stars or the Moon, which have a direction from any given point on the surface of the Earth. It can be calculated precisely for a given date and time, and is not influenced by local variations in the magnetic field due to local deposits of minerals such as iron ore. Burt's instrument allows surveyors to determine the true north direction in reference to the Sun rather than being influenced by the Earth's magnetic field. It is made of brass and therefore has no magnetic influence on a compass needle, as it was originally a small attachment to a standard surveyor's common compass. Application of the solar compass requires knowledge of the apparent motion of the Sun around the Earth, relative to the Earth as the center of the frame of reference, and more specifically, relative to the position of the instrument when set up to use in a survey. An understanding of the latitudinal and seasonal declination and the longitudinal variation with time of day are necessary, as the compass has specific sub-assemblies to take each of these variables into account. At the Earth's equator at the equinoxes, the Sun has no seasonal declination, rises due east, and sets due west. At noon the Sun is at its highest point, directly overhead. It is at its lowest point at midnight, and appears to move in the plane of the equator. At locations away from the equator, the noon altitude of the Sun will be reduced by the angle of the Earth's local horizontal to the polar axis the latitude so at a latitude of 10° south or north, the noon altitude will be at the equinoxes. This angle is known to the surveyor and is set on the latitude arc of the instrument so that with the base leveled, and the compass aligned with true north, the axis of the hour arc will be parallel to the polar axis. The rotational axis of the Earth is tilted from the perpendicular to the plane of its orbit around the Sun. This angle causes the altitude angle of the Sun to vary with the seasons by an amount which depends on the direction of the misalignment. It varies predictably throughout the year, increasing and decreasing smoothly at a calculable rate, and is constant for everywhere on Earth at the same time. This value is also known to the surveyor, as it is