parallel to CO and crossing the line AB.
11 .From points A and B we extend the arms of the angles separating their arcs so that they intersect the lines JK and LM.
12.From point O we draw lines connecting O with the places where the arms of the angles crossed the lines GH, JK and LM.
The hour lines of the horizontal sundial are now ready. Here is how to mark the hours on the plotted clock (photo 5, the order of their application must be clockwise).
In the example above you can see the arc drawn around the point O, where the hour lines end.It is not necessary, but thanks to it we avoid the thickening that arises when so many lines are concentrated in one place. This is purely an aesthetic question. We set the finished sundial so that the CO line coincides with the local meridian line, which we marked earlier at the place where the clock was installed. Point C and the end of the gnomon must face north, point A west, and B point east. Point O is important for another reason: it is the so-called center of the sundial, in which the bottom tip of the gnomon must be located.
Dariusz OczkiEditor of the Gnomonika.pl website
