page_title: Burt's solar compass
section_title: Construction and operation
hierarchical_section_title: Burt's solar compass / Description / Construction and operation
caption_attribution_description: English: Sketch of Burt's solar compass axes
context_page_description: Burt's solar compass or astronomical compass is a surveying instrument that makes use of the sun's direction instead of magnetism. William Austin Burt invented his solar compass in 1835. The solar compass works on the principle that the direction to the sun at a specified time can be calculated if the position of the observer on the surface of the Earth is known, to a similar precision. The direction can be described in terms of the angle of the sun relative to the axis of rotation of the planet.
  This angle is made up of the angle due to latitude, combined with the angle due to the season, and the angle due to the time of day. These angles are set on the compass for a chosen time of day, the compass base is set up level using the spirit levels provided, and then the sights are aligned with the sun at the specified time, so the image of the sun is projected onto the cross grating target. At this point the compass base will be aligned true north–south. It is then locked in place in this alignment, after which the sighting arms can be rotated to align with any landmark or beacon, and the direction can be read off the verniers as an angle relative to true north.
context_section_description: 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 magnetic 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 the day adjusted for longitude of the location. The sights to set alignment by the sun are mounted on the movable arm of the declination arc and have a small lens for focusing an image of the sun's disc on the target grating. The upper plate is aligned with the sun and remains stationary after polar alignment, while bearings are taken with the sights on the lower plate. The lower plate carries the surveying sights and can be rotated relative to the upper plate, and may be clamped in any position to the upper plate. There is a graduated ring on the lower plate which displays the relative rotation between the north-aligned top plate and the surveying sight-line on the bottom plate, and has verniers to allow precise reading of the angle.
  The latitude arc is attached perpendicular to the upper plate. The hour arc is fixed perpendicular to the movable upper part of the latitude arc, and the declination arc swivels on a polar axis over and perpendicular to the declination arc. The positions of the arcs can be finely adjusted by screws and the angles read with a vernier. Clamp screws are provided where necessary to lock the components in place. At one end of the adjustable limb of the declination arc there are small lenses set up to focus an image of the sun's disc onto a target plate inscribed with parallel pairs of perpendicular lines to frame the image when correctly aligned.
  The operation is as follows:
  Set the sun's declination for that day, obtained by means of tables, on a scale attached perpendicular to the time arc.
  Set the latitude of the location on a scale in the alidade.
  Set the approximate local time on the arc that rotates on a polar axis.
  Orient the instrument, while it remains level, so the image of the sun appears between four scribed lines on the screen opposite to the lens.  The time dial is fine adjusted to bring the image between a second pair of scribed lines perpendicular to the first pair.  The main axis of the upper plate will then point to the pole.
  The pinnula (sighting vanes) may then be aligned with a terrestrial object and its bearing read from the angle scale.
  The magnetic declination may be read from a compass attached to the base plate.
caption: Burt's solar compass axes
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