If you are starting out with a telescope or a set of binoculars for astronomy, and are interested in finding your way around the night sky you might come across terms used in astronomy you’ve never heard before. So, we’ve compiled the following astronomy terms and definitions to help.
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Astronomy is “the scientific study of all objects in space” 1 or otherwise defined as “the study of celestial objects and phenomena” 2.
Objects in space include planets, which you can see clearly at different times of the year.
What are some space terms?
The following list of words* are terms used in reference to space and the Universe. As an amateur astronomer, you may come across these words in technical speak or writings on the subject or just in day to day use of astronomical terms.
Astronomy for beginners – Space terms: You’ll find these ‘words to do with space’ listed in alphabetical order and their meanings provided alongside.
Aberration of starlight
Change appearing in the star’s position
An object is at its farthest from the sun at this point
An orbiting star is at its farthest from the sun at this point
A model of objects in the sky designed in a spherical framework of rings to show the relationships between them on the celestial sphere
A minor planet that orbits around the Sun and which is no more than 600 miles (1000 km) in diameter (some are much smaller). Composed of metals and rocky material, they differ to comets, which involve ice and dust.
A belt of asteroids in the Solar System. It is a torus-shaped region situated roughly between the orbits of Jupiter and Mars. It contains numerous solid but irregular-shaped celestial bodies that range across many sizes but which are much smaller than planets.
The angle between an object’s rotational (running pole to pole, equatorial) axis with respect to its ecliptic plane when orbiting the Sun. (AKA obliquity) For example, the Earth orbits the Sun tilted about 23.4º from its equatorial plane. This slant is the reason for the seasons on Earth.
The horizontal bearing of a celestial object. It is measured clockwise from a given direction.
Two stars connected through gravitational attraction.
In space, an area where the gravitational pull is so strong that not even light can escape it.
Shortening of the wavelength towards the blue spectrum as the celestial object moves closer too us
The sphere around the Earth that is imagined for the purpose of finding or identifying the position of celestial bodies
The occasion when two celestial bodies align on the celestial sphere
A group of stars in a recognized order or pattern. Today 88 constellations are recognized by astronomers.
A small Solar System body of ice and dust that is sometimes seen with a tail of gas and dust particles as it vaporizes on passing near the sun. Comets differ from asteroids, which are comprised of metals and rocky material.
The faint light seen as a halo around a star, and as seen around the Sun and the Moon
Another name for universe.
The study of the cosmos – deals with the origin and evolution of the Universe. A branch of astronomy.
The phase of the moon when only a small arc is illuminated. Seen at the start of the first quarter or the end of the last quarter.
Angular measure of a star’s position in degrees north and south of the celestial equator
Also known as outer space. There is no definite official delineation of the space beyond the Earth’s atmosphere. However, ‘deep space objects’ is a term often used to describe objects beyond the limits of the Solar System.
See binary star.
Pluto is a dwarf planet. A dwarf planet orbits the sun. They are smaller than planets and their orbital path is not cleared of neighboring bodies.
Where we live. Third planet from the Sun in the solar system, the Earth rotates once per 23h 56m 4s relative to distant celestial bodies, and about once per 24 hours in respect to the Sun.
Where light from one celestial body is obscured by the crossing of another between either it and you (the observer) or it and the source of illumination
A circle representing the annual path of the Sun, apparent on the celestial sphere in relation to other stars
An orbit with an oval-shaped path. As an example, the Earth’s orbit around the Sun is elliptical.
A lunar phase where you will observe the moon fully illuminated. Usually occurs once per month. Where it occurs a second time in a calendar month, called a “blue moon”.
Not really a star. It is an illuminated path of a meteoroid as it enters Earth’s atmosphere
What keeps you on Earth. The gravitational force of Earth or other body pulls objects toward its centre. It is also the reason the planets of the Solar System remain in orbit around the Sun.
An example of a galaxy is the Milky Way. We live in the Milky Way. A galaxy is made up of thousands to billions of stars and other celestial objects clustered together by gravity.
Seen a few days either side of a full moon, i.e., when the full moon is waxing or waning and the illuminated part is less than a full circle.
This is when you only see half of the moon illuminated. Half-moons occur at the first-quarter and last-quarter lunar phases.
In astronomy, inertia refers to a measure of an object’s ability to resist change when in motion. Newton’s law of inertia: “Every object continues in a state of rest, or uniform motion in a straight line, unless acted upon by external forces.”
These are the two closest to the Sun, Mercury and Venus. They differ to the other planets in that they can never reach opposition.
The four planets of the Solar System closest to the Sun — Mercury, Venus, Earth, and Mars, which are all solid and made of rock material.
Planet of the Solar System, fifth from the Sun. It is the largest planet in our solar system. It is a gas planet, mostly hydrogen but also helium. Know for it’s Great Red Spot, which is a large anti-cyclonic storm. From Earth, it can be seen by the naked eye and is the fourth brightest object in the sky (after Sun, Moon, and Venus), though competes with Mars for this position at times. It has numerous moons (79 known at the time of writing).
The term “lens” in the context of gravitational light deflection means a distortion and magnification of light from distant galaxies created by the gravitational field of a huge amount of matter, like a cluster of galaxies.
The term “lens” in the context of a telescope can refer to the telescope optics that reflect and focus the image or the eyepiece lens that magnifies that image.
The distance light travels in a year and used as an astronomical measurement because of the huge distances involved. One light-year equals 5.87849981 × 1012 miles (or 9.4605384 × 1012 kilometers).
Of the Moon.
A celestial body’s brightness as a measure, apparent or absolute.
Planet of the Solar System, fourth from the Sun. It has two moons, Phobos and Deimos. The Red Planet, Mars is the Red Planet featuring polar caps that are permanent. The polar caps can be seen through a telescope.
Planet of the Solar System, closet to the Sun. It is the smallest of the Solar System planets. Difficult to see with the naked eye, mercury is best observed in the tropics and subtropics (especially of the southern hemisphere).
A meteoroid entering the Earth’s atmosphere will produce a streak of light and this is a meteor, which starts to burn from the heat of friction.
A rock or metal chunk from space of a size that can pass through the atmosphere to reach the Earth’s surface without burning up.
A natural satellite orbiting a planet or other celestial body. Earth has one moon, which we call the Moon. Mars has two. Jupiter has numerous.
The point in the sky directly under the observer or diametrically opposite the Zenith.
A galaxy dust or gas cloud.
Planet of the Solar System, eighth from the Sun and farthest known in the Solar System. No visible to the naked eye. Seen as a small blue disk through a telescope or astronomy binoculars. Considered an “ice giant”.
When a slight “nodding” of the Earth’s axis occurs.
A star that brightens suddenly to several times its normal magnitude of brightness.
Open star cluster
A group of up to a few thousands of stars loosely bound by gravitational attraction and formed from the same giant molecular cloud.
In astronomy, “opposition” means that it is on the opposite side of the Earth from the Sun (it’s opposite to the Sun).
The outer planets in the Solar System are the gas planets. These are Jupiter, Saturn, Uranus, and Neptune, which sit outside the asteroid belt within the Solar System.
The lighter outer part of the shadow cast on an object as seen during a partial eclipse.
A point when an orbiting object is closest to Earth.
A point when an object orbiting the Sun is closest to the Sun.
A planet is a large celestial body that orbits a star. There are eight planets in our Solar System that orbit the Sun.
In order from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Here is a phrase as a mnemonic to help you remember this order: My Very Earthly Mother Just Sat Up and Noticed)
Dwarf planet of the Solar System, situated in the Kuiper belt, which is beyond Neptune, the farthest planet of the Solar System. It is not one of the 8 planets (though at one time it was considered a planet bringing the total to 9). It is not visible to the naked eye.
A star that rotates rapidly and transmits a regular radiation flash
An object of intense brightness, possibly an energetic nucleus of a far off galaxy.
A phase between the full and new moons where you will see half the moon illuminated. (See half-moon)
A type of star.
Lengthening of the light wavelength towards the red spectrum as a celestial object moves further away.
Planet of the Solar System, sixth from the Sun. It’s known for its rings, which can be seen through a telescope. Saturn is visible to the naked eye as a bright yellow light point.
There is an infinite density of matter at this point in space-time.
Not really a star. See ‘falling star’.
Of the Sun.
The Earth is one of 8 planets orbiting the Sun in the Solar System. The Solar System comprises the Sun, the planets and their moons, and all other bodies (e.g., dwarf planets, asteroids, comets, meteoroids, and Kuiper belt objects) that orbit the Sun, being the star at the center of the Solar System.
A solar wind is a rapid outflow or ejection of protons, electrons, and heavier metal atoms from the Sun.
The farthest point either north or south of the equator that the Sun reaches each year.
The term “space” in astronomy refers to the region outside the Earth’s atmosphere, i.e., it starts about 100 kilometers or 60 miles above Earth – a vast 3-dimensional region of blackness with too few oxygen molecules to produce the sky blue effect we see when the Sun illuminates Earth’s atmosphere. It is a vacuum because molecules are too far apart to transmit sound.
A spherical mass of plasma held together by gravity. There are several types of stars. See Know Your Star Types.
An end of life star exploding.
A celestial body is either in opposition to or in conjunction with the Sun at this point in its orbit.
Darkest part of a shadow. Where you would observe a total eclipse.
Planet of the Solar System, second from the Sun. It can be seen by the naked eye and often referred to as the “evening star”. It is the third brightest natural object in the sky, after the Sun and the Moon.
In reference to the Moon, progressively less is being illuminated and so its size is appearing to decrease.
In referent to the Moon, progressively more is being illuminated and so its size is appearing to increase.
The point in the sky that is directly above the observer. In the coordinate system of alt-az, it is altitude (alt) +90o.
*Sources: Reader’s Digest 1989, ‘Astronomy Terms’ in Reverse Dictionary Reader’s Digest Association Ltd.
Branches of astronomy
In our list of outer space terms, we included one branch of astronomy – Cosmology.
If you are interested in a career in astronomy you might want to consider one of the following 17 branches that fall into four subfields: Astrophysics, Astrometry, Astrogeology, and Astrobiology. 2
The study of the laws of physics as it applies to stars and celestial bodies.
The study of the origin and evolution of the universe.
The study of light reflection, absorption, and transfer between matter.
The study of luminous astronomical objects in relation to electromagnetic radiation
The study of internal structure of stars through observing their oscillations.
The study of the composition of stars in terms of their interior structure and dynamics by way of observing their surface waves
The study of the laws of physics as it applies to the Sun.
The study of the Sun’s constant and dynamic radiation effect on its surrounds in space.
The study of celestial bodies in terms of their position and how they move in space.
The study of planets in terms of how they form, including their composition and dynamics in history.
The study of planets that exist outside our solar system.
The study of the geological composition of Mars.
The study of the Moon’s physical features, e.g. the luna maria, craters, and mountain ranges.
The study of geology relating to moons, asteroids, meteorites, comets, and other celestial bodies. (also known as Planetary Geology)
ASTROBIOLOGY The study of the origin and evolution of life in the Universe.
The study of the likelihood of life in space.
The study of the interactions of biota with space environments.
The study of chemical substances in celestial bodies, stars, and interstellar space
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