September 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 September)

The Sun

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.

The Planets

Mercury is visible at the start of the month, rising at 05:30. On the 1st, it is mag -1.4 and lies 1° N of Regulus (Alpha (α) Leonis, mag +1.4). It soon moves too close to the sun to be visible and is at superior conjunction on the 10th.

Venus is visible in the morning skies this month. At the start of the month, it rises at 02:25 and by month’s end rises at 03:25. It fades from mag -4.3 to mag -4.1 during the month. It passes within 3° to the S of M44 – The Beehive Cluster on the morning of the 13th and lies 4° to the W of Regulus (Alpha (α) Leonis, mag +1.4) at month’s end.

Mars is not visible this month.

Jupiter is visible in the evening sky by month’s end. At the start of the month, it rises at 23:10 and by month’s end rises at 21:25. It brightens from mag -2.3 to mag-2.5 during the month.

Saturn is not visible this month.

Uranus is visible as an evening object this month in Pisces and is at opposition on the 29th. It rises at 20:50 at the start of the month and during daylight hours by month’s end. At the time of opposition, it lies 8° to the SE of Omega (ω) Piscium (mag +4.0). On the night of the 22nd, it passes 1 arcminute from 44 Piscium (mag +5.7). It maintains its brightness at mag +5.7 during the month.

Neptune is visible as an evening object this month in Aquarius and is visible as soon as darkness falls during the month. It sets at 03:55 by month’s end and maintains its brightness at mag +7.8 during the month. It lies 1° to the E of 38 Aquarii (mag+5.4) which lies between Iota (ι) (mag +4.3) and Theta (θ) Aquarii (mag +4.2).

The Moon

The last quarter moon is on the 8th with the new moon on the 16th. The first quarter moon is on the 22nd with the full moon on the 30th.

On the evenings of the 1st, 2nd and 3rd the waning gibbous moon lies near to Uranus. On the 1st, it lies to the NW of the planet, then 8° N of it on the 2nd and to the E of it on the 3rd. On all evenings, look at around 22:00.

On the evening of the 6th, the waning gibbous moon lies to the SW of M45 – The Pleiades at around midnight.

On the evenings of the 7th and 8th, the moon lies near to Jupiter and Aldebaran (Alpha (α) Tauri, mag +0.9). On the 7th, the waning gibbous moon lies 6° to the NW of the planet and 8° to the N of the star. On the 8th, the last quarter moon lies to the SE of both objects. On both evenings, look at around midnight.

On the mornings of the 12th and 13th, the waning crescent moon lies near to Venus. On the 12th, it lies 7° to the W of the planet and on the 13th, to the S of it. On both mornings, look at around 04:00.

On the evening of the 26th, the waxing gibbous moon lies 8° to the NW of Neptune at around 22:00.

On the evening of the 29th, the waxing gibbous moon lies to the N of Uranus 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.

There are no major meteor showers visible this month.

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

Asteroids

Asteroid (11) Parthenope is at opposition on the 3rd at mag +9.0 in Aquarius. It moves SE during the month.

Asteroid (2) Pallas is at opposition on the 25th at mag +8.3 in Cetus. It moves S during the month and passes 15 arcminutes from Iota (ι) Ceti (mag +3.6) on the evenings of the 28th and 29th.

Finder charts are available at www.rasnz.org.nz in the source list below.

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. 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.

General Notes

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. 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.

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.