Measuring Sun Altitude
For various calculations, such as the hour angle and deriving the time from it, we need the altitude of the Sun, measured as an angle in degrees from the horizon.
There are two ways to determine the altitude of the Sun with the Quadrant Telemeter.
1. Sun altitude via plumb line and weight
With this method we hold the Quadrant Telemeter upright and aim at the Sun, the plumb line measures the angular height from the vertical.
To measure the altitude of the Sun, we need a workaround since we cannot look into the Sun directly. For maximum precision, install cotter pins, or small metal or wooden rods, or similar in the holes parallel to the sighting edge so that on the quadrant side, two equal parts are sticking out. Alternatively, for models without holes, two binder clips or a straw can be aligned on the sighting edge.
Sun altitude via plumb line and weight. Never look directly into the Sun; use this back-sighting technique instead. A short cut to quick-and-easy Sun sighting is to hold up the quadrant as is and rotate and tilt until the orientation with the smallest shadow possible is reached.
- Roughly align the instrument so that the Sun is in the direction alongside the sighting edge.
- Rotate and tilt the instrument in order for the sunlight to cause the smallest shadow possible.
- Align the shadows of the two clips in such a way that they are perfectly congruent.
- Read the altitude of the Sun where the string crosses the degree scale.
(This method is also used to read the time directly on the horary quadrant with calendarium.)
Example
In the picture above, the Sun is at approx. 61°, read from 0°.
2. Sun altitude via gnomon
With this method, we place the Quadrant Telemeter with the long edge on a level surface; a horizontal gnomon projects the position of the Sun onto the quadrant scale.
This type of Sun altitude sighting was already described by Ptolemy in his Almagest in the 2nd century with the prototype of a quadrant. We need a gnomon (for example a straightened paperclip), which we install perpendicularly in the vertex of the quadrant, which remains static for the measurement.
Sun altitude via gnomon. The quadrant is positioned on a level surface, the shadow cast by the gnomon shows the altitude of the Sun.
- We rotate the instrument until the Sun shines alongside the quadrant.
- To do this, it is advisable to turn the quadrant from the shadow "into the Sun" until a shadow is just becoming visible.
- We read the Sun altitude where the shadow crosses the degree scale.
Example
In the picture above, the Sun is at approx. 43° (90° - 47°), read from 90°.