The night sky is majestic during springtime. It presents the stargazer with a great variety for the eyes to feast on, as winter’s chilly grip is replaced by milder April evenings.
As last month’s featured constellation, Gemini, sets in the South-West, this month we can focus our attention on another sign of the zodiac – Cancer the Crab. Alongside our aquatic companion, we also have a Pink Supermoon to see, plus a meteor shower to catch….
Stars and constellations
Featured constellation: Cancer the Crab
We remain focused on the signs of zodiac this month, picking up in the South-West region of the sky, where during March we explored the constellation of Gemini. The focal point of Gemini was its two bright stars, Castor and Pollux, and it will be those two stars that will guide us to the neighbouring constellation of Cancer the Crab.
Using Castor and Pollux as your guide (see image of Gemini below), start by drawing a mental line from Castor, the higher of the two stars in Gemini. Then, follow a diagonal line through Pollux, continuing at an angle, until you arrive at the brightest star in Cancer: Altarf. You may need several attempts to locate Altarf but try and be patient, allowing your eyes to properly adapt to the part of the sky you are observing. As your eyes become more sensitive to the darkness, this should allow you to generally see more stars and given a little time, you will locate Altarf. The star can be found right at the bottom of what essentially appears to be an inverted ‘Y’ shape, which makes up the constellation of Cancer.
Remember to give yourself the best view of the South and South-West horizon as possible, and try and make sure you are in the darkest location possible – naturally keeping safe, and adhering to any social distancing guidelines that are in place.
While the majority of Cancer’s stars are somewhat faint, Cancer is the 31st largest constellation out of a total of 88 that make up the entire number of constellations seen from both the Northern and Southern hemisphere.
Cancer the Crab in mythology
In Greek mythology, Cancer is associated with the crab from the story of the Twelve Labours of Heracles, in which Heracles is forced to perform twelve labours as a penance for killing his family. Heracles was one of Zeus’ illegitimate children and is represented in the night sky by the constellation of Hercules.
In the myth, Hera, Queen of the Gods in Mount Olympus, harboured a great dislike for the mighty Heracles, and sent a giant crab named Karkinos to distract Heracles while he was fighting the Hydra: a serpent-like beast with many heads and poisonous breath (represented in the night sky by the constellation of Hydra). The fight was the second of the twelve labours Heracles was being made to undertake.
During his battle with the Hydra, Heracles discovered that every time he cut off one of the Hydra’s heads, two more grew in its place. However, Heracles finally figured out that if he put fire to the wound, it would sear and seal it shut, so that new heads could not grow up through the old wound. Having finally defeated the Hydra, Heracles turns his attention to the crab and crushes it with his foot.
Hera though, took kindly to the crab, and as a reward for the Karkinos’ obedience and sacrifice, she placed the crab’s image among the stars. However, Hera placed the crab in a region of the sky that has no bright stars because despite the crab’s efforts, the crustacean was not successful in accomplishing the task that he had been dispatched to perform.
Additional myths relating to Cancer the Crab
In Chaldean and Platonic philosophy Cancer was called ‘The Gate of Men’ where souls were said to descend to Earth from heaven through the constellation, to take up their abodes in the bodies of new-born babies. Likewise, they would then ascend to heaven through ‘The Gate of the Gods’ in the constellation of Capricorn the Goat.
In Babylonian culture, Cancer was viewed as being unfortunate because it marked the entrance to the underworld.
In China, the four stars surrounding a cluster of stars named Praesepe (a star cluster in the constellation that we’ll cover later on), were known as Gui, literally meaning ‘Ghosts’, referring to the spirits of the deceased. Praesepe itself was called Jishi, a group of corpses. Praesepe and its four surrounding stars were sometimes seen as a ghost being carried in a sedan chair; hence this quadrilateral was also given the alternative name Yugui Ghost Wagon.
Romans saw the star cluster as a manger from which two donkeys – the adjacent stars in the constellation, Asellus Borealis and Asellus Autralis respectively – are eating.
Observing Cancer the Crab
The Main Stars
Altarf is the brightest star in the constellation and comprises two stars, the largest of which is 53 times bigger than our Sun, and around 660 times brighter.
At 290 light-years away from Earth, Altarf (or just Tarf), represents the crab’s southern hind foot. Altarf derives from the Arabic for ’The End’.
Asellus Australis is the second brightest star in the constellation. The star is 10 times bigger than our Sun, 53 times brighter, and 180 light-years away from Earth.
The star is also notable for its less famous name, Arkushanangarushashutu, which is the longest of all the known star names. It means ‘The South East Star in the Crab’ in ancient Babylonian. The more commonly used name, Asellus Australis, means ‘southern donkey colt’ in Latin.
Acubens is the third brightest star in the constellation and comprises several stars. Acubens is 23 times brighter than our Sun, and 173 light-years away from Earth.
The star is sometimes also called Al Zubanah or Sertan. The name Al Zubanah comes from the Arabic, az-zubānah, which means ‘claws’, and marks the lower end of the crab’s left leg. Sertan is derived from saraţān, which means ‘The Crab.’
Other stars in Cancer the Crab
Asellus Borealis is a solitary star, 158 light-years away from Earth. The star’s name means ‘northern donkey’ in Latin.
55 Cancri is made up of two stars 41 light-years away from Earth. 55 Cancri is of special interest to astronomers, as in 2010, five planets were detected orbiting the main star in the system.
Tegmine – Cancri (Zeta Cancri)
Tegmine comprises at least four stars, 83 light-years away from Earth. Tegmine, means ‘The Shell of the Crab’ in Latin.
Piautos (Lambda Cancri)
Piautos is made up of two stars 419 light years away from Earth. The star’s name means ‘The Bright Fire’ in Chinese.
Nahn (Xi Cancri)
Nhan is made up of two stars, 381 light years away from Earth. The star’s name in Persian means ‘The Nose’.
Cancer the Crab: Special objects
Cancer contains several special objects, and by these, we mean objects such as star clusters and galaxies, or in the case of Orion the Hunter, nebulae, which are clouds of dust and gas.
These special objects are commonly referred to as ‘Messier objects’; a set of astronomical phenomena catalogued by French astronomer Charles Messier, (1730–1817). Messier was the first to compile a systematic catalogue of such objects in the night sky, and the catalogue is still used in modern day astronomy.
Cancer’s most famous Messier object is M44 (M for Messier– 44th object recorded), also known as ‘The Beehive Cluster’, is a small star cluster that resembles a swarm of bees. M44 contains about 50 stars, and is also referred to as Praesepe (pronounced pree-seep-ee), which means ‘manger’ or ‘cradle’ in Latin.
The star Asellus Australis marks the position of the cluster, which at 577 light years away from Earth, is one of the closest clusters to us. Observable with the naked eye as a fuzzy patch under clear conditions, it is estimated that the cluster is 600 million years old.
The Greek astronomer Ptolemy (100–170 AD), observed the cluster and called it ‘The Nebulous Mass in the Breast of Cancer’. Galileo (1564–1642) was the first to see the cluster through a telescope in 1609, and Charles Messier added the cluster to his catalogue in 1769.
April’s Pink Supermoon
April’s full moon will be a ‘supermoon’, which will appear slightly larger and brighter in our skies during the last week of April.
A supermoon is a new or full moon closely coinciding with perigee – the point at which the moon is closest to Earth in its monthly orbit. The Moon’s orbit around the Earth isn’t circular, more egg-shaped, so at certain times it can be even closer or more distant than it would normally be.
On average, the moon orbits at 238,000 miles from Earth. But at perigee, it’s about 226,000 miles away — or about 12,000 miles closer to Earth. Therefore, if the moon is full at this point, it appears up to 30% brighter and up to 14% larger than a full moon (which occurs when the moon is at its most distant point from Earth in its elliptical orbit). The point at which the moon is furthest away, is known as lunar apogee.
A full moon in April is also referred to as a ‘Pink Moon’ – named after the pink flowers (phlox) that bloom in the early spring in some parts of the world. Other names for the April full moon include ‘Sprouting Grass Moon’, ‘Fish Moon’, ‘Hare Moon’, and the Old English/Anglo-Saxon name is ‘Egg Moon’. It is also known as the ‘Paschal Moon’ because it is used to calculate the date for Easter.
April’s supermoon presents the first in a “season” of three full supermoons, and gives us the second-closest full moon of the year.
The two innermost planets in the Solar System, Mercury and Venus, begin to emerge in the evening sky during April. Watch above the West-Northwest horizon for the lovely pairing of Venus and a thin crescent moon on Monday, April 12th.
Then on Sunday, April 25th, look in the same West-Northwest direction to see Venus and Mercury appearing close together. Venus will be the brighter of the two, with Mercury positioned up and over to the right. When you locate Venus, imagine the planet as the centre of a clock face, with the small hand pointing toward Mercury.
Mars is best spotted in the West after sunset, although, as its distance from Earth increases, it looks smaller, and during April appears lower in the sky. However, with its reddish appearance, Mars can still be seen with the naked eye.
Both Jupiter and Saturn are visible in the South-East morning sky throughout April. Jupiter will appear as the brighter of the two planets. A waning crescent moon sits beneath Saturn on the morning of Tuesday, April 6th. On the following morning, Wednesday, April 7th, the moon will be positioned to the South of Jupiter.
Lyrids meteor shower
Throughout the year, we have several displays of meteors: such as when the Earth on its 365 day orbit around the Sun, passes through debris trails left by comets – or perhaps when a pile of rubble is left behind by an asteroid that has disintegrated.
In April, we have a Lyrids meteor shower, which will present us with a fine opportunity to view some ‘shooting stars’ with the naked eye. During the shower (which takes place each year), the Earth passes through debris, which has been left by Comet Thatcher. The ‘radiant’ of the shower (the region of the sky that the meteors appear from) can be located at the constellation of Lyra (Latin for lyre, a small harp-like instrument).
The Lyrids are one of the oldest known meteor showers, having been observed for over 2,700 years. The first recorded sighting of a Lyrid shower can be traced back to China in 15 BC and 687 BC, when prominent displays of meteors were recorded. Also, in 1136, a report from Korea chronicled the shower with the words ‘many stars flew from the Northeast’. Similarly, impressive shower sightings were recorded in 1922 in Greece, 1945 in Japan, and 1982 in the United States.
History of Comet Thatcher
Comet Thatcher is a long-period comet, meaning that it takes many years to make one orbit of the Sun. In the case of Thatcher, this is a period of 415 years. Comet Thatcher was discovered by American amateur astronomer A.E Thatcher in 1861, whilst observing the night sky from his home in New York.
The Comet Thatcher is not expected back in our region of space until roughly the year 2283, following its last closest approach to the Sun on June 3rd 1861.
Observing the Lyrids in April
Between April 16th and April 30th, the Earth starts to plough through the debris left by Thatcher, with April 22nd seeing our planet pass through the densest part of the rubble. It is this particular night that we need to concentrate on – or a day or so either side – as it is here that we will see the most meteors.
Meteors are best seen in the skies after midnight and toward dawn, as the leading edge of the Earth turns into the direction of the incoming debris – much like turning a car into the direction of the rain, so that the rain appears more fierce on the windscreen as you face into it.
Therefore, under clear skies with your gaze turned to look high in the Eastern sky, you should be able to spot the Lyrid meteors. When just a couple of meteors are seen, it is possible to draw a mental line back to where the meteor appeared to be coming from, and this will lead the observer to Lyra constellation, where the radiant of the shower lies. Based on observations over the decades, the Zenith Hourly Rate (the number of meteors expected per hour), is around 18.
Despite the moonlight blotting out some of the fainter meteors, there should still be a fair number as the rubble hits our atmosphere at 30 miles a second: causing friction that burns the rock and creates a blaze of light, just like a ‘shooting star’.
Remember to prepare to view the shower from dark but safe surroundings, and even though it is April, do remember to wrap up warm as the nights can still be chilly.
April’s Moon Phases
To understand why we see phases of the Moon, we have to understand the relationship between the Earth, the Moon, and the Sun. The phases result from the changing angles of the Moon and Sun that can be seen from Earth. As the Moon orbits the Earth every month, the angle of it alters, revealing different portions of the moon’s surface.
During the Moon’s journey around the Earth, the same side of the moon always presents itself to us, a phenomena known as ‘tidal locking’. The Moon is tidally locked to the Earth because it rotates in exactly the same time that it takes to orbit the Earth, which is 29.5 days, or one lunar cycle.
There are four main phases of the moon. Together, these four phases create the outline for a lunar cycle that begins with the new moon, then the first quarter moon, next the full moon, and then finally the third quarter (or last quarter) moon, before the cycle starts all over again.
To be able to more clearly see and understand the Moon’s phases, it can help to take a note of the dates below; where on a clear night, you should be able to see each phase.
Dates for April’s Moon phases
Third Quarter April 4th; New Moon April 12th; First Quarter April 20th; Full Moon April 27th.
April’s sunrise/sunset times
Start of April: Sun rises at 6.47 a.m. Sets at 7.55 p.m.
End of April: Sun rises at 5.35 a.m. Sets at 8.54 p.m.
Start of April: Sun rises at 6.47 a.m. Sets at 7.46 p.m.
End of April: Sun rises at 5.46 a.m. Sets at 8.34 p.m.
Start of April: Sun rises at 6.35 a.m. Sets at 7.34 p.m.
End of April: Sun rises at 5.34 a.m. Sets at 8.22 p.m.
Jonathan is a contributor to the BBC Sky at Night magazine. He has written three books on astronomy, Cosmic Debris; Rare Astronomical Sights and Sounds (which was selected by ‘Choice’ magazine as an Outstanding Academic Title for 2019); and From Cave Art to Hubble, all of which are available from Amazon. Jonathan worked at BBC Radio Wales as their astronomy correspondent and is currently a columnist at the South Wales Argus, and presenter on Astro Radio UK. He has also written a book on castles, Fortress Wales, and was part of the writing team for the BBC Television show, ‘The Fast Show’, which won a BAFTA.