Analemmatic Sundial - Epping Forest, Essex, UK
N 51° 39.841 E 000° 02.485
31U E 295395 N 5727815
This "human" sundial is located in Epping Forest between the Epping Forest Visitor Centre and Epping Forest Field Centre. Small, knee high posts point the way.
Waymark Code: WMJ7MC
Location: London, United Kingdom
Date Posted: 10/07/2013
Views: 1
The "path" with the months marked on it is orientated in a north south
direction with the "hour" marker stones set around it. Close west is an
information table that tells us:
This is an analemmatic sundial and the hour posts are
set out according to unique calculations prepared for this locality.
Instructions for telling the time by the analemmatic sundial
1. Stand on the paving stones along the north/south centre line according to
today's date with your back to the sun and raise one arm above your head.
2. Now look at your shadow which will be pointing towards the hour posts.
This will show the local sun time.
3. The sun's rays fall here at High Beach 10 seconds before they reach
Greenwich. As the difference is so small, it is really the same as Greenwich
Time. (Other places further east or west will be aout by a greater amount).
4. Because the earth goes round the sun in an elliptical path, the sun
appears to vary its speed throughout the year and it can be up to a quarter
of an hour fast or slow compared with an accurate clock. To work out the
true time, you must apply the correction called the Equation of Time, as
shown in this graph, to obtain Greenwich Mean Time.
5. Of course we have to add an hour to give British Summer Time.
This sundial was designed and donated to the Epping Forest Centre by Comdr.
Richard Andrewes.
The
Maths website tells us about an analemmatic sundial:
We often take the problem of telling the time or of
finding the day's date for granted, but finding solutions to both of these
questions is vital to our civilisation. Early men and women needed to have
an accurate notion of the seasons in order to know when to plant and harvest
their crops. As civilisation developed it became important to know the time
during the day. Two times were obvious as everyone was aware of when the sun
rose (dawn) and when it set (dusk). More accurate observations of the sun
then showed that having risen in the East, it climbed to its highest point
in the sky (due South in the Northern hemisphere) before descending towards
its setting point in the West. The time of the highest point reached by the
sun (which is when the shadows were shortest) is called Noon. Knowing dawn,
Noon and dusk gave us morning, afternoon and night.
Every civilisation has been interested in the way the sun moves, and
astronomy and mathematics have always developed alongside each other. Better
observations called for quicker and more efficient mathematical methods.
More detailed theory drove people to make more accurate observations. In
this way both theory and practice evolved together. In this article we give
some theory about the way the sun moves and then concentrate on a practical
method for building a type of sundial known as an analemmatic sundial. We
choose initially not to give too much justification for the mathematics
behind the construction of this dial. Details of the theory of this dial can
be found in a later section of this article.
Note that the following article assumes that you are in the Northern
Hemisphere. If you want to build a sundial that is to work South of the
equator you will need to make some changes, but the following method is easy
to adapt.
What is an analemmatic sundial?
An analemmatic sundial is a particular kind of horizontal sundial in which
the shadow-casting object is vertical, and is moved depending on the date,
or to be more precise, depending on the declination of the sun on a given
day. The time is read from the dial by noting where the shadow cast by the
vertical pin crosses hour points laid out on an ellipse. If we make the dial
on the ground and large enough, we can use the shadow cast by a person. This
makes it very different from the traditional sundial we see often in parks
and gardens where the shadow is cast by a triangular shaped wedge. The
analemmatic sundial is perfect as a piece of large mathematical sculpture.