December 2010 Observing Guide

Hi all,

December's guide is below. Enjoy, Merry Christmas and lets hope for a clear morning for the eclipse.

OBSERVING GUIDE
(NB: all times are UT and are based on the location of Belfast and covers December)

The Sun

At the start of the month, the Sun rises at 08:25 and sets at 16:05. By month’s end, it rises at 08:45 and sets at 16:05.

The Planets

Mercury may be visible as an evening object very low in the SW at the start of the month. It is mag -0.4 and sets at 17:00 at this time. It undergoes inferior conjunction on the 20th and is better placed for observation during the last week of the month as a morning object, low in the SE before sunrise. It rises at 07:35 on Christmas Day and at 07:00 by month’s end and is mag +0.1.

Venus is visible again as a morning object. It rises at around 04:35 during the month and fades from mag -4.9 to mag -4.7 during December.

Mars is not observable this month.

Jupiter continues to be well placed for observation this month, although by month’s end it sets at 23:20. It lies to the South of the Circlet asterism and is in Pisces, fading from mag -2.6 to mag -2.3 during the month. It is at Eastern Quadrature on the 17th and at month’s end, lies less than 1° to the S of Uranus. The South Equatorial Belt is starting to reappear in recent times; also its four Galilean moons are worth a look in a small telescope.

Saturn is visible as a morning object and rises at 03:00 at the start of the month and at 01:10 by month’s end. It maintains its brightness at mag +0.8 during the month and can be located in Virgo.

Uranus is well placed for observation this month, although by month’s end it sets at 23:25. It fades from mag +5.8 to mag +5.9 during the month, lying to the South of the Circlet asterism in Pisces. It is at Eastern Quadrature on the 19th and lies less than 1° to the N of Jupiter at month’s end. Don’t expect to see much detail - it will be like a green-blue star.

Neptune sets by 20:30 by month’s end and fades from mag +7.9 to mag +8.0 during the month. At the start of the month, it lies within 1° of Mu (μ) Capricorni (mag +5.1). It also lies to the NE of Deneb Algedi (Delta (δ) Capricorni, mag +2.9). You’ll see even less detail than on Uranus: It will appear like a faint bluish star.

The Moon

The new moon is on the 5th with the first quarter moon on the 13th and the full moon on the 21st. The last quarter moon is on the 28th.

There is a total lunar eclipse on the morning of the 21st. The moon starts to enter the umbra at 06:32 when it will be 20° above the horizon in the NW and is fully inside the Earth’s inner shadow by 07:40, 7° above the horizon. Mid-Eclipse is at 08:17 when the moon is in the N of the umbra and the S limb should appear darker than the N limb and it lies only 3° above the horizon. The brightening pre-dawn sky and the low altitude of the moon will add difficulty to observing the event, but take advantage if you can as the next lunar eclipse in which mid-totality will be visible from Ireland is not until 2015.

In addition, there may be a chance with this eclipse to observe a phenomenon called a Selenelion. This occurs when the sun and the eclipsed moon can be observed at the same time. This can only happen just before sunset or just after sunrise, and both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being referred to as a horizontal eclipse.

It happens during every lunar eclipse at all those places on the Earth where it is sunrise or sunset at the time. For this one, sunrise is at 08:44 and the moon sets at 08:55, so have a go at it.

On the morning of the 1st, the waning crescent moon lies 8° to the W of Saturn at around 06:00.

On the morning of the 2nd, the waning crescent moon lies 8° to the S of Spica (Alpha (α) Virginis, mag +1.0) and to the W of Venus at around 06:00.

On the evenings of the 10th and 11th, the waxing crescent moon lies near to Neptune. On the 10th, it lies to the SW of the planet and on the 11th; to its N.

On the evening of the 13th, the first quarter moon lies 6° to the W of Uranus and also to the NW of Jupiter at around 21:00.

On the evenings of the 18th and 19th, the waxing gibbous moon lies near to M45 –The Pleiades. On the 18th, it lies 4° to the SW of the cluster and on the following evening, lies to its E.

On the evening of the 20th, the waxing gibbous moon lies 8° to the W of M35. On the following night, the waning gibbous moon lies to the E of the cluster.

On the evening of the 22nd, the waning gibbous moon lies to the S of Pollux (Beta (β) Geminorum, mag +1.1) at around midnight.

On the evening of the 23rd, the waning gibbous moon lies 6° to the W of M44 –The Beehive Cluster at around 22:00.

On the nights of the 24th /25th and 25th/26th, the waning gibbous moon lies near to Regulus (Alpha (α) Leonis, mag +1.4). On the 24th, it lies to the NW of the star and on the following evening to its S.

On the morning of the 28th, the last quarter moon lies to the W of Saturn and to the NW of Spica (Alpha (α) Virginis, mag +1.0) at around 03:00.

On the morning of the 29th, the waning crescent moon lies 5° to the W of Spica (Alpha (α) Virginis, mag +1.0) and to the S of Saturn at around 04:00.

On the mornings of the 30th and 31st, the waning crescent moon lies near to Venus. On the 30th, it lies to its W and to its SW on the following morning. Look about 06:00 on both mornings.

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 Monocerotids peak on the 9th with a ZHR of 2. This is a minor shower with the radiant visible from 23:00. The moon is below the horizon so viewing conditions are perfect.

The Geminids peak on the morning of the 14th and have a ZHR of 120. The meteors are typically slow and can produce fireballs. The radiant is visible from 20:00 on the 13th; however any viewing of this shower will have to wait until the first quarter moon in Pisces sets around 00:55 on the 14th. After this viewing conditions will be perfect for this shower.

The Ursids peak on the 22nd with a ZHR of 10. The radiant is circumpolar, but the bright waning gibbous moon in Gemini will wash out this shower unfortunately.

Comets

Comet 103P/Hartley is now past its peak in December and starts the month in Puppis before heading W into Canis Major. It lies E of Sirius (Alpha (α) Canis Majoris, mag -1.5) during the month and is visible from about midnight at the start of the month. By month’s end, it is visible about 22:00. Current estimates put the comet at mag +6 and fading so it is likely to be beyond naked-eye visibility and that a visual aid i.e. binoculars/telescope will be needed to observe it.

Comet C/2010 V1 (Ikeya – Murakami) was only discovered last month by Japanese astronomers and is a morning comet. It is currently around mag +8 and fading. At the start of the month, it lies to the N of Venus. It moves SE during the month and heads out of Virgo into Libra and is visible from about 06:00 during the month.

Finder charts and further information about the above and other 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 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. In Auriga there are three open clusters M36, M37 and M38 and also M35 in Gemini. Taurus has the excellent Pleiades – M45, the Hyades and also M1 – The Crab Nebula. Orion returns to our skies with M42 – The Great Orion Nebula and also Cancer with M44 – The Beehive Cluster.

General Notes

Always keep an eye out for Aurorae. The winter solstice is on the 21st which sees the shortest day of the year and after this date the nights shorten and the days lengthen.

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;
Sky Guide 2010 – South Dublin Astronomical Society; Philip’s Stargazing 2010;
Patrick Moore’s 2010 Yearbook of Astronomy; www.heavens-above.com ; www.spaceweather.com ;
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.

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.