First of all, what is a constellation?
Before we take a closer look at the Cetus constellation, let’s get a grasp of what a constellation is. Basically, it’s a group of Stars and objects in the night sky. A constellation is an area on the celestial sphere (an imaginary sphere) where a group of visible stars and objects are located.
These stars typically form a pattern or outline, which we perceive to represent an inanimate object, (like a question mark), an animal (like the fish) a mythical person (like Poseidon, the god of the sea, the Hero Perseus, King Cepheus and Queen Cassiopeia and their daughter Andromeda or even a type of creature (like Pegasus, the winged-horse, Medusa, or Cetus, the Sea Monster), from Greek mythology.
It is also an astronomy term used to describe a variety of groups of stars that have been given a specific name such as –
Constellations are constantly moving and move in the direction from East to West.
The Cetus Constellation – Facts in brief:
What is it? –
Constellation Cetus is named after Cetus, the sea monster that was a popular character from Greek mythology.
The Cetus Constellation is one of the oldest known constellations, and ranks as the 4th largest Constellation in the night sky and as one of the original 48 Constellations listed by Greek Astronomer Ptolemy, in the 2nd century.
Cetus is also one of the official International Astronomical Union (IAU) listed 88 modern constellations as seen in the night sky from Earth and listed as the 4th largest Constellation overall, filling around 2.99% of the night sky.
Cetus, abbreviated to Cet, or Ceti (its Latin name) is quite easy to recognize, as it clearly resembles the outline of the great sea monster, from Greek mythology.
The Constellation of Cetus is a member of The Perseus family, of Constellations, which also includes:
Where is it located? –
The Cetus Constellation is positioned in the first quadrant of the Southern Hemisphere, north of the ecliptic. It is sometimes referred to as being located in the SQ1 Quadrant.
FACT: The ecliptic is the imaginary line tracing the route that The Sun, the Moon, and the Planets take across the sky, over the year.
The Cetus Constellation is located in an area of the sky known as ‘The Water’.
There are 3 Constellations found within ‘The Water’ area of the night sky:
- Constellation Eridanus – represented as ‘the river’
- Constellation Aquarius – represented as ‘the water bearer’
- The Constellation of Pisces – represented as ‘the pair of fishes’
The Constellation Piscis Austrinus is another Constellation in the Southern Celestial Hemisphere and associated with The Water area of the sky.
The name Piscis Austrinus is Latin meaning ‘The Southern Fish’.
Where can it be seen?
Co-ordinates of a right ascension, or left ascension and their declination are used to locate all of the Constellations, like the Constellation of Cetus.
Cetus is most prominent in the Southern Hemisphere, from January to February. It is not a circumpolar Constellation (meaning it is visible all year round).
The Constellation of Cetus lies at 2-hour right ascension and a Declination of 10 degrees South.
It’s visible in the Southern Hemisphere at latitudes between +70 degrees and -90 degrees and covers an area of 1231 square degrees in the southern sky.
The Cetus Constellation is bordered by several other Constellations:
- Pisces – to the North
- Fornax, Phoenix and Sculptor – to the South
- Aquarius – to the East
- Eridanus to the West
It is also visible in the Northern Hemisphere in Autumn time and early in Winter.
Diphda (beta Ceti) is the brightest star in the Constellation of Cetus has an apparent visual magnitude of 2.04.
The closest star in Cetus is Tau Ceti, it’s a Sun-like Star that’slocated around 11.9 light years from Earth. It is visible to the naked eye and has an apparent visual magnitude of 3.5.
The furthest star within the borders of Cetus (but actually part of the constellation) is HIP 6396 and estimated to belocated around 326163.4 light years from the Earth.
There are 15 Stars that make up the shape of the main Cetus Constellation.
How can you identify The Constellation of Cetus?
The simplest method for spotting any particular Constellation from Earth is to first of all locate the brightest star in that Constellation, and then look at the neighboring illuminations, to see if you can identify a recognizable pattern.
There are many different stars located within the Constellation of Cetus.
The named Stars in any Constellation are listed in order of luminosity from the brightest Star to the faintest Star. The ‘Alpha’ letter is normally allocated to the brightest star, then ‘Beta’ and so on in decreasing order through the letters of the Greek alphabet.
Although in the case of the Constellation of Cetus, the brightest Star has not been allocated the Alpha name.
The brightest Star in Cetus is called Diphda, the alpha Star, known as alpha Ceti. Then the second brightest Star in Cetus is called Menkar and is actually the beta Star, known as beta Ceti.
The Cetus Constellation can be identified in the night sky if you find its 2 brightest stars, Diphda (beta Ceti) and Menkar (alpha Ceti) and imagine the outline of the sea monster from Greek Mythology.
While Cetus, is often connected with this image of the sea monster, it’s also known in some cultures as the ‘Whale’, although it doesn’t really resemble the shape of a whale.
It is shaped more like a hybrid creature that is part sea serpent with large jaws and big forefeet.
The brightest stars of Cetus can be viewed from Earth, from a northern location, by the naked eye.
The Star System within Cetus
The Constellation of Cetus has about 15 main Stars making up the imaginary outline of the sea monster.
The main bright stars that form the ‘sea monster’ shape, listed from brightest Star to fainter:
- Diphda (Beta Ceti) – an Orange Giant Star
- Menkar (Alpha Ceti) – a Red Giant Star
- Dheneb Algenubi – an Orange Giant Star
- Kaffaljidhma – a Triple Star System
- Mira (Omicron Ceti) – a Binary Star System
- Tau Ceti – a Yellow Dwarf Star
- Deneb Kaitos Shemali – an Orange Giant Star
- Theta Ceti – an Orange Giant Star
- Baten Kaitos – an Orange Giant Star
- Upsilon Ceti – an Orange Giant Star
- Delta Ceti – a Blue-White Sub-giant Star
- Mu Ceti – a Binary Star System
- Xi-2 Ceti – a Blue-White Giant Star
- Lambda Ceti – a Blue-White Giant Star
There are many named Stars within Cetus that have been approved by the International Astronomical Union (IAU):
Location of Cetus
Cetus can be spotted in the Northern Hemisphere, also referred to as the Northern sky.
The Constellation Cetus is surrounded by various recognizable Constellations such as Aries, Pisces and Eridanus.
The Cetus Constellation is a well-known guide-point in the sky used by astronomers and amateur stargazers to identify certain Deep Sky objects; like the neighboring Andromeda Galaxy.
When to see the Constellation Cetus
The best months to spot the Constellation of Cetus in the Northern Hemisphere are January and February.
How was it formed, found and named?
Cetus is one of the original 48 Constellations catalogued by Ptolemy, and one of the 88 Constellations listed in the official IAU chart published by the International Astronomical Union (IAU).
It fills an area of 1231 square degrees.
- Initially, the shapes of their star patterns informally categorized the Constellations in the sky.
- Eventually, the IAU published the official listing of constellation boundaries. This maps the constellations by their sky coordinates not by the line patterns and shapes they are referred to by.
The shape of a strange sea monster representing the Constellation of Cetus comes from the position of its main Stars. You will have to use your imagination using the following Stars as part of the outline of this asterism.
The constellation of Cetus is sometimes sketched as having the head of a squid with its tentacles representing a beard, with the body of an athletic looking man and the tail (or arm) of an octopus or big fish:
- Lambda Ceti, Mu Ceti, Xi-2 Ceti represent the top of the skull of the monster
- Kaffaljidhma (Gamma Ceti) – represents the base of the head of this asterism image (imagine a Whale or a strange looking sea monster.
- Menkar (Alpha Ceti) – represents the nose of this asterism image
- Mira (Omicron Ceti) and Delta Ceti – represent the neck of this asterism
- Baten Kaitos (Zeta Ceti) – represents the belly of this asterism
- Deneb Algenubi (Eta Ceti) – represents the northern tail of of this asterism
- Dheneb Kaitos Shemali (Iota Ceti) – represents the southern tail of this asterism
- Diphda (Beta Ceti) – represents the end of tail of this asterism
This image works when you imagine the constellation of Cetus as an unusual sea monster with bits of sea creatures and bits of human. If you think of Cetus as a whale this image would not work so well.
What’s within the Cetus Constellation?
The Constellation of Cetus is formed by of a number of different components.
The different components housed by the Constellation Cetus are mainly Stars, Deep Sky Objects and Messier objects (galaxies).
The Cetus Constellation contains:
- 15 main Stars
- 110 bright Stars
- 14 Stars with known planets
- 1 star that hosts an exoplanet (HD 4747)
- 3 associated meteor showers known as the ‘Cetids Meteor Showers’
- ‘The October Cetids’
- ‘The Eta Cetids’
- ‘Omicron Cetids’
- 1 Messier Object –
- Messier 77, (M77, is a barred spiral galaxy and also referred to as NGC 1068
There are many different types of Stars in the star system categorized by size, lifespan and luminosity.
Generally, larger Stars have a shorter lifespan.
Stars are formed from clouds of interstellar gas and include:
Red Dwarf Stars
Most of the stars in the galaxy are Red Dwarf Stars. They are small in size measuring about 40-50% of the mass of The Sun. They are cool and their luminosity has only about 10% of the brightness of the Sun (our brightest Star), and they live for longer.
Brown Dwarf Stars
These are known as failed stars that form like other stars but don’t reach the mass, heat or density to begin the nuclear fusion process. They are only about 8% of the mass of the Sun and are red not brown, and not easy to spot in the night sky.
Red Giant Stars
These are giant luminous stars that have a low or medium mass. Red Giant Stars are formed when a star expands its volume by fusing all of its hydrogen into helium, and then burning the helium to produce carbon and oxygen to expand.
These are giant, bright stars that range from 10-100 times the size of the Sun and are 1000 times brighter. They are big and hot and therefore burn out quickly. The biggest are called Blue super giants or hyper giants. The biggest ever discovered was about 10 million times brighter than the Sun,
These are main-sequence stars like the Sun, but only 80% of its size, and are bright stars,
These are small burnt out husks of stars, about the same size as the Earth. White Dwarfs are dense and represent the final state of evolution for a star, like most stars in the galaxy.
These are the remains of a White Dwarf after it cools and darkens. This is likely to happen after about 10 billion years of life.
These are also main-sequence stars like the Sun, but twice the size, and are bright stars and hot.
Other types of stars include Neutron stars, Variable Stars and Binary Stars
The first variable Star
In 1596, a German Astronomer called David Fabricus, was the first person to discover a variable Star. He found a third magnitude Star within the Constellation of Cetus but believed it was a nova, as it disappeared.
It was Johann Bayer who gave this variable Star the name Omicron Ceti, and due to its variability it was named Mira meaning miraculous.
What is a Bright Star?
The sky is home to various bright stars.
The brightness of a star is measured by a value called its magnitude (apparent magnitude) and they come in different sizes, composition, mass and color.
Their vast distance away from us is measured in light years from the Earth, the Sun or even the Milky Way.
The lower the magnitude value the brighter the star appears in the night sky when viewed from Earth.
FACT: The Sun is considered to be the brightest star in the sky.
Cetus has 15 formally named stars that have been officially approved by the International Astronomical Union (IAU):
As well as Stars, the Cetus Constellation also has deep sky objects and galaxies (or even globular clusters or open clusters).
Deep Sky Objects
The Constellation of Cetus has 9 deep sky objects:
- Messier 77 (M77, NGC 1068),
- NGC 1055,
- NGC 1087,
- The NGC 1073 barred spiral galaxy,
- NGC 45,
- The NGC 17 spiral galaxy,
- NGC 47 (NGC 58),
- NGC 1042
- Galaxy NGC 247
FACT: A Deep Sky Object is an astronomical object, that is not a solar system object like the Sun, Moon, Comet or a Planet. An individual Star is not considered to be a Deep Sky Object.
Deep Sky Objects are faint objects that can still be observed by the naked eye in the night sky from Earth. They include Galaxies, Star Clusters and Nebulae.
- Star clusters – such as Globular Clusters of Stars or Open Clusters of Stars
- Dark Nebula, Planetary Nebula, Diffuse Nebula, and Supernova remnants
- Galaxy Groups, Galaxies, Gravitational Lenses and Quasars.
Galaxies in Cetus
The Darth Vader Galaxy, (NGC 936), is a barred lenticular galaxy deep space object, located close to the 75 Ceti Star in the constellation of Cetus.
Messier 77, (NGC 1068), is a spiral galaxy located around 47000 light years from earth with an apparent visual magnitude of 10.00 and cannot be seen by naked eye from earth.
Generally a minimum visual magnitude of 6.0 is required for any space object to be visible by the naked eye.
NGC 247 – is one of several galaxies bound by gravity to the sculptor galaxy, NGC 253, close to the Milky Way. This galaxy has an apparent visual magnitude of 9.9.
What is a Nebula?
A Nebula is a massive cloud of gas and dust in Space.
Some Nebulae are formed when a star explodes and then dies, as is the case with a Supernova. Sometimes they can act as Star nurseries and are the areas where new Stars are forming.
The Nebulae are the spaces in between the stars referred to as interstellar space.
There are several types of Nebulae:
- Bright Nebulae,
- Emission Nebulae,
- Reflection Nebulae,
- Dark Nebulae
- Planetary Nebulae
FACT: a ‘reflection nebula’ is an interstellar cloud that should be a dark nebula (a molecular cloud) however its dust reflects light from a nearby bright star and it reflects the light, hence the name.
Images of the Nebulae have been captured using professional Space telescopes, such as the NASA Spitzer Space Telescope, operated by NASA, and the famous Hubble Space Telescope.
What is a Messier object?
A Messier is a cluster of Stars
There is one Messier objects within Constellation Cetus called Messier 77 (M77, NGC 1068), a barred spiral galaxy.
It was Charles Messier, a French astronomer, whois credited with cataloging each of the Messier Star clusters, around 1764.
He is famous for publishing an astronomical catalogue that lists 110 nebulae and star clusters, known as the New General Catalogue (used in its abbreviated form NGC and numbered).
These later became known as the Messier objects.
FACT: A star cluster is a large group of Stars that can be Globular Clusters or Open Clusters:
- Globular Clusters:
A global cluster is a spherical collection of ‘Old Stars,’ numbering hundreds to millions, that are tightly bound by gravity and orbits a galactic core.
- Open Clusters:
An open Cluster is a looser formation of ‘Young Stars’ that generally has less than a few hundred Stars.
The Messier Marathon
The best time of year to view all 110 Messier objects at the same time, if the night sky conditions are positive, is between mid to late March and early April.
A supernova remnant is the structure that’s left after a star explodes in a supernova.
There are 3 types of supernova remnants: shell-like, composite and mixed-morphology (or thermal composite).
What is the Milky Way?
The Milky Way is a Spiral Galaxy, containing over 200 billion Stars, and actually forms part of the Constellation of Sagittarius and is not itself, a Constellation of Stars.
The Milky Way is the Galaxy that contains our solar system and it gets its name from the fact that it looks like a hazy swirl or river of milk across the sky, when viewed from earth.
It is made up of gas, dust and stars, with spiral arms wrapped around it, and a massive black hole in the center of the Galaxy.
Not all of the Stars in the Universe are contained within the Milky Way.
It is at its brightest if looking towards the galactic center in the direction of Sagittarius.
The Stars that make up the Milky Way are many light years away and cannot be individually identified by the naked eye.
Background & Facts:
The Greeks were the first ancient culture to name the 88 Constellations in the sky.
It was the Greek Astronomer – Ptolemy, who first cataloged the Constellation of Cetus as one of 48 early constellations, in the 2nd Century (2 AD.).
Ptolemy listed the various constellations in his Almagest (a book recording astronomical data).
In 1603, The German Astronomer – Johann Bayer, systematically assigned names to the brightest stars in Cetus and cataloged them in his Star atlas – ‘Uranometria’. Bayer depicted Cetus as a ‘dragon fish’.
The Bayer designations are stellar designations where the stars within Constellations are initially identified by a name or a letter from the Greek Alphabet from Alpha through Omega (in order of brightness).
1. The names of the Stars begin with a letter of the Greekalphabet starting with– Alpha, then beta, gamma, delta, epsilon, zeta etc.
2. Followed by the genitive form of their parent constellation’s Latin name – ‘Cet’ with ‘I’, makes the name ‘Ceti’
3. Giving the first Cetus Star the name Alpha Ceti (although the brightest star is beta Ceti)
The main stars of Cetus are named by their apparent magnitude (luminosity) from the brightest to faintest star in decreasing order:
1. Diphda (Beta Ceti) – an apparent visual magnitude of 2.04
2. Menkar (Alpha Ceti) – an apparent visual magnitude of 2.54
3. Mira (Omicron Ceti) – an apparent visual magnitude of 3.04
4. Deneb Algenenubi (Eta Ceti) – an apparent visual magnitude of 3.46
5. Kaffaljidhma, from Arabic, (Gamma Ceti) – an apparent visual magnitude of 3.47
6. Thanih al Naamat, from Arabic, (Tau Ceti) – an apparent visual magnitude of 3.60
7. Deneb Kaitos Shemali, from Arabic, (Iota Ceti) – an apparent visual magnitude of 3.56
8. Thanih al Naamat (Theta Ceti) – an apparent visual magnitude of 3.60
9. Baten Kairos (Zeta Ceti) – an apparent visual magnitude of 3.74
10. (Upsilon Ceti)- an apparent visual magnitude of 3.99
11. Al Kaff al Jidhmah (Delta Ceti) – an apparent visual magnitude of 4.08
12. Al Sadr al Kaitos (Pi Ceti) – an apparent visual magnitude of 4.24
13. Al Kaff al Jidhmah (Mu Ceti) – an apparent visual magnitude of 4.27
14. Al Kaff al Jidhmah II (Xi-2 Ceti) – an apparent visual magnitude of 4.30
15. Al Kaff al Jidhmah I (Xi-1Ceti) – an apparent visual magnitude of 4.36
The importance of the Constellations such as Cetus dates way back to the times of the Babylonians who identified constellations with bright Stars
How do the Bright Stars of Cetus form the shape of a sea monster?
The Bright Stars
If you look up and into the night sky you can imagine the recognizable outline of the Constellation of Cetus, which is a large sea monster or whale
This main constellation is made up of 15 main Stars
Stars with Planets
Cetus has 1 Star with an exoplanets orbiting around it in the solar system but it is unlikely to be able to support life forms.
The furthest exoplanet discovered was actually in the Andromeda Galaxy, not in the Milky Way.
Not all the stars within the Cetus Constellation are visible to the naked eye but with telescopes and modern imagery techniques is it possible to glimpse all of the stars.
FACT: An exoplanet (also referred to as an extrasolar Planet) is a planet that orbits a Star that is not located within our Solar System (exoplanets do not orbit our Sun)
Nothing stands still in the sky.
Planets are continually being discovered and lists updated.
The Constellations change their positions throughout the year as the Earth rotates around the Sun.
This means our position in space is forever changing and as a result our view of what’s in space changes too, and will continue to do so.
Why and what is the purpose of Cetus? –
In ancient times the dots, bright lights and perceived objects in the sky were of great interest and the makings of folklore to a great range of people from seamen to farmers.
From children to the elderly, we have had an ongoing fascination, with our solar system and star system. Perhaps it’s because the enormity and variety within it makes us realize just how large and exciting the universe is.
FACT: The Star System or Stellar System is a small number of stars that orbit around each other and are bound together by gravity.
When it becomes a large group of stars, again bound together in the same way, by gravity, it is known as a Galaxy or Star Cluster.
Whether they contain small groups of stars or larger groups of start the both come under the classification of ‘Star System’.
The Cetus celestial pole
The celestial pole defines the poles of the celestial equatorial coordinate system.
An object at the Celestial pole has a declination of 0 degrees.
- The declinations for the north celestial pole is +90 degrees
- The declinations for the south celestial pole is -90 degrees
The celestial poles are not permanently in a fixed position against the background of the stars as everything moves in Space.
Cetus is located in the northern celestial sky at a +70 degree north and -90 degrees south declination, and a 2-hours right ascension.
Navigational tools in the sky
The many Constellations in the night sky were a useful navigation tool and guide as well as the subject of legends and myths.
42 of the Constellations have been named after animals with a story behind each name.
Historical significance: the legends, and myths surrounding Constellation of Cetus
When it comes to the many recognized constellations in the sky, Cetus is one of the largestConstellations.
When Ptolemy, the Greek astronomer, catalogued Cetus as far back as the 2nd century, it became a topic of great interest in Greek mythology and in other ancient civilizations.
However the origins of the earliest Constellations probably date back to prehistory.
Many ancient civilizations have related the Constellations in the sky to suit their beliefs and creations itself. They have been the subject of folklore and experiences for a very long time.
There are many Greek myths and legends surrounding the origin and names of the constellations.
One myth surrounding the constellation of Cetus involves how the hero Perseus rescued Andromeda
The myth begins with Cassiopeia, the wife of King Cepheus, of Ethiopia, who was a vain queen. Together they had a daughter Andromeda. Cassiopeia boasted that she was more beautiful than all of the 50 Nereids (the Sea Nymphs). The angry Sea Nymphs asked Poseidon, the god of the sea, who was married to a Sea Nymph(Amphitrite), to punish Cassiopeia. He obliged and sent the Sea Monster Cetus to ravage the kingdom.To appease the Sea Monster, Cassiopeia tied her daughter Andromeda to a rock for him to eat.
The hero Perseus, on his winged horse Pegasus rescued Andromeda just in time, from being devoured by Cetus. He slayed Cetus and they married and lived happily ever after. The gods were so happy at the outcome that they placed them all in the heaven as stars. Cassiopeia was tied to a chair and put in the heavens where she revolves around the celestial pole,sometimes in an upside down position. Cetus the sea monster who tried to devour Andromeda was also banished to the skies.
Ancient associations with the constellations
The Greeks, the Romans and the Sumerians all had an interest in the constellations in the sky.
The Sumerians were the first literate civilization of the Ancient Mesopotamia (an area occupying parts of Turkey and the Syria of today, Iraq, Iran). The Sumerian civilization was not unified like the ancient Greek or Roman civilizations it was bonded by a common attitude.
Their belief systems featured many deities. They regarded their gods as being responsible for everything and as such held them in great respect. Many stories arose as a result.
For thousands of years, various cultures around the world have identified and named the constellation we know and see in the night sky as Cetus.
The Mesopotamian civilization (the first known civilization) identified constellations like Cetus. In Mesopotamian culture the ‘whale’ would have had mythical status.
FACT: The ancient lands of the Mesopotamians now stretches across Turkey, Syria, Iraq and Kuwait
Cetus and the other constellations in the sky were not only the subject of legends but they had a practical use too.
The ancient Mesopotamians used the orientation of the constellations to set the seasons for sowing crops and harvesting.
The Babylonians also recorded details of various bright stars within the constellations in their Babylonian star catalogues before 100BCE.
What is the difference between a constellation and an asterism?
An asterism is a group of stars that appear to form a pattern in the night sky but with no officially determined boundaries.
It can make up part of a constellation or cross the boundaries of an official constellation or even a defunct constellation.
An asterism is a more vague assembly of stars than a recognized constellation.
The meteor showers of Cetus
There are 3 annual meteor showers associated with the Constellation of Cetus; they are known as the ‘Cetids’
‘The Cetids’ meteor showers includes –
- The October Cetids, The Eta Cetids and The Omicron Cetids, and are best seen between May 7 and June 9 each year.
The location is at a right ascension of 2 and a declination of -4.
Its peak around May 14 every year and is visible in the Southern Hemisphere.
The closest Star to the key point of the Cetids meteor showers is Mira (Omicron Ceti).
The meteor shower is associated with the constellation of Cetus because the point at which they appear is located in that direction.
Fun Facts about Constellations – Did you know that?
- The Constellation of Cetus is not one of the 12 constellations of the Zodiac.
- Cetus is represented as a huge and unusual sea monster with parts of sea creatures and large forefeet. It is also represented as a Whale in certain cultures.
- David Fabricus, a German astronomer was the first person to discover a variable Star, known as Mira
- There are over 4000 known exoplanets, with another 5000 awaiting classification
- The scale of a Constellation is measured in square degrees
- The Constellation of Cetus is the 4th largest and occupies almost 3% of the night sky
- Charles Messier the French Astronomer who cataloged the Messier objects has a crater on the Moon named after him.
- Constellations like Cetus are not part of our Solar System; they are groups of stars that appear to form shapes that are visible with the naked eye from Earth.
- The largest Constellation is called Hydra and the smallest Constellation is called Crux
- The Sun does not belong to any constellation.
- A Constellation does not actually exist as a fixed object, it is a group of bright stars that happen to be in a random place and are light years apart and ever moving. We see the pattern of their presence.
- The center of a Galaxy does not contain a Giant Star it contains a massive Black Hole.
- Red Dwarf is not a Dwarf Planet it is a Star. Most common Stars are Red Dwarf (cool Stars)
- Spiral Galaxies make up about two third of all the Galaxies in the Universe
Commonly Asked Questions
Q. What is the celestial sphere?
A. In astronomy and navigation terms, the celestial sphere is imaginary.
This virtual sphere has a large radius that is concentric with Earth.
We can imagine all objects in the night sky as being projected upon the inside of this celestial sphere, as if it has images placed inside a dome.
Q. What’s the difference between a Constellation and an asterism?
A. The stars that make up a Constellation have a definite position and form, whereas an asterism is a collection of stars without a fixed position
Q. What prevents us seeing the Stars and Constellations in the night sky?
A. Light pollution, fog, city lights and artificial lights all limit our visibility of the objects in the sky at night.
Q. Will the Constellations change over time?
A. The Constellations are continually on the move.
The images we form in our imagination to make objects, shapes and patterns out of the constellations have already shifted over time.
As we view the night skies from Earth they are likely to continue to shift and possibly in time the images may look very different.