Galactic quadrant
Overview
Circular sector
A circular sector or circle sector, is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the...
s in the division of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy.
In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system
Galactic coordinate system
The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
, which places the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
as the pole of the mapping system. The Sun is used instead of the galactic center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
for practical reasons since all astronomical observations (by human
Human
Humans are the only living species in the Homo genus...
s) to date have been based on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
or within the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
Quadrants are described using ordinals
Ordinal number (linguistics)
In linguistics, ordinal numbers are the words representing the rank of a number with respect to some order, in particular order or position . Its use may refer to size, importance, chronology, etc...
—for example, "1st galactic quadrant" "second galactic quadrant," or "third quadrant of the Galaxy." Viewing from the north galactic pole with 0 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s (°) as the ray that runs starting from the Sun and through the galactic center, the quadrants are as follow:
- 1st galactic quadrant – 0° ≤ longitude (ℓ) ≤ 90°
- 2nd galactic quadrant – 90° ≤ ℓ ≤ 180°
- 3rd galactic quadrant – 180° ≤ ℓ ≤ 270°
- 4th galactic quadrant – 270° ≤ ℓ ≤ 360° (0°)
Due to the orientation of the Earth with respect to the rest of the galaxy, the 2nd galactic quadrant is primarily only visible from the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
while the 4th galactic quadrant is mostly only visible from the southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
.
