Sextant by Jesse Ramsden

Made:
c. 1772 in London
maker:
Jesse Ramsden

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Jesse Ramsden sextant. Full view, graduated matt black perspex background.
Science Museum Group Collection
© The Board of Trustees of the Science Museum

Jesse Ramsden sextant. detail view of mirrors and coloured filters.
Science Museum Group Collection
© The Board of Trustees of the Science Museum

Jesse Ramsden sextant. Detail view of measurment scale. Black background.
Science Museum Group Collection
© The Board of Trustees of the Science Museum

Jesse Ramsden sextant. Cropped view for gallery use. Graduated matt black perspex background.
Science Museum Group Collection
© The Board of Trustees of the Science Museum

Sextant, 15-inch with two removable viewing telescopes, by Jesse Ramsden, London, c 1775. Type of Hadley's sextant
Science Museum Group
© The Board of Trustees of the Science Museum, London

Sextant, 15-inch with two removable viewing telescopes, by Jesse Ramsden, London, c 1775. Type of Hadley's sextant
Science Museum Group
© The Board of Trustees of the Science Museum, London

Sextant, 15-inch with two removable viewing telescopes, by Jesse Ramsden, London, c 1775. Type of Hadley's sextant
Science Museum Group
© The Board of Trustees of the Science Museum, London

Sextant, 15-inch with two removable viewing telescopes, by Jesse Ramsden, London, c 1775. Type of Hadley's sextant
Science Museum Group
© The Board of Trustees of the Science Museum, London

Brass sextant by Jesse Ramsden, Piccadilly, London, 1770-75. Sextant has polished brass 120° scale (-2° to 136°) with 20’ divisions and brass vernier (30”), three index-filter shades (red & green), one horizon filter (red) and ‘Maskelyne flap’ for greater accuracy when taking a sight of the Sun or Moon. Fitted with threaded telescope bracket for sighting telescope (130mm –inverted image). The instrument has its original fitted mahogany keystone case.

Sextants were navigational instruments used to measure angular altitudes of celestial bodies whilst at sea, enabling mariners to determine latitude and local time. They were also used to calculate longitude at sea using the so-called ‘lunar distance’ method, carried out by comparing an accurate measurement of the angular distance between the Moon and a fixed star to a set of precalculated tables.

Sextants underwent a number of design improvements throughout the 18th century. Astronomer Royal Nevil Maskelyne made recommendations for ensuring accuracy despite unavoidable imperfections in the grinding of glass when taking sights of the Sun or Moon, which included the introduction of a hinged brass ‘Maskelyne flap’ as seen on this instrument. This sextant design by Jesse Ramsden was a similar type supplied to Captain Cook for his voyages in 1772 and 1776.

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Details

Category:
Astronomy
Object Number:
1928-924 Pt2
Materials:
brass (copper, zinc alloy), glass, mahogany (wood), metal
Measurements:
overall (flat): 110 mm x 440 mm x 435 mm, 3 kg
type:
sextant
taxonomy:
  • furnishing and equipment
  • measuring device - instrument
credit:
Mr Thomas H Court

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