WHAT'S UP THIS MONTH - APRIL 2020

(Link to What's Up May 2020)

(Link to What's Up March 2020)

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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: Uranus and Venus in the early evening with Mars, Saturn and Jupiter early morning.

EXPLORING THE NIGHT SKY THIS MONTH

The Southern Night Sky during April 2020 at 20:00 BST

The chart above shows the night sky looking south at about 20:00 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 (in red) 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 brightest stars often appear to form a group or recognisable pattern; we call these ‘Constellations'. Shown (in yellow) are the ‘Deep Sky' Messier Objects [Galaxies, Clusters and Nebulae].

Constellations through which the ecliptic passes this month are: Aries (the Ram) just moving over the western horizon, Taurus (the Bull), Gemini (the Twins), Cancer (the Crab), Leo (the Lion), Virgo (the Virgin) and Libra (the Scales) rising over the eastern horizon.

Still prominent in the south west is the constellation of Taurus (the Bull). It sits on the Ecliptic and looks like a squashed cross ‘X'. The most obvious star in Taurus is the lovely Red Giant Star called Aldebaran. It appears slightly orange to the ‘naked eye' but it is very obviously orange when seen using binoculars or a telescope. Aldebaran is located at the centre of the ‘flattened' X shape formed by the brightest stars in Taurus.

At the end of the top right (upper west) arm of the ‘X' is the beautiful ‘naked eye' Open Star Cluster Messier 45 (M45) known as the Pleiades (or the Seven Sisters). It really does look magnificent using binoculars. Venus is shining very brightly in the constellation of Taurus this month. Venus will pass through the bright and ‘naked eye' open star cluster Messier 45 the Pleiades (Seven Sisters) on 3 rd April. This will be a special event to see and for those interested in astro-imaging to possibly take a picture. See page 6 for details.

Following Taurus along the Ecliptic is the constellation of Gemini (the Twins). The two brightest stars in Gemini are Castor and Pollux that are named after mythological twins and they are so alike they do look like twins. There are lines of fainter stars linked to Pollux and Castor and extending to the south west (down to the right). There is a lovely Open Cluster called Messier 35 (M35) just off the end and above the upper line of stars emanating from the star Castor. M35 will need a telescope to see well.

Following Gemini along the Ecliptic is the rather faint constellation of Cancer (the Crab). It does need a dark and unpolluted sky to see with the naked eye. In a good sky the faint stars can be seen with a nice Open Cluster of stars at its centre. The cluster is called Messier 44 (M44) or ‘the Beehive Cluster' because of its resemblance to an old straw built beehive with a swarm of stars looking like bees around it. It looks best using binoculars.

Following Cancer along the Ecliptic is the constellation of Leo (the Lion). It does actually look a little like a resting male African lion but perhaps more like the Sphinx in Egypt. Below Leo are some relatively bright galaxies M65, M66, M95 and M96 but we will need a telescope to see the galaxies. The sky around Leo and particularly between Leo and Virgo hosts a cluster of nearby galaxies. Our Galaxy (the Milky Way) is actually a member of a small ‘Local Group' of galaxies that forms part of this larger cluster of galaxies.

To the south of Taurus and Gemini is the spectacular constellation of Orion (the Hunter). Orion dominates the southern sky and is one of the best known constellations. It also hosts some of the most interesting objects for us to seek out.

THE LYRID METEOR SHOWER 21st – 22nd April

 

The Lyrid Radiant Point at 02:00 (2 hours after the peak of activity)

The Lyrid meteor shower takes place annually between 16th April and 25th April and in this year it will peak late in the evening of 21st April and the morning of the 22nd April. This particular shower is visible in both the Northern and Southern Hemispheres, offering stargazers a chance to see up to 18 meteors per hour during its peak.

Lyrid meteors are typically as bright as the stars in Ursa Major the plough (or the Big Dipper to the Americans). Some are much more intense, even brighter than Venus, the brightest object in the night sky after the moon. Called ‘Lyrid fireballs', these may be bright enough to cast shadows for a split second and leave behind trails that sometimes linger for minutes.

So what causes the Lyrid meteor shower?

The ionised gas in the meteor trail burns up as it enters the Earth's atmosphere, creating the glow which can be seen streaking across the night sky. The shower occurs as the Earth passes through the dust left over from Comet Thatcher (C/186 G1) which makes a full orbit of the Sun once every 415 years. Comet Thatcher last visited the inner solar system in 1861 (before the widespread use of photography) and is not expected to return until the year 2276 (hence there are no photographs of it).

Flakes of comet dust (called Meteorids), most no bigger than grains of sand, strike Earth's atmosphere travelling 49 km/s (110,000 mph) and disintegrate as streaks of light.

The Lyrid shower meteors appear to radiate out from a point in the sky close to the star Vega, the brightest star in the constellation Lyra the Harp, from which it takes its name. Vega is a brilliant blue-white star about three times the diameter of our Sun and 25 light years away. This Radiant Point occurs in the same way that raindrops or snowflakes appear to radiate from a point ahead of a car when driving into a shower. Earth is careering into these Meteoroids at about 100,000 kilometres per hour as it orbits the Sun so they appear from a point in the direction that Earth is moving, just like the raindrops in front of a car.

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A meteor shower 'Radiant Point' (Geminid Shower)

About Meteors, Meteoroids and Meteorites

On any clear night if you sit back and look up into the night sky for a while you will more than likely see a streak of light speed across the sky - this will be a METEOR or shooting star. These random meteors are called ‘Sporadic Meteors' others appear in showers. These are not stars at all they are just a small speck of dust known as a METEOROID entering the Earth's atmosphere at very high speed. Just as the space shuttle or other space craft become very hot as they re-enter the atmosphere at about 30,000 km/h . However these dust particles get even hotter at their re-entry speed of up to 270,000 km/h. At this speed the dust is vaporised by the heat and the surrounding air is also heated until it glows in a similar way to a fluorescent light.

There are two types of Meteor, the first is thought to originate from the large lumps of rock and iron left over when the planets formed , known as ASTEROIDS. Most asteroids orbit the Sun in a belt between Mars and Jupiter. The huge gravitational forces exerted by Jupiter may have pulled the rocks apart before they could accumulate into a planet. Very rarely two asteroids may collide but when they do, chips of rock and Iron are thrown off and occasionally head towards Earth. These may be a few millimetres across or up to tens or even hundreds of metres across. These are quite rare and are seen as individual ‘fireballs' sometimes impacting the ground as METEORITES and if big enough may even cause craters.

A very bright 'Fireball'

The second type of meteor originates from a comet and is much more common. Comets are large lumps of ice, typically between ten and thirty kilometres across that reside beyond the orbits of the planets. There are millions of these objects just sitting there quietly orbiting around the Sun at enormous distances. Occasionally one of these objects may be nudged out of its orbit by a close encounter with another object and may begin to move in towards the Sun. As a comet which can be thought of as being like a giant dirty snowball, approaches the Sun, the water and frozen gases begin to boil off and are blown away by the radiation from the Sun. This gas and dust will form the familiar twin tails associate with comets.

Dust particles released by the melt are heavier and therefore continue more or less on the same orbit. These particles spread out along the orbital path and may eventually form a complete ring around the orbit. Once or twice a year Earth may pass through this stream of particles that then collide with the atmosphere as Meteors. Meteoroid dust particles from comets are usually small, very light and generally have the consistency of cigarette ash.

A comet leaves a particle trail around its orbit and Earth can pass through the trail

The clarity of the sky will make a significant difference to the number of meteors that can be seen. Any mist or hazy cloud will severely reduce the chance of seeing the fainter meteors especially if observing from a light polluted area. If it is cloudy , there is of course less chance of seeing any meteors at all. It is never possible to predict exactly when the maximum peak might appear and sometimes it may not appear at all. This i s because the dust from the comet that produces the meteors moves through space in wisps and filaments. All depends on whether Earth passes through a filament and how thick that filament is.

Before rushing out into the garden to look for meteors, there are a few things to consider for your own comfort, the first and most important is clothing. The nights, even in summer, can be cold so it is essential to dress in warm clothes. A garden ‘lounger' chair is an excellent piece of equipment especially the type that can be reclined into a near horizontal position. This will help avoid neck and back ache when looking up.

So let us think about where to position ourselves to see the meteors. For the Lyrids a clear view towards the south to south east is required so set up your lounger seat with your feet facing south to east. Use a patio or path if possible, they are more comfortable and less prone to dampness from dew. Obviously try to set up away from lights, trees or buildings but this may not be possible so set up in the best place to view, you can always move to another position later.

Direct your gaze up between 45º and directly overhead (don't use binoculars). Sit and wait an hour or more to see a number of meteors. The only thing that is predictable about meteor showers is they will always be unpredictable. Just hope for clear skies and a good shower.

THE SOLAR SYSTEM THIS MONTH

MERCURY will just be observable this month but it will be very close to the horizon in the east before the Sun rises at 06:15. The innermost planet will be at Greatest Western Elongation (furthest position from the Sun) on 24th March, see the chart below. Mercury is small but quite bright although its brightness is rather overwhelmed by the brightness in the sky from the rising Sun. It is best seen using binoculars or telescope but we must make sure the Sun is below the horizon before sweeping the sky using binoculars to find Mercury.

Neptune, Mercury, Saturn, Jupiter and Mars at sunrise

VENUS has been moving out from behind the Sun and will reached its Greatest Eastern Elongation (appearing furthest from the Sun on 24th March, see the March issue of this Magazine). Venus will achieve its maximum brightness on 28th April. See the Uranus chart below.

On 3rd April Venus will pass through Messier 45 (M45) the Pleiades Star Cluster (Seven Sisters). The image below shows the position of Venus at about 20:00 on 3rd April. Venus will appear to move slowly through M45 towards the stars Atlas and Pleione but the movement will not be detectable in real time but Venus may been seen to have moved over a period of a few hours.

Venus in Messier 45 (M45) on 3rd April

Venus has been moving out from behind the Sun and reached Greatest Eastern Elongation (appearing furthest from the Sun on 24th March). Venus has looped out from behind the Sun along its orbit inside the orbit of Earth and our vantage point and is moving towards us. It will now appear to move back towards the Sun. Venus will now appear larger as it moves towards us and will become a narrower crescent shape as it appears to move back towards the Sun. As Venus appears to steadily become larger but develops a thinner crescent its overall brightness remains almost the same. It will actually be at its brightest on 28th April but it will not be noticeably brighter.

Venus imaged on 21st March 2020 by Steve Harris

MARS will be observable (with difficulty) this month, low in the east before sunrise. Mars is still a long way from us on the other side of the Solar System so it looks small at just 5.9" (arc seconds). See the chart below.

Mars, Saturn and Jupiter at sunrise at 04:30 on 30th April

JUPITER is moving away from the Sun in the early morning sky in the east. It will be very low in the sky and looking large, (40.0”) but disappointing in the dirty and turbulent air close to the horizon. See the chart above and the Mercury chart.

SATURN will be low in the south east as the sky brightens before the Sun rises over the eastern horizon. Saturn is very low and in the murky and turbulent air close to the southern horizon. It will be in the bright dawn sky and will require a clear view to the eastern horizon. It may still just be possible to see the ring system although it will appear unstable due to the air movement close to the horizon. See the chart above.

URANUS will not be visible this month as it is moving into conjunction with the Sun on 26th April (appearing to pass behind the Sun). See the chart below.

Uranus and Venus at Sunset on 15th April

NEPTUNE was in conjunction with the Sun on 8th March (appearing to pass behind the Sun). It will not be observable this month. See the Mercury chart.

THE SUN

The Sun rises at 06:30 BST at the beginning of the month and at 05:45 BST by the end of the month. It will be setting at 19:40 BST at the beginning and 20:30 BST by the end of the month. There have been no sunspots during March. Sunspots and other activity on the Sun can be followed live with day to day images of the Sun by visiting the very good SOHO website at: http://sohowww.nascom.nasa.gov/ .

THE MOON PHASES IN APRIL

First Quarter will be on 1st April

Full Moon will be a Super Moon on 8th April

Last Quarter will be on 14th April

New Moon will be on the 23rd April

 

ANOTHER ‘SUPER MOON' ON 8th April

 

The Moon at Perigee (closest to Earth)

We hear a lot about the ‘Super Moon' these days, mainly from the television news or in the popular news papers. So what does this mean and does it have any significance for astronomers? The simple answer is no, it is of no real scientific interest to astronomers but it is of general interest. There are two factors that produce the effect we call the Super Moon. One is a physical effect and the other is illusionary.

The first effect is to do with the orbit of the Moon around Earth. Like most orbiting bodies the orbit of the Moon is elliptical and not circular. This means the Moon will be closer to Earth at one point that we call ‘Perigee' and furthest away at the point we call ‘Apogee'. At Apogee the Moon can be up to 406,700 km away from Earth but at Perigee can be as close as only 356,500 km.

Comparison of size Perigee to Apogee

With this change in distance the Moon will actually look larger at Perigee (closest) and smaller at Apogee (furthest away). The difference in apparent diameter is up to 14% and the difference in the Moon's reflective area is about 30% so this does make a difference.

The second Super Moon effect is an optical illusion and is most noticeable during the summer months. It is caused by the Moon appearing low in the summer night sky. The 23.4º tilt in Earth's axis of rotation results in our view of the sky appearing to be tilted by the same amount. So during the summer the Sun appears higher in the daytime sky but the Moon and planets appear low in the night sky as they move along the Ecliptic (the equator of the Solar System).

A full Moon always rises in the east as the Sun is setting in the west. So as the Moon rises over the horizon our eyes try to compare the size of the very distant Moon to the nearer features on the horizon. Our eyes are confused and try to relate the apparent size of the Moon to the features near the horizon and an optical illusion causes the Moon look larger.

The Super Moon Effect in San Francisco

So if a summer Full Moon coincides with the Moon's orbital Perigee then the Moon can look particularly large as it rises over the eastern horizon. This due to it actually appearing larger combined with the optical illusion.

If a Full moon occurs when the Moon is less than 361,885 km from Earth it is regarded as a ‘Super Moon'. The Full Moon on 9th March 2020 occurred when the Moon was 357,404 km from Earth so was officially a ‘Super Moon'. This Full Moon on 8th April will look even larger than last month as it will be just 357,035 km from Earth which is almost 400 km closer than on 9th March.

So have a look at the Full Moon on 8th April and if possible try to get a photo that emphasises this effect

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