If you are starting out with a telescope or a set of binoculars for astronomy, and wanting to find your way around the night sky, you might come across astronomy terms you’ve never heard before. So, I’ve compiled the following to help.
What is astronomy?
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 manuals or in day to day use of astronomical terms in forums.
Astronomy for beginners – Space terms: I’ve listed the meanings alongside these ‘words to do with space’, which are in alphabetical order.
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 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. This can be the best time to stargaze, as I explain in my article on Moon phases.
Also known as outer space. There is no definite official delineation of 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.
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.
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.
These are the two closest to the Sun, Mercury, and Venus. They differ to the other planets in that they can never reach opposition.
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.
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. Known for its 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). See my article on Jupiter through a telescope and you can also learn about identifying Jupiter’s Moons.
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. Refractor telescopes (vs reflectors) use lenses.
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).
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. See my article on Mars through a telescope.
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 when using astronomy binoculars. Considered an “ice giant”.
A group of up to a few thousand stars loosely bound by gravitational attraction and formed from the same giant molecular cloud. Pleiades is an example of an open star cluster, known as the Seven Sisters.
In astronomy, “opposition” means that it is on the opposite side of the Earth from the Sun (it’s opposite to the Sun).
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.
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. See my article on Saturn through a telescope.
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.
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.
Darkest part of a shadow. Where you would observe a total eclipse.
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.