Light angles

Before installation, conservators check and document the condition of incoming loans. Here, conservator Andrew Pearce talks about preparing Colonel Light’s sextant from Adelaide City Council Archives.

For an Adelaidian like me, Colonel Light is a very significant figure. The layout of Adelaide, its extensive parklands and the width of its streets were all fundamental parts of Light’s design, and unlike many cities where the original planning vision has been changed over time (in many cases for the worse), Light’s concept for the city of Adelaide remains intact.

Detail from Light's 1837 plan Adelaide. Click to see the whole plan. (source:

Working on Light’s sextant was, for me, a great thrill. It’s a stunningly beautiful example of the instrument maker’s art, but more than that, it’s a direct link with the man himself. This is “his”. He held it in his hands and carried out his surveying with it. Could you imagine walking in Neil Armstrong’s moon boots, writing at Jane Austen’s desk, playing Coltrane’s saxophone? Looking through Light’s sextant is a little like that.

Sextants are a bit like an abacus to many of us. We know that you can use an abacus to perform more advanced mathematics than simple addition and subtraction, but most of us have no idea at all how, it’s pretty much a lost art. The use and function of a sextant is the same. We know that it’s a surveyor’s or captain’s “angle measuring thing”, but for most of us, the details are part of an arcane past, especially in these days of GPS. In inspecting and documenting Light’s sextant, I had to undertake enough research to be able to name its parts, and this led to a basic understanding of the method of operation.


Left: The half-silvered mirror. Right: The curved scale. (Photos: Andrew Pearce)

The key component of a sextant is a half silvered mirror. That is, a mirror where only half of the glass is reflective, while the other half remains transparent. The user looks through a telescope at a distant object through the transparent half of the mirror while simultaneously looking at the image of another object (usually the sun or a star) in the reflective half of the mirror. Tilting the moving arm attached to the top mirror on the sextant enables the viewer to align the images of the two objects in each side of the half silvered mirror. The angle between the two can then be read on the curved scale. Beyond that, it all comes down to trigonometry.

One Response to “Light angles”

  1. Isa says:

    Thanks for this insight Andrew. I must admit that I’m in the ‘angle measuring thingy’ camp, but your sense of looking through the same eyepiece as the man himself is a familiar one. Working with historical objects is a marvel!