A Brief History of the Analemma Sundial
The instrument was designed by Ronald
Rosensweig and Hans Muller when they were engineering students at the
How
the Sundial Works
Important Features
The working
section consists of two circular bronze strips mounted perpendicular to
one-another. The analemma engraving is
mounted on one bronze circle, while the hour engraving is made on the other
circle. A small focusing lens is mounted
in the junction of the circular strips opposite the analemma engraving (in the
top photograph, it is the junction against a window). The working section can rotate about an axis
indicated by the stainless steel fixtures.
The Hour Circle is calibrated with
reference to Greenwich Mean Time. There
are 12 engraving lines per hour, so each engraved unit represents 5 minutes of
time.
Reading time with the Sundial
On a sunny day, a visitor who wishes to
read the local time rotates the working section until the lens focuses an image
of the sun on the analemma at the current month and day. The visitor then reads the time, directly to
5 minutes and estimating the nearest minute, by using the Index Arc.
Improved Resolution
If the Index Arc were built closer to the
Hour Circle, a vernier scale could be engraved on the Index Arc, and time could
be read directly to the minute, with an estimate of a fraction of a minute. This improvement is scheduled to be installed
on the original instrument in 2005, and it will be included in the replica
installed at Blandair.
Technical Details
The analemma is a means of accounting for
the change in the location of the apparent path of the sun across the sky
during the year. It is formed from
images of the sun at its highest point projected onto a horizontal surface at
the equator. This change is indicative of the fact that the earth’s
axis of rotation is not perpendicular to the plane of its orbit about the sun.
Note that the two lobes of the analemma are
not the same size. This is because the
earth’s orbit is elliptical and not truly circular. During the months indicated by the smaller
lobe, the earth is closer to the sun than during the months indicated by the
larger lobe. Since the orbital speed of
the earth increases as it gets closer to the sun, “solar days” are shorter on
the smaller lobe than they are on the larger lobe. Thus, local solar time, as measured by blade
sundials, requires further correction to correspond with solar time as measured
by mechanical, electronic, or atomic clocks, and there are tables for making
this correction. However, after initial
calibration, an analemma sundial reads local solar time directly, since the
analemma makes this correction automatically.
Aligning and Calibrating the Sundial
The axis of rotation of the sundial is to
be aligned parallel to the earth’s axis of rotation. One way of doing this is described here. First, place the instrument on a horizontal
surface and raise what will be the northerly end of the instrument’s axis and
lower what will be the southerly end until the instrument’s axis is inclined
with the horizontal by the local latitude (in the northern hemisphere); this
can be set in the manufacturing process.[1] Next, the instrument’s axis must be contained
in a plane that contains the geographic north and south poles. This can be done at the installation
site. Ensure that the base on which the
dial is to be set is horizontal. In the
absence of the dial, suspend a plumb line so that its shadow falls across the
base when the sun is on the overhead meridian (12:00 noon LST),[2]
lightly scribe the line of the shadow across the base. This line is oriented
North-South. Mount the dial on the base
with its axis parallel to the scribed line.
Once the sundial’s axis of rotation is
aligned with that of the earth, the Index Arc can be engraved to read local
solar time. Focus the image of the sun
on the analemma at the current month and date.
Reading the time from the calibrated watch, engrave the index line on
the Index Arc opposite the local solar time on the Hour Circle. Complete engraving the vernier scale on the
Index Arc with the length of 5 vernier scale units equal to the length of 4
units on the Hour Circle.
Funds for the Garden
Donations may be sent to:
The Blandair Foundation
6425 Little
Analemma Sundial T-shirts are available
from our web site store
The Blandair Foundation is a 501(c)(3), non-profit
organization.