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.
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.
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.
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.
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.
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
Ecliptic
A circle representing the annual path of the Sun, apparent on the celestial sphere in relation to other stars
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”.
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.
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.”
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.
J
Jupiter
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.
Light- Year
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).
A celestial body’s brightness as a measure, apparent or absolute.
Mars
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.
Mercury
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).
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”.
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.
A point when an object orbiting the Sun is closest to the Sun.
Planet
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)
Pluto
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.
Lengthening of the light wavelength towards the red spectrum as a celestial object moves further away.
S
Saturn
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.
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.
The farthest point either north or south of the equator that the Sun reaches each year.
Space
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.
Star
A spherical mass of plasma held together by gravity. There are several types of stars. See Know Your Star Types.
Darkest part of a shadow. Where you would observe a total eclipse.
Uranus
Planet of the Solar System, seventh from the Sun. Considered an “ice giant”, like Neptune. It has 27 moons. It is visible to the naked eye, but it is very dim. In telescopes with the magnification for planet viewing, Uranus appears as a pale cyan disk.
V
Venus
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.
W
Waning
In reference to the Moon, progressively less is being illuminated and so its size is appearing to decrease.
Waxing
In referent to the Moon, progressively more is being illuminated and so its size is appearing to increase.
Z
Zenith
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
ASTROPHYSICS
Astrophysics
The study of the laws of physics as it applies to stars and celestial bodies.
Cosmology
The study of the origin and evolution of the universe.
Spectroscopy
The study of light reflection, absorption, and transfer between matter.
Photometry
The study of luminous astronomical objects in relation to electromagnetic radiation
Asteroseismology
The study of internal structure of stars through observing their oscillations.
Helioseismology
The study of the composition of stars in terms of their interior structure and dynamics by way of observing their surface waves
Solar Physics
The study of the laws of physics as it applies to the Sun.
Heliophysics
The study of the Sun’s constant and dynamic radiation effect on its surrounds in space.
ASTROMETRY
Astrometry
The study of celestial bodies in terms of their position and how they move in space.
Planetology
The study of planets in terms of how they form, including their composition and dynamics in history.
Exoplanetology
The study of planets that exist outside our solar system.
ASTROGEOLOGY
Areology
The study of the geological composition of Mars.
Selenography
The study of the Moon’s physical features, e.g. the luna maria, craters, and mountain ranges.
Exogeology
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.
Exobiology
The study of the likelihood of life in space.
Astroecology
The study of the interactions of biota with space environments.
Astrochemistry
The study of chemical substances in celestial bodies, stars, and interstellar space
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