Sextant – Sextans Astronomicus Trigonicus proo distantiis rimandis

by Joan Blaeu


Date of first edition: 1662

Date of this edition: 1664

Dimensions (not including margins): 26,5 x 40cm

Condition: Mint. Sharp copper engraving printed on paper.

Condition rating: A

Verso: blank

Map reference: Van der Krogt 2:621

From: Grooten Atlas, oft Werelt- Beschryving, in welcke ‘t Aerdryck, de Zee, en Hemel, wort vertoont en beschreven. Van der Krogt P-Brahe 12:2.


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The Sexant

Latitude is geographic coordinate that specifies the north–south position of a point on the Earth’s surface towards the Equator. So, latitude is an angle which ranges from 0° at the Equator to 90° at the poles. Lines of constant or equal latitude, or parallels, run east–west as circles parallel to the equator. Width determination was done on the basis of the own position and the movement of the stars in the sky.

Below is a list of a few cities having more or less the same northern latitude:
– 52° N: Amsterdam, Berlin, Warschau and Irkutsk
– 51° N: Brussels and Calgary
– 45° N: Lyon, Venice, Milan, Montreal and Ottawa
– 43° N: Nice, Florence, Toronto, Vladivostok and Saporro
– 41° N: Rome, Barcelona, Istanbul, Cleveland and Chicago
– 40° N: Madrid, Naples, New York and Beijing
– 37° N: Athens, Granada, San Francisco and Seoul
– 35° N: Valetta and Tokyo
– 31° N: Alexandria and Shanghai

These parallel lines were perfectly projected on the famous world-map by Mercator in 1569, still known and used as the Mercator Projection (see map, House of Congress, Washington DC).

The calculation of latitude was relatively simple. Already in ancient times one could calculate the latitude. The sextant replaced the more simple, but less reliable Jacob’s staff, also known as cross-staff (see design). It would become THE instrument to measure latitude and thus, to accommodate east-west travel in the age of exploration.

The sextant (see drawing) is a doubly reflecting navigation instrument that measures the angle between two visible objects. Having a framework of 1/6th part of a circle (hence its name; so there are also the quadrant and the octant) it measures the altitude (or angle) between the horizon and a common celestial body (usually the Sun).

This sextant was designed by Tycho Brahe in 1584 and it had a radius of 155 cm. Johannes Blaeu published this design in his Atlas major as an appraisal both to his father Willem and to Tycho Brahe, where his father was an apprentice.