Monthly Sky Guide
OBSERVING GUIDE by Neill McKeown
(Please note all times are ST and are based on an observing location of Belfast and covers the month of September)
At the start of the month, the Sun rises at 06:30 and sets at 20:15. By month’s end, it rises at 07:25 and sets at 19:00.
Mercury is at greatest eastern elongation on the 14th, but is not visible this month.
Venus is visible very low in the evening sky, moving from Virgo to Libra during the month. At the start of the month, it sets at 21:05 and by month’s end, it sets at 19:55. It brightens from mag -3.9 to mag -4.1 during the month.
Mars is not visible this month.
Jupiter is visible in the evening sky in Capricornus during the month. During the month, it is visible as soon as darkness falls and by month’s end, it sets at 03:00. It fades from mag -2.7 to mag -2.6 during the month.
Saturn is visible in the evening sky in Capricornus during the month. During the month, it is visible as soon as darkness falls and by month’s end, it sets at 01:30. It fades from mag +0.3 to mag +0.5 during the month.
Uranus is visible in the evening sky in Aries during the month. At the start of the month, it rises at 21:55 and by month’s end, it rises at 20:00. It maintains its brightness at mag +5.7 during the month.
Neptune is at opposition on the 14th and is visible in the evening sky in Aquarius during the month. At the start of the month, it rises at 20:35 and by month’s end, it is visible as soon as darkness falls. It maintains its brightness at mag +7.8 during the month. It lies to the East of Phi (φ) Aquarii, mag +4.2.
The new moon is on the 7th (01:52) with the first quarter moon on the 13th (21:39). The full moon is on the 21st (00:55) with the last quarter moon on the 29th (02:57)
16th pm the 81% waxing gibbous lies SW of Saturn at 21:00.
17th pm the 89% waxing gibbous lies SE of Saturn and SW of Jupiter at 21:00.
18th pm the 95% waxing gibbous lies SE of Jupiter at 21:00.
20th pm the near full moon lies SE of Neptune at 21:00.
24th pm the 86% waning gibbous lies SE of Uranus at 22:00.
25th pm the 78% waning gibbous lies SW of M45 – The Pleiades at 22:00.
26th pm the 70% waning gibbous lies NE of Aldebaran (Alpha (α) Tauri, mag +0.9) at 23:00.
The best time to observe meteor showers is when the moon is below the horizon; otherwise its bright glare limits the number you will see especially the fainter ones. Below is a guide to this month’s showers.
There are no major meteor showers this month.
There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section. The ZHR or Zenithal Hourly Rate is the number of meteors an observer would see in one hour under a clear, dark sky with a limiting apparent magnitude of 6.5 and if the radiant of the shower were in the zenith. The rate that can effectively be seen is nearly always lower and decreases as the radiant is closer to the horizon. The Zenith is the overhead point in the sky.
Asteroid (2) Pallas is at opposition on the evening of the 11th at mag +8.5. It can be seen in Pisces and is visible as soon as darkness falls.
Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.
There are no bright comets visible this month.
Finder charts and further information about the above and other fainter comets can be found in the below Information Sources and Links Section. Any of the above estimates are based on current information at the time of writing the guide and can be wrong – “Comets are like cats; they have tails, and they do precisely what they want”, David H Levy. “If you want to have a safe gamble, bet on a horse – not a comet”, Dr Fred Whipple.
On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. In Hercules, two globular clusters – M92 and the excellent M13 can be observed and in Lyra – M57 – The Ring Nebula can be observed. In Vulpecula – M27 – The Dumbbell Nebula can be found. In Andromeda, M31 – The Andromeda galaxy can be observed along with its satellite galaxies M32 and M110. In Perseus, there is the open cluster M34 and the excellent Double Cluster – NGC 869 and 884. In Triangulum, there is the galaxy M33. Finally Auriga is reappearing with its three open clusters M36, M37 and M38 as is Taurus with the excellent Pleiades – M45 and the Hyades.
Always keep an eye out for Aurorae. The autumn equinox is on the 22nd, which sees the end of summer and the beginning of autumn. This is the day where the length of day and night is the same and after this, the night will take over cumulating with the shortest day of the year on the winter solstice in December. Other interesting naked eye phenomena to look out for include the Zodiacal Light and the Gegenschein. Both are caused by sunlight reflecting off dust particles, which are present in the solar system.
Zodiacal Light can be seen in the West after evening twilight has disappeared or in the East before the morning twilight. The best time of year to see the phenomenon is late-Feb to early-April in the evening sky and September/October in the morning sky – it is then that the ecliptic, along which the cone of the zodiacal light lies, is steepest in our skies. The Gegenschein can be seen in the area of the sky opposite the sun. To view either, you must get yourself to a very dark site to cut out the light pollution. When trying to observe either of these phenomena, it is best to do so when the moon is below the horizon. A new appendix has been added explaining some of the more technical terms used in the guide.
Information Sources and Links
Sky at Night Magazine Observing Guide – All Rounder
Stardust Magazine – All Rounder
https://in-the-sky.org/– All Rounder
http://www.nightskyhunter.com/ – All Rounder
Philip’s Stargazing 2020 – All Rounder
Collins 2020 Guide to the Night Sky – All Rounder
www.heavens-above.com – All Rounder
Sky Safari App – All Rounder
Stellarium App – All Rounder
http://www.timeanddate.com/astronomy/ – All Rounder
www.irishastronomy.org – Irish Federation of Astronomy Societies Website and Calendar – All Rounder
http://irishastro.org.uk/– Irish Astronomical Association website – All Rounder
www.niaas.co.uk – Northern Ireland Amateur Astronomy Society – All Rounder
http://eco.mtk.nao.ac.jp/cgi-bin/koyomi/cande/phenomena_en.cgi – Sun/Planets/Moon Only
International Meteor Organisation – https://www.imo.net/files/meteor-shower/cal2020.pdf – Meteors Only
http://britastro.org/computing/charts_asteroid.html – Asteroids Only
www.cobs.si – Comet Observation Database
www.aerith.net – Comets Only
www.ast.cam.ac.uk/%7Ejds/ – Comets Only
http://astro.vanbuitenen.nl – Comets Only
http://theskylive.com/ – Comets/Asteroids
http://messier.seds.org/ – The Messier Catalogue website – Deep Sky Only
www.spaceweather.com – Aurorae Forecasts/Naked Eye Atmospherics
The radiant is the point in the sky, from which (to a planetary observer) meteors appear to originate, i.e. the Perseids, for example, are meteors which appear to come from a point within the constellation of Perseus. When the radiant is quoted as “circumpolar”, it is never below the horizon and visible all night, otherwise the times quoted are when the constellation in which the radiant lies rises above the horizon in the East.
A fireball is defined by the International Astronomical Union as a meteor brighter than any of the planets, i.e. magnitude -4 or brighter. The International Meteor Organisation alternatively defines it as a meteor which would have a magnitude of -3 or brighter at the zenith.
The full moon’s width when viewed from the Earth is 30 arc minutes or ½ a degree. This should give an idea for judging any distances quoted in the guide.
An asterism is a collection of stars seen in Earth’s sky which form simple patterns which are easy to identify, i.e. the Big Dipper. They can be formed from stars within the same constellation or by stars from more than one constellation. Like the constellations, they are a line of sight phenomenon and the stars whilst visible in the same general direction, are not physically related and are often at significantly different distances from Earth.
A conjunction is when two objects appear to be close to each other in the sky according to the perspective of the observer.
Mag is short for magnitude which is the measure of an object’s brightness. The smaller the number, the brighter the object. The brightest object in the sky is the Sun at mag -26, the full moon is mag -12 and Venus the brightest planet is mag -4. The brightest stars are mag -1. If there is a 1 mag difference between two objects – there is a difference in brightness of a factor of 2.5 between the two objects. For example the full moon is eight magnitudes brighter than Venus on average which means it is 1,526 times brighter than Venus. Objects down to mag +6 can be seen with the naked eye under very dark skies.
Local time is always quoted in the guide and this means for November – February – universal time (UT)/GMT is used and for April to September – daylight savings time (DST, = GMT+1). For the months of March and October when the clocks go forward/back respectively, both times will be used and attention should be paid to any times at the end of these months for that change.
Deep Sky Objects such as galaxies, nebulae and star clusters are classified in catalogues such as the Messier catalogue for objects like M44 – M for Messier. Another example of a catalogue would the New General catalogue whose objects have the prefix NGC. There are links for websites to both catalogues in the section above.
Perihelion is the point in the orbit of a planet, asteroid or comet where it is at the nearest point in its orbit to the sun. It is the opposite of Aphelion, which is when the object is at the farthest point in its orbit from the sun. For the earth, the comparative terms used are perigee and apogee and for the moon, pericynthion and apocynthion are sometimes used.
From Earth – Mercury and Venus are the inner planets in the solar system and Mars, Jupiter, Saturn, Uranus and Neptune are the outer planets. Below is a short guide as to how both the inner and outer planets move around the sun. The above pictorial guide should hopefully help in this.
The Inner Planets
These are best seen when at Greatest Eastern/Western elongation and are not visible when at either Inferior/Superior conjunction. Greatest Eastern elongation is when the inner planet is at its furthest point east from the sun as seen from Earth and visible in the evening sky in the West after sunset, Western elongation is when it’s at its furthest point west from the sun as seen from Earth and visible in the morning sky in the East before sunrise. Inferior conjunction occurs when the inner planet is between the Sun and the Earth. Superior conjunction occurs when the inner planet is on the other side of the Sun as seen from Earth.
From our Northerly latitudes, the ecliptic, along which the planets move, lies at a very shallow angle to the horizon after sunset in the autumn and before sunrise in the spring. This means that any of the planets will be difficult to see when fairly close to the Sun in the evening sky in the autumn or in the morning sky in the spring. In particular, Mercury is more or less invisible from here when at Eastern elongation in the autumn or at Western elongation in the spring, because it lies so close to the horizon and is never above the horizon except in daylight or bright twilight.
The normal cycle for an inner planet is Superior Conjunction – Greatest Eastern Elongation – Inferior Conjunction – Greatest Western Elongation – Superior Conjunction. After superior conjunction, the planet moves away from the Sun as seen from Earth and becomes visible in the evening sky after a period of time. It then moves past the point of Greatest Eastern Elongation and moves back towards the Sun as seen from Earth until a point when it is not visible and at Inferior Conjunction. After this the planet appears in the morning sky for a time, before again slipping into the Sun’s glare as seen from Earth. The duration of this cycle will depend on the planet’s closeness to the Sun, i.e. Mercury completes the above cycle in around 4 months.
The Outer Planets
These are best seen when at opposition and are not visible when at conjunction. Opposition occurs when the earth is between the sun and the outer planet. It is the best time to observe them because the planet is visible all through the night and it is due south and at its highest at about midnight. The planet is also at its closest point in its orbit to Earth – making it appear brighter. Conjunction occurs when the outer planet is on the other side of the Sun as seen from Earth.
If the planet is at or near it furthest point south along the ecliptic, then it won’t get very high in the sky even at opposition – just as the Sun never gets high in the sky in midwinter. This happens when opposition occurs near midsummer when the planet is opposite the Sun in the sky and in midsummer the Sun is high, so the planet will be low. The opposite of course applies in winter.
The normal cycle for an outer planet is Conjunction – Western Quadrature – Opposition – Eastern Quadrature – Conjunction. After conjunction, the planet moves away from the Sun as seen from Earth and becomes visible again. The planet from this point on rises earlier and earlier in the morning sky and eventually becomes visible in the evening sky. At Western Quadrature it is at its highest at sunrise and by opposition it is in the same position by midnight. By Eastern Quadrature, it is past its best and is at its highest at sunset, meaning it is rising in daytime and setting earlier and earlier until a point when it sets too close to the Sun as seen from Earth and is no longer visible. The duration of this cycle will depend on the planet’s closeness to the Sun, i.e. Jupiter completes the above cycle in around 13-14 months.