12:00 GMT

The mirror and the camera were both placed on the prime meridian near March, Cambridgshire, in the  UK. With the camera faceing north the flim shows the sun  passing over head at midday reflecting the sunlight into the lens. 

When the sun is at its highest point exactly above the Prime Meridian, it is 1200 noon at Greenwich. GMT which is still widely used as the standard time against which all the other time zones in the world are referenced. A prime meridian is a line of longitude in as geographic coordinate system at which longitude is set at 0˚. This great circle divides the earth into two hemispheres, called the Eastern and Western Hemispheres.