April 2012 Sky Guide PDF Print E-mail

This page is dedicated to the practical observer including :

Observing highlights
By Neill McKeown

Have a look for these objects below and let us know about them on the NIAAS Forum

OBSERVING GUIDE

(Please note all times are ST unless otherwise stated and are based on an observing location of Belfast and covers the month of April) 

The Sun

At the start of the month, the Sun rises at 06:55 and sets at 20:00. By month's end, it rises at 05:50 and sets at 20:55.

 

The Planets

Mercury is at greatest western elongation on the 18th, but is not visible this month as the ecliptic, along which the planets move, lies at a very shallow angle to the horizon before sunrise in the spring.

Venus is visible in the evening sky during the month. At the start of the month, it sets at 00:50 and by month's end sets at 01:20. It brightens from mag -4.5 to mag -4.7 during the month. Between the 1st and 4th, it lies close to M45 – The Pleiades. On the evening of the 1st, it lies 3° to the S of the cluster, by the following evening, the gap is 1°. On the evening of the 3rd, it lies at the SE edge of it and is 1° to the NE of it by the following evening.

Mars is Leo during the month and is visible as soon as darkness falls. It sets at 04:30 by month's end and fades from mag -0.7 to mag -0.1 during the month.

Jupiter is an evening object moving from Cetus into Aries during the month. By month's end, it sets at 21:45 and is lost to the sun's glare. It fades from mag -2.1 to mag -2.0 during the month.

Saturn is at opposition on the 15th and is an evening object this month in Virgo. At the start of the month, it rises at 21:10 and by month's end, it rises during daylight hours. It maintains it brightness at mag +0.3 during the month.

Uranus is not visible this month.

Neptune is not visible this month.

 

The Moon

The full moon is on the 6th, with the last quarter moon on the 13th, the new moon on the 21st and the first quarter moon on the 29th.

On the evening of the 1st, the waxing gibbous moon lies to the W of M44 – The Beehive Cluster at around 22:00.

On the evening of the 3rd, the waxing gibbous moon lies to the SW of Mars and to the S of Regulus (Alpha (α) Leonis, mag +1.4) at around 22:00.

On the evening of the 6th, the full moon lies 5° to the W of Spica (Alpha (α) Virginis, mag +1.0) and to the W of Saturn at around 23:00.

On the morning of the 10th, the waning gibbous moon lies to the N of Antares (Alpha (α) Scorpii, mag +1.0) at around 03:00.

On the evening of the 22nd, the waxing crescent moon lies 4° to the N of Jupiter at around 21:00.

On the evening of the 23rd, the waxing crescent moon lies 7° to the SE of M45 – The Pleiades at around 22:00.

On the evening of the 24th, the waxing crescent moon lies to the S of Venus and to the N of Aldebaran (Alpha (α) Tauri, mag +0.9) at around 22:00.

On the evening of the 25th, the waxing crescent moon lies to the E of Venus at around 23:00.

On the evening of the 30th, the waxing gibbous moon lies to the S of Regulus (Alpha (α) Leonis, mag +1.4) and to the SW of Mars 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 Lyrids peak on the morning of the 22nd at around 05:30 with a predicted ZHR of 18, however the rate can be as high as 90 depending on the year. The radiant is located near to Vega and is circumpolar, although it is very low in the sky at the start of the evening and gets higher as the night goes on. There is no lunar interference, so this is a good year for this shower. They are typically average speed meteors with rates of 30 miles/second. This is in comparison with the Perseids who have speeds of 42 miles/second.

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

 

Comets

Comet C/2009 P1 (Garradd) passes into Lynx from Ursa Major in April. It is visible as soon as darkness falls, low in the N. It gets higher in the sky as the night goes on. 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 , http://cometchasing.skyhound.com , http://www.ast.cam.ac.uk/%7Ejds/ , http://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. 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

www.skyviewcafe.com - Used for the Sun and Planets section. Also partly used for the Moon Section 

Sky at Night Magazine Observing Guide and CD

www.aerith.net and http://cometchasing.skyhound.com – Used for the Comet Section for information and finder charts 

http://www.ast.cam.ac.uk/%7Ejds/ - BAA and SPA Comet page

http://kometen.fg-vds.de/fgk_hpe.htm - German Comet page

http://www.rasnz.org.nz – Royal Astronomical Society of New Zealand website – good for asteroids

BAA Handbook

Stardust Magazine

Sky Guide 2010 – South Dublin Astronomical Society

www.heavens-above.com – For the latest ISS passes, Iridium Flares and Shuttle launches

www.irishastronomy.org – Irish Federation of Astronomy Societies Website

www.stronge.org.uk – Excellent weather site including Space Weather

www.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.


ISS Passes

For detailed sightings information and sky charts, check out Heavens Above or Calsky .

Iridium Flares

Every night there are a number of Iridium flares of varying brightness, some can be up to Mag-8 which are very impressive to watch. Check out Heavens Above or Calsky for the latest predictions.