WHAT'S UP THIS MONTH - APRIL 2018
(Link to What's Up March 2018)
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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY
Click HERE for a downloadable PDF white chart - (Close PDF file to return here)
The chart above shows the whole night sky as it appears on 15th April at 21:00 (9 o'clock) 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 quite easy to find. This month it is almost directly 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 in the evening sky: Venus. Mars, Jupiter and Saturn will be in the early morning sky.
EXPLORING THE NIGHT SKY THIS MONTH
The Southern Night Sky during April 2018 at 21:00 BST (9:00 pm)
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 at the upper 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 constellations through which the ecliptic passes are known as the constellations of the ‘Zodiac'.
Constellations through which the ecliptic passes this month are: Aries (the Ram), Taurus (the Bull), Gemini (the Twins), Cancer (the Crab), Leo (the Lion) and Virgo (the Virgin) just appearing over the eastern horizon.
The Milky Way (our Galaxy) appears to rise up from the southern horizon. It continues up through the constellations of Monoceros, Orion, Gemini, Auriga, Perseus and into Cassiopeia (located just off the top right of the chart).
This time of year (Spring) is sometimes referred to as the season if the galaxies because it is the best time to look for galaxies. A larger telescope is required to see any of the galaxies except M31 the Great Spiral Galaxy in the constellation of Andromeda. M31 can be seen using a smaller telescope or even a good pair of binoculars. It is unfortunately disappearing over the western horizon this month and cannot be seen.
There are many galaxies in the Virgo Cluster that is centred on the space between the constellations of Leo, Virgo and Coma Berenices on the center left of the chart. This time of the year is a good time to look for galaxies. It is time when we are looking up and out of the spiral structure of our galaxy so we have a clearer view of the deep sky. Some of the Virgo Cluster galaxies are shown on the chart above. They are marked with their Messier number in yellow.
Orion is still prominent the evening sky and is easy to find in the south western sky. The familiar shape of Orion the Hunter is followed across the sky by his hunting dogs Sirius and Procyon.
To the north of Orion are the fairly obvious constellations of Taurus and Gemini. To the North West and sitting astride the ecliptic is the constellation of Taurus (the Bull). The Taurus asterism (shape) looks like a squashed cross ‘X'. At the centre of the cross is a large, faint and dispersed Open Cluster, the Hyades. It has the bright Red Giant star Aldebaran in the centre. The real beauty of Taurus is the naked eye Open Cluster M45 the Pleiades, or the Seven Sisters.
Aldebaran in the Hyades and M45 the Seven Sisters
The Seven Sisters named
Most people with fairly good eyesight can see the brightest six stars in the cluster. Binoculars will reveal about forty bright stars and the view will be magnificent. In fact the best view is obtained using binoculars. A small short focal length telescope will also give a wonderful view but the whole cluster cannot fit into the narrow field of view of most telescopes. There are a host of smaller stars in the cluster that are too faint to be seen and identified in amateur astronomer's telescopes. It is thought there could be more than 1400 stars in total in the cluster. This month will be our last chance to see this beautiful cluster before it disappears over the western horizon to appear again towards the end of the year.
To the north of Taurus is the constellation of Auriga with its beautiful bright star Capella. A good pair of binoculars will just reveal a line of three Open Clusters listed in Charles Messier's Catalogue as M36, M37 and M38. They can be seen as small patches of light through binoculars but do require a telescope to see as clusters.
Following Taurus along the ecliptic is Gemini (the Twins). The twin stars Pollux and Castor are easy to find. There is a lovely Messier Open Cluster M35 in Gemini just off the end of the line of stars emanating from the bright star Castor. Castor is a double star when seen in a telescope.
To the east of Gemini is the faint and rather indistinct constellation of Cancer (the Crab). The asterism (shape) of Cancer looks quite uninteresting. However the Open Cluster Messier 44 (M44) ‘Praesepe' or the ‘Beehive Cluster' looks beautiful and like a swarm of bees around an old style straw hive, when seen using binoculars.
Following Cancer is Leo (the Lion) which was the ‘constellation of the month' last month.
CONSTELLATIONS OF THE MONTH – Virgo and Coma Berenices
The constellations of Virgo (the Virgin) and Coma Berenices (the Hair of Berenices)
Virgo is located on the ecliptic (the imaginary line along which the Sun, Moon and planets appear to move across the sky). This means the Sun, Moon and planets can appear to pass through Virgo . Jupiter can be seen at the lower left of the chart above, just above the ecliptic in the neighbouring constellation of Libra.
Spring time is regarded as the season of galaxies. Th is is because there is a group of Galaxies located between the constellations of Virgo , Coma Berenices and Leo. Leo was the constellation of the month in the March What's Up. We saw then how Leo has four lovely bright (Messier) galaxies of its own, these are known as: M65, M66, M95 and M96. They can be seen on the chart above marked in yellow just below the shape of Leo the ‘lion'.
When we look in the direction of Virgo we are looking up and out of our Galaxy (the Milky Way). We are not looking through the main disc structure so our view it not obscured by the multitude of stars and thick clouds of gas and dust in our galaxy. With this clearer view out of the Milky Way we are able to see the other galaxies that surround our galaxy. Some of the brighter of these local galaxies called the ‘Virgo Cluster' are marked in yellow on the chart above. The more detailed image below shows many of the brighter galaxies in the Virgo Cluster which are labelled.
The Virgo cluster is the immediate area around our ‘Local Group' of galaxies. Our ‘Local Group' is dominated by the Great Spiral Galaxy in Andromeda and our own Milky Way Galaxy. The other members of the Local Group are smaller spiral galaxies and Irregular Galaxies. Some of the Irregular Galaxies are very small and are referred to as Dwarf Galaxies. It is thought there are between 55 and 60 members in our Local Group. The centre of mass of the Local Group is located between the two Giant Spiral Galaxies and all the smaller members are dominated by these two larger members. In fact the two Giant Spirals are also dominated by this centre of gravity. The gravity of the two giants is pulling them towards each other. Inevitably they will meet and collide in about 4 billion years time which coincidently is about the same time that our Sun will start to run out of its Hydrogen fuel.
With two such enormous objects hurtling towards each other at a combined speed of 400,000 km/h we could be excused for thinking that there was an unimaginable disaster ahead. This is not necessarily going to be as bad as we might think. Before the two giant galaxies actually meet they will make an incredibly beautiful sight in the sky. We are not sure about the exact way the two giant spirals will meet but it will not be all bad. The stars in the galaxies are very far apart compared to their size so the space between the individual stars is enormous. Consequently the galaxies will pass through each other and very few stars, if any will collide. The combined gravity will cause them to loop around each other and eventually combine into one larger galaxy.
An artist's impression of M31 approaching the Milky Way
Our local Group is also part of the larger group of galaxies that we can see in the Virgo Cluster. The Virgo cluster is in turn part of the vast network of filaments comprised of billions upon billions of galaxies stretching across the Universe.
An image of the Virgo Cluster showing the brighter galaxies
Virgo is one of the ‘Spring Constellations' because it enters the night sky in the early months of the year as it begins to rise over the eastern horizon in the evening. Most of its stars are not bright but Spica is the exception. It is a variable star with an apparent brightness (known as magnitude) that varies between +0.97 and -1.4. It is classified as the 16 th brightest star in the night sky.
Spica is listed as a Spectroscopic Binary star. This means it is a double star but the two stars are so close together that they cannot be separated using a telescope. It was found to be a double star when the spectrum of the star's light was found to be the combined spectra of two different stars. The pair is so close together that the gravity has caused them to be elliptical (egg shaped) as the part of each star facing the other is pulled towards the other by their gravity.
The two Spica stars are both larger and hotter than our sun but are only 18 million kilometres apart. That is very close for stars. For comparison, Earth's distance from our S un is 150 million kilometr e s. They are so close to each other that t heir mutual gravity distorts the star s to produce a bulge pulled towards the other as they whirl around . They orbit around their common centre of gravity in just four days.
An artist's impression of the Spica pair
THE SOLAR SYSTEM APRIL 2018
MERCURY rises at 05:00 at the beginning of the month and 04:00 by the end of the month. It will be very difficult to see in the brightening pre-dawn sky.
Mercury, Venus, Uranus and Neptune at Midday
VENUS is visible in the west as the Sun sets and is very bright at magnitude -3.9 but it will require a clear view to the western horizon. The telescopic view is not very good because Venus is on the opposite side of the Sun and appears small. It will become larger but narrower over the next few months. It still requires a Moon filter to reduce the dazzling and sparkling effect and improve the appearance while it is low in the sky. See the chart below.
Venus in the west at 20:00
MARS will rise in the south east at 02:00, following Jupiter but will be lost in the brightening sky by 04:00. The Red Planet still appears small at 9.5 arc-seconds in diameter but will brighten to magnitude -0.2. Mars is low and in the turbulent air but is starting to look larger. Earth is gaining on Mars as the two planets move around their orbits.
Mars will be very close to Saturn in the early morning sky in the east at the beginning of April.
Saturn and Mars on 4th April just before sunrise
JUPITER is now a good late evening and early morning object. It rises over the eastern horizon at about 21:00 and will be observable in the south east from 23:00. A pair of binoculars will reveal the four brightest of Jupiter's moons, Io, Europa, Ganymede and Callisto. A small telescope will allow the moons to be seen very clearly.
Jupiter is also observable in the early morning sky in the south until the sky brightens. It actually sets in the west at 07:00 at the beginning of the month and at about 05:30 at the end of the month. However the sky will be bright before then as the Sun rises at around 05:00 so it will be best before 04:00. At this time Saturn and Mars will also be observable in the south east.
Mars, Saturn and Jupiter at 02:00
Jupiter at midnight on 15th April
SATURN will be visible in the brightening dawn sky close to the south eastern horizon. The ringed planet rises at about 01:00 this month, this about 4 hours before the Sun. The view of Saturn will not be good this year as it will be close to the horizon. It is observable in the south east from 02:30 until the sky begins to brighten at about 04:00 which is about an hour before sunrise.
URANUS will not be observable this month as it is too close to the Sun. See the Mercury chart above.
NEPTUNE will not be easily visible this month as it rises over the south eastern horizon at 04:00 so it will be in the brightening sky before the Sun rises at 05:00. See the Mercury chart.
There have been virtually no sunspots to see over the last few months as the active phase of the Solar Cycle is over.
The Sun rises at 05:30 at the beginning of the month and at 04:40 by the end of the month. It will be setting at 18:40 at the beginning and 19:30 at the end of the month. Any Sunspots and other activity on the Sun can be followed live and day to day by visiting the SOHO website at : http://sohowww.nascom.nasa.gov/ .
THE MOON PHASES IN APRIL
Last Quarter will be on 8th April
New Moon will be on 16th April
First Quarter will be on 22nd April
Full Moon will be on 30th April
LYRID METEOR SHOWER
There will be a meteor shower on the morning of 23rd April at around 3 o'clock. The shower is known as the Lyrid Meteor Shower. It is not one of the most impressive showers with a maximum hourly rate of just 10 meteors.
The radiant point of the Lyrid Meteor Shower
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