An equinoctial ring (or universal ring ) is a relatively coarse measuring instrument, which is considered the solar watch of the sailor, and can also serve as a compass.
In its principle and its functioning it is the equivalent of an armillary sphere of observation, limited to the observation of the Sun, put in the form of a foldable instrument, robust but not very precise, a pocket instrument which can also be Wear collar.
The instrument is mainly composed of:
- An outer “meridian” ring, bearing latitude graduation and adjustable suspension point;
- An internal “equatorial” ring mounted on the meridian ring, which bears the graduation of the hours;
- From a central axis mounted on the equatorial ring, which carries a mobile pinna and the graduation of dates (in practice, often, signs of the zodiac ).
In models with better precision, the central axis can pivot on itself.
The time scale can sometimes be equatorial, allowing in this case to adjust the difference between the legal hour and the solar time .
Before using it, it must be set for the date and place:
- On the large outer ring (which represents the meridian plane), it is necessary to slide the ring of hanger to adjust the point of attachment on the latitude of the place;
- On the central axis style (which represents the pole axis), the date must be set by sliding the moving pinna (which represents the height of the Sun on the celestial equator ), so that the cursor is placed on the month corresponding.
- Some models can be used to slide the time ring, so as to take into account the offset between the legal hour and the solar time ; This adjustment must be made according to the longitude of the place.
To use it, one unfolds the circle of hours (which represents the plane of the celestial equator ) at right angles to the meridian circle. If the central style can be rotated, it is then necessary to orient the pinna according to the estimated time, so that it is roughly perpendicular to the direction of the Sun (if it is incorrect, this setting can be resumed Thereafter, to improve the accuracy of the reading). The ring is then ready for use.
The ring is then exposed to the Sun. A small hole in the pinnule lets pass the light of the Sun. The ring is then oriented so that the light beam passing through this hole strikes the equatorial ring.
The light spot must be centered on the equatorial ring, on the line representing the equator (when it exists). In this position, the height of the Sun makes it possible to determine both the time and the geographic north :
- The graduation or stopping of the luminous point marks the solar time ;
- The meridian plane is correctly oriented, and thus marks the North-South axis.
The solar time differs from the legal time, on the one hand because of the longitude deviation from the time zone reference meridian (which causes a one-hour shift for 15 °, ie four minutes per degree And by the time equation which translates an advance or a delay of the Sun on the average time, which can reach up to a quarter of an hour of lag (late in February, and in advance in November).
As with all devices based on the Sun’s height, it is not possible to distinguish between one hour in the morning and one in the afternoon: to distinguish between nine o’clock in the morning ( And three in the afternoon, you have to know whether you are in the morning or afternoon (which is generally the case), or have a good idea of the direction of the north. Because of this indeterminacy, the instrument is especially useful at the beginning and at the end of the day, and is less precise as we approach the solar noon.
It remains theoretically possible to use the instrument around noon if it can be oriented correctly, for example on an alignment that has been identified beforehand; But such alignment is generally impossible at sea.
The larger the rings, the bigger the rings, but the larger the rings are incompatible with the portable character of the instrument (slipped into the pocket or worn as a collar). Under good conditions (beginning or end of the day, equatorial or temperate latitudes) and for rings of the order of four to five centimeters in diameter, the precision obtained can be of the order of a quarter of an hour on the hour circle .
This accuracy is generally sufficient for local navigation and appreciation of the tide. For its part, the precision on the orientation may be a few degrees on the north-south axis, but the use of the instrument as a compass is generally irrelevant.
Illustration of latitude and longitude
The instrument can also be used to set the latitude (roughly) if the time or direction of the geographic north is known , by adjusting the suspension ring so that the light beam falls on the equatorial ring, Either on the right time if known, or by orienting the meridian ring in the North-South axis if known.
It can also serve to appreciate the longitude if one knows a time reference, by the difference between the legal hour and the solar time.
In either case, the very imprecise result makes the instrument totally unsuitable for astronomical navigation.
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