The Cancer constellation represents the ‘crab’ that represents the Zodiac month from June 21 – July 22. However, in modern times the sun passes through the Cancer constellation approximately 1 month later. It is both a difficult constellation to find and to identify the shape because it lacks bright stars. The bright stars of Gemini, Leo and Canis Minor are critical in helping to find Cancer constellation.
The Cancer constellation is one of the original 48 constellations listed by the Egyptian mathematician Ptolemy, who lived under Roman rule in Alexandria in the 2nd century. These constellations formed the basis for the modern list agreed by the IAU. Constellations from the southern hemisphere, unable to be seen from the Mediterranean, represent the majority of additions to the list.
As the earth moves in its orbit around the sun, each night you are looking at a different portion of the sky. When looking at stars it is important to be aware of what is within your view. Northern hemisphere stargazers can group constellations into 3 groups; circumpolar, summer and winter constellations. The circumpolar constellations are in the north sky, appear to move around the north star and are visible throughout the year. The constellations in the south sky are only visible for part of the year and are grouped as either summer or winter constellations. Each is visible from between 4 to 10 months.
It is important to be aware of the specific time of year and hour of the night when deciding what to look for. These pages below show the constellations in each group so you can find the constellations that interest you.
CIRCUMPOLAR (year around) – Ursa Major – Cassiopeia – Ursa Minor – Draco – Cepheus
WINTER – Pegasus – Pisces – Aries – Auriga – Taurus – Orion – Canis Major – Canis Minor – Gemini – Lynx – Cancer – Leo – Winter Hexagon
SUMMER – Virgo – Libra – Scorpius – Bootes – Hercules – Lyra – Ophiuchus – Sagittarius – Aquila – Cygnus – Capricornus – Aquarius
Or use this guide to easily see which constellations are easiest to find right now:
Cancer Constellation Quick Facts:
Brightest star: Tarf, 3.53 magnitude (297th brightest star in the night sky)
Stars brighter than 3 magnitude: 0
Primary stars: 5 (The constellation is a Y shape with3 stars in a line representing the body and 2 stars at 45 degree angles representing the claws)
Latitude: 10 – 30 degrees north
Northern Hemisphere Season: Winter (February – April)
Which months can you see Cancer constellation?
Cancer constellation can be seen from September to June but can be found high in the sky at 21:00 during March:
- Early evening viewers (before 21:00) can find the constellation from January in the eastern sky until June in the western sky. Cancer constellation will be visible overhead in April.
- Mid evening viewers (21:00-23:30) can find the constellation from December in the eastern sky until May in the western sky. Cancer constellation will be visible overhead in March.
- Late evening viewers (after 23:30) can find the constellation from November in the eastern sky until April in the western sky. Auriga will be visible overhead in January.
- Early morning viewers can find the constellation from September in the eastern sky to January in the western sky. Auriga will be visible overhead in November.
As time passes the constellation will gradually appear earlier in the day with ranges below showing the window of opportunity in each month. Cancer constellation sits between 10-30 degrees north latitude. Therefore, the further south your position the further north it will appear in the sky. At its maximum range it is possible to see Gemini at latitudes between +90 and -50.
Best time to see Cancer constellation:
Best visible at 21:00 in March
September: appear on the eastern horizon at 03:00, it will move west until sunrise when it will be 30 degrees above the eastern horizon.
October: appear on the eastern horizon at 01:00, it will move west until sunrise when it will be 60 degrees above the eastern horizon.
November: appear on the eastern horizon at 23:30, reaching its peak at04:30 75 degrees above the southern horizon. It will continue moving west until sunrise when it will be 75 degrees above the western horizon.
December: appear on the eastern horizon at 21:30, reaching its peak at 02:30 75 degrees above the southern horizon. It will continue moving west until sunrise when it will be 50 degrees above the western horizon.
January: appear on the eastern horizon at 19:30, reaching its peak at 24:30 75 degrees above the southern horizon. It will continue moving west until sunrise when it will be 25 degrees above the western horizon.
February: 30 degrees above the eastern horizon at sunset, reaching its peak at 22:30 75 degrees above the southern horizon. It will continue moving west until 04:00 when it will begin to be only partially visible on the western horizon.
March: 60 degrees above the eastern horizon at sunset, reaching its peak at 21:00 75 degrees above the southern horizon. It will continue moving west until 02:00 when it will begin to be only partially visible on the western horizon.
April: 75 degrees above the southern horizon at sunset at its peak. It will continue moving west until 01:30 when it will begin to be only partially visible on the western horizon.
May: 50 degrees above the western horizon. It will continue moving west until 23:30 when it will begin to be only partially visible on the western horizon.
June: Visible breifly after sunset 20 degrees above the western horizon. It will only be partially visible after 21:30.
Trajectory: The constellation is between 10-30 degrees north which means that for those in low latitudes of the northern hemisphere is will pass in the high southern sky, for those in higher latitudes of the northern hemisphere it will appear in the lower southern sky. Its peak trajectory in Wadi Rum is 75 degrees above the southern horizon from February through April
Not the right time for Cancer constellation? Have a look what constellations you can see tonight.
How to find Cancer constellation?
The Cancer constellation is difficult to find because it is made up of faint stars the are not easy to locate. The basic shape of the constellation is a Y and once discovered the shape is clear but to find the relatively faint stars initially is a challenge. The best starting point is to identify the beehive cluster which is a cluster of stars at the centre of the constellation and the most recognisable feature.
Fortunately Cancer is surrounded by bright constellations so there are several pointer stars to help find it. As a zodiac constellation is it between Gemini to the west and Leo to the east, but also has nearby Canis Major to assist in determining its location.
The beehive cluster which you will be looking for to initially identify the constellation will appear as a cloud of light rather than a point of light such as you see from single stars. This cluster is the most prominent light source in the constellation and is possible to recognise because of the lack of other nearby bright stars.
Option 1: Gemini
The 2 stars that represent the heads of Gemini, Castor and Pollux are near to Cancer. Imagine a line that dissects the 2 stars and follow it south east. This line dissects the Cancer constellation and you will find the beehive cluster north of this line. The beehive cluster is 4 times the distance between the heads of Gemini away from the nearest head.
Option 2: Canis Minor
Canis Minor is close to the Cancer constellation and because of the bright star Procyon at the tail of the constellation it is helpful to locate Cancer. Procyon is directly south of Castor and Pollux stars in Gemini and they frame the constellation to the west. The beehive cluster is north east of Procyon and when combined with Castor produces a equilateral triange.
Option 3: Leo the Lion
The constellation of Leo is east of Cancer following the suns path. When Cancer is in the west sky and the convenient use of Gemini and Canis Minor is not possible, Leo is the best reference to find Cancer. Once you are familiar with the Leo constellation, the line from the ear to the nose points south west towards the southern end of the Cancer constellation.
If you aim 45 degrees left of this line in a west direction you will find the beehive cluster. The beehive cluster is around 6 times further from the nose of Leo than the ear is from the nose.
Learn how to form the shape of Cancer constellation
The cloud of light that is the beehive cluster which you will need to identify using one of the pointer asterisms above, is at the centre of the Cancer constellation.
From this starting point you can identify the Y shape that makes the official Cancer constellation. The Y will appear in the southern sky upside down for viewers in the northern hemisphere.
- there is a very close star to the south east of the beehive cluster that represents one eye of the crab
- there is another very close star to the north east of the beehive cluster that represents the other eye of the crab.
- From the eyes if you move north approximately 3 times the distance between the eyes you will find another star.
- From the eyes similarly move south approximately 3 times the distance between the eyes to find the 4th star of the constellation.
These 4 stars running north south and are symmetrical form the primary part of the cancer constellation.
- From the southern eye move to the south west around 6 times the distance between the eyes and you will find the final official star of the constellation. This star is the furthest from the others but also the brightest. These 5 stars identify the inverted Y shape for viewers in the northern hemisphere.
Imagining the shape of the Cancer crab
The 2 stars at the centre of the constellation mark the eyes, the 2 equidistant stars north and south of the eyes mark the claws. The beehive cluster represents the body of the crab and the bright star to the south west represents the tail or legs of the crab.
Although not an official star of the constellation, there is a star within the boundary of the constellation which helps to form an image of the crab. When imagining a line north from the tail, west from the northern (right) eye and the imaginary line that dissects the heads of Gemini. These 3 lines will intersect at a star. This star in combination with the tail star forms a polygon that represents the body.
Cancer constellation in the Ancient World
The representation of the the water sign of Cancer originates from ancient Babylon. Both crabs and snapping turtles had the same name in ancient Babylonian language and research of ancient script and artifacts indicates that the Babylonian reference was of a snapping turtle as various depictions associated with the constellation of turtles (usually in pairs) have been discovered and no reference to crabs. It is thought that Greek astronomers later misinterpreted the Babylonian term.
Cancer constellation represents the crab who was killed by Hercules during his second labour. He was required to kill nine headed Hydra in the swamp. The crab was considered a protector of the creatures of the swamp. Hera, the queen of the gods and wife of Zeus, did not enjoy the attention that Hercules received. She was jealous of him as he was the Demi-god son from Zeus’ relationship with a human. She immortalised the crab in the sky in an attempt to embarrass
Main stars in Cancer constellation
Tarf (+3.53m, 290ly, 227k suns, #297)
Tarf is from Arabic for ‘edge’ (as in the limit). It is the 297th brightest star with an apparent magnitude of +3.53. The distance from earth is 290 light years. It is around 227,ooo times the size of the sun. Tarf marks the tail of the crab.
Asellus Australis (+3.94m, 131ly, 300 suns)
Asellus Australis from the Latin for ‘southern donkey’. It has an apparent magnitude of +3.94. The distance from earth is 131 light years. It is 1330 times bigger than the sun. Asellus Australis marks the left eye of the crab.
Decapoda (+4.02m, 330ly, double star)
Decapoda is the Latin term meaning ‘ten footed’, it refers to all species of crustacean with ten legs incuding lobster, crayfish, prawns and crabs. It is double star with an apparent magnitude of +4.02. The distance from earth is approximately 330 light years. What appears as a single form of light to the naked eye is a double star system. Decapoda marks the right claw of the crab.
Acubens (+4.20, 164ly, double star)
Acubens is from the Arabic for ‘the claws’. It is a double star with a third smaller star in its orbit with an apparent magnitude of +4.20. The distance from earth is 164 light years. Acubens marks the left claw of the crab.
Asellus Borealis (+4.65m, 181ly, 15 suns)
Asellus Borealis from the Latin for ‘northern donkey’. It has an apparent magnitude of +4.65. The distance from earth is 181 light years. It is 1330 times bigger than the sun. Asellus Australis marks the right eye of the crab.
Deep Sky Objects
Beehive cluster (Messier 44) – A cluster of over 50 stars that was one of the first objects analysed by Galileo following the development of the telescope where he identified 40 seperate stars within the cluster. In Roman times it was part of an asterism with the 2 stars at the head and body of the crab with the cluster identifying a manger and the 2 stars approaching donkeys. The stars have retained these names until this time. Messier 44 represents the body of the crab.
Wadi Rum is one of the best locations in the world to see the full beauty of the stars. Combining high altitudes, clear skies and no light pollution. You may be surprised how many stars are visible to the naked eye. Come with us and spend a night under the stars.