August 2010 night sky guide and podcast

To help you learn about the southern night sky, Sydney Observatory provides a monthly audio guide/podcast, transcript of that audio, and a sky map or chart (links below). This month’s audio sky guide is presented by Melissa Hulbert, a Sydney Observatory Astronomy Educator. You can listen online, or download the audio onto your ipod or mp3 player.

The free monthly night sky map PDF (below) shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.

August 2010 night sky map

For a year’s night sky maps and much more information, you can buy ‘The Australian sky guide’ book by Dr Nick Lomb at Sydney Observatory, Powerhouse Museum, good bookshops or through Powerhouse Publishing.

Read the transcript of the audio podcast. Hear the audio podcast:

 

 August 2010 night sky guide – 17 mins 10 secs: Play Now | Play in Popup | Download

Harry observes the strange morphing sunspot group AR11089

The sunspot group AR11089 as it first appeared on the east limb or edge of the Sun. Drawing. Drawing and copyright Harry Roberts ©, all rights reserved

AR11089 is not the name of an underground rock band, but the label applied by the US NOAA organisation, charged with keeping track of sunspots – as counting spots is still the key way of comparing solar activity across the centuries. Was the sun really spotless during the 17th century? Was its activity in the mid 20th century a four hundred year peak? Such questions are answered mainly by looking at the sunspot count.

The second half of the 20th century was the age of satellites; they imaged the sun in a range of wavebands particularly EUV (extreme ultraviolet). But satellites have short lives (one was even ‘shot down’ by the USAF) and normalising their data with the classic spot count isn’t easy. In fact, the best view of any sunspot is still the one in your trusty 3” or 4” amateur ‘scope with an aperture filter!

While this is titled AR11089 I want to briefly recall AR11085 – the spot group that “nobody saw” (except NOAA and the lucky writer). This small bipolar group perhaps lasted less than ten hours, and then disappeared – but when its locality next returned to the sun’s east limb, there was a large active-looking spot group at the exact site. And now everyone could see it, renumbered AR11089.

There are several odd things about this new group, apart from its Lazarus like reappearance. It’s roughly triangular in layout while most spot groups are bipolar and align E-W on the sun. And AR11089 sits in a large area of faculae (WL) and plage (Hα) that has another such activity patch only a few degrees west of it; this latter area is void of spots. Are they related in some way?

Solar Dynamics Observatory views of sunspot region AR11089, annotated by Harry Roberts

They are; the new SDO satellite views show magnificent arches (field transition arches) connecting both plage regions – the whole being one huge magnetic entity (Fig 3, main spots arrowed). And yet while fields within AR11089 briefly reached a ‘strong-ish’ 2500G the group has produced no flares >GOES C1. Curiously it also has few of the usual dark active region filaments (arf) that attend even the smallest sunspots.

The writer first saw the group at the east limb early in its development amid brilliant faculae – having six spots, the largest sited at –21/204 (Fig 1). The Fig shows changes from 19th to 21st (UT) – the group developing substantial penumbrae and ten or more umbrae, still in a triangular layout.

Sunspot group AR11089 on 23 July 2010. Drawing and copyright Harry Roberts ©, all rights reserved

Twenty-four hours later saw an increase to 20+ spots, with many tiny ones in short chains. The preceding spot (p) had a field of R25G (red 2500 gauss) and the larger following (f) spot to the SE had V24. Both are strong fields; but the trailing spot of the ‘triangle’ was now fading, with a few small spots in a chain. Overall the group seems to be ‘morphing’ into a more conventional E-W arrangement, and the remaining (p) and (f) components with 6º N-S separation may soon feel the pull of the Hale-Nicholson force and undergo some dramatic rearranging.

Harry Roberts is a regular contributor to this blog and a member of the Sydney City Skywatchers.

Mars is coming……….in 2018!

A map of Mars centred on the giant volcano Olympus Mons. Taken from Microsoft World Wide Telescope, credit NASA/USGS/MalinSpace Science Systems/JPL.

An email, often based on a PowerPoint presentation, has been circulating for the last six years or so and tells people that Mars is approaching in August and will be seen as big as the full Moon. To the disappointment of many of the recipients this email is a hoax. It recounts the events of August 2003 when Mars was exceptionally close to the Earth, but in a highly exaggerated fashion. Mars was not near Earth in August 2004, 2005, 2006, 2007, 2008 or 2009 and will not be near in August 2010. The next time we will have a good opportunity to view Mars will be in late July 2018.

Oppositions of Mars from 2003 to 2018. The central yellow disc represents the Sun, the next circle the path of the Earth and the outer circle or oval represents the path of Mars. Drawing Nick Lomb.

Why is Mars sometimes relatively near the Earth? Mars and the Earth both circle the Sun with Mars taking 687 days and the Earth 365 days to do so. That means Earth circling inside the path of Mars, will periodically “lap” the slower planet on the outside. On average this will happen every two years and two months with Mars being at “opposition”, that is on the opposite side of the Earth to the Sun, on those occasions. At oppositions Mars is relatively close the Earth.

To complicate matters, Mars has a much more oval-shaped path around the Sun than the Earth. As can be seen from the diagram above that means that some oppositions of Mars are more favourable, that is closer, than other ones. The one in August 2003, which triggered off the hoax email, was a favourable opposition while the next favourable opposition is the one on 31 July 2018.

Even at favourable oppositions Mars only appears like a bright red pinpoint of light to the unaided eye and certainly does not appear like the wonderful image at the top of this post. Through a telescope, like the the ones at Sydney Observatory, some detail can be discerned on the surface of the planet at those favourable oppositions. And even then the amount of surface detail is subject to the vagaries of atmospheric conditions above the telescope and on Mars itself. There have been disappointing Mars oppositions when planet-wide duststorms on Mars completely hid its surface from view.

While we wait for the opportunity that the favourable opposition of 2018 will provide to view the planet, we have unprecedented opportunities to study and become familiar with the surface of Mars on any personal computer. Numerous spacecraft have circled Mars sending back images and many are continuing to do so. Much of the mapping of Mars has been collected and is available through the Microsoft World Wide Telescope/Mars. With this web experience you can see Mars as you could during a favourable opposition or in much greater detail or in ways that are impossible from Earth such as looking down on the north or south poles of the planet.

As well as having fun with Mars on your computer, you can often see Mars in the sky with your unaided eye without having to wait for an opposition, favourable or otherwise. For example, in August 2010 Mars will be one of four planets giving a sky show by bunching up in the western sky after sunset. From 17 to 21 August it will appear close to Venus, the brightest planet.

Neptune almost back to the discovery position – in 2010 the outermost planet has nearly completed a circuit of the sky

The positions of the planet Neptune on 23 September 1846, 2010 and 2011 in relation to selected stars in the constellations of Capricornus and Aquarius. Drawing Nick Lomb

The planet Neptune is now regarded as the outermost planet of the solar system. It was discovered in somewhat controversial circumstances by the German astronomer Johann Gottfried Galle on 23 September 1846 at Berlin Observatory.

What was the controversy about? Galle did nothing controversial – he just searched the sky for a predicted new planet at a position sent to him by the director of Paris Observatory – a position calculated by the French scientist Urbain Le Verrier. This was all brilliant work, but the problem that arose was that an Englishman John Couch Adams had also made a similar prediction for the location of the new planet and the English scientists wanted Adams to share some of the credit for the discovery.

According to the story of Adams’ supporters he wanted to give the position that he had calculated to the Astronomer Royal at Greenwich Observatory, but due to the indifference of the Astronomer Royal George Airy no search was made for the planet. Recent historical research, helped by the rediscovery of files “borrowed” from the archives by an ex-director of Mt Stromlo Observatory Olin Eggen, suggests that Adams is due little credit as, unlike Le Verrier, he had not calculated the path of the predicted planet around the Sun.

How did Le Verrier and Adams know that there was an unseen planet in the sky? They knew that the planet Uranus discovered by William Herschel in 1781 was not quite following the path predicted by Newton’s Law of Gravity. Sometimes the planet was moving a little too fast and sometimes a little too slowly. A simple and correct explanation was provided by the gravitational pull of an unseen planet.

The planet takes 164.79 years to circle the Sun. How do we know the period so accurately? After the discovery of the planet, researchers found a number of pre-discovery observations made by people who had observed Neptune without realising that they were looking at a planet. The great Italian scientist Galileo observed Neptune at least twice, on 28 December 1612 and on 27 January 1613, without realising that he was seeing a planet, probably because his tiny telescope did not allow him to make out the disc of the planet.

A full period from the discovery date of 23 September 1846 takes us to the middle of 2011. However, as can be seen from the diagram above Neptune is already close to where it as first observed, a position in the vicinity of the star Deneb Algedi.

When is Ramadan in 2010?

By Drs Henry Woodruff and Andrew Jacob,
Acting Curators

The calendar we use in civil society (the ‘Gregorian’ calendar) is a solar one – based on the time it takes for the Earth to orbit the Sun. Many religious calendars, however, are based on the phases of the Moon. These include the Catholic, Jewish and Islamic religious calendars. The dates of festivities, holidays and important events in the lunar calendar move by about 10 days every year within the Gregorian calendar.

The ninth month of the Islamic calendar, known as Ramadan, is the Islamic month of fasting. The Hilal, or crescent Moon, marks the beginning of the fasting period. However, there are differences of opinion on how to define ‘crescent’. While some simply demand an unaided sighting by eye of the crescent moon, others are leaning towards using astronomical calculations to avoid confusion.

The following astronomical data concern the new moons in August and September of 2010.

The new moon in question will happen at 1:08 pm on Tuesday, August 10th (all times AEST, i.e. Sydney time). On that day the Sun will set at 5:22pm, and the Moon at 5:34pm, giving an elongation (angular separation in the sky) of only 3 degrees. This is not likely to be enough for the crescent Moon to be visible. On Wednesday, August 11th, the Sun will set at 5:22pm and the Moon at 6:47pm, with an elongation of 21 degrees. In this case the crescent Moon should be visible if the western sky is cloud free. The following new moon (marking the end of Ramadan and thus the beginning of Eid-ul-Fitr) occurs on Wednesday, September 8th at 8:30pm – after sunset. On Thursday the 9th the Sun sets at 5:42pm and the Moon sets at 6:45pm with a 15.75 degree elongation. If the crescent Moon is not visible on this night it should be visible the following night.

Ramadan Mubarak!

Harry observes a long-lived sunspot AR 11084 and considers if it is the return of a previously seen spot

Two sketches of sunspot AR11084, drawing Harry Roberts

While many Cycle 24 (C24) sunspots are hard to see and short lived, AR11084 was easy to see and long-lived. In fact, it is probably the return of AR11078 that first appeared in the sun’s SW quadrant on June 7th, when it emerged as a pair of tiny spots preceding (p) a pair of larger spots with some penumbra around them – a bipolar spot group.

It was next recorded on June 11 close to the solar SW limb – now both (p) and following (f) spots had grown larger and developed penumbrae – Helio freeware sited the group at –19/140 (i.e. solar latitude 19º south, longitude 140º) Bright faculae threaded between the (p) and (f) components. Umbral fields showed the bipolar group “broke” the Hale Nicholson Law – having reversed polarity for a C24 group, with violet polarity preceding red – the second reversed group of C24. Would the group survive a transit of the sun’s far side? Partly, it did!

June 26 showed a fair sized single spot at the sun’s SE limb with very bright faculae following 8º behind (Fig 1) – there was only the one penumbral spot present. H-alpha showed a low bright prominence above the ‘new’ spot and filaments and plage surrounding it. The ‘new’ spot was sited at –19/146. It slowly “dawned” on the writer that the new spot was the return of AR11078!

Two clues prompted this: one is positional. AR11078’s preceding component had a longitude of ~143º, (as well as the same latitude as the new spot 19ºS). Allow a degree or two of western drift that all (p) spots undergo and we see that the ‘new’ spot matches 11078’s western component exactly.

Another clue was polarity – the two parts of 11078 had reversed polarity, with the (p) spots having violet polarity (as stated) and the single monopolar spot of 11084 had violet polarity also, with bright faculae following. This makes AR11084 Hale class “Alpha preceding” – the bright faculae having persisted since first seen in AR11078 14 days earlier. Mt Wilson magnetograms showed that it was the following spots of the earlier group that had disappeared, leaving the (p) spot and faculae intact.

An extreme ultraviolet view of AR11084 from NASA’s Solar Dynamics Observatory, annotated by Harry Roberts

Alpha class sunspots have only one polarity with no other visible spots nearby. Hale first suggested (1920’s) they were bipolar BUT the spots of opposite polarity were invisible. This is in fact the case; and field transition arches (fta in Figs) from the single spot fan out and connect to multiple small polarities in the associated faculae; polarities too “warm” to be visible as spots in any ‘scope. A fine image from the SDO satellite shows exactly this in the corona above AR11084 (Spaceweather Archive for July 2nd – take a look. Note: the fta are NOT visible in H-alpha unless a flare has occurred when they show as post flare loops. SDO can detect them in EUV)

A sketch of spiral filaments around sunspot AR11084, drawing Harry Roberts

AR11084 was remarkably stable – growing a little to just over 100 area units as it crossed the sun’s face, followed by extensive dark filaments and plage –at one stage dubbed the “spiral” sunspot due to filaments that like the arms of a galaxy encircled the spot (Fig 2). The spot produced very weak flares– despite its umbral field slowly rising to V24 on June 30 15:00UT. The spot will surely survive to pass behind the sun’s SW limb a second time – and may yet reappear at the SE limb around July 23.; who knows?

Keep a close watch on the sun – anything can happen!

Harry Roberts, a frequent contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.

Sydney Observatory Eclipse Trip 2010: The Saros Cycle

Their first ever total solar eclipse, I believe. Photo: Andrew Jacob

A total solar eclipse occurs when the Moon passes between the Sun and Earth. The Moon’s shadow (a small black spot) sweeps across the face of the Earth and anyone under the shadow spot experiences the eclipse for a few minutes each. See Dr Woodruff”s blog. The characteristics of each eclipse can be predicted by something called the Saros cycle.

In fact, these days we just use a computer and an astronomical planetarium program to predict how an eclipse will look. But in the past keen observers noticed that similar eclipses repeated every 18 years 11 days and 8 hours – the Saros cycle. The cycle is used to predict when & where an eclipse will occur and how it will appear. This has been known since ancient times. I suspect, but have no evidence, that the Inca would have been aware of the cycle.

Each cycle is numbered, although arbitrarily from the second millenium BC. The total solar eclipse of July 11 2010 was the 27th eclipse, and the 5th total solar eclipse in Saros cycle number 146. There will be 76 eclipses in all in this cycle. It began on Sept 19 1541 and will end on Dec 29 2893! How is that for precise long term astronomical prediction?

Within each even-numbered series the first eclipse occurs over the antarctic and every 18 years each eclipse appears further north until they pass over the arctic. Each eclipse is offset by 120 degrees around the Earth due to the eight-hour part of the cycle. This figure shows this nicely.

None of the previous eclipses of cycle 146 were visible to the Inca nor from Rapa Nui (Easter Island). However, several occured over the Inca empire during their time. One in particular passed north of Cusco on February 24 1514 (if my computer is correct), long before the Conquistadors arrived. Hardly a precursor of disaster it was just a natural, if spectacular, celestial event.

For more information on the Saros cycle see Fred Espenak’s excellent site.

As far as I can determine there has been no previous total solar eclipse over Rapa Nui, at least in the time it has been inhabited.

Planets Crowd into Virgo in August

by Henry Woodruff Acting Curator

Here’s another event to watch out for, since it will only happen again in 2040!

It all begins quite harmlessly in early August. Looking West, when it’s dark enough after sunset, you’ll first see a bright object, probably the first dot in the darkening sky. This is the gorgeous Venus, second planet from the Sun and aptly named after the Roman goddess of love and beauty. Venus is also known as both the Evening Star and the Morning Star (which is quite deceptive, given that it’s not a star), depending on when you can see it.

As it gets darker, a red and a lemon coloured dot become visible just above Venus. These are the planets Mars (fourth from the Sun, Roman god of War) and Saturn (sixth from the Sun, father of Jupiter), respectively. As the month progresses, watch the bright Venus glide up to the other two planets. Now we have 3 planets in the constellation of Virgo.

Below them a very faint Mercury struggles to be seen before it sets below the horizon…
But behold! Enter stage left: The Moon, only a crescent, zooms past Mercury to join the party at Virgo’s on August 13th. We can now see 4 solar system objects at one go, a must do for those of you who like a good bang for your buck.

Now what does it all mean? If you ask me, it means that’s worth while rugging up, turning off your TV, going out and looking at the sunset and just afterwards enjoying the show the Solar System is putting up for Earth’s inhabitants. It costs nothing, you can invite all your friends and family and it happens a lot less often than the soccer World Cup.

Sydney Observatory Eclipse Trip 2010: The Inca World

The Sacred Valley of the Inca – fertile and full of Inca archaeological sites. Photo: Andrew Jacob.

Before arriving in Peru for the Sydney Observatory Eclipse tour of 2010 my knowledge of the Inca civilization was limited: they worshiped the Sun, built extraordinary stone walls and their empire was crushed by Spanish conquistadors a few hundred years ago.

These are some of my observations and recollections of our guides’ comments – please keep in mind I am no expert on South American civilizations or their astronomy.

The Inca were not an ethnic group but a ruling class. They kept their Quechua people uneducated and taxed. In return they supplied a trickle of astronomical information for agricultural use.

However, they built on centuries of previous knowledge acquired by numerous pre-Inca civilizations and developed a substantial and extensive administrative network. With a population of 16 million they had plenty of labour on call to build their impressive palaces, lined with gold & silver and their temples, terraced hillsides & irrigation canals.

Only 15% of the population was Inca and only they held specific and detailed knowledge about their rituals, beliefs and constructions. The arrival of the conquistador Francisco Pizarro in 1532 changed everything. Using a dispute between two half-bothers who ruled the empire and a dose of treachery he supplanted Inca rule for his own. In the process the Inca were destroyed and their knowledge lost.

Today we are left with buildings apparently aligned to various celestial events including solstice sunrises and sunsets. We know of a few bright and dark constellations. However, with no written record we have no idea of the true purpose of many buildings, carved rocks and the strange nubbins of rock jutting out of many walls.

The Sun Temple at Machu Picchu showing the summer solstice window, masterful stonework and mysterious rock nubbins. What were they thinking? Photo: Andrew Jacob.

It is reasonable to assume the alignment of a window or rock towards a solstice sunrise or sunset is intentional because of the importance of these dates for agriculture. However, with no written records or true understanding of a lost cultures perspective on the world it is easy to overinterpret the evidence. And an alignment is not proof of intention.

Sydney Observatory, for instance, is aligned with the cardinal directions and has decorative architectural features. The only important ‘alignments’ however are that of the Transit Telescope (north-south) and the mount of the South Dome refractor. Without information about 19th century architecture and engineering skills or the concept of science a future civilization would not understand our building.

There seems to be much speculation and little certainty regarding the Inca. Everyone seems to have their own interpretation. An unsolvable mystery is the most enticing of all.