Sky notes for April 2012

jfSDAS

April 2012 sky notes

I came across a suggestion some time ago that the saying "March comes in like a lion, goes out like a lamb" may relate to Aries (the Ram) setting during the late evening at this time of year while Leo, the Lion, approaches the meridian (due south). It's an interesting observation though the true quote, author unknown, runs;

The March wind roars
Like a lion in the sky,
And makes us shiver
As he passes by.

When winds are soft,
And the days are warm and clear,
Just like a gentle lamb,
Then spring is here.

The planets
Jupiter and its flock of moons are being shepherded towards the lingering twilight earlier each evening during April and the planet will be more or less lost to view by the end of the month. The gas giant is slowly plodding across Aries and passes through solar conjunction in May, by which time it will near the celestial ram's border with Taurus. One of your last views of the planet before it is swallowed up in the solar glare may be on the evening of April 22nd. The Moon will be only 36 hours after New and Jupiter will lie less than three degrees below the slender lunar crescent. A clear and unobstructed western horizon is a must to catch them as the pair will be quite low.

Venus on the other hand still grabs the attention. It's brilliant light shines above the western skyline long after Jupiter has set. The planet glides past the beautiful Pleiades star cluster in Taurus on the evening of April 3rd and is close to the group for a couple of days either side of this date. The view in binoculars will be spectacular. Catalogued as M45 in Charles Messier's famous catalog (a guide to the deep sky's celestial showpieces), the cluster is also known as the Seven Sisters in Greek myth and lies about 425 light years distant.

After passing Jupiter on the 22nd the Moon will lie near Venus on April 24th. In a small telescope you'll see the phase of Venus is changing from a little under half illuminated at the beginning of the month to just 27% lit at the end of April. It will be interesting to contrast the growing lunar phase (waxing) against that of Venus which is waning.

Mars peaked in brightness last month when at opposition and its orange glow is immediately obvious under the prone "figure" of Leo the Lion. Just to the planet's right is Regulus, brightest star of Leo, which Mars creeps towards the first half of April. The Red Planet then does an about turn on the 15th and its motion is eastward (or direct) for the rest of the month. What is going on? Well, it's due to a phenomenon exhibited by all the outer planets known as retrograding. A simple analogy is that of an observer standing on the inside field of an athletics track watching a runner on the inside lane catching up on the bend and overtaking a slower participant on the outside lane.

The Earth, being closer to the sun than Mars and moving faster, catches up with and overtakes the planet. The effect we see is that of Mars moving westward, stopping, and then moving in the opposite direction over a period of time. Retrograding created all sorts of problems for early observers of the sky as they puzzled over its cause and led to them to introduce the infamous epicycles of Ptolemy's cosmology.

Summer is warming the northern hemisphere of Mars now and a telescope will show the north polar cap retreating. Wispy clouds float in the planet's atmosphere (a blue filter enhances the view of these) while dusky marking paint the surface. As the gap between us and Mars widens so the apparent diameter of its disk shrinks (to less than 10 arc-seconds by the end of April). Similarly, it fades in brightness from magnitude -0.7 to 0.0 in the same period. The almost full Moon lies close to Mars on the evening of April 3rd.

Saturn comes to opposition on April 15th when it will be visible throughout the hours the darkness and is best placed for viewing this year. Using the historic Grubb telescope at Dunsink Observatory recently we could see the butterscotch colour of Saturn's disk and the magnificent ring system tipped at a favourable angle. Titan, the planet's largest moon, was easily visible while closer in we were delighted to spot Rhea, Saturn's second brightest moon that has recently had a very tenuous oxygen and carbon dioxide atmosphere detected around it. Rhea is visible in small telescopes using a moderate magnification eyepiece. Saturn is currently in Virgo and to the unaided eye the planet has a yellowish tint. At magnitude 0.3 the Ring World is a little bit brighter than first magnitude Spica, the brightest star in Virgo -- the Full Moon is near both objects on the night of April 6th.

Mercury, Uranus, and Neptune are not visible this month.

The Moon
The Moon is Full on April 6th, at Last Quarter on the 13th, New on the 21st, and at First Quarter on April 29th. Easter Sunday this year is on April 8th and is generally taken as the first Sunday after the first Full Moon following the Spring Equinox. The computation is a little more complex than this but it’s a useful rule of thumb.

The Sun
A poster on Boards.ie recently enquired what were the dark spots they saw on the setting sun when they viewed it through binoculars. After the sound of jaws collectively dropping on hearing of such an irresponsible move died away the hapless observer was soon put right about the dangers of viewing the sun along with an admonition to see an optician as soon as possible.

In short, never, ever view the sun through any optical instrument unless it is suitably filtered -- and by this, I mean specialist astronomical filters, not those largely useless and dangerous eyepiece filters sold with some cheap telescopes. In fact, the ONLY safe way to view sunspots is to project the image on to a piece of card.

Although the sun may appear dimmed and safe enough to view when low down, it is not just the visible light that will harm your eyes but the sun's infrared radiation that we don't see. Prolonged viewing through an unfiltered instrument like binoculars will cause irreparable damage to your retina. A discussion on safe solar viewing has been penned by Prof. Ralph Chou, a respected world authority on eye safety and observing the sun. The article can be read at http://www.transitofvenus.org/june2012/eye-safety/280-viewing-the-transit-eye-safety

Minor Planets
No bright minor planets are well placed for observing this month.

Comets
Comet C/2009 P1 (Garradd) passes into Lynx from Ursa Major in April and is visible low in the northern sky as soon as darkness falls. The comet does gets higher in the sky though as the night goes on. It is now past its peak and is expected to be magnitude +7, fading slowly as the month progresses. Finder charts and further information about this comet and other fainter ones can be found at http://www.aerith.net or http://www.ast.cam.ac.uk/~jds/

Meteors
New Moon on the 21st means that the sky will be dark for the peak of the Lyrid meteor shower which is predicted for the morning of April 22nd at around 05:30. Rates are low to average normally with about 15 to 20 seen per hour given perfect observing conditions. In practice, the number seen is usually lower but the rate can be as high as 90 meteors per hour if the shower is in outburst as has happened in the past. The radiant, or point of origin in the sky when you trace the meteor trails back, is located near to the bright star Vega and is visible throughout the night. The radiant is very low in the sky however at the start of the evening and gets higher as the night progresses. Details of additional minor showers during April can be found at http://meteorshowersonline.com/calendar.html or http://www.imo.net/calendar/2012.


Morning passes of the International Space Station are currently taking place. The 3rd European Automated Transfer Vehicle (ATV) docked with the ISS recently (see http://en.wikipedia.org/wiki/Automated_Transfer_Vehicle). Predictions set for Terenure are linked to at http://www.heavens-above.com/PassSummary.aspx?satid=25544&lat=53.3&lng=-6.283333&loc=Terenure&alt=65&tz=GMT but your own site selection can be modified from the Heavens-Above front page.


Another aurora display was spotted on the night of March 27/28 from Ireland. Martin McKenna captured some stunning images of the lights and you can read more on his site at http://www.nightskyhunter.com


The following web sites are also a great resource for information about what’s on view in our skies during the month;

www.skymaps.com — superb charts you can download
www.jb.man.ac.uk/astronomy/nightsky/
www.guardian.co.uk/science/series/starwatch
www.telegraph.co.uk/news/newstopics/nightsky/
www.heavens-above.com — space station predictions
http://dcford.org.uk/index.php -- the Digital Astrolabe
http://earthsky.org/astronomy-essentials -- absolutely essential reading for what's up

www.jodcast.net — monthly night sky tour for MP3 players

www.spaceweather.com – details of transitory events during the month as well as aurora alerts

http://spaceflightnow.com — mission launches and other news (click “Launch Schedule” too)

www.skymania.com – astronomy and space news
www.universetoday.com – more astronomy and space news
www.nightskyobserver.com/the-sky-this-month.php -- Gary Nugent’s astronomy and space news

NUTZZ

Excellent post, very informative.

gkell2

jfSDAS wrote: »

Mars peaked in brightness last month when at opposition and its orange glow is immediately obvious under the prone "figure" of Leo the Lion. Just to the planet's right is Regulus, brightest star of Leo, which Mars creeps towards the first half of April. The Red Planet then does an about turn on the 15th and its motion is eastward (or direct) for the rest of the month. What is going on? Well, it's due to a phenomenon exhibited by all the outer planets known as retrograding. A simple analogy is that of an observer standing on the inside field of an athletics track watching a runner on the inside lane catching up on the bend and overtaking a slower participant on the outside lane.

The Earth, being closer to the sun than Mars and moving faster, catches up with and overtakes the planet. The effect we see is that of Mars moving westward, stopping, and then moving in the opposite direction over a period of time. Retrograding created all sorts of problems for early observers of the sky as they puzzled over its cause and led to them to introduce the infamous epicycles of Ptolemy's cosmology.

So,tell me after that fine description of the main Western argument for the Earth's orbital motion developed by Copernicus what you think of the technical non sequitur of Newton ? -

"For to the earth planetary motions appear sometimes direct, sometimes
stationary, nay, and sometimes retrograde. But from the sun they are
always seen direct,..." Newton


What you basically practice is homocentricity which is one step below geocentricity and one step away on the others side from astronomical oblivion.Here is what you do,when you watch your goto telescopes whir around their own individual axis as they track the stars in circumpolar motion,remember they are not following the rotation of the Earth but creating their own homocentric axis of rotation.You probably think the telescope's tracking/rotation is following the axis of rotation of the Earth but I assure you it does not.Unless you and your telescope are exactly at the North or South polar coordinates,you are watching homocentricity in action as the scope turns to keep track with circumpolar motion.You will figure it out .....eventually.

coylemj

Another good thread hijacked by this nutter. Will people please join me in reporting him and suggesting to mods that his posts are pure mumbo-jumbo and completely inappropriate in a science forum. I repeat, this is a science forum, I suspect this guy would even be run out of the Conspiracy Theories forum!

Otherwise we can just abandon this forum to the religious fantastists. That guy has only posted in this forum and in the 'Christianity' forum, specifically in a thread called 'The Bible, Creationism, and Prophecy (part 2)' - I rest my case.

gkell2

coylemj wrote: »

Another good thread hijacked by this nutter. Will people please join me in reporting him and suggesting to mods that his posts are pure mumbo-jumbo and completely inappropriate in a science forum. I repeat, this is a science forum, I suspect this guy would even be run out of the Conspiracy Theories forum!

http://www.youtube.com/watch?v=BYy0EQBnqHI

It is a simple question - does a goto telescope tracking a star in stellar circumpolar motion reflect the rotation of the Earth ?.

The answer is no,it does not while the Ra/Dec observing community imagines it does.You can enjoy the Ra/Dec homocentric convenience for what it is but you have to be an astronomer to know its limitations.

jfSDAS

many thanks for the kind words folks.

I guess the poster gkell2 is entitled pose a question or two but I was concerned if it became a to-and-fro discussion it could have diluted the original post, which was purely just to highlight what was happening in the sky this month.

Ideally, any topic arising out of the notes that merits lengthy debate should be on a separate thread. People are good enough judges to know when this is warranted. A separate thread created with its own subject line suggests a topic that may be interesting to read in its own right ... and also prevents alleged "hijacking"

john


I forgot to say that I also provide these notes to Carl O'Beirnes who posts them on the Balbriggan community site www.balbriggan.info (there's a scrolling "menu" on the right side of the home page and when "Balbriggan Observatory" appears click on this) -- Carl has been mooting the idea of an astronomy club in the Balbriggan and north county Dublin area.