To make any kind of measure in archaeoastronomy, it is in principle advisable always to use a surveyor transit, which enables measuring azimuths and the corresponding altitudes at the horizon with an optimal accuracy (within 1 arc minute or less). There are times when the accuracy required in archaeoastronomical studies must be very high, such as when ancient monuments were oriented with astonishing accuracy, for example, the pyramids of Giza, or when they exhibit perfectly conserved and measurable architectural features, as with many Greek temples (for the practical use of the transit in archaeoastronomy, see Aveni 2001). However, in many situations the careful use of a very simple, hand-held instrument, such as a combined clinometer (to measure altitudes) and magnetic compass, may suffice (for further discussion, see Belmonte and Hoskin 2002, Hoskin 2001). To use the magnetic compass properly, it is important to remember that the earth acts like a magnet. Therefore, an iron needle rotating freely on the earth's surface points toward the poles of this magnet, in a direction called magnetic north. While geographical north (sometimes called true north in this context) is defined invariably by the projection of the earth's axis into the sky, magnetic north depends on a number offactors, and although it may chance to coincide with true north, it generally varies according to the place one is in, as well as the time. The magnetic compass, therefore, gives an indication of the direction north, which has to be corrected by using (readily available) magnetic declination values; further, magnetic anomalies, which are special geological features of the site (or the presence of modern iron gates in temples) have to be taken into account.
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