WHAT'S UP THIS MONTH - APRIL 2019

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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY

The chart above shows the night sky as it appears on 15th April at 21:00 (9 o'clock) in the evening British Summer Time (BST). As the Earth orbits the Sun and we look out into space each night the stars will appear to have moved across the sky by a small amount. Every month Earth moves one twelfth of its circuit around the Sun, this amounts to 30 degrees each month. There are about 30 days in each month so each night the stars appear to move about 1 degree. The sky will therefore appear the same as shown on the chart above at 10 o'clock BST at the beginning of the month and at 8 o'clock BST at the end of the month. The stars also appear to move 15º (360º divided by 24) each hour from east to west, due to the Earth rotating once every 24 hours.

The centre of the chart will be the position in the sky directly overhead, called the Zenith. First we need to find some familiar objects so we can get our bearings. The Pole Star Polaris can be easily found by first finding the familiar shape of the Great Bear ‘Ursa Major' that is also sometimes called the Plough or even the Big Dipper by the Americans. Ursa Major is visible throughout the year from Britain and is always easy to find. This month it is almost overhead. Look for the distinctive saucepan shape, four stars forming the bowl and three stars forming the handle. Follow an imaginary line, up from the two stars in the bowl furthest from the handle. These will point the way to Polaris which will be to the north of overhead at about 50º above the northern horizon. Polaris is the only moderately bright star in a fairly empty patch of sky. When you have found Polaris turn completely around and you will be facing south. To use this chart, position yourself looking south and hold the chart above your eyes.

Planets observable: Mars. Venus, Saturn and Jupiter are observable in the early morning.

EXPLORING THE NIGHT SKY THIS MONTH

The Southern Night Sky during April 2019 at 21:00 BST

The chart above shows the night sky looking south at about 21:00 BST on 15th April. West is to the right and east to the left. The point in the sky directly overhead is known as the Zenith and is shown, marked in red' at the top and centre of the chart. The curved brown line across the sky at the bottom is the Ecliptic or Zodiac. This is the imaginary line along which the Sun, Moon and planets appear to move across the sky. The brightest stars often appear to form a group or recognisable pattern; we call these ‘Constellations'.

Constellations through which the ecliptic passes this month are Taurus (the Bull), Gemini (the Twins), Cancer (the Crab), Leo (the Lion) and Virgo (the Virgin) rising over the eastern horizon.

The Milky Way (our Galaxy) flows up from the south horizon in Puppis, through Orion and Gemini . It continues up through Perseus and Cassiopea and on to Cygnus which is on the northern horizon.

Mars is still in a reasonable position, for observing during the early evening but is looking small now, in the constellation of Taurus. Earth overtook Mars a few months ago as it moved faster along its smaller orbit inside the orbit of Mars. Mars in now being left behind by Earth and appears smaller as it becomes further away. Although it is looking smaller it still appears bright.

Taurus is easy to find thanks to the presence of Mars and its location on the Ecliptic. The stick figure representation of Taurus resembles a squashed ‘X' with the bright orange coloured Red Giant star Aldebaran at its centre (see the chart below). Aldebaran is a lovely star to look at especially using binoculars or a telescope and is noticeably orange.

Following the North western (upper right) arm of the ‘X' shape of Taurus guides us to the beautiful Pleiades ‘naked eye' Open Star Cluster. This bright Open Cluster with its seven brightest stars is known as Messier 45 (M45), the Pleiades or ‘the Seven Sisters'.

The constellation of Taurus

Messier 45 (M45) the Seven Sisters

Attached to the upper left star of Taurus, called ‘Elnath', is the constellation of Auriga the Charioteer. Auriga appears like an odd shaped pentagon with the beautiful bright white star Capella on the opposite side to Elnath.

To the east of Taurus along the Ecliptic is the constellation of Gemini (the Twins). The twin stars Castor and Pollux are easy to identify. To the east is the constellation of Cancer with a lovely Open Cluster M44 which is best seen using binoculars.

The constellation of Cancer

Messier 44 (M44) the Beehive Cluster

Below Taurus and Gemini is Orion the second most recognisable constellation after Ursa Major (the Great Bear). See the Constellations of this month – Ursa Major and Ursa Minor.

Orion is depicted as a hunter with two hunting dogs called Sirius and Procyon. The two stars that represent Orion's hunting dogs are also called Sirius and Procyon. Sirius and Procyon are the brightest stars in the constellations of Canis Major (the great dog) and Canis Minor (the little dog). Sirius is the closest and brightest star visible from the UK.

Orion is one of the easiest constellations to recognise and dominates the southern sky at this time of the year. There are many depictions of Orion shown on many different star charts. Some old pictures of Orion are very beautifully drawn in fact some are so beautiful that the artists even moved the positions of some of the stars so they would fit the image they had drawn.

 

Orion the Hunter appears in the winter sky, with his club held over his head and his shield (sometimes shown as a lion's skin) held out in front of him. His hunting dogs, Canis Major (the star Sirius) and Canis Minor (the star Procyon) following behind him.

Orion is one of the few constellations that does look (with a little imagination) like what it is named after. The most obvious feature is the line of three stars, called Alnitak, Alnilam and Mintaka that make up Orion's belt. From his belt we can see two bright stars called Saiph and Rigel below. These define the bottom of his ‘skirt like' tunic. Above the belt are two stars Betelgeuse and Bellatrix that denote the position of his shoulders. Above and between his shoulders is a little group of stars that mark out the head. From his right shoulder (Bellatrix) he holds out a shield. From his left shoulder (Betelgeuse) a club is held above his head. It almost looks as if Orion is fending off the charge of the great bull Taurus who is located above and to the west (right) of Orion.

Down from Orion's, very distinctive, belt there is a line of stars, ending at the star Nair al Saif that looks very much like a sword attached to his belt. Here can be found the main interest in Orion, Messier 42 (M42) the Great Orion Nebula.

M42 with the Trapezium superimposed

M42 the Great Orion Nebula

If an imaginary line is traced down from the belt for about six belt length towards the south eastern horizon, a bright twinkling star will be seen. This is Sirius, Orion's Large Hunting Dog in the constellation of Canis Major. See the chart above. It is the brightest and closest star to be seen from the UK at just 8.6 light years from us. To Orion's left (east) of Betelgeuse a quite bright star in a rather large empty area of sky can be seen. This is Procyon in Canis Minor, Orion's Small Hunting Dog. Coincidentally both of these ‘dog stars' are double stars that have an invisible White Dwarf companion.

Further to the east (left) of Gemini is the constellation of Leo (the Lion). Leo is quite distinctive with the ‘Sickle' shaped pattern of stars looking much like the head of the lion that Leo represents. In fact the traditional ‘stick figure' shape of Leo as shown on the chart above does look rather like the lion's body or the Sphinx in Egypt. The ‘Sickle' shape is also described as looking like a backwards question mark (?).

Following Leo is the less obvious constellation of Virgo but it does have one fairly bright star called Spica.

CONSTELLATIONS OF THE MONTH – URSA MAJOR & URSA MINOR

 

The constellations of Ursa Major (the Great Bear) and Ursa Minor (the Little Bear)

The Constellation of the Month, this month, is probably the best known of all the constellations and is certainly the most recognisable . It is Ursa Major (the Great Bear) also known as the ‘Plough' or ‘the Big Dipper' in the USA. However it has very little resemblance to a bear and looks very much like a ‘Saucepan'.

Ursa Major is directly overhead at this time of the year as can be seen by the chart above. The point in the sky directly overhead of the observer is called the ‘ZENITH' and is shown in red on the chart. The chart shows the sky as it will appear at 21:00 on 15th April.

Ursa Major is a circumpolar constellation, this means it never disappears below the horizon from the UK and so is always visible somewhere in the night sky throughout the year. All the stars in the night sky appear to rotate around a point in the sky that we call the ‘North Celestial Pole'. This point is located very close to the star Polaris in the constellation of Ursa Minor (the Little Bear). Polaris can be found by following the two stars opposite the handle of the ‘saucepan shape', up out of the pan and about five times the distance between the pointer stars. See the arrow on the chart above.

Our planet Earth rotates around the North Celestial Pole once a day (24 hours). As Earth rotates the sky appears to rotate above us. The sky appears to rotate so Ursa Major and the other constellations will appear to move around the North Celestial Pole in an east to west direction. It will appear to move left as shown on the chart above (handle to the front). The movement is slow and not perceivable in real time.

The axis of rotation of our Earth is tilted over at 23.4º to the axis of rotation of our Solar System. So we see our axis of rotation 23.4º to the north of our Zenith (the point directly overhead). We do not normally notice that we have a slightly odd view of the sky due to this tilt of Earth. Astronomers are acutely aware of this strange perspective we have of the sky as we need to take account of it when we are observing the moving sky. However a long exposure using a camera will show the stars as trails as they rotate around the Pole Star.

Star trails imaged by James Lowenthol

So the two bears in our sky are quite important constellations. Ursa Minor because it hosts the North Celestial Pole and Ursa Major because it is used to help us to find Polaris and the North Celestial Pole close by.

Ursa Major is also host to some lovely galaxies for those who are fortunate to have access to a larger telescope. These are known as M81, M82, M101 and M51 (M51 is not, strictly speaking, in Ursa Major but close by in the neighbouring constellation of Canes Venatici.)

Messier 81 (M81)

Messier 82 (M82)

Messier 101 (M101)

Messier 51 (M51)

Messier 81 (M81) is a beautiful giant spiral galaxy similar to our Milky Way Galaxy. It is 11.7 light years away from us and slightly smaller in diameter than our Galaxy at about 90,000 light years (Milky Way 100,000ly).

Messier 82 (N82) and M81 appear close together in the sky and are gravitationally associated. M82 is seen edge on but slightly tilted from our vantage point. It appears to be ‘Cigar' shaped and is sometimes called the Cigar Galaxy. M82 has a very active core at its centre. There is an intense area of a starburst (star creation) in the core that is causing powerful radiation to stream out from the core. There is thought to be a rare Intermediate Mass Black Hole orbiting only 600 light years away from the Super Massive Black Hole at the centre that may be causing the Starburst.

Messier 101 (M101) is a beautiful ‘face on' Spiral Galaxy also known as the ‘Pin Wheel Galaxy'. It is larger than our Milky Way with a diameter of 170,000ly compared to the Milky Way at 100,000ly. M101 has beautifully formed spiral arm that look spectacular in long exposure images. It is thought to contain up to a trillion stars, the Milky Way is thought to have 200 billion or maybe more.

Messier 51 (M51) is not actually in Ursa Major, it is in the neighbouring constellation of Canes Venatici (the Hunting Dogs). However the easiest way to finding it is to follow the handle of the saucepan shape of Ursa Major to the star Alkaid at the end of the handle. M51 can then be found just below the star Alkaid.

Messier 51 is also called the Whirlpool Galaxy and was the first Galaxy (or any other object) to be proved to reside outside our Galaxy. Until this discovery by William Parsons the 3 rd Earl of Rosse in 1845 it was generally accepted that the Milky Way might be the total extent of the Universe. So with this discovery the Universe became unimaginable larger than it had ever been thought to be and stretches out for billions of light years.

There is another special feature of Messier 51 that can give us an insight into how galaxies formed. M51 has had a close encounter with another galaxy and we can see the affect it had on the two galaxies. The image above shows that the larger spiral galaxy has had gas, dust and even stars pulled off by the smaller galaxy as it brushed past. The spiral arms have been pulled off and dragged along behind the smaller galaxy.

It is now thought the combined gravity of the two galaxies will pull them back together for further encounters. They will pirouette around each other and eventually combine into one larger galaxy. In the process very few stars will actually collide due to the vast distances between stars. This does give us an insight into the future of our own Galaxy ‘the Milky Way'. Our neighbouring Giant Spiral Galaxy ‘M31 in Andromeda' is hurtling towards us and is expected to pass lose to or even collide with the Milky Way in about 4.5 billion years time. The two Giant Spiral Galaxies will probably pirouette around each other and eventually combine into one larger Elliptical Galaxy in much the way we see M51 doing. Very few stars will collide but the interaction of the gravity of the two galaxies will certainly cause a massive burst of star formation.

THE SOLAR SYSTEM APRIL 2019

The positions of the planets at 08:00 in the middle of the month (the sky has been darkened to reveal the planets)

MERCURY will be at greatest elongation from the Sun on 11th April but will be close to the eastern horizon before dawn and very difficult to see.

VENUS rises over the eastern horizon at about 05:20 and will be very bright in the south as the sky brightens at dawn. Venus is very bright at magnitude -4.0 but is moving back towards the Sun. It will move into Superior Conjunction (pass behind the Sun) on 14th August.

MARS is still observable in the early evening but is lower now and getting close to the southern western horizon in turbulent and smoggy air. The Red Planet is moving away from Earth and looking smaller at 4.4 arc-seconds in diameter but still quite bright at magnitude +1.5.

JUPITER is moving further away from the Sun before sunrise. It now rises over the eastern horizon around 01:00 nearly 5 hours before sunrise. It is well worth getting up early to see. See the chart below.

Saturn and Jupiter at 05:00

SATURN is moving steadily away from the Sun before sunrise and is becoming easier to see before the sky brightens. It will be observable in the south east until the Sun rises in the east. A telescope will show the ring system quite well. See the chart above.

URANUS will not be observable this month as it will be in conjunction (appearing to pass behind the Sun) with the Sun on 22nd April.

NEPTUNE is still close to the Sun after passing through conjunction last month and not visible.

THE SUN

The Sun rises at 06:27 at the beginning of the month and at 05:44 by the end of the month. It will be setting at 19:40 at the beginning and 20:15 at the end of the month. There has been no activity for some months.

 

THE MOON PHASES IN APRIL

New Moon will be on the 5th April

First Quarter will be on 12th April

Full Moon will be on 19th April

Last Quarter will be on 26th April

 

LYRID METEOR SHOWER

There will be a meteor shower during the night of 22nd and 23rd April with a peak of activity around 1 o'clock BST (00:00 GMT). This is a minor shower (with just 10 meteors per hour) but does sometimes produce the occasional bright meteor known as a fireball. To observe the shower, look high towards the East to North EastThe Radiant point of the Lyrid meteor shower is always located to the west (right) of the familiar shape of Lyra and its very bright star Vega. Although Lyra does not appear over the eastern horizon until 10 o'clock BST in the evening of 22nd April, meteors may be seen rising up over the horizon before the constellation appears. The number of meteors is usually at its highest after midnight because at this time Earth is crashing headlong into the particle stream.

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