Skynotes March and April 2006

From South Dublin Astronomical Society

Current moon phase. Image from USNO.

These sky notes are updated every two months and give you the low down on what's up in the skies during the period. If there is any aspect of the notes you do not understand, or would like more comprehensive details of a particular event, then feel free to contact the author, John Flannery, at .

Links to web sites where you will find more detailed information about particular phenomena are highlighted at the end of this page. An archive of previous months skynotes is availble.

Contents 1 Sky Notes for March and April 2006 1.1 Total Solar Eclipse 1.1.1 Eclipse Safety 1.2 Easter Sunday and Clock Changes 1.3 The Moon 1.3.1 Lunar Occultations 1.4 Eclipse of the Moon 1.5 The Planets 1.6 Asteroids 1.7 Meteors 1.8 Comets 1.9 Websites where you can get more detailed information

Sky Notes for March and April 2006

Spring in the northern hemisphere officially begins with the Spring Equinox on March 20th at 18h. To the ancient Celts the equinox marked the victory of the Sun God Bran over the forces of darkness with the Sun’s rays beginning to bring warmth and a burst of new life to the northern climes.

Total Solar Eclipse

Darkness of a different kind will be eagerly awaited by astronomical tourists on March 29th when the Moon’s shadow sweeps across northern Africa and through Turkey giving us a total eclipse of the Sun.

Outside of this narrow swathe we’ll see a partial eclipse with about 18% of the Sun covered from the Midlands during mid-eclipse at 10:28am. The first views of the Moon’s silhouette nicking the edge of the Sun will be had at 9:49am with the event completely over by 11:06am.

Eclipse Safety

Under no circumstances should you look directly at the Sun through any kind of optical instrument. To do so risks instant blindness. Many cheap telescopes come with a so-called sun filter. Often they are nothing more than pieces of smoked glass and are generally designed to fit over the eyepiece of the telescope. This does nothing to dim the full power of the sun and the filter can shatter without warning because of the intense heat concentrated at the eyepiece, driving shards of glass into your eyes. Neither do they filter out the more harmful invisible solar rays. Such filters are therefore NOT SAFE and should be immediately discarded.

A more practical method is to project the Sun’s image on to a piece of white card. Care should be used in case someone tries to take a quick glance through the eyepiece. Some telescopes may not be suited to using the projection method; one potential problem may be where the lens cement of eyepieces could melt. If in doubt, check with your local astronomical society.

Easter Sunday and Clock Changes

Easter Sunday falls this year on April 16th, about the latest possible date for the feast day (April 18th being the latest). Summer Time begins on March 26th when clocks go forward one hour. Astronomers use Universal Time (Winter Time) all year round however when stating the time a celestial event occurs.

The Moon

The course of the Moon through the sky during 2006 almost mirrors the sine wave of the ecliptic (the plane of the Earth’s orbit traced in the sky and the path which the Sun appears to follow during the year). The ecliptic is inclined 23.5° to the celestial equator and this coupled with the Moon’s slightly tilted orbit about the Earth — an angle of five degrees — means the attentive observer will notice some unusual characteristics to the Moon’s appearance this year.

Our natural satellite is at its maximum southerly declination on the celestial sphere on March 22nd when at Last Quarter and so will appear quite low above your local horizon when on the meridian. Indeed, it has been rather low the last couple of months and will continue to be so right through to May. Half an orbit later, the most northerly moonrise (measured in azimuth on the horizon) of April 4th means the Moon traces out its largest arc across the sky for the year. The First Quarter Moon after sunset that evening will appear unusually high. The phenomenon is known as a major lunar standstill. Later in September we will have a repeat event except this time it’s the most southerly moonrise of the year, leading to the Full Moon appearing quite low as it travels across the sky. Because this will be close to Harvest Moon – and the closest Full Moon of the year – expect September’s Moon to appear huge! We’ll have more details about that in the next few months.

A particularly pretty sight on the evening of April 1st will see the three day old Moon breathtakingly close to the beautiful Pleiades star cluster in Taurus. The Moon will actually occult, or pass across and briefly hide, the stars of the cluster for observers in North America. A similar conjunction between the Moon and the Pleiades takes place on the evening of March 5th but the glare from the six day old Moon will interfere somewhat.

Lunar Occultations

Lunar Occultations
Time	Star	Mag.	Ph.	P.A.°
Mar 1 18h 03m 19s	PPM 143346	6.2	d	15
Mar 5 19 05 52	HD 24802	6.2	D	106
Mar 9 19 43 21	76 Gem	5.3s	D	143
Mar 9 20 47 12	76 Gem	5.3s	r	245
Mar 17 02 32 09	PPM 196621	6.3	R	331
Apr 4 21 28 00	49 Aur	5.3s	D	80
Apr 4 22 30 31	49 Aur	5.3s	r	304
Apr 6 19 28 01	Lambda Cnc	5.9	D	80
Apr 11 03 35 02	89 Leo	5.8	D	95

Occultations listed are for Dublin-based observers. That is not to say a particular occultation will be unobservable from other parts of Ireland; rather, the time quoted is when Dublin-based observers will note the object’s disappearance (D) or reappearance (R). The magnitude (Mag.) and position angle (P.A.°) are also listed. The position angle is defined as the angle between the lunar north pole, the centre of the Moon and the position of the star at the moment of occultation. The angle increases eastward and is expressed in degrees.

Eclipse of the Moon

There is a penumbral eclipse of the Moon on the evening of March 14th. This is not a total lunar eclipse but occurs when the Moon just dips into the outer edges (penumbra) of the Earth’s shadow cast in space.

Generally, the magnitude of a penumbral eclipse has to be greater than 0.7 for an observer to notice a dimming of the lunar disk. The Moon is fully immersed in the penumbra this time round and at mid-eclipse (23h 48m) you should see a slight darkening of the southern hemisphere of the Moon as our satellite passes to the north of the Earth’s shadow. There won’t be a similar type eclipse where the Full Moon is completely immersed solely in the penumbral shadow until August 29th 2053.

(the following information is supplied by Fred Espenak, NASA GSFC) First and last penumbral contacts occur at 21:22 UT and 02:14 UT (Mar 15), respectively. The whole Moon will lie completely within the penumbral shadow from 23:18 UT to 00:18 UT (Mar 15).

Greatest eclipse occurs at 23:48 UT with a penumbral magnitude of 1.0565. At that instant, the Moon will stand midway in the penumbral shadow. The Moon's northern limb will lie 1.6 arc-minutes from the shadow's outer edge while the southern limb be 1.6 arc-minutes from the edge of the umbra.

Penumbral eclipses are difficult to observe, especially during the early and late stages. Nevertheless, a subtle yet distinct shading should be visible across the southern half of the Moon, especially during the two hour period centred on greatest eclipse.

The Planets

Mercury will be visible as a magnitude 0.5 object low above the western skyline in the evening twilight glow until towards the end of the first week of March. Thereafter, it becomes too close to the Sun to be seen as it heads towards inferior conjunction on March 12th. The planet then moves into the morning sky but will be poorly placed for observers to catch a glimpse of it from our latitude.

Venus heralds the dawn and though low, is a beautiful sight as the world stirs for another day. Did anybody follow up my suggestion of spotting the planet in both the morning and evening sky in January? I succeeded in spying Venus in the morning sky the very last day of the month when clear skies favoured an early rise on January 31st to glimpse the planet as a diminutive crescent in 20-power binoculars. The evening sky sightings earlier the same month were far easier but equally splendid.

Venus reaches greatest western elongation on March 25th (47°) but the low angle the ecliptic makes with the horizon in the morning sky at this time of year means the planet is not that high up at all. A small telescope will show the planet at exactly half phase on that date. Uranus will be within the same eyepiece field as Venus (19 arc-minutes separating the pair) on the morning of April 18th. It will be a difficult observation though with the sky rapidly brightening before sunrise and both planets being rather low.

Mars continues to decline in brightness from magnitude 0.8 to magnitude 1.5 throughout the two month period covered by these notes. It was quite interesting to see it pass close to the Pleiades star cluster in mid-February with the planet’s motion obvious from night to night. At high power in a telescope, the planet’s phase is decidedly gibbous.

The planet’s progress through Taurus brings it within ½° of the open star cluster NGC 1746 on March 22nd. It’s a little known stellar association that is visible in binoculars so take the opportunity to become acquainted with one of the Bull’s hidden treasures. The cluster comprises about 20 true members and lies approximately 1,350 light years distant.

Speeding onward, Mars can be found 2½° to the lower left of the Moon on the evening of April 3rd. But what is even more interesting is that it is equidistant from the two stars marks the “horns” of the Bull, Beta and Zeta Tauri that same night. Mars crosses into Gemini in mid-April and can be found within a one degree circle of the rich open star cluster M35 at the feet of the Heavenly Twins. This particular rich group is easily seen in binoculars and the scene explodes with stars in a small telescope.

Careful observation may also reveal NGC 2158, a cluster five times more distant than M35. While the planet’s disk is far too small to reveal any surface detail, these close encounters may introduce you to other rewarding celestial sights.

Giant Jupiter has been purely for early risers so far this year but is now rising around midnight with it appearing above the horizon a little before 8pm by the end of the period as it nears its May 4th opposition date.

It's brightness increases from magnitude -2.2 to -2.5 over the next two months while the disk measures 41 arc-seconds in diameter by the end of April. The planet begins March within 3 arc-minutes of magnitude 5.2 Nu Librae and plods eastward for a few days before reaching its stationary point on March 5th and then commencing its retrograde loop which will carry it slowly westward for the rest of the period.

Binoculars will let you follow its four largest moons as they circle the planet but two interesting satellite phenomena for telescope users are March 15th when at 3am you can see a transit of Io and Ganymede across the disk, while on March 29th at 5:30am the shadows of Io and Ganymede will be seen against the planet’s cloud tops. There’s something else happening the same date but I can’t quite put my finger on it at the moment ;-)

Saturn continues to enthral and the planet is a fine sight close to the famed Beehive star cluster in the constellation Cancer. The Ring World continues its westward motion fleeing the bee swarm as it retrogrades. Saturn is stationary on April 5th and resumes its eastward track, heading back towards another encounter with the stellar association.

What I found remarkable as a naked eye sight was how the Beehive’s misty glow was overpowered by the light of Saturn but from a dark site in recent weeks the cluster has become visible again as Saturn moved away from the field.

Saturn declines slightly in brightness from magnitude -0.1 to magnitude 0.3 during the period and the rings open up just a fraction more. Their southern aspect is now tipped 20° Earthward. The next couple of months will let telescope users see the shadow of Saturn cast on its rings, lending the scene a striking three-dimensional appearance.

Details for the rest of the outer planets are beyond the scope of these notes for the moment but we hope to expand on them in the near future.

Asteroids

A daylight occultation of 1 Ceres by the Moon on March 25th is recorded here just for completeness. To all intents and purposes the observation will be impossible.

A small telescope or large binoculars will let you spot 9 Metis when at opposition on March 2nd. The 158-km diameter world can be found as a magnitude 9.1 speck of light within 1½° of Theta Leonis. Metis is even more remarkable in that it is the only such body to have been discovered from Ireland. It was found on April 26th, 1848 by the astronomer Andrew Graham while observing from Markree Castle, Co. Sligo — the ancestoral home of the Cooper family and once a renowned observatory.

Meteors

March is characterised by low rates of meteors with only the Virginid complex of radiants producing some activity throughout the month.

The cluster of minor showers loosely classed as Virginids are quite diffuse and may in fact be associated with the anthelion — a point nearly opposite the sun in the sky that produces low meteor rates throughout the year. These meteors are in low inclination orbits and circle the Sun in the same direction as Earth. An occasional bright Virginid may be noted but membership is difficult to distinguish from the sporadic background. The anthelion is centred on Virgo in March and drifts slowly through the ecliptic constellations during the year.

The Lyrid meteor shower climaxes on the night of April 22nd/23rd with the skies relatively Moon-free until the early hours of the morning. The Moon is just a day after Last Quarter and rises around 03h 30m. Lyrid numbers are rather low (the theoretical number of meteors you will see per hour is 15) but an unusual characteristic from International Meteor Organisation data is the variability of the length of the shower’s peak which ranges from 15 to 62 hours.

Comets

March 13th marks the twentieth anniversary of the European Space Agency’s Giotto spacecraft’s encounter with comet Halley. I remember the occasion well; nervously awaiting the first images of the comet’s nucleus to be broadcast. To date, probes have visited a number of these interplanetary vagabonds, returning many spectacular close-ups of the nucleus of each, with the Stardust mission recently returning precious grains of comet material which will keep Solar System theorists busy for years.

While no longer the superstitious stuff of yore, comets continue to enthral the human imagination. A trio of these ephemeral visitors will be within reach of small telescopes and binoculars during the period.

C/2005 E2 (McNaught) is a magnitude 10 evening sky object and appears as a slight patch of light that will spend most of March crossing the dim zodiacal constellation of Pisces. It is getting lower in the sky for us now and will be effectively lost to view by April.

C/2006 A1 (Pojmanski) was discovered on photographic plates on January 1st this year and has only been visible from the southern hemisphere. It is moving northward however and brightening at the same time. The comet will be a morning sky object for us in early March when it is expected to be magnitude 5 making it visible to the unaided eye from a dark site.

Binoculars will easily show it as a marked haze. The comet will then pass through Delphinus and Cygnus before ending the month in Lacerta, all the while remaining visible before dawn. The brightness begins to fade thereafter and will slowly fade below magnitude 8 by the end of the period.

Our final comet for the period is actually a number of objects. Comet 73P/Schwassmann-Wachmann 3 is a periodic comet that underwent a major outburst when at perihelion in 1995 with the result that the nucleus fragmented into a number of components. These pieces were recovered at the last return in 2001 and the comet is next at perihelion in early June this year.

What has astronomers especially excited is the fact that one of the fragments will pass just 11 million kilometres from Earth on May 11th and may be magnitude 3 at the time. A smaller piece might just be a couple of magnitudes dimmer, giving the possibility of two naked-eye comets very close together at the same time. Incidentally, this comet was a target of the ill-fated CONTOUR spacecraft that was destroyed in an engine firing mishap in August 2002 (the other comets it was due to rendezvous with were Encke and d’Arrest).

It is difficult to say how 73P/Schwassmann-Wachmann 3 will appear to observers during the period because the fragile nature of the comet pieces may mean they could undergo further disruption and fizzle out. I would therefore recommend keeping an eye on the web sites mentioned below to glean a better picture of what you will see. During the next two months the comet will track through Boötes, Corona Borealis and on in to Hercules in the late evening sky.

Websites where you can get more detailed information

The International Meteor Organisation can be found here where you can also download their annual meteor shower calendar.

Comprehensive details and elements for the comets mentioned in these notes can be found from the BAA comet section or from Seiichi Yoshida's comet information homepage.

Click here for Fred Espenak’s extensive web site on eclipses.

An interesting article on the lunar standstill phenomenon is on the Wiki website here. The web site of Sky and Telescope magazine also has an article on the highest Full Moon. The page is here

Details of spacecraft missions to comets can be found here