Astronomy On-Line

What is Local Noon?

Information about gnomons, sundials and related matters

In order to determine your geographical longitude by means of a Lunar Eclipse, you must also know the time of your Local Noon. But what does this term mean?

The following text will help you to understand this astronomical term and will also tell you how you can measure the exact time of Local Noon with a very good accuracy. Just follow the instructions given below!

Note that, if you want to return to the text about the Lunar Eclipse on September 27, 1996, you just click on the corresponding icon to the left.

Local Noon

Local Noon is the time when the Sun crosses an imaginary North-South circle in the sky. That is the circle that begins at the South point on the horizon, passes the point directly over your head in the sky and ends at the North point on the horizon. This circle is known to astronomers as the Meridian and the point directly above is called the Zenith. By the way, the point directly below you in the sky - which you cannot see because of the Earth - is called the Nadir. The words `Zenith' and `Nadir' come from the Arab language, as do many star names.

In Europe, Local Noon occurs when the Sun is seen exactly in the South direction. This is also the time when it reaches its highest point in the sky. Note, however, that in more southernly geographical regions, Local Noon may occur when the Sun is seen directly towards Zenith or even to the North.

Rotation of the Earth

You have seen this picture before:

As you know, our Globe makes one full rotation (360 degrees) every 24 hours. Thus, it rotates 15 degrees every hour. In other words, Local Noon at two geographical locations which are separated by a longitude difference of 15 degrees occurs with a time difference of 1 hour.

About Sundials

The Earth circumference (that is the length of the Equator) is approximately 40 000 kms. Let us now take a closer look at two European cities, Stockholm in Sweden and Barcelona in Spain.

Until the last century, most European countries, even individual towns, used to indicate the local time on the basis of a well-defined astronomical measurement with a sundial.

Sundials were usually positioned so that they indicated 12 o'clock, when the Sun was seen directly South, that is at the moment of Local Noon. The local time of a town was therefore regulated in such a way that it was 12 o'clock at Local Noon. The figure to the left shows one type of sundial - click on it if you want to see a larger version [Watch out! It is a 184k-file, so it may take some time to download it!].

Try this exercise: Stockholm is situated at a geographical longitude of 18 degrees 3 minutes East, and Barcelona at a longitude of 2 degrees 8 minutes East (Here, 'East' means East of 0 degrees longitude, which corresponds to the longitude of the Greenwich Observatory near London in England). Show that the Local Noons in Barcelona and Stockholm occur with a time difference of more than one hour. In which of the two cities does Local Noon occur first? Can you also tell what is the exact time difference of Local Noon in Barcelona and Stockholm (in hours, minutes and seconds)?

The Earth circumference is approximately 40 000 km - corresponding to 24 hours - see the drawing above. Show also that two cities, both situated on the equator, and just 25 kms apart, have a local time difference of about one minute.

You may also show that two other cities, both situated on the 60th parallel (geographical latitude +60 or -60 degrees), and just 25 kms apart, have a local time difference of TWO minutes.

Unified time in Europe

Of course, in a modern society you cannot have different time indications across a country, and you should not have too many different time zones on a continent. So, in 1894, the countries of Europe agreed to `synchronise' their watches (this means that they show the same time) and to use a unified indication of time. Since then, in continental Europe, with a few exceptions, you no longer have to correct your watch when you pass from one country to another; they all use the same time, which is refered to as Central European Time (CET). This is clearly a great advantage. Nevertheless, back in those days many sundial owners considered this time unification a catastrophe!

During recent years, it has been decided to shift the time during the summer period. Thus, each year from late March to late September, the time used in Central Europe is the Central European Summer Time (CEST) which is equal to Central European Time plus 1 hour. Similar changes are made in many other regions. In 1996, the time will change back to CES in late October. (In earlier years, the summer time period in Europe ended in late September; this year, the period has been prolonged by one month.)

Sundials are based on astronomical concepts, and before this time unification, the Sun passed the meridian of Barcelona at exactly 12 o'clock local time - per definition.

The same was the case in Stockholm, also per definition.

However, after the introduction of unified time, in Stockholm the Sun no longer passes the meridian at exactly 12 o'clock. For instance, on September 26, 1996, it does so at 12 hours 39 minutes Central European Summer Time.

The same is true in Barcelona. Here the shift is even larger. While the Sun previously was seen in South at 12 o'clock local time, this now happens at 13 hours 43 minutes.

It is no wonder, that many sundial owners felt that their sundials lost any relation to reality. Some of them became rather desperate and there are cases known where they tried to adjust their instrument with a hammer! The results of such desperate actions may still be seen on some European church sundials!

Local Noon and the Lunar Eclipse

When we use this unified European time indication, all European observations of the Lunar Eclipse on September 27 will also be `unified' in an `unfortunate' way.

This is because observers in Stockholm and Barcelona will both observe the Lunar Eclipse to end at about 5 hours 29 minutes Central European Summer Time, even though they do not have the same geographical longitude.

So, when we want to compare astronomical observations of the lunar eclipse and to use this to determine the longitude of an observer, we have to work in a different way.

We can do so by a classical, astronomical approach which is based on the old sundial time frame, as used before 1894. By means of this method, we only have to compare the time difference between a well determined moment of the lunar eclipse (for instance the begin or end of the totality), and the moment of Local Noon.

Here is an example to illustrate what this means: In Stockholm, the Sun passes South (Local Noon) at 12 hours 39 minutes Central European Summer Time. The Lunar Eclipse ends at 5 hours 29 minutes the next morning, that is 16 hours 50 minutes later.

Try this exercise: In Barcelona, the Sun passes South at 13 hours 43 minutes (Central European Summer Time). Show that the totality of the Lunar Eclipse, as seen from Barcelona, ends at 15 hours 46 minutes after Local Noon.

It is clear that this difference, that is the time interval between Local Noon and the end of totality, is not the same in the two cities, because Barcelona and Stockholm have different geographical longitudes.

If we turn this around, it is now obvious that may we measure the difference in longitude between the two cities by measuring the difference of the time interval between Local Noon and the end of totality, as observed in the two cities. So, if we know the geographical longitude of one of the cities, we can then determine the longitude of the other.

This is the principle of longitude determination by means of lunar eclipses.

How to determine the time of Local Noon

Fortunately, it is not difficult to measure the time of Local Noon. To do so, you find the direction towards the South and then note the time, when the Sun is seen in this direction. (If you are south of the equator, you must find North in the same way).

You can of course use a magnetic needle, but it is not very accurate. To obtain a more precise determination, there are better methods, based on measurements of the Sun itself. The important point is, that the Sun reaches its highest point in the sky, exactly at Local Noon. At this time, the shadows are the shortest.

Here we show a shadow-keeper, a Gnomon (a Greek word). It is just an ordinary pole :

In late September, the end of the shadow of the gnomon will wander along a straight line. The exact time for Local Noon may be found as indicated above.

There is another method, which will work at any time of the year, and may be even more precise :

Here you simply draw a circle around your gnomon. The Local Noon can be determined from the times when the end of the shadow crosses this circle, before and after noon. This idea dates back to ancient Egypt.

In the example above, point B is passed at 11 hours 15 minutes and point A at 13 hours 15 minutes, so the Local Noon is at 12 hours 15 minutes. (This type of method is also used for nautical navigation with a Sextant).

The next drawing shows yet another useful method. Look at the angle A-Gnomon-B, from A to G(nomon) to B.

If you draw two circles with the same radius and with their centres in A and B, you can divide the angle AGB into two halves of the same size. You just draw a line from the Gnomon to the point of intersection of the two circles, as shown. This line defines the North-South direction.

This may have been the method that enabled the ancient Egyptians to align the pyramides so accurately to the North-South-East-West directions.

The photo to the left is an aerial view of two pyramids near Cairo. Click on it to obtain a larger version (JPEG,16k). They were not placed randomly by their builders. Modern measurements indicate a North-South orientation with very high accuracy - in many cases to within a few tenths of a degree. This is an amazingl feat, considering that many of the pyramids are nearly 5000 years old. This shows the high level of astronomy in ancient Egypt!

A final word

Thus, if you are able to measure the moment of Local Noon at your location on September 26 (or September 27, or even on one of the following days), and also the moment of the beginning (or end) of totality of the Lunar Eclipse, you will be able to determine your geographical longitude with a very high accuracy, especially if you compare your measurements with those of other Groups.

On the small photo to the left (click on it to obtain a larger version [JPEG,17k]), you will see two students from the Soenderborg Amtsgymnasium (High School) in Denmark, using a sextant to measure the Sun's position in the sky. This instrument gives very accurate results, but remember, you do not need such sophisticated methods to participate in this project!

However, if you do not find the time to perform these daytime measurements, or if the weather does not permit, never mind, the Astronomy On-Line organisers will still tell you what the time of Local Noon would have been at your location, so please do not hesitate to transmit your observations of the Lunar Eclipse to us.

You are welcome to send comments about this text to:
EAAE European Student Project Group

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