Light measurements
during the Total Solar eclipse of August 11th, 1999

at Quatre-Champs near Vouziers (France).

Please send comments to Felix Verbelen

During the total solar eclipse of August 11th, 1999 light measurements were carried out,
using homemade automatic registration equipment.
One of the purposes of these measurements was to evaluate the light variations during solar
eclipses as seen by ancient observers.

The equipment
The light measuring device was essentially made up of a LDR (light dependent resistor)
through with a capacitor was loaded.
The loading time of the capacitor in such a RC-circuit equals t = R.C, (t seconds, R Ohm, C Farad).
The resistance of the LDR varies as aL-b where L = illumination in Lux, a and b being constants.
So,   t = a.C.L-b.

The RC-circuit was connected to the COM-port of a Toshiba T1950 Laptop.
The loading time t of the capacitor was measured at intervals of about 1 second and written to a file on
the hard disk, by a self-written QuickBasic-program

According to E.H. Weber's law of sensation (see "Photometry" by John W.T.Walsh, Dover Ed.),
the human eye's reaction to light variation is proportional to log(L).
Going back to our value for t we find that

      log(t) = log(a.C) - b.log(L)
     log(t)/b - log(a.C)/b = -log(L)

Neglecting for the time being the constants in this last equation, we find that log(t) ~ -log(L),
i.e. that log(t) is inversely proportional to the eye's reaction to light variations .
For this reason we used -log(t) as the scale for the light intensity in our graphs.

Since the main purpose of our measurements was to obtain a graph of the variations of the light intensity
as experienced by an ancient or a casual observer, the setup was designed in the first place to measure the general
reflected light intensity.
At previous eclipses we had directed our light detector towards the pole of the ecliptic, to the zenith, to some
other particular point of the sky, or to the ground.
We felt however that this was perhaps not the best way to measure what was actually seen by an ordinary
observer since normal viewing is done by watching reflected light.
Therefore, the light detector was pointed to a white plane that was permanently kept at a right angle to the
incident light of the Sun. The light detector was placed in such a way that at no instant there could be
interference by any shadow, other than by clouds or indeed the Moon's shadow.
A translucent white filter placed in front of the LDR further dispersed the reflected light.

Location and circumstances of observation
The place of observation was near Quatre-Champs (France), at coordinates 445'36" East,
4927'15" North, Altitude 198 meters.

Calculated times of contact (UT) were as follows:

      1st contact: 9h07m36s.5 (elevation Sun 43)
      2nd contact: 10h25m41s,5
      maximum eclipse : 10h26m46s.8 (elevation Sun 52)
      3rd contact: 10h27m52s.2
      4th contact: 11h49m00s.5 (elevation Sun 56)

Pictures and Graphics
The registrations started at 6h52m38s UT and were stopped at 12h13m54s UT.
Hereafter some typical pictures are given of the site and the equipment as well as some selected
graphs of the light measurements.
As one can see, the measurements are in a way affected by clouds, although less after than before totality,
but contain nevertheless some interesting elements.

* * Click on picture to enlarge * *

Observation of partial phases

Our observation site

Partial phase projected


Light Measuring Equipment

Complete setup

LDR directed to white plane

Selected graphs

Light variations between 9 h and 12 h UT

Light variations between 10 h and 11 h UT

Light variations between 10h20m00s and 10h21m00s UT

Light variations at totality