Astronomy Principles and Practice Fourth Edition.pdf

The Sun as a position finder 407
Figure 24.3. A simple noon-marker.
falls on the vertical line through the centre of the figure, whereas on those days when the departure
from a line is greatest then the equation of time has its maximum value. This curve is an alternative
way of displaying the equation of time. Its form is sometimes to be found on world maps or on globes.
Using a simple device similar to that depicted in figure 24.3, and after orienting it correctly in a
similar manner as for the horizontal sundial, the analemma may be drawn during the course of a year
and the variations of the equation of time deduced. Conversely, if the analemma has previously been
drawn on to the vertical plane, the device may be used for announcing the passage of the local noon.
Comparison with the civil time noon may be made after applying the correction for longitude.
24.3 The Sun as a position finder
At any particular instant, the apparent position of the Sun in the sky depends on the longitude and
latitude of the observer. If positional measures of the Sun are used to determine the location with some
degree of accuracy, an observer needs to use an optical instrument such as a theodolite or sextant and
corrections such as those for refraction effects of the Earth’s atmosphere must be applied. Nowadays,
an observer’s location on the Earth can be obtained to extreme accuracy by using GPS devices. Previous
to the advent of such satellite technology, location fixes were derived from measurements by optical
instruments.
It is of interest to note just how much GPS technology has improved the accuracy in respect of
the location determination. Consider a simple situation of measuring the altitude of the Sun, α ⊙ ,when
it is exactly on the meridian. The determination of the latitude of the observer based on this altitude