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Astronomical Glossary - Terms & Definitions

Astronomical terms can be a little technical and difficult to understand. Below is a list of definitions and explanations that may make it easier.

Albedo
In astronomy, the term albedo refers to the brightness of an object in space. Derived from Latin, albedo means "whiteness" (albus= "white").
Albedo is measured on a scale, where 0 refers to an object that is completely dark i.e. it does not reflect any light. 1 on the scale refers to a perfectly reflective object. The Moon has an Albedo of 0.12, while Earth's average albedo is 0.3.
Asterism
A pattern of stars recognizable to observers from Earth. Asterisms may or may not be part of a constellation. The Big Dipper is one of the most well known Asterism - its stars belong to the constellation Ursa Major.
Axis
An axis in astronomy refers to the (imaginary) line that an object, usually a planet, rotates around. The Earth's rotational axis is an imaginary straight line that runs through the North and South Poles (see illustration).
Illustration showing Earth's axis drawn as a red line.
Earth's axis is illustrated by a red line.
Earth's axis is illustrated by a straight, red line that runs through the Poles.
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Axial Tilt – Obliquity
Earth's axis is tilted about 23.4 degrees to the perpendicular to the orbital plane (see illustration).
The Earth rotates the Sun at a slant, which means that different amounts of sunlight reaches the Northern and Southern Hemispheres thoughout the year. This is the reason we have different seasons on Earth.
Celestial Equator
The celestial equator is the imaginary extension of the Earth's equator infinitely into space.
Celestial Pole
The celestial poles are imaginary lines that trace the Earth’s rotation axis in space. The Earth has two celestial poles – the Northern Celestial Pole and the Southern Celestial Pole. At the moment, these poles correspond to the geographical North and South Pole. Because of this, objects that lie on the celestial pole do not seem to move at all for observers on Earth, while all other objects, mostly stars, seem to move in a circle around the pole.
Celestial Sphere
The celestial sphere is an imaginary sphere that extends infinitely into space with the Earth at its center.
Circumpolar stars
Circumpolar stars never set or go below the horizon for observers from specific latitudes. They are visible to observers from these latitudes throughout the year because of their proximity to the celestial pole. Circumpolar objects lie within the circumpolar circle, and stars circumpolar to latitudes in the Northern Hemisphere are not visible in the Southern Hemisphere, and vice versa.
Constellation
In the present day, a constellation is a two dimensional area in space as defined by the International Astronomical Union (IAU). Today, there are 88 officially recognized constellations. This is differ ent from the historical understanding of a constellation dominantly used in astrology which is defined as a group of stars easily identifiable by a pattern to observers on Earth.
Ecliptic
The ecliptic is the imaginary line that traces the Sun's apparent path in the sky. In other words, it is the projection of the Earth's orbit into the celestial sphere. Because the ecliptic marks the annual path of the Sun, it is also where eclipses occur. Any constellations on this line are known as Zodiacal or Zodiac Constellations.
Ecliptic Plane – Orbital Plane
See Orbital Plane
Great Circle
A Great Circle is any circle that is formed by a plane that passes through the center of the Earth. The Equator and the circles created by the meridians form Great Circles. A straight line connecting two Great Circles is the shortest distance between them. Because of this, they are important to study migration and other human interactions and activities including shipping and airline routes.
Light Year
A light year (also spelled: light-year or lightyear) is a unit of distance and is defined as the distance traveled by light in a vacuum during a Julian year. In astronomy, a Julian year (symbol: a) is a time unit defined as exactly 365.25 days of 86,400 seconds each. The distance is approximately 9.5 trillion kilometers or 6 trillion miles.
Local Noon
Local noon occurs when the Sun crosses the meridian above the horizon.
Local Midnight
Local midnight occurs when the Sun crosses the meridian below the horizon.
Meridian
the circle on the celestial sphere that passes through the zenith and both celestial poles.
Orbit
An orbit is the path of an object around a point or another object in space. In general, this path is repeatedly followed by the object, though, in some cases, different celestial forces like gravitation can change its orbit. An object in orbit is called a satellite. Orbits are formed due to two opposing forces - the orbiting object's momentum and the force of gravity that pulls it towards the object it is orbiting. These two forces have to balance each other for an orbit to be sustained.
Orbital Plane – Ecliptic Plane
The orbital plane of a satellite or an orbiting object is the geometric plane on which the orbit lies. The orbits of all the Sun's planets are coplanar, meaning that they all lie on the same orbital plane.
Reflection and Refraction
Reflection occurs when light bounces off the surface of an object. On the other hand, refraction occurs when light enters an object and bends.
Obliquity – Axial tilt
See "Axial Tilt"
Zodiacal Constellations
Zodiacal constellations are the constellations located within the Earth's ecliptic. There are 13, not 12, zodiacal constellations. The 13th constellation is Ophiuchus.

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