How to read them

Introduction

Hour reading

The Equation of Time

Season reading

The local noon

The motto

Introduction

In the following pages I am describing the most common form of vertical sundials : a dial with french (astronomical) hour lines showing the local time, or the time zone mean time, with a polar style.

Other types of dials are explained in the "in depth" chapter.

Hour reading

You can read the time from a sundial in a similar way as reading time from a common watch.

In a sundial there is one only hand that is the shadow the style casts on the wall where the hour lines are drawn : the shadow marks the hours and there's no hand for the minutes.

When the shadow is on a hour line you know that the time is what is written near the hour line.
When the shadow is between two hour lines, the time is some value between the two times written near the hour lines; with some common sense  it is possible to estimate the minutes. Sometimes half hour or quarter hour lines are drawn and they can help in the reading.

The style shadow rotates counter-clockwise around the point where the style is fixed to the wall, it so rotates on the contrary with respect to a watch hand. In the morning the shadow is in the left side of the dial, it moves to the bottom center where it reaches the noon line, then in the afternoon it proceeds to the right side where it will disappear at the sunset.

So the sundial is similar to a watch, it contains 24 hours instead of 12, and the shadow moves in a counter-clockwise direction.

For instance, the dial that is shown in the previous picture shows that it is a few minutes past 9, that is about 9:05.

Of course the sun does not know anything about Daylight Saving Time !
So in summer, when DST is applicable, you have to add one hour to the time shown on the sundial. In the same previous example the time would be about 10:05.

The Equation of Time

Don't be afraid of this scientific term, actually it is simply the error that exists between every sundial and a classical watch.  

A sundial only shows the correct time 4 days in the year¹ (April 15^th, June 13^th, September 1^st and December 24^th) and in all the other days it can be slow up to 14' 16'' (on February 11^th) or to be in advance up to 16' 29'' (on November 3^rd).
Here is the explanation.

It is well known that the earth rotates around itself and it performs one complete rotation in 24 hours.
However the apparent movement of the sun in the sky is not so constant, being its speed slightly different day to day.
This is due to the inclination of the earth axes with respect to the earth orbit and to the elliptical shape of the orbit itself.
So every day the sun adds some seconds of delay (or anticipation) giving as a result a difference with respect to the expected mean value that can reach about plus or minus 15 minutes.

This error is so unavoidable. However it is possible to correct the sundial reading by summing or subtracting this known error from the reading.

The value of this error in the year is usually shown in the form of a graph or in a table and it is called Equation of Time. Sometimes the graph or the table are drawn on the dial so that it becomes easy to find the current value and to correct the reading.

The following table shows the approximate values of the Equation of Time during the year :

Table 1 — Approximated values of the Equation of Time

(add the value to the sundial reading in order to obtain the mean time)

The value that is shown in the table must be added to the time shown in the sundial : a positive value means that the dial is late, a negative value means the dial is in advance².

If for instance the previous picture had been taken at October 11^th then, being the EoT equal to -13'15", the current time would be about 9:05-13'15" @ 8:52 that is about 10 minutes to nine.

It is evident that it's a fuzzy topic to correct the dial time with the EoT : sundials should be used as a long time ago in order to have just an idea of the current moment in the day, to know if it's lunch time, dinner time or if the sunset is near to come, and they should help us to live our life with that frame of mind that was usual in the ancient natural world.

It is worthy to note that some sundials include the correction for the EoT by means of a curved 8-shaped line that is called Analemma. More on this topic in the following pages.

¹  Actually a sundial when well designed can be very precise and "correct time" in this context means with respect to the artificial time that man invented.  

²   Thre is no standard definition of the EoT so it is sometimes defined with the opposite sign. In this case the EoT value has to be subtracted from the sundial time.

Season reading

Hour lines in a sundial are always included and day lines, that are used for season reading, are optional and they can be present or absent in a dial (a similar situation applies to classical watches where time is always shown and the current date is an option).

Date or daily lines, when included in the dial, give a measurement of sun declination (angle between sun rays and the equatorial plane) and, being months and seasons strictly related to sun declination, they show the current season in the year.

Zodiac signs, instead of months or seasons, are usually drawn in a dial as these elements are strictly related to well defined astronomical events.

Equinox (spring or fall) line is usually drawn. This is a straigth line that crosses the hour lines. It is horizontal in a south facing dial, it is inclined to the left (to the right) for west (east) declining walls.

Other common lines are related to winter (December 21^st) and summer (June 21^st) solstices. These are two curved lines (hyperboles) cross the hour lines respectively above and below the equinox line. They mark the minimum and maximum height of sun in the sky.

Sometimes the other 4 zodiac lines are drawn too.

How to read season time on a dial ? Now it's not the position but the length of the shadow that shows the season. Look at the tip of the shadow. When it reaches exactly one of these lines, then we are at the solstice or at the equinoz or at the zodiac sign start date. When the tip of the shadow is somewhere between two lines, we again have to approximately estimate the date. Of course only the two solstice line mark two exact dates while all the other lines apply to two different dates in the year. So the seven date lines apply to : 21/12 (winter solstice), 21/11 & 21/1, 21/10 & 21/2, 21/9 & 21/3 (equinoxes), 21/8 & 21/4, 21/7 & 21/5, 21/6 (summer solstice).

It is worthy to note that sometimes the date mark is not provided by the tip of the shadow but by the shadow spot created by a nodus placed in the style (as in the previous picture). Rarely this nodus is replaced by a disc with a hole (gnomonic hole) so that a light spot allows you to read both the date and the time.

For instance, from the previous picture it can be assumed that the picture was taken some days before the spring equinox (end of february / beginning of march) or some days after the fall equinox (about at the end of october).

The local noon

Starting from the end of 1800 (in Italy since 1893) the world has been divided into 24 time zones, each one including the countries where the same time applies. This means that the true noon, that is the time when the sun reaches the maximum height in the sky, is such at the center of the time zone only, while in other places it happens earlier or later with respect to the mean noon time applicable to all the time zone (4 minutes of difference for each degree of longitude).

For instance in Castellamonte (To) - Italy - this difference is 29 m 9 s and, being this place at the west of the reference meridian, true noon happens at 12:29:09 (without taking into account the Equation of Time !).

In a vertical sundial the true noon is always drawn on a vertical line starting from the base of the style. Sometimes this line is drawn even on dials showing the mean (time zone) time and it is marked with the letter M or with the symbol of a bell (see previous picture).

The motto

Although this is not useful for reading time or date, a motto is always present on a dial.

Sometimes it teaches ancient principles, sometimes it is ironical, it often expresses religious concepts related to vanity of life and certainty of death.

In my experience it is often more difficult to find the motto than to design the dial.

Fig. 1 — Sundial showing the TMEC (Central Europe Mean Time = GMT+1)  

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updated on venerdì 06 marzo 2015