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The entire celestial sphere is divided into two hemispheres, northern and southern, by the encircling line of the celestial equator. Each of these two hemispheres can be separated further into four quadrants, creating a system that divides the entire sky into a total of eight regions of equal size and shape.
Each quadrant measures from the equator to one of the celestial poles, north or south, spanning a range of declination from 0° to either +90° or -90°. Running west to east along the equator, the sky is divided into twenty-four hours of right ascension, and each of the four quadrants covers six of these hours. So, the first quadrant runs from 0h00 to 6h00, the second from 6h00 to 12h00, and so on. Each of the eight quadrants has its own identifying abbreviation: as an example, the code 'SQ3' identifies Southern Quadrant 3, the area between 12h00 and 18h00 right ascension in the southern celestial hemisphere.
Because each quadrant describes a longitudinal quarter of the sky, and the Sun takes a year to complete its apparent circuit of the celestial sphere, each quadrant becomes fully visible in sequence, dominating the night sky in periods three months apart. This means that the quadrants line up approximately with the seaons on Earth, with NQ1 (for example) being most visible from autumn to winter in the northern hemisphere, followed by NQ2 from winter into spring, and so on. Constellations in the far north of the northern quadrants, and the far south of the southern quadrants, are visible all year round from their corresponding hemispheres on Earth.
Segments of the sky described by the four northern quadrants are more visible from Earth's northern hemisphere, and those in the southern quadrants are more visible from the southern hemisphere. Constellations around the celestial equator, where the quadrants meet, are commonly visible from both hemispheres, though their particular visbility will depend on the observer's latitude on Earth.
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Right ascension: 0h00 to 6h00
Declination: 0° to +90° (northern hemisphere)
Optimum visibility: October to January
Constellations in this quadrant are centred around Perseus, and include Taurus to the southeast and Cassiopeia to the northwest. The brilliant star Capella falls within this quadrant, as do Betelgeuse and Bellatrix in Orion. The large but generally faint constellation of Pisces runs eastward from the First Point of Aries, filling much of NQ1's southwestern area.
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Right ascension: 6h00 to 12h00
Declination: 0° to +90° (northern hemisphere)
Optimum visibility: January to April
Gemini falls in the southwestern part of this quadrant, and Leo in the southeastern part, with the bright star Procyon in Canis Minor falling near the equator on its southern boundary. The northern regions of NQ2 are occupied by the large by relatively sparse constellations of Lynx and Camelopardalis, but the more prominent Ursa Major extends into the quadrant from the northeast. Ursa Major's two Pointer stars point the way to Polaris at the Northern Celestial Pole, the meeting point of all four northern quadrants.
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Right ascension: 12h00 to 18h00
Declination: 0° to +90° (northern hemisphere)
Optimum visibility: April to July
The constellation of Ursa Minor, the Little Bear, runs down from the Northern Celestial Pole into NQ3. Southward from Ursa Minor, though still in the north, the handle of the Plough or Big Dipper extends into this quadrant from NQ2 to the west. The remainder of the quadrant is dominated by four large constellations: Hercules, Boötes, Canes Venatici and Coma Berenices. Arcturus in Boötes stands out as the brightest star in this part of the sky.
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Right ascension: 18h00 to 0h00
Declination: 0° to +90° (northern hemisphere)
Optimum visibility: July to October
The eastern parts of this quadrant are dominated by three bright stars: Deneb in Cygnus, Vega in Lyra and Altair in Aquila. Together these form the Summer Triangle, a formation that outlines a dense region of the Milky Way during northern summer nights. To the southeast of the quadrant is another distinctive shape, the four stars that make up the Square of Pegasus. In the northern region of NQ4, its area is divided between the large constellations of Cepheus and Draco.
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Right ascension: 0h00 to 6h00
Declination: 0° to -90° (southern hemisphere)
Optimum visibility: October to January
Most prominent among the constellations of this quadrant is Eridanus, which runs from its northern extent down to the star Achernar at declination -57°, far into the sky's southern hemisphere. Cetus the Whale occupies much of the quadrant's northwestern area, with numerous smaller and fainter constellations filling the remainder of this region. Both the Magellanic Clouds lie in the southern parts of quadrant SQ1.
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Right ascension: 6h00 to 12h00
Declination: 0° to -90° (southern hemisphere)
Optimum visibility: January to April
The central regions of this quadrant are dominated by the three constellations that combine to form Argo Navis, representing the Ship Argo. Northeastward from the Argo, the immense constellation of Hydra occupies much of the northern reaches of the quadrant, and stretches on eastward into SQ3. Sirius and Canopus, the two brightest stars in the night sky, both lie along the western edge of quadrant SQ2.
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Right ascension: 12h00 to 18h00
Declination: 0° to -90° (southern hemisphere)
Optimum visibility: April to July
In the northern parts of SQ3, where the quadrant approaches the celestial equator, the line of the Ecliptic passes in sequence through Virgo, Libra, Scorpius and Ophiuchus, with Scorpius' brightest star, red Antares, standing out against the backgrop of the Milky Way. Further south, the body of this quadrant is dominated by Centaurus, with Alpha Centauri shining at the Centaur's foot and pointing the way, with neighbouring Hadar, toward the shape of the Southern Cross in the quadrant's southwestern regions.
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Right ascension: 18h00 to 0h00
Declination: 0° to -90° (southern hemisphere)
Optimum visibility: July to October
The Milky Way runs through Sagittarius in the northwestern corner of this quadrant, in a region filled with star clusters and nebulae. Eastward from Sagittarius, the Ecliptic continues through Capricornus and Aquarius, before leaving SQ4 at the First Point of Aries in Pisces, which also defines the quadrant's northeastern corner. Southward, the quadrant is composed of a patchwork of small constellations until it reaches Octans around the Southern Celestial Pole.
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A similar approach, dividing the sky into quadrants like this, is sometimes applied using galactic coordinates in place of equatorial or celestial coordinates. This system maps the sky using the alignment of the Galaxy as its reference, rather than the Earth's equator and poles. The result is a series of eight galactic quadrants, labelled NGQ1, NGQ2 and so on. These work on the same principle as celestial quadrants, but describe eight different sections of the sky that meet along the axis of Milky Way rather than the celestial equator.
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