WHAT'S UP THIS MONTH - OCTOBER 2021

(Link to What's Up September 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 October 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 above the Northern horizon. 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, Jupiter, Neptune and Uranus later.

British Summer Time (BST) ends on 31st October so the clocks go back one hour.

 

THE SOUTHERN NIGHT SKY THIS MONTH

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

The chart above shows the night sky looking south at about 22:00 BST on 15th October. 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: Sagittarius (the Archer), Capricornus (the Goat), Aquarius (the Water Carrier), Pisces (the Fishes), Aries (the Ram) and Taurus (the Bull).

Prominent in the southern sky is the Summer Triangle that dominates the Summer Sky and was described in detail previous pages. 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.

To the west (right) of the Summer Triangle and almost overhead is the constellation of Hercules (the Strong Man). Hercules has a 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 the Great Globular Cluster, Messier 13 (M13) that can be found in the western vertical imaginary line forming 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.

To the west of Hercules and close to the western horizon 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 developing 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 and Planetary Nebula.

To the East of the Summer Triangle is the constellation of Pegasus (the Winged Horse). The main feature of Pegasus is the square formed by the four brightest stars. This asterism (shape) is known as the Great Square of Pegasus. The square is larger than might be expected but once found is easier to find again . There is a very nice Globular cluster in Pegasus it is known as Messier 15 (M15). It is a lovely sight to see in a telescope.

Coming into view in the south east is the constellation of Taurus (the Bull). 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.

The Ecliptic is low in the sky during the summer months so the Moon and planets appear close to the southern horizon. Saturn and Jupiter are well placed 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.

 

OBSERVING VENUS THIS MONTH

Venus will be at Greatest Eastern Elongation on 29th October 2021

Venus was in Superior Conjunction with the Sun on 26th March this means it was passing behind the Sun from our point of view. When Venus emerged from its Superior Conjunction (behind the Sun) it would have appeared to the east of the Sun in the Evening sky at sunset. Venus would have appeared fully illuminated and would have appeared at its smallest diameter as it was at its furthest distance from us. However it was much too close to the Sun for us to have seen it.

Over the last six months has Venus moved further away from the Sun until it has now moved far enough to be seen to the east (left) of the Sun after sunset. It has been difficult to see because it has been very low and close to the western horizon. During this time it appeared small but full, close to the western horizon and in the bright sky after sunset. Its appearance this year has occurred in the summer when the Ecliptic is low in the sky.

As Venus appeared to move further east and away from the Sun it also moved towards Earth. It began to appear larger in diameter but less was illuminated by the Sun so it developed a Gibbous shape (between full and half full).

When Venus emerged from its Superior Conjunction, behind the Sun, it appeared to the east of the Sun in the Evening sky at sunset. It was small but ‘full', close to the western horizon and in the bright sky after sunset. It is sometimes called the Evening Star at this time. As Venus appeared to move further east and away from the Sun it was also be moving towards Earth. It began to appear larger in diameter but less was illuminated by the Sun.

As Venus moved out from Superior Conjunction it appeared as a full disc because it is on the opposite side of the Sun to us. From our point of view we see the whole surface of Venus illuminated. Venus then moved on its orbit around the Sun and appeared to us to move away from the Sun to the east (left). As it moved further away from the Sun its orbit also brought it towards us so some of the illuminated surface began to disappear from our view. See the images below.

The phases of Venus from Gibbous to thinnest Crescent imaged in 2004

The images above show the development of the phases of Venus as seen in 2004 and show how the planet appears bigger as it moves towards Earth. It also becomes narrower as less of the illuminated part of the surface is visible. Despite the obvious changes in size and illumination the overall brightness remains about the same. In fact Venus is the brightest object in the sky except for the Sun and the Moon. Jupiter is about magnitude -2.8 and Venus was about -4.0 on 1st September, -4.1 on 15th and -4.2 on 30th September.

Magnitude is the measure of brightness used for astronomical objects. The larger the magnitude number the fainter the object will appear. So a minus magnitude indicates the object is brighter than most of the stars seen in the sky. The bright star Vega in the constellation of Lyra (in the Summer Triangle) is a star that has a magnitude of 0.0 so is a good comparison star. A difference of one magnitude means there is an increase or decrease in brightness of 2.5 times (actually 2.512). The brightest star visible from the UK is Sirius at -1.46.

A telescope is required to see Venus as a phased shape but it also appears very bright in a telescope so even a small telescope will make it appear quite dazzling. Observing Venus does require some slightly special observing techniques. Most importantly Venus is often close to the Sun so care must be taken to make sure the Sun light does not enter the telescope. If the Sun is close by then we must wait for the Sun to move below the horizon or behind a building or other immoveable object.

This year Venus appears very low in sky and in the murky and turbulent air close to horizon. It is best to start with a low power eyepiece (25mm) when observing Venus then use a higher power (magnification) eyepiece (10mm) to have a closer look. If the image is too bright then a Moon filter can be used. Alternatively the Dust cap can be fitted to the telescope and the small ‘Moon' cap removed to reduce the glare and have a better view.

If the image looks good then a Barlow Lens can be used to effectively double the magnification of a 10mm eyepiece. When Venus is low in the sky and we are looking through more of the atmosphere some colour distortion can be seen as red and blue fringes. No surface features can be seen so the main interest in observing Venus is just the phases.

GREATEST EASTERLY ELONGATION IS ON 29th OCTOBER

After about 56 (Earth) days Venus will have moved a quarter of its way around the Sun and will be at its apparent furthest point from the Sun as we see it so we call this Greatest Easterly Elongation. At this point Venus will be about the same distance from us as the Sun so we will see the half of Venus facing the Sun illuminated and appearing ‘Half Moon' shaped.

Up to this point more than half of Venus would have been illuminated and the apparent shape would have been more than half, a shape we call ‘Gibbous'. Actually the Greatest Easterly Elongation occurs after the point when Venus is a quarter way around its orbit and occurs when Venus is at 90º from the Sun as we see it from Earth as shown on the diagram below (dotted lines).

The orbit of Venus around the Sun

After Greatest Easterly Elongation, Venus will appear to start moving back towards the Sun in a westerly direction. As it is still moving closer to Earth it will appear to increase in diameter. Less and less of the illuminated side of Venus will be visible to us so it develops into a narrowing crescent. However the overall perceived brightness will remain about the same.

Venus as it will appear on 1st October

Venus as it will appear on 29th October

 

THE AUTUMN CONSTELLATIONS

Chart showing the Autumn Constellations

Last month on the 22nd September it was the day of the Autumn Equinox. This was the time when we were half way between midsummer 21st June (Summer Solstice) and midwinter 21st December (Winter Solstice). This also means that at this time the length of the day and night are the same, 12 hours. The term Equinox actually means Equal Night.

The chart above shows the Autumn Night Sky on and around 15th October at about 21:00 BST. North is to the top, South at the bottom, East to the left and West to the right. The point in the sky directly overhead is known as the Zenith and is shown (in red) at the upper centre of the chart.

Mapping the night sky is actually rather difficult as there are only the stars to use as reference features and stars all look quite similar. The only difference to the untrained eye is that some stars appear brighter than others. With our unaided eyes (astronomers call ‘naked eye') we can see about 6000 stars in a good dark sky. The brighter stars do seem to form (all be it, sometimes indistinct) groups and patterns in the night sky.

From ancient times these patterns have been recognised by different cultures around the world and given special names. The names have traditionally been taken from characters in mythological stories and are often very old. In 1922, the International Astronomical Union (IAU) standardised the constellation names and adopted the modern list of 88 universally recognised ‘Constellations'.

At the centre of the chart is the Summer Triangle. The corners of the imaginary triangle are positioned on the 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 easy to find so once it is identified we can use it to lead us to other constellations around it and we can follow the constellations to find other constellations. This can help us to become familiar with the night sky and lead us to the many interesting things to search out and observe.

A number of other constellations are quite easy to find and identify and once found can also lead us to other constellations. One of the best known is Ursa Major (the Great Bear) that we sometimes call ‘the Plough' and the Americans call ‘the Big Dipper'. It actually looks remarkably like a large saucepan. At this time of the year it is ‘sitting' on the northern horizon.

Ursa Major can be found at top (North) of the chart above. The seven brightest stars of Ursa Major make a pattern (that we call an Asterism) that looks very much like a saucepan. Ursa Major is ‘circumpolar, that means it can be seen in the sky all through the year.

The sky appears to rotate due to the rotation of Earth and its orbit around the Sun. As Earth rotates once a day its northern axis points to a point in the sky close to the star Polaris in the constellation of Ursa Minor (the Little Bear). This point is very important and can be found by following the two stars of the ‘pan' opposite the handle in the saucepan shape of Ursa Major. We call these two stars ‘the Pointers' [to the Northern Celestial Pole]. Follow the arrow on the chart.

 

THE SOLAR SYSTEM - OCTOBER 2021

The planets at 22:00 on 15th October

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 planets to the east of the Sun (left) will be visible in the early evening sky after sunset as above.

MERCURY will be very close the Sun after sunset and will not be visible this month.

Mercury and Mars close to the Sun 29th October 07:45

VENUS will be visible in the early evening sky 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. Venus is completely covered in dense clouds so no surface features can be seen. Venus has an orbital period (year) equivalent to 225 Earth days but its axial rotation (day) is equivalent to 243 Earth days. This means a day on Venus is, very oddly, longer than its year. However this makes no difference on the surface because the Sun is never visible due to the very thick Carbon Dioxide (CO 2 ) clouds . It will appear at its Greatest Easterly Elongation on 29th October when it will be at it furthest distance from the Sun. See the previous article about Observing Venus.

Venus, Jupiter and Saturn at sunset on 29th October

MARS has now moved out of view and will not appear in the evening sky again until September 2022. It can be seen very close to the Sun on the Mercury chart but it will be too close to the Sun and too small in diameter to be observable.

JUPITER will be rising in the east at about 16:30 and will be visible in the south as the sky darkens. Jupiter was at opposition and its best on 20th August. See the September What's Up for more advice on observing Jupiter.

SATURN will be rising in the east at about 18:00 but will be more difficult to observe than Jupiter in the turbulent air close to the horizon. Saturn rises before Jupiter in the east and was at its best for this year on 2nd August when it was at opposition.

URANUS will be observable this month and will be best at 02:00 when it will be due south and at its highest point above the horizon but is small and faint at +5.7.

NEPTUNE will be just visible to the east of Jupiter (see the chart above). It is small a difficult to see at only 2.4 arc-seconds in diameter and at magnitude +7.7.

THE SUN

The Sun rises at about 07:00 at the beginning of the month and 07:45 by the end. It sets at 18:35 at the beginning of the month and 17:40 at the end of the month. It reached its highest point in the sky on 21st June which was the Summer Solstice and will be at the Autumn Equinox on the 22nd September.

There have been a few Sunspots between July and September. The following image is from NASA's Solar and Heliospheric Observatory (SOHO) and shows some spectacular sunspots. These are features on the visible surface of the Sun caused by the strong magnetic forces present on the Sun. Where lines of magnetic force enter of exit the surface they cause a disturbance the causes a depression that exposes a ‘less hot' layer below.

Sunspots appear darker than the rest of the surface because they are about 1000ºC cooler. If these spots could be seen in isolation they would bright and white as they still have a temperature of about 4000ºC.

The Sun can be observed by projection its image on to a screen or using a special solar filter that blocks most of the light and heat. Never look directly at the Sun especially using binoculars or a telescope without a solar filter fitted.

Sunspots imaged by SOHO on 8th September 2021

 

THE MOON PHASES DURING OCTOBER

New Moon will be on 6th October

First Quarter will be on 13th October

Full Moon will be on 20th October

Last Quarter will be on 28th October

 

OBSERVING JUPITER AND SATURN - OCTOBER 2021

Jupiter and Saturn at 22:00 on 15th October

Jupiter will look brilliant and interesting on any clear night but some nights there may be something special to see. The computer generated chart below shows Jupiter on 7 th October with Io in transit (passing in front of Jupiter) and its shadow projected on to the surface. Io starts its transit at 20:47 and reaches the mid-point of the transit at 21:49 when Io's shadow also reaches the edge of Jupiter. Io will finish the transit at 23:00. The shadow will leave the face of Jupiter two minutes after midnight.

Moon Io and shadow in transit on 7th October

Jupiter and Saturn can be seen using a smaller telescope of the sort that a beginner to astronomy might have been advised to buy as a first telescope by experienced astronomers. It will not be possible to see the moons when they are in transit (passing in front of Jupiter) because they will be hidden by the glare of the brightness of Jupiter. The moon shadow will appear black against Jupiter and will be quite easy to see so it can be tracked and timed as it crosses the face of Jupiter.

A planetarium application like Stellarium (free to download) can be used to search for these special events and the times can be predicted by the application. We must however ensure our computer time has been set accurately.

The movement of Jupiter's moons can be predicted using a Planetarium Application on a computer. We will be able to predict when a moon will pass in front (transit) or behind the planet (occultation). An accurate clock will also be required if we want to monitor the follow the times of these events.

Saturn is more difficult to observe so a high magnification must be used and on a good clear and still night when the view will be very rewarding. These charts show the sort of things that can be seen using telescopes but a better view will always be obtained when using a larger aperture telescope. A telescope with a longer effective focal length will also produce a larger image.

Saturn and its brightest moons

As Saturn is twice as far away as Jupiter it appears only half the diameter and quarter of the area. At twice the distance of Jupiter the amount of sunlight (per unit of area) reaching Saturn is a quarter of what Jupiter receives. Also the amount of light that we see reflected back from Saturn is also four times less that reflected by Jupiter. In total this means that Saturn not only appears much smaller but also receives and reflects less sunlight therefore appears 16 times fainter (4 x 4) than Jupiter. This equivalent to 3 magnitudes (2.512 x 2.512 x 2.512) = 15.85 times fainter. See the brief explanation about ‘Magnitudes' on related to Venus. This month the brightness of Jupiter will be about -2.6 and Saturn is about +0.5 a difference of 3.1 magnitudes. Click for more on Magnitudes.

So that is the bad news but this is compensated for by the magnificent ring system of Saturn and the ring does add a little to the brightness so it can be seen even using a smaller telescope.

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