Sextant to Second

A sextant uses two mirrors to superimpose the image of a celestial body onto the horizon. The navigator looks through the eyepiece and sees the horizon directly through a half-silvered mirror, while a second mirror on a movable arm reflects the Sun or star down into the same field of view. You swing the arm until the star appears to sit exactly on the horizon, then read the angle off the graduated arc. The double-reflection design means the arc only needs to span 60° (one sextant) to measure angles up to 120°.

The name refers to the arc of the instrument, not its measurement range. A sextant's arc is one-sixth of a circle (60°), but thanks to the double-reflection principle — where the angle of reflection doubles the arc angle — it can actually measure angles up to 120°. Similarly, an octant (one-eighth of a circle, 45° arc) measures up to 90°. The naming convention describes the physical shape of the tool, not its capability.

Yes, and navies worldwide still require it. The US Naval Academy reintroduced mandatory celestial navigation in 2015 after a decade-long hiatus, citing concerns about GPS vulnerability to jamming, spoofing, and satellite failure. A skilled celestial navigator with a sextant, an accurate clock, and a nautical almanac can determine position to within about 1–2 nautical miles — good enough to make port safely. Several solo round-the-world sailors carry sextants as backup specifically because they have no electronics to fail.

The sextant itself couldn't solve longitude — that required an accurate clock (John Harrison's marine chronometer, completed in 1761). But the sextant was the other half of the solution. A navigator used it to measure the Sun's altitude at local noon to find the exact time of solar noon at their position. Comparing this to Greenwich time on the chronometer gave the time difference, and since Earth rotates 15° per hour, that time difference directly yielded longitude. Sextant + chronometer = position anywhere on Earth.

Sixty degrees is the interior angle of an equilateral triangle — the simplest regular polygon after the square. Honeycomb cells are hexagons (six 120° angles, each the supplement of 60°) because hexagonal packing is the most efficient way to tile a plane. Carbon atoms in graphene and diamond form 60° and 109.5° angles respectively. The 60° angle appears everywhere in nature because it's the geometric consequence of close-packing equal-sized spheres or circles.

One arcsecond of latitude corresponds to roughly 31 meters (about 101 feet) on the ground. This is why high-precision GPS coordinates are quoted to fractions of arcseconds — a shift of just 0.01″ means about 30 cm. Longitude arcseconds cover less ground as you move toward the poles because the meridians converge; at 45° latitude, one arcsecond of longitude spans about 22 meters.

Stellar parallax is the tiny apparent shift of a nearby star against distant background stars as Earth orbits the Sun. Even the closest star, Proxima Centauri, shifts by only 0.768 arcseconds over six months — far too small for the naked eye. The parsec (parallax-arcsecond) is defined as the distance at which a star would show exactly 1″ of parallax. No star is close enough to reach that threshold, which gives you a sense of how mind-bogglingly far away even our nearest neighbors are.

Hubble resolves details down to about 0.05 arcseconds — roughly the angular size of a coin seen from 80 km away. At that resolution it can distinguish individual stars in nearby galaxies, spot the discs of Pluto and large asteroids, and detect gravitational lensing arcs. Ground-based telescopes are blurred to about 0.5–1″ by atmospheric turbulence unless they use adaptive optics, which is why space telescopes remain essential for sharp imaging.

Arcseconds per pixel is the standard metric for imaging sensors in astronomy because it directly links detector geometry to sky coverage. A telescope with 0.3″/pixel resolution can separate objects that close together on the sky. Photographers encounter this too — the resolving power of any long telephoto lens is ultimately limited by atmospheric seeing (typically 1–2″), which is why even a perfect 600 mm lens produces soft images of distant objects on a hazy day.

The average human eye resolves about 60 arcseconds (1 arcminute) under good conditions, though some people with exceptional vision reach 30″. This is why the standard eye test chart (Snellen chart) defines 20/20 vision as the ability to resolve details that subtend 1 arcminute. For comparison, Jupiter at its brightest subtends about 50″, just below that threshold — which is why it looks like a bright dot to the naked eye, not a disc.