WHAT'S UP THIS MONTH - AUGUST 2021

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

The chart above shows the whole night sky as it appears on 15th August at 22:00 (10 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 9 o'clock BST at the beginning of the month and at 11 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 high in the North West. 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 early evening with Saturn and Jupiter later.

 

THE SOUTHERN NIGHT SKY THIS MONTH

The night sky looking south at about 22:00 BST on 15th August

The chart above shows the night sky looking south at about 22:00 BST on 15th August. 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'.

Constellations through which the ecliptic passes this month are: Virgo (the Virgin), Libra (the Scales), Scorpio (the Scorpion) Sagittarius (the Archer), Capricornus (the Goat) and Pisces (the Fishes) rising in the east.

The constellation of Perseus appears in the north east (upper left) of the chart above but by midnight it will have moved to the northern horizon. During the summer months Perseus is located in the north close the horizon so that is where it can be found this month, when we are looking out for the Perseid Meteor Shower. During the winter however it is located almost overhead and during November and December is actually host to the Zenith (the point in the sky directly over head).

In the west is the less obvious constellation of Virgo but it does have one fairly bright star called Spica. Virgo gives its name to a large cluster of Galaxies that is also spread over into the neighbouring constellations of Coma Berenices (Berenices' Hair) and into Leo.

To the north of Virgo is the bright orange coloured star called Arcturus in the constellation of Boötes. Arctaurus is a star similar to our Sun but more advanced and is developed into a Red Giant star that is nearing the end of its ‘life' as a normal star. It has used almost all of its Hydrogen fuel and has expanded to become a Red Giant, 25 times the diameter of our Sun. At the moment it shines 115 times brighter than our Sun but it is destined to collapse and become a White Dwarf.

Almost overhead is the constellation of Hercules (the Strong Man). Hercules has a rather distinctive distorted square shape, at its centre, called the ‘Keystone'. This is due to its resemblance to the centre stone of an arch or bridge. The jewel of Hercules is without doubt is the Great Globular Cluster, Messier 13 (M13). M13 can be found in the western (right) vertical imaginary line of the ‘Keystone'. It is just visible using a good pair of 9 x 50 binoculars. The spherical cluster, of about a million stars can be seen using a 90mm f 10 telescope but will look even more impressive when using a larger telescope.

Prominent in the southern sky is the Summer Triangle that dominates the Summer Sky and was described in detail in the July magazine. The triangle is defined by three obvious bright stars: Deneb in the constellation of Cygnus, Vega in Lyra, and Altair in Aquila. The Milky Way (our Galaxy) flows through the Summer Triangle and passes through Aquila and Cygnus. The Summer Triangle is bigger than may be expected but once it has been found it is very easy to find again.

As the Summer Triangle is so easy to find it is very useful to use as a starting place for finding our way around the night sky.

The Ecliptic is low in the sky during the summer months so the Moon and planets at appear close to the southern horizon. Saturn and Jupiter are starting enter the night sky but due to their low altitude will not be at their best for observation this year. The thick, murky and turbulent air will cause the planets to appear quite unsteady.

 

THE CONSTELLATION OF PERSEUS

Chart showing the constellation of Perseus

Perseus is our constellation of special interest this month because it is host to the Radiant of the Perseid Meteor shower. The other articles in this what's Up give the details of the Perseid Meteor shower but the constellation of Perseus has interesting things to see as well.

The asterism (stick figure shape) looks rather like a horse riding stirrup. The two brightest stars Algol and Mirfak along with the fainter star Atik mark out the ‘V' main shape with another line of stars pointing from Mirfak towards the very obvious ‘W' shape of Cassiopeia.

Other constellations around Perseus are: Cassiopeia to the north, Andromeda to the west (right) Auriga to the east (left) and Taurus to the south (below). Continuing the line of stars from Mirfak to Atik the beautiful Open Cluster of stars Messier 45 (M45) the Pleiades also called the Seven Sisters can be found in the constellation of Taurus (the Bull).

During the summer months Perseus is located in the north close the horizon so that is where it can be found this month. During the winter it is located almost overhead and during November and December is actually host to the Zenith (the point in the sky directly over head).

Perseus is also host to two Messier ‘deep sky' objects Messier 34 (M34) and Messier 76 (M76). M76 is a rather nice Planetary Nebula which is sometimes called the Little Dumbbell. This is a star similar to our Sun that has collapsed to become a White Dwarf and has developed a ‘bubble' of gas around it. It does need a medium sized telescope to see.

Messier 34 is a fairly bright Open Cluster comprised of about 80 stars. It can be seen as a small ‘fuzzy' patch of light using binoculars but does need a telescope to see as a cluster of stars. However there is another Open Cluster or rather two clusters called the Double Cluster that are listed in the New General Catalogue as NGC 869 and NGC 884 and shown in the image below.

NGC 869 and NGC 884 the Double Cluster

The Double Cluster can be seen on a clear dark night with the ‘Naked Eye' as a ‘fuzzy patch of light in the line of stars leading from the star Mirfak up towards the constellation of Cassiopeia. It is best seen using binoculars or a small telescope fitted with a low power eyepiece (25mm or 32mm). It is not clear whether this is a true associated double cluster or just a ‘line of sight' coincidence.

 

THE PERSEID METEOR SHOWER

 

The Perseid Meteor Shower Radiant at 01:00 13th August looking north

Meteor showers are notoriously unpredictable. The exact time of any spectacular increase in numbers or if the meteors will be bright is difficult to predict as is the clear weather needed to see them. However every year on the evening of the 12th and early morning of 13th August there is always a spectacular display from the Perseid Meteor Shower.

Fortunately this year the 3 to 4 day old Moon will setting in the west soon after sunset on the 12th August so even the fainter meteors may be seen in the dark sky away from street lights. The meteors of a shower appear to radiate from a point in the sky that is called the ‘Radiant'. The meteors of this particular shower appear to originate from a ‘Radiant' point in the constellation of Perseus. See the chart above.

If the trail of any meteor that is seen can be tracked back and found to have originated from this radiant point it will be a Perseid. A few meteors might appear to originate from other directions so these are the meteors that might be seen randomly and not part of any named shower. These are known as Sporadic Meteors.

From a clear dark site, the constellation of Perseus can be clearly seen, in the northern sky, as a line of stars stretching from the very distinctive ‘W' shape of Cassiopeia and down towards the bright star Capella. The brighter stars of Perseus appear to mark out the rough shape of a horse riding spur. If the sky is clear the Milky Way (our galaxy) can be seen rising up from the northern horizon passing through Perseus, Cassiopeia and right across the sky though Cygnus and the Summer Triangle. The bright star Capella in the constellation Auriga will be twinkling noticeably close to the northern horizon.

Observing can start as soon as it is dark but there is likely to be more meteors after midnight. Position a garden lounger chair so the feet are pointing to the north. Elevate the lounger so the northern horizon can be seen. Look at about 45º above the horizon and anywhere between west, through north and to the east. Meteors will appear as a fast streak of light flashing across the sky. One or two meteors every five to ten minutes may be seen. Some might be faint and difficult to see especially from a well-lit area in the towns. Any bright meteors will be seen even from fairly light polluted skies. These may appear anywhere in the sky from close to the radiant in the north to directly overhead. With a clear sky it may be possible to follow the tracks back through the constellations they passed through to the radiant point in Perseus.

SO WHAT IS A METEOR?

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 also called a ‘ shooting star ' . It is not a star at all it is just a small speck of dust known as a METEOROID entering the Earth's atmosphere at very high speed.

We all know how a space capsule or other space craft become very hot as they re-enter the atmosphere at about 27,000 km/h . However these meteoroid 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 and are 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 nother planet.

Very rarely two asteroids may collide but when they do, chips of rock and Iron are thrown off and occasionally may head towards Earth. These can be a few millimetres across or up to tens or even hundreds of metres across. They are quite rare and are seen as individual ‘fireballs' . Large ones can sometimes impact the ground as a METEORITE and may even cause craters.

The second type of meteor originates from a comet and is much more common. Comets are large lumps of ice, typically between five and thirty kilometres across that reside beyond the orbits of the outer 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. A comet can be thought of as being like a giant dirty snowball. As it spirals in towards the Sun, the water ice 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. See the picture below.

Dust particles released by the melt are heavier and therefore continue more or less on the same orbital path. These particles spread out along the orbital path and may eventually form a complete ring around the orbit.

Comet Hale Bopp 1998

Once or twice a year Earth may pass through this stream of particles that then collide with the atmosphere as Meteors. Meteoroid dust particles are usually small and very light and generally have the consistency of cigarette ash but are travelling very fast (>50 km/sec.). As Earth ploughs into the stream of meteoroids they appear to enter the atmosphere at a single point in the sky. This point is in the direction that Earth is travelling along its orbital path. The meteors will appear to radiate out in all direction from this point, very much like driving a car into a snow storm, see the images below.

Driving into a snowstorm

A meteor shower radiant

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 meteor showers 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.

With a combined impact velocity between 11 and 76 km per second meteors have a lot of kinetic energy (energy due to velocity) and burn up in the atmosphere at a height of about 100 km. Only the largest rocky or metal meteors from asteroids reach the ground. So all the meteors originating from comets burn up in the upper atmosphere and present no danger to us.

 

THE SOLAR SYSTEM - AUGUST 2021

The outer planets in the evening at 22:00 on 15th August

The chart above shows the location of the planets along the Ecliptic. The sky has been darkened to make the planets visible. The planets to the west of the Sun (right) will be visible late evening and early morning sky before sunrise. The inner planets are to the east of the Sun (left) and will be visible in the early evening sky after sunset.

MERCURY will be very close the Sun after sunset. Experts may be able to find it in the bright evening sky but it will require a clear view to the western horizon.

VENUS will be visible in the early evening sky as soon as possible after sunset. It will be easy to find but will it require a clear view to the western horizon. Venus is emerging from its excursion behind the Sun when it was in ‘Conjunction' with the Sun. It will appear at its smallest diameter and will be fully illuminated because it is still beyond the Sun from our point of view.

Venus, Mars and Mercury at sunset on 15th August

MARS can still be seen in the evening sky after the Sun has set and the sky darkens. However it is looking small now at about 3.6 arc-seconds as Earth pulls further away. Mars sets at about 21:00 and is moving ever closer to the south western horizon.

JUPITER AND SATURN AT OPPOSITION - AUGUST 2021

The planets Jupiter, Saturn (and Neptune) at midnight on 15th August

The two brightest ‘superior' (outer) planets Jupiter and Saturn are at their best positions this month for observing. Both planets will be at opposition during August. Saturn will be rising in the east at about 20:00 on 2nd August when and will be at opposition due south at midnight. Jupiter will be rising in the east at about 20:30 and will be at opposition on 20th August.

Opposition is the time when Earth will pass the outer planets on their orbits around the Sun. At this time the planets will appear due south at midnight 24:00 GMT (01:00 BST).

JUPITER will be rising in the east at about 20:30 and will be visible in the south east by midnight. It is the largest planet and appears much larger and brighter than Saturn because it is only half the distance away from us. Jupiter is 778.3 million km away from us compared with 1429 million km to Saturn. Jupiter will be at opposition at midnight on 20th August.

Jupiter at opposition at 01:00 BST (00:00 GMT) on 20th August

Any good quality telescope will reveal the brown and white cloud bands on the surface of Jupiter. The four brighter moons Io, Europa, Ganymede and Callisto will also be observable using any telescope. It is very interesting to follow the movement of the four Galilean moons from night to night and even during the course of one observing period. When the inner moons Io and Europa move close to Jupiter they can be seen to move in short periods of time. They can be watched as they approach Jupiter and disappear in front (a transit) or behind (an occultation) of the planet. The times of these events can be predicted using a planetarium application on a PC or laptop.

Jupiter and its four brightest moons on 20th August 2021

 

SATURN will be rising in the east at about 19:45 but will be more difficult to observe than Jupiter in the turbulent air close to the horizon. Saturn will be at its best this year on 2 nd August when it will be at opposition and will be due south at midnight.

Saturn can be seen using a small to medium sized telescope (90mm to 120mm). This will enable us to see the ring system and the brightest moons. A high magnification must be used and on a good clear and still night the view will be very rewarding. The picture below shows the sort of image that can be expected in one of these first telescopes along with an example of the view through a larger aperture telescope and with a longer effective focal length.

The view through a 150mm and a 200mm telescope

However this year the ‘seeing' will be unsteady due to the murk and turbulence close to the horizon. The two or three brightest moons may be visible using a 90mm aperture telescope (with some difficulty) but a 120mm or larger telescope will show another 2 or 3 fainter moons. The ring system will be discernible but colours and detail will be difficult to see.

Saturn at opposition at 01:00 BST (00:00 GMT) on 2nd August

Saturn and some of its Moons on 2nd August 2021

Up to six of Saturn's moons may be visible, distributed around the planet. The largest moon Titan is the easiest to see. Some of the others may be difficult (but possible) to see in a smaller telescope. On a good clear night it may be possible to see Titan and two or even three others in a 90mm refractor or a 113mm reflector. The other two will need a larger telescope to see.

URANUS will not be observable this month as it will mainly be in the sky during daylight. This month it will rise at 23:00 and will be in the brightening morning sky.

NEPTUNE will be just visible this month to the east of Jupiter (see chart). It will be difficult to see in the brightening morning sky as it is only magnitude +7.7.

 

THE SUN

The Sun rises at about 05:30 at the beginning of the month and 06:10 by the end. It sets at 20:45 at the beginning of the month and 20:00 at the end of the month. It reached its highest point in the sky on 21st June which was the Summer Solstice and is heading towards the Autumn Equinox on the 22nd September. There have been a few small Sunspots during July.

 

THE MOON PHASES DURING AUGUST

New Moon will be on 8th August

First Quarter will be on 15th August

Full Moon will be on 22nd August

Last Quarter will be on 30th August

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