published in a set of tables in an almanac. A correction for this declination is made on the declination arc of the compass, which is mounted to rotate on the polar axis on top of the latitude arc, as the latitude and declination angles are additive. The angle of the Sun due to time of day is set on the hour angle arc, which is perpendicular to the polar axis. This angle is calculated from longitude and GMT, also tabulated for the convenience of surveyors and navigators, as the calculations are tedious to perform in the field, and any error could have extensive effects. Assuming a spherical Earth, if a straight line were drawn from the rising to the setting sun, and from the sun at noon and at midnight on the equinoxes, both of these lines would pass through the Earth's center and the equator would intersect these lines. This is not so when the sun has north or south declination because its apparent motion will be at an angle to the equatorial plane, equal to the amount of the sun's declination north or south, so that when the sun has north or south declination, and the Earth is regarded as the center of its revolutions, the line from the Sun to the center of the Earth describes a cone. This conical motion of the Sun can also be illustrated by the dished spokes of the wheel of a covered wagon with the rim representing the Sun's apparent path, the hub representing the Earth, and the spokes being lines drawn from the Sun's path. It may be seen that a line drawn from the Sun to the Earth's center would pass north or south of the equator, equal in degree to its declination north or south. The instrument has an equatorial movement, with a mechanical attachment for sighting a star as a reference. Construction and operation Burt's solar compass consists of a main plate mounted to the tripod stand and leveled using the two orthogonal attached spirit levels. It carries a common compass needle box, having divisions for the north end of the needle of about 36 degrees, with a vernier to read the needle's variation, and the three adjustable arcs of the solar instrument: one is set for the latitude of the location; another for the seasonal declination of the Sun; and the third for the hour of