August 12 Observing Guide

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 21:25. By month's end, it rises at 06:30 and sets at 20:15.

The Planets

Mercury is visible during the second half of the month as a morning object. It is at greatest western elongation on the 16th. At the end of the month, it rises at 05:25 and is mag -1.3.

Venus is visible in the morning skies this month. It is at greatest western elongation on the 15th. At the start of the month, it rises at 02:20 and by month’s end rises at 02:20. It fades from mag -4.6 to mag -4.3 during the month.

Mars is in Virgo during the month and is visible as soon as darkness falls. It sets at 21:35 and is lost to the evening twilight by month’s end. It fades from mag +1.1 to mag +1.2 during the month. It passes near to Saturn on the evenings of the 13th and 14th.

Jupiter is visible in the morning sky this month. At the start of the month, it rises at 01:05 and by month’s end rises at 23:15. It brightens from mag -2.2 to mag-2.3 during the month.

Saturn is in Virgo during the month and is visible as soon as darkness falls. It sets at 21:40 and is lost to the evening twilight by month’s end. It maintains its brightness at mag +0.8 during the month.

Uranus is visible as a late night object this month in Pisces. It rises at 22:55 at the start of the month and at 20:55 by month’s end. It brightens from mag +5.8 to mag +5.7 during the month.

Neptune is visible as an evening object this month in Aquarius. It is at opposition on the 24th. It rises at 22:00 at the start of the month and is visible as soon as darkness falls by month’s end. It maintains its brightness at mag +7.8 during the month. It lies 4° to the S of Ancha (Theta (θ) Aquarii, mag +4.2) and 4° to the NE of Iota (ι) Aquarii (mag +4.3) at opposition.

The Moon

The full moon is on the 2nd and the 31st with the last quarter moon on the 9th. The new moon is on the 17th with the first quarter moon on the 24th.

On the evenings of the 2nd and 3rd, the moon lies near to Neptune. On the 2nd, the full moon lies to the NW of the planet and on the 3rd; the waning gibbous moon lies to the NE of the planet. On both evenings, look at around 23:00.

On the evenings of the 5th and 6th, the waning gibbous moon lies near to Uranus. On the 5th, it lies to the NW of the planet and on the 6th; it lies to the E of the planet. On both evenings, look at around midnight.

On the mornings of the 10th, 11th and 12th, the waning crescent moon is near to Taurus. On the 10th at 01:00, it lies to the W of M45 – The Pleiades. On the following morning, it lies to the S of the cluster at 02:00. At the same time, it lies to the NW of Jupiter and to the N of Aldebaran (Alpha (α) Tauri, mag +0.9). On the morning of the 12th, it lies 5° to the S of Jupiter and 6° to the E of Aldebaran around 02:00.

On the mornings of the 13th and 14th, the waning crescent moon lies near to Venus. On the 13th, it lies to the NW of the planet and on the following morning, it lies 7° to the SE of the planet. On both mornings, look around 04:00.

On the mornings of the 15th and 16th, the waning crescent moon lies near to Mercury. On the 15th, it lies to the NW of the planet and on the following morning, it lies to the SW of the planet. On both mornings, look at around 05:00 and MAKE SURE THAT THE SUN IS BELOW THE HORIZON.

On the evenings of the 21st and 22nd, the waxing crescent moon lies near to Virgo. On the 21st, it lies to the S of Saturn, 5° to the S of Spica (Alpha (α) Virginis, mag +1.0) and to the SW of Mars. On the following evening, it lies 8° to the E of Mars and to the E of Saturn and Spica. On both evenings, look at around 21:00.

On the evening of the 24th, the first quarter moon lies 8° to the NW of Antares (Alpha (α) Scorpii, mag +1.0) at around 21:00.

On the evening of the 30th, the waxing gibbous moon lies to the N of Neptune at around 22:00.

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 between 13:00 and 15:30 on the 12th with a ZHR of around 100. The nights of the 11th/12th and 12th/13th are the times to observe this shower. The radiant is visible as soon as darkness falls. Lunar conditions are relatively good with the moon in its waning crescent phase. On the morning of the 12th, it rises at 00:35 in Taurus and on the following morning, it rises at 01:25 in Auriga.

There are additional minor showers this month, details of which can be found at meteorshowersonline.com/calendar.html or www.imo.net/calendar/2012

Comets

There are no bright comets this month.

Finder charts and further information about the above comet and other fainter comets can be found at www.aerith.net , cometchasing.skyhound.com , www.ast.cam.ac.uk/%7Ejds/ , kometen.fg-vds.de/fgk_hpe.htm and www.rasnz.org.nz in the source list below.

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. Between May and the middle of August, Astronomical twilight is present at night. This is when the sun is between twelve and eighteen degrees below the horizon. This time of year is very good for observing the numerous satellites and other objects in orbit above us.

Watch out for NLCs - Noctilucent Clouds during August. They are mostly visible between the Northern latitudes of 50 to 65 degrees, however sightings have been made as far South as Iran – 38 degrees. Look to the North-West for a white/silvery glow with the best times between 22:30 and Midnight and to the North-East between 02:00 and 03:30. They can sometimes be faint, sometimes bright. 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.

The 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 Used and Links

www.skyviewcafe.com ; Sky at Night Magazine Observing Guide and CD; www.aerith.net ; cometchasing.skyhound.com ; www.ast.cam.ac.uk/%7Ejds/ ; kometen.fg-vds.de/fgk_hpe.htm ; www.rasnz.org.nz ; Stardust Magazine;
Philip's Stargazing 2012; Patrick Moore's 2012 Yearbook of Astronomy; www.heavens-above.com ; www.spaceweather.com ; meteorshowersonline.com/calendar.html ; www.timeanddate.com/astronomy/ ; www.imo.net/calendar/2012 - International Meteor Organisation; seds.org/messier/ - The Messier Catalogue website; www.seds.org/messier/xtra/ngc/ngc.html - NGC Catalogue website; www.irishastronomy.org - Irish Federation of Astronomy Societies Website; irishastro.org.uk - Irish Astronomical Association website; www.eaas.co.uk - Northern Ireland Amateur Astronomy Society;

Appendix

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.

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 ° symbol in the guide is that for degrees. A degree is two full moon widths to give an idea for judging any distances quoted in the guide. There are 60 arcminutes in a degree.

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.

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.

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 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 its at its furthest point west from the sun as seen from Earth and visible in the morning sky in the East before sunset. 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.

Clear Skies

Neill