May 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 May)

The Sun

At the start of the month, the Sun rises at 05:45 and sets at 21:00. By month's end, it rises at 05:00 and sets at 21:50.

The Planets

Mercury is at superior conjunction on the 27th and is not visible this month.

Venus is lost to the sun’s glare by month’s end. At the start of the month, it sets at 01:20, but by month’s end, sets at 22:35. It fades from mag -4.7 to mag -4.1 during the month.

Mars is Leo during the month and is visible as soon as darkness falls. It sets at 02:35 by month’s end and fades from mag +0.0 to mag +0.5 during the month.

Jupiter is at conjunction on the 13th and is not visible this month.

Saturn is in Virgo during the month and is visible as soon as darkness falls. It sets at 03:40 by month’s end and fades from mag +0.3 to mag +0.5 during the month.

Uranus is visible by month’s end when it rises at 03:00 in Pisces and is mag +5.9.

Neptune is in Aquarius during the month and is at western quadrature on the 23rd. It rises at 04:05 at the start of the month and by month’s end, it rises at 02:05. It maintains its brightness at mag +7.9 during the month.

The Moon

The full moon is on the 6th, with the last quarter moon on the 12th, the new moon on the 21st and the first quarter moon on the 28th.

On the evening of the 1st, the waxing gibbous moon lies to the SE of Mars at around 23:00.

On the evenings of the 3rd and 4th, the waxing gibbous moon lies near to Spica (Alpha (α) Virginis, mag +1.0) and Saturn. On the 3rd, it lies to the W of the two objects and on the 4th; it lies to the S of them. On both evenings, look at around 23:00.

On the mornings of the 7th and 8th, the waning gibbous moon lies near to Antares (Alpha (α) Scorpii, mag +1.0). On the 7th, it lies to the NW of the star and on the following night, 7° to the E of it. On both mornings, look at around 01:00.

On the mornings of the 13th and 14th, the waning crescent moon lies near to Neptune. On the 13th, it lies to the NW of the planet and on the 14th, to the NE of it. On both mornings, look at around 04:00.

On the evenings of the 22nd and 23rd, the waxing crescent moon lies near to Venus. On the 22nd, it lies to the SE of the planet and on the 23rd; it lies to the E of it. On both evenings, look at around 22:00.

On the evening of the 26th, the waxing crescent moon occults the mag +7 open cluster M67 in Cancer. The disappearance of the cluster behind the moon at around 21:00 is not visible; however the reappearance of the cluster should be visible at around 22:50.

On the evenings of the 27th-29th, the moon is in Leo. On the 27th, the waxing crescent moon lies to the S of Regulus (Alpha (α) Leonis, mag +1.4). On the following night, the first quarter moon lies to the SW of Mars and to the SE of Regulus (Alpha (α) Leonis, mag +1.4). Finally on the 29th, the waxing gibbous moon lies to the SE of Mars. On all three evenings, look at around 23:00.

On the evening of the 31st, the waxing gibbous moon lies 5° to the SW of Spica (Alpha (α) Virginis, mag +1.0) and to the SW of Saturn at around 23:00.

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 Eta Aquarids peak on the morning of the 5th. The ZHR predictions for this shower are around 65. However given that the radiant only rises at 05:00 in the morning twilight and will be very low, a much lower rate should be expected. The waxing gibbous moon in Virgo sets at 04:50, giving a short observing window before sunrise at 05:35. They are typically fast, bright meteors and are of a similar speed to the Perseids. Some will leave persistent trains.

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

Comet C/2009 P1 (Garradd) passes into Cancer from Lynx in May. It is visible as soon as darkness falls, heading South during the month, setting at 03:00 by month’s end. It is now past its peak and is expected to be mag +7, fading slowly.

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 Leo, we have several galaxies on view including The Leo Triplet - M65, M66 and NGC 3628. M95, M96 and M105 can also be observed in Leo. The place to really find galaxies is in Virgo. The Virgo Super Cluster can be found here with numerous galaxies on view. Also in Virgo, M104 - the Sombrero Galaxy can be found. In Coma Berenices, there is M64 - the Black-Eye Galaxy. Also 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. Finally there are some excellent open clusters in Cancer - M44 – The Beehive Cluster and M67.

General Notes

Always keep an eye out for Aurorae. The night sky does not get fully dark this month. 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 May. 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 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.