August 2021 Observing Guide

Hi all,

What to look out for the night sky in August? We have the return of dark skies and Jupiter and Saturn are at opposition which means they are well placed to be spotted. In addition we have a good year for the Perseid Meteor Shower which peaks on the night of Thursday 12th/Friday 13th. The waxing crescent moon sets at around 10:45pm that night with dark skies (end of Astronomical twilight) from around Midnight.  

OBSERVING GUIDE
(Please note all times are ST and are based on an observing location of Belfast and covers the month of August)

The Sun

At the start of the month, the Sun rises at 05:35 and sets at 21:20. By month's end, it rises at 06:30 and sets at 20:15. 

The Planets 

Regular Stuff 

Mercury is at superior conjunction on the 1st and is not visible this month.

Venus is visible very low in the evening sky, moving from Leo to Virgo during the month. At the start of the month, it sets at 22:25 and by month’s end it sets at 21:05. It brightens from mag -3.8 to mag -3.9 during the month.

Mars is visible very low in the evening sky at the start of the month in Leo when it sets at 22:05 and is mag +1.8. It is not visible after mid-month.

Jupiter is at opposition on the 20th and is visible in the evening sky, moving from Aquarius to Capricornus during the month. At the start of the month, it rises at 22:05 and by month’s end, it is visible as soon as darkness falls. It maintains its brightness at mag -2.7 during the month. 

Saturn is at opposition on the 2nd and is visible in the evening sky in Capricornus during the month. At the start of the month, it rises at 21:25 and by month’s end, it is visible as soon as darkness falls. It fades from mag +0.2 to mag +0.3 during the month. 

Uranus is at western quadrature on the 6th and becomes visible in the evening sky in Aries by month’s end. At the start of the month, it rises at 23:55 and by month’s end it rises at 22:00. It brightens from mag +5.8 to mag +5.7 during the month.

Neptune becomes visible in the evening sky in Aquarius by month’s end. At the start of the month, it rises at 22:40 and by month’s end it rises at 20:40. It maintains its brightness at mag +7.8 during the month. 

The Moon

The new moon is on the 8th (14:50) with the first quarter moon on the 15th (16:20). The full moon is on the 22nd (13:02) with the last quarter moon on the 30th (08:13). 

Regular Stuff 

1st am the 46% waning crescent lies S of Uranus at 01:00. 

2nd am the 36% waning crescent lies SW of M45 – The Pleiades at 01:00. 

3rd am the 27% waning crescent lies SE of M45 – The Pleiades and N of Aldebaran (Alpha (α) Tauri, mag +0.9) at 01:00. 

10th pm the 6% waxing crescent lies NW of Venus at 22:00. 

11th pm the 12% waxing crescent lies NE of Venus at 22:00.   

12th pm the 21% waxing crescent lies NW of Spica (Alpha (α) Virginis, mag +1.0) at 22:00. 

13th pm the 30% waxing crescent lies NE of Spica (Alpha (α) Virginis, mag +1.0) at 22:00. 

16th pm the 64% waxing gibbous lies N of Antares (Alpha (α) Scorpii, mag +0.9) at 22:00. 

20th pm the 97% waxing gibbous lies S of Saturn at 23:00. 

21st pm the 99% waxing gibbous lies SW of Jupiter at 23:00. 

22nd pm the just past full moon lies SE of Jupiter at 23:00. 

23rd pm the 97% waning gibbous lies SW of Neptune at 23:00. 

24th pm the 93% waning gibbous lies SE of Neptune at 23:00. 

27th pm the 72% waning gibbous lies W of Uranus at 23:00. 

28th pm the 63% waning gibbous lies SE of Uranus and SW of M45 – The Pleiades at 23:00. 

30th am the 53% waning gibbous lies N of Aldebaran (Alpha (α) Tauri, mag +0.9) and S of M45 – The Pleiades at Midnight. 

31st am the 44% waning crescent lies NE of Aldebaran (Alpha (α) Tauri, mag +0.9) at Midnight. 

Meteors

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. 

The Perseids peak on the night of the 12th/13th with a ZHR of 110. The radiant is visible as soon as darkness falls on the evening of the 12th, lunar conditions are good for 2021 with the waxing crescent moon setting at 22:45 on the 12th.  

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. 

Asteroids

Asteroid (349) Dembowska is at opposition on the evening of the 12th at mag +9.8. It can be seen in Piscis Austrinus low in the South and is visible from midnight. 

Asteroid (43) Ariadne is at opposition on the evening of the 19th at mag +9.6. It can be seen in Aquarius and is visible from midnight. 

Asteroid (89) Julia is at opposition on the evening of the 22nd at mag +8.9. It can be seen in Aquarius and is visible from midnight. 

Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.

Comets

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. 

Deep Sky

On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. Check out the constellation Canes Venatici with the globular cluster - M3 and several galaxies including M51 - the Whirlpool Galaxy and M63 - the Sunflower Galaxy. 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. Finally in Triangulum, there is the galaxy M33.

General Notes

Always keep an eye out for Aurorae. We have the return to dark skies in August with Astronomical twilight no longer dominating the night. 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's 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.

Clear Skies

Neill McKeown

Information Sources and Links

Sky at Night Magazine Observing Guide – All RounderStardust Magazine – All Rounder
in-the-sky.org/– All Rounder
www.nightskyhunter.com/ - All Rounder
Philip's Stargazing 2021 – All Rounder
Collins 2021 Guide to the Night Sky – All Rounder
Night Sky Almanac: A Stargazers Guide to 2021 – All Rounder
Yearbook of Astronomy 2021 – All Rounder
www.heavens-above.com – All Rounder
Sky Safari App – All Rounder
Stellarium App – All Rounder
www.timeanddate.com/astronomy/ - All Rounder
www.irishastronomy.org  - Irish Federation of Astronomy Societies Website and Calendar – All Rounder
irishastro.org.uk/- Irish Astronomical Association website – All Rounder
www.eaas.co.uk  - Northern Ireland Amateur Astronomy Society – All Rounder
eco.mtk.nao.ac.jp/cgi-bin/koyomi/cande/phenomena_en.cgi – Sun/Planets/Moon OnlyInternational Meteor Organisation -
www.imo.net/files/meteor-shower/cal2021.pdf - Meteors Only
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
astro.vanbuitenen.nl – Comets Only
theskylive.com/ - Comets/Asteroids
messier.seds.org/ - The Messier Catalogue website – Deep Sky Only
www.spaceweather.com – Aurorae Forecasts/Naked Eye Atmospherics Appendix

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.  

The Planets

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 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.