Astronomy Principles and Practice Fourth Edition.pdf

The Sun as a position finder 409
Figure 24.4. The simple determination of latitude.
24.3.2 Theodolite observations
This exercise involves the use of a theodolite to obtain the azimuth of the Sun at a particular time.
Knowing the hour angle of the Sun at the time of the observation, the angle between the Sun and the
meridian can be calculated. The position of the meridian may, in turn, be related to the observer by
means of the azimuth of a fixed reference object and it is then determined once and for all.
Check that the theodolite is level and adjust if necessary. Having taken the precaution of
checking that the filters are fitted to the telescope, direct the theodolite towards the Sun and record
the following times:
• the moment when the limb of the Sun approaching the cross-wire makes contact with it; and
• the moment when the opposite limb breaks contact with the cross-wire, after the solar image has
trailed across the field of view.
The mean of these two observations gives the time at which the centre of the Sun’s disc was at
the cross-wire. Note the angle on the horizontal circular scale of the theodolite at which the Sun was
observed. Note also the angle of some distant fixed reference object.