Astronomy terms explained

how to learn stargazing

Accretion disk

Circular disk of stellar matter captured by a large celestial body, like a star or a black hole.

Achromatic

Said of a lens that receives light without separating the colors that compose it. Its opposite is apochromatic – which breaks down light into red, blue, green…

Anthelion

Optical phenomenon where a halo of light appears opposite the Sun.

Altazimutal

Telescope mount orientable both vertically (azimuth) and horizontally (altitude).

Annular eclipse

It is a form of eclipse where the Moon does not completely cover the Sun because it is at its furthest position from Earth, thus revealing a thin luminous ring.

Aperture

Diameter of the front lens or main mirror of a telescope, usually expressed in inches or millimeters.

Apoapsis

Point of the orbit of a star orbiting another where the distance between the two is maximum.

Apogee

Term used to describe the apoapsis for the Earth, applied to the Moon and artificial satellites in orbit.

Asterism

Remarkable pattern formed by easily recognizable stars that can belong to one or more constellations. Although these groups of stars are not official, they are often well known in popular culture (“the pan”, “the summer triangle” or “the Orion’s belt” for example).

Asteroid

Asteroids are small planets. They should not be confused with comets. They are found mainly in the internal solar system, orbiting the Sun.

Asteroid belt

Region of space between Mars and Jupiter containing millions of asteroids orbiting the Sun with low eccentricities.

Astronomical horizon

Unlike the real horizon, the astronomical horizon is the imaginary horizon drawn perpendicular to the direction of gravity.

Astronomical nutation

Slight periodic movement undergone by the axis of rotation or the orbit of a planet.

Astronomical unit

Average distance between Earth and the Sun, defined just below 150 million km.

Aurora

When charged particles from the Sun meet a planet, they are channeled to its poles by its magnetic field. The luminous veils forming the aurora are due to the penetration of these particles into the upper atmosphere.

Azimuth

Azimuth measures the angle of an object on the observer’s horizon line from true north (at zero degrees).

Binary (star)

Couple of stars that orbit around a common center of gravity.

Blue filter (80A)

Color filters are good tools for astronomers who want to enhance the relative contrast of certain surface characteristics. Each color has a number, the 80A representing a medium blue. It has also become very popular for observing Jupiter. It is sometimes called the “Jupiter filter”. It darkens the appearance of Jupiter’s belts as well as the small details around larger storms.

Blue Moon

The second full moon in the same month. This term can also designate the blue tint of the Moon caused by volcanic activity.

Blueshift

When an object approaches you, the wavelengths of the light it emits shift towards blue in the visible spectrum.

Brightness

Measurement of brightness, more precisely the total amount of energy emitted by a celestial body.

Brown dwarf

Object endowed with all the material necessary to become a star but whose mass is not sufficient to achieve it.

Catadioptric

Telescope using refraction and reflection techniques to form an image.

CCD

Charged Coupled Device (CCD) refers to an electronic chip that detects photons to produce digital images.

Celestial equator

It is an imaginary line separating the northern and southern hemispheres and corresponding to the projection onto the celestial sphere of the Earth’s equator.

Celestial pole

Like the celestial equator, the celestial pole is an imaginary line towards which the axis of rotation of the Earth points.

Cepheid variables

Pulsating stars whose quantity of light emitted changes. Their speed of change is proportional to their brightness. We can thus calculate their distance by measuring their brightness and the speed of their pulsations.

Chromatic aberration (CA)

If a telescope has only a poor quality lens or doublet, bright objects may be surrounded by a red or bluish halo.

Circumpolar

Said of a celestial body which never seems to rise above or descend below the horizon. If you lived at one of the Earth’s poles, all of the visible stars would be circumpolar. Conversely, if you lived at the equator, none would be.

Collimation

Unintentional hits can change the alignment of the primary and secondary mirrors of a Dobsonian type telescope. You can adjust the two mirrors by changing their orientation. This alignment process is called “collimation” and those who use a reflector must collimate their telescope from time to time. Better to do it in the light. By looking inside the eyepiece holder, you can identify the primary mirror at the base of the tube and the secondary mirror on its support. Although it is elliptical, the secondary mirror appears circular because it is inclined at 45°. The purpose of collimation is to center and orient these two elements perfectly. Collimation instructions are generally provided with the most recent Dobsonian type telescopes. Astronomical equipment suppliers also sell a number of collimation accessories, such as the Cheshire eyepiece and the collimation laser, which provide more precise collimation than a simple estimate.

Color index

Value to measure the brightness of a star on different frequency bands of the electromagnetic spectrum.

Commensurability

Ratio comparing the orbit of two objects gravitating around the same body, like Saturn whose period of revolution is 5/2 that of Jupiter.

Conjunction

A rare phenomenon that occurs when two celestial objects appear very close to each other in the sky.

Corrector plate

The corrective plate modifies the light that passes through a telescope to compensate for the deformations created by the spherical primary mirror. This deformation is called “spherical aberration” and would render the images unusable without the correction effects provided by this plate specially designed for this.

Counterweight

There is a very wide variety of eyepieces and astronomical accessories that can be used with a Dobson telescope, including heavy ultra-wide field eyepieces, Barlow lenses, filters and filter wheels as well as camera holders. All this can add weight and unbalance the instrument. One of the best ways to rebalance it is to wrap a small bungee cord around the tube near its base and insert an object of appropriate weight, the counterweight.

Craters

We now know that the craters of the Moon were caused by impacts of asteroids and meteorites. There is no atmosphere on the Moon, so no wind or rain to erode the remains of the impacts that occurred at the beginning of its history.

Crater radius

Circular traces delimiting a crater due to an impact with an object which released small particles of matter on the surface.

Depression & high pressure

Jupiter is covered with swirling vortex, storms often extending over several thousand kilometers. Whether they are low pressure or high pressure depends on their latitude and their direction of rotation linked to the Coriolis force. A storm turning counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere is a depression. An anticyclone corresponds to the reverse of this description.

Disturbance

Force that can influence the orbit of a celestial body orbiting another gravitational body. It can also create resistance or misshapen bodies.

Doppler (effect)

Change in frequency of a wave for an observer moving relative to its source. This gives a redshift or a blueshift.

Doublet

Association of two lenses. Each lens in the system has a different shape, one being convex (curvature facing outward) and the other concave (curvature facing inward). This brings the light from the blue and red ends of the spectrum back to the same focal point.

Dwarf planet

According to the official definition, it is a body in direct orbit around the Sun, large enough for its shape to be controlled by gravity but which has not cleaned its orbital zone.

Dwarf stars

These are small stars like our Sun, and which oppose giant stars, like Betelgeuse.

Eccentricity

Eccentricity indicates the deviation of an orbit from a perfect circle, which is the case of an elliptical orbit.

Eclipsing binaries

Two stars orbiting their common center of gravity which can pass one in front of or behind the other. When this happens, they are “eclipsed” and the usual combined light of the stars is weaker.

Ecliptic

Imaginary line drawing a large circle in the sky. It goes through each of the twelve Zodiac constellations and a thirteenth constellation, Ophiuchus. The ecliptic represents the path of the Sun as it appears in the sky throughout the year.

Electromagnetic (spectrum)

Celestial objects emit radiation in a range of frequencies that make up the EM spectrum, from gamma rays to visible light and radio waves.

Ephemeris

Table giving the position of celestial objects in the night sky at any time.

Epoch

Reference moment used to determine the coordinates or the orbital elements of a celestial body.

Equinox

Point in Earth’s orbit where the apparent declination of the Sun is zero: the Sun is at its zenith at the Earth’s equator. The equinox is the intersection of the ecliptic (line of the apparent trajectory of the Sun on the celestial sphere) and the terrestrial equator.

Evolutionary track

Prediction of the behavior of a solitary star over its entire lifespan depending on its mass and composition.

Exoplanet

Planet located outside the solar system, orbiting around other stars.

Extinction

Absorption and diffusion of electromagnetic radiation by dust and gases located between the object and the observer.

Eyepiece holder

In order to correctly see and enlarge the image coming from the mirrors, you need to see it through an eyepiece. It is placed in a removable tube called “eyepiece holder”, adjustable using the focus wheel so that the observer has a clear and precise vision.

Field galaxy

Galaxy not belonging to a cluster of galaxies.

Field stars

Stars located in the field of vision of associated stars under study which not only have no relation, but which can also affect the results of the study.

Floccule

Particular region of the solar surface that can be observed through certain wavelengths of the EM spectrum.

Galilean moons

The four largest moons of Jupiter were discovered by Galileo Galilei in 1610: Io, Europa, Ganymede and Callisto.

Gas giant

Refers to a large planet which is mainly composed of gases. The four planets furthest from the Sun in our solar system are good examples.

Geocentric

Said of an orbit centered around the Earth, like the Moon or any of our artificial satellites.

German equatorial mount

This type of telescope mount was invented by Joseph von Fraunhofer. With a simple T-shaped piece, this mount is very easy to assemble while being versatile and precise. It is portable enough to be transported for observations in a remote location. However, you need a counterweight.

Gibbous

Contrary to a crescent, when a planet or a moon is between its first quarter and its full phase.

Globular cluster

Star groups that revolve around a galaxy, like satellites, linked by gravity.

GoTo

Some modern telescopes are equipped with computers that will “search” for any object in their database when instructed to do so on a device.

Gravitational lens

Effect of the gravitational forces which are exerted on the light and deflects it between the source and the observer.

Halo

Optical phenomenon caused by ice crystals in the atmosphere which produce a circle of light around the Sun or the Moon.

Heliocentric

While a geocentric object orbits the Earth, a heliocentric object orbits a central star, such as our Sun.

Heliopause

Boundary of the heliosphere, where the solar winds stop and the interstellar medium begins.

Heliosphere

Space bounded by the heliopause containing the Sun and all the bodies of the solar system. In this area of space, the energy density of the solar wind is higher than that of the interstellar medium. It thus protects the planets from harder radiation from space.

Horizon

The true horizon of planet Earth, as opposed to that defined by the gravity of the observer.

Interstellar cloud

A denser than average area of space made up of plasma and dust, almost like a nebula.

Interstellar medium

Space between two stars, outside the heliopause of a star.

Irregular variables

The brightness of these stars varies at random intervals unlike many variable stars, the variations of which always follow a regular pattern.

Kuiper belt

Area beyond Neptune composed of asteroids and other small objects. The Kuiper belt is twenty times wider than the asteroid belt.

Layers of clouds

Saturn has no solid surface. It is made up of gases, some of which are frozen, which rotate around the planet as it rotates on its axis. Because of this rotation, the gases form visible “layers” because they are slightly different colors.

Light-year (ly)

Astronomers measure cosmic distances according to the time it takes for light, which travels at 300,000 km/s, to reach us. A light-year is equivalent to ten trillion km. Our galaxy, the Milky Way, is 100,000 light-years across. The nearest large galaxy, the Andromeda Galaxy, is about 2.6 million light-years away.

Local group

Our local group of galaxies, of which the Milky Way is a part. It is made up of a total of 54 galaxies.

Local standard of rest (LSR)

Average movement of matter in the Milky Way around the Sun, a movement that is not perfectly circular.

Local/Virgo Supercluster

It is the superama which contains the Virgo cluster and the Local Group, which themselves contain the Milky Way and the Andromeda Galaxy.

Lunar libration

The Moon oscillates slightly when it rotates around the Earth, a phenomenon called “lunar libration”, which means that we can sometimes see behind the “edges” of craters, mountains and other characteristics that are not normally visible from very sharp angles.

Lunar mare

We know there is no water on the Moon, but a long time ago it was thought that the dark, smooth areas visible to the naked eye were seas. We now know that these areas are actually plains of lava that formed while the Moon was young and warm.

Magnification

Lower magnifications, like those of binoculars, offer a relatively wide field of vision while the magnification of a telescope can be adjusted by changing the eyepiece. Originally, the term only designated the magnification capacity of an element in an optical system (like a lens or a mirror) but now it has by extension become the whole system, with the objective and the eyepiece.

Magnitude

The brightness which one perceives from a celestial object is called “apparent magnitude”. The brightest star is Sirius, of magnitude -1.4, while the least bright stars visible to the naked eye in a night sky have a magnitude of about +6.0. 10×50 binoculars allow you to observe objects up to a magnitude of +11.0.

Mascon

Contraction of “mass concentration”, this term indicates an area of a planet or a lunar crust which presents gravitational anomalies like a local increase of its gravitational field.

Main sequence

Category of stable stars undergoing nuclear fusion with standard temperature and brightness.

Meridian

As on Earth, a meridian is a vertical line which crosses the apparent center of the disc of Jupiter. This is used to predict when interesting phenomenon, such as the Great Red Spot, are on the meridian, because this is where they are most visible. When a phenomenon is on the meridian, not only is it visible from your position but it is also the time to take your telescope and observe it!

Messier (catalog)

List of astronomical objects described by Charles Messier in the 1700s in which bodies are called Ml, M2, M3, etc.

Meteorite

If a meteoroid survives its entry into the atmosphere and hits the ground, what is left is called a meteorite.

Meteoroid

Refers to a small rocky or metallic object moving in space. Meteoroids are much smaller than an asteroid.

Minor planet

An object gravitating directly around the Sun and which is neither a planet nor a comet.

Molecular cloud

Type of interstellar cloud that has the right conditions to form molecules, including hydrogen.

Mons

Term meaning “mountain” in Latin and traditionally used to name extraterrestrial mountains, such as Olympus Mons on Mars.

Moving group

A group of stars, usually formed in the same cloud, and moving together in space.

Moving shadow

When a moon of a planet casts its shadow on the surface of this planet, it moves slowly along the disc when it revolves around it. This is called a “moving shadow”.

Nadir

Vertical direction towards the center of gravity of an object, opposite the zenith.

Natural satellite

Our Moon is a natural satellite: a body not created by man orbiting a planet.

Nebula

Interstellar cloud composed of dust, hydrogen, helium and other ionized gases. Stars and planets can form there.

Neutron star

Remains of a dead star almost entirely made up of neutrons. They are extremely hot.

Node

Point where an orbit crosses a reference plane, as when the Earth’s orbit crosses the celestial equator of the Sun.

North

The classic North Pole, the point around which the Earth turns. It is used to distinguish it from the celestial pole.

North Star

It is the star which is currently almost exactly in the extension of the axis of rotation of the Earth, at the North Pole. If we extend the North Pole in space, we reach the celestial North Pole. In the northern hemisphere, all the stars seem to revolve around this point.

Nova

Cataclysmic nuclear explosion, characterized by a sudden brilliance, of a white dwarf which attracted matter from a nearby star.

OB association

Group of massive stars which move freely together in space without being linked by gravitation.

Observable universe

Extent of the universe that we can see because its light has had enough time since the Big Bang to reach us.

Occultation

Complete disappearance of a celestial object, masked by another closer to the observer.

Oort (Cloud of)

Spherical cloud composed of small frozen objects at the limit of the sphere of influence of the Sun.

Open cluster

A group containing up to a few thousand stars that formed in the same cloud at the same time.

Parsec (pc)

A parsec is a cosmic unit of measurement based on parallax. Parallax is the change in the apparent position of an object relative to other more distant objects that occurs when you change the angle of view. Although the stars are at incredible distances, the Earth’s orbit around the Sun is almost 300 million km in diameter, so the nearby stars have a small but measurable parallax on the celestial sphere for six months (from one side of the orbit to the other). Since we know the value of our unit of measurement, we can determine the distance of stars that have a measurable parallax. A parsec is the distance between the Sun and an object with a parallax angle of 1 second, or 3.262 light years. Proxima Centauri, the closest star, is 1.29 parsec (4.2 light years).

Periapsis

Point where an object is closest to the body around which it revolves.

Perigee

Term used to describe the periapsis in the case of Earth applied to the Moon and other satellites in orbit.

Phase

The Moon and the planets, as observed from Earth, go through specific phases when they move in space: full, half, new, etc.

Phase angle

Angle formed between an orbiting body and the Sun from the point of view of an observer. It makes it possible to determine to what extent the observed object is visible or in the shade.

Polar finder

The largest German equatorial mounts have often a polar finder that attaches to the mount along the polar axis. The polar finder has a reticle which generally shows the W of Cassiopée and “the Plough” for the Northern hemisphere and a trapezoid of brighter stars in the Octant for the Southern hemisphere. In both cases, you must turn the polar finder so that it corresponds approximately to what you see in the sky, in order to be able to adjust the pole by moving only the latitude, the altitude and the azimuth of the mount.

Precession

Because of the precession, the role of the North Star is assumed by different stars. When the Earth rotates, its axis of rotation changes slightly creating a cone in space, like a peak. The axis between the poles also rotates by describing a circle which it traverses in approximately 26,000 years. During this period, the position of the Sun at the equinoxes moves west through the Zodiac constellations.

Primary mirror

Main mirror in a Newtonian reflecting telescope. The light of the objects observed first strikes this mirror. Its diameter as well as its quality determine their details and their brilliance for the observer. It is housed on an adjustable support called “cell”.

Proper motion

Angular displacement or change of position of an object over time as observed from the solar system.

Protostar

The beginnings of a star, when it acquires its mass from the contraction of an interstellar cloud.

Pulsar

Type of neutron star formed during the death of a supernova star, having a strong magnetic field and rotating very quickly. It emits radio waves which, due to rotation, seem to pulsate.

Quasar

Very bright and distant celestial object, with a very active galactic nucleus, which surrounds a supermassive black hole.

Radial velocity

Velocity of an object along the line of sight of the observer, with a positive value for objects which move away.

Radiant

Shooting stars in a meteor shower seem to come from a single point, called the “radiant”. Since meteorites deploy from the radiant, we do not see any that crosses it, unless they are sporadic.

Red giant

Star which has exhausted all its fuel, which has become enormous and has cooled, which gives it this red-orange hue. Red giants are among the largest stars in the universe.

Redshift

As an object moves away from the observer, the wavelength of light and other electromagnetic waves it emits increases.

Refraction

When light passes through matter, such as glass, it is deflected or refracted. We discovered that by controlling the shape of the glass (the lens), we could change the point where the image formed behind the lens. This is called the focal length of the lens and this has a direct influence on its magnification.

Reverse image

Many telescopes show reverse images.

Right ascension and declination

In astronomy, the sky is projected onto a sphere, like a terrestrial globe. Instead of using longitude and latitude, the coordinates are given in right ascension and declination. The poles have a declination of 90° and -90°. The maximum arc of right ascension is the celestial equator. Because of the rotation of the Earth, everything seems to be moving. A right ascension axis correctly aligned with the polar axis compensates for the movement of the Earth by driving the telescope in the opposite direction. The position of stars and deep sky objects is basically fixed in the night sky, so we can find them using the coordinates provided by a star map and using graduated rings of a good equatorial mount.

Rille

A rille is a narrow line on the surface of the Moon similar to a channel or a river. They can come from the collapse of old lava tunnels.

Roche limit

Radius corresponding to the distance at which orbiting bodies dislocate because the tidal forces of the object around which they gravitate are stronger than their internal cohesion.

Second

Seconds and minutes are the units of measure of the angular distance of a body north or south of the equator.

Secondary mirror

The secondary mirror in a Schmidt-Cassegrain telescope is suspended in a “mirror holder” held in place by the corrector plate. This mirror does not only reflect the light towards the hole of the primary mirror and towards the eyepiece holder, it also contributes to flatten the field of vision of the image so that it is not curved. You can also correctly align the secondary mirror with the optical axis.

Semi-major axis

Circles have a constant radius, unlike ellipses. The semi-major axis corresponds to the maximum length of the radius.

Semi-minor axis

The shortest distance between the edge of an elliptical orbit and its center, the opposite of the semi-major axis.

Semi-regular variables

Giant or supergiant stars whose shine changes follow a defined pattern but which is sometimes interrupted.

Sheperd moons

Small moons in orbit near the rings of a planet or in the space between the planet and its rings. They help define the shape of the rings.

Sidereal time

Time measurement system based on the Earth’s rotation speed to track the movement of stars.

Solar apex

Direction towards which the Sun is moving with respect to the local standard of rest. Its destination is fictitious and unattainable since it revolves around the center of the Milky Way.

Solar filters

There are several types of solar filters that can be used with telescopes, binoculars, and camera lenses. Telescope manufacturers often produce metallized glass filters suitable for their instruments. To get one of good quality at a lower cost, you can create one with a film sheet “Astrosolar Safety”. This filter, which is sold in A4 format and looks like aluminum foil, is made of metallized polymer. You can also buy filters that use this material in metal rings suitable for opening the telescope. Specialized solar telescopes use Hydrogen-Alpha filters.

Solar mass

It is a unit of mass based on that of our Sun and used to weigh astronomical objects. It is (1,988 4 ± 0,000 2) × 1030 kg

Solar wind

Charged particles emitted by the Sun, made up of electrons and protons, at the origin of the aurorae in our atmosphere and protecting the solar system from cosmic rays.

Spectroscopic binaries

Binary star which can only be found using spectroscopy and not through visible light through a telescope.

Spectroscopy

Using a prism to separate the light of a star into a “rainbow” of colors can tell us a lot about the nature of the chemical elements of a star and its speed. We can also determine if there is more than one star, even if the stars are too close to each other so that we could observe them individually.

Spherical primary mirror

The spherical primary mirror is pierced with a hole in its center to allow the light reflected by the secondary mirror to concentrate behind the telescope.

Sphere of influence (SOI)

Area around a celestial body where its gravitation is most influential.

Spider

The secondary mirror of a telescope is suspended by a device that originally had four arms or “supports”, and therefore called a “spider”. You can also find it with two or three arms. They hold the secondary mirror in the center of the primary and allow it to be aligned and adjusted in an operation called collimation.

Stellar atmosphere

Outer layer of a star, representing only a small part of its mass and sometimes a large part of its size.

Sunspots

Areas with complex magnetism on the Sun. They appear as dark spots with a dark center and lighter outer areas, either by projecting the image through a telescope, or by using a “white light” filter. If these sunspots are darker, it is because they are colder. They travel through the solar disk as it rotates, growing and shrinking as the magnetic fields change.

Sunrise azimuth angle

It is the horizontal angular distance between the azimuth of a celestial body at sunrise and the east.

Super moon

It is a full moon or a new moon that takes place while it is closest to Earth.

Synodic period

Time it takes for one object to go around another. It is calculated in relation to large fixed stars.

Syzygy

Alignment of three bodies in a gravitational system, like during a solar or lunar eclipse.

Terminator

As the light of the Sun moves on the face of the Moon, we can see the border between day and night on its surface, This is called the terminator and it is a very good place to observe through a telescope, wherever it is on the lunar surface, because the grazing light of the Sun projects on the lunar surface the shadows of the reliefs.

Ternary systems

System with three stars gravitating around each other, as for a binary star.

Terrestrial planets (or telluric planets or rocky planets)

The four rocky planets that revolve around the Sun, inside the asteroid belt: Mercury, Venus, Earth and Mars.

Tidal acceleration

Effect of tidal forces. The gravitational attraction of two objects in orbit contributes to their acceleration or deceleration.

Tidal locking (or gravitational locking)

Because of the tidal acceleration, most moons undergo a gravitational lock on the part of their planet: they rotate at the same speed as they orbit, which means that we only see one side from the moon.

Transneptunian objects

Objects located after Neptune in the solar system, for example Pluto and the Kuiper belt. They are sometimes called plutoids.

Transit

It is the opposite of an occultation. It occurs when a smaller body passes in front of a larger body. For example, when a planet passes in front of the Sun.

Variable star

At first glance, all the stars seem to shine at constant brightness. However, the light emitted by many of them varies, increasing or decreasing their brightness.

Variation period

The time it takes a star to go from its strongest brightness to its weakest brightness. For some variable stars, it may take a few days or even hours. For others, it can go up to years.

Visual binaries

Couple of stars easily observable with the naked eye and whose stars are relatively distant from each other.

Vortex

Powerful whirlwind of a gas or fluid around an axis.

White dwarf

Star at the end of its life and, following its collapse, becoming a small ball of hot gas with a mass comparable to that of our Sun.

Zenith

Point of the sky located just above the observer’s head, and therefore the one at 90° altitude. It is the opposite of the nadir.

Zone of avoidance

Designates the area of ​​the night sky obscured by the Milky Way and which limits what can be observed in this direction.