Where does declination collide?

In positional astronomy, declination is one of the two coordinates that fix a point on the celestial sphere: it is the angle of that point north or south of the celestial equator, playing the same role for the sky that latitude plays for the Earth, and it pairs with right ascension the way latitude pairs with longitude. Smart's Textbook on Spherical Astronomy (6th edition, revised by Green, 1977) sets out how declination is measured along the hour circle through a body, running from 0 degrees at the equator to 90 degrees at either celestial pole. The referent is a fixed sky coordinate of a star or other body, read against the equator of the heavens, not the angle between a compass needle and true north, and not the tilt of a sundial's wall away from due south.[1]

Unit

A body's angle north or south of the celestial equator, from 0 to 90 degrees toward either pole

Recovery means

Not applicable

Measured by

Read from a telescope's setting circles or catalogued from precise astrometry, referred to the celestial equator of a stated epoch

In geomagnetism and navigation, declination, also called variation by mariners, is the angle at a place between magnetic north, the direction a compass needle points, and true or geographic north, the direction of the Earth's rotation axis. It is reckoned east or west of true north, and because it comes from the Earth's internal field it differs from place to place and drifts year by year, so a chart or model must state the date. Alken and colleagues' International Geomagnetic Reference Field: the thirteenth generation (2021) gives the global model from which declination is computed anywhere on Earth. The referent is a local compass correction that changes with position and time, not a body's fixed coordinate on the celestial sphere, and not the orientation of a sundial's face.[2]

Unit

The east or west angle between compass north and true north at one place, in degrees

Recovery means

Not applicable

Measured by

Computed from a geomagnetic field model such as the IGRF, or measured directly by comparing a compass bearing with a known true bearing

In gnomonics, the craft of designing sundials, the declination of a wall or dial is the angle by which a vertical dial's face turns away from due south, in the northern hemisphere, reckoned around to the east or west. A wall that faces exactly south has zero declination; any other vertical wall is a declining dial, and its declination must be known before the hour lines can be laid out correctly. Waugh's Sundials: Their Theory and Construction (1973) works through how this angle is found and how it enters the geometry of a declining vertical dial. The referent is the orientation of a physical dial surface relative to the meridian, not a compass needle's departure from true north, and not a body's angle above the celestial equator.[3]

Unit

The angle a vertical dial's face turns from due south, east or west, in degrees

Recovery means

Not applicable

Measured by

Determined on site from the wall's orientation to the meridian, historically by timing the sun across noon or sighting a known bearing