A comparison between sunspot group AR12109 on 10 July 2014 and its returned version as AR12130 on 30 July and 1 August 2014. Sketch and copyright Harry Roberts ©, all rights reserved
These reports have often noted the return of sunspot groups from the Sun’s far side, which make a second transit of the visible hemisphere. They are mostly large, long-lived groups. Sometimes we see a less obvious kind of return – as was the case for AR12130.
In mid-July the large spot group, AR12109, was monitored as it crossed the disc – growing larger and more complex – attaining Hale magnetic class Beta-gamma-delta, but producing no great flares. We speculated it might return at the eastern limb in early August. It did; but was not recognized for some time. Figure1 shows the earlier group 12109, on July 10, and the new group 12130 on July 30 and on August 1(Figs 2 & 3), when it had assumed a rather different shape. The three logs are plotted on the same longitude scale and all have the same average latitude.
The major feature of 12109 was its very large preceding (p) spot sited at –8,223, with an umbral field of red 2600G (R26) on July 10.
When the group returned 21 days later it was very different, as the old (p) spot was now a small ‘following’ (f) component in a modest sized (seemingly) bipolar group (Fig2). But when Mt Wilson polarities were published it was realized that the new group comprised red polarities only: there were no violet ones! And the (f) spot of this new unipolar group was sited just 2º west of the old red (p) of AR12109.
The new group AR12130 had a large red (p) spot of 2100G polarity which gave this group the ‘looks’ of a typical bipolar or beta class group. This group is plotted in Fig 2 on the same scale as 12109 (Fig1) and we see that it retains only the dominant (p) spot of 12109, but it is now in the following (f) position. Ahead (west) of this old spot are new red spots. This new unipolar group hosted a GOES M2.5 on the 31st at 11:14 UT. Note that the 2º of westward drift in longitude of the old spot over 14 days is normal for a long-lived (p) sunspot. Why were there no violet spots in this new group?
Recall that old 12109 had only minor violet followers in its earlier stages, and they faded during their first disc transit, leaving mostly red spots to pass behind the western limb on July 14.
Fig 3 shows the first appearance of some meager violet polarities on August 1. Why did 12130 not emerge as a normal bipolar or Beta class group? There is just one small V13 a bit north of the original (returnee) red follower at –8,226. However, new violet flux has also emerged in following position well behind the returnee, -7,219 (ff in Fig) some 7º to the east. Mt Wilson workers made this a new spot group, but its following polarity and some small spots in between suggest it is the normal appearance of (f) spots. NOAA agreed and made the very elongated entity, 13º end to end, a single large spot group: AR12130. Note the small red spot south of the V14 followers (ff): what’s it doing there?
Following logs showed more small spots emerging at the east end of this group; but Mt Wilson was under cloud and detail polarities were not available.
Unipolar sunspots are common, but they are mostly single Zurich class H spots: Hale class Alpha’s. In such cases preceding polarity is seen in the single spot and EUV coronal images show the flux loops emerging from that spot ‘earth’ to the photosphere, usually in areas of opposite (i.e. f) polarity east of the single spot.
In the case of a cluster of unipolar spots like 12130 the EUV images show coronal loops connecting to (f) spots in another group some 18º to the west (AR12127)– and also to a range of spotless surface sites of opposite sign east of the group: where bright plage and faculae were seen. Some, undoubtedly, were the sites of earlier (f) spots from AR12109.
By the 4th the group had grown to 28 spots – but then declined rapidly as the west limb approached. Apart from the M2.5 cited, no major flares were logged. So, it seems unlikely we will see the old red spot at longitude 226 next rotation; still, you can never be sure!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.
On this day in 1877 American astronomer Asaph Hall discovered Deimos, the first of the two moons of Mars. Hall worked at the United States Naval Observatory located at Foggy Bottom in Washington DC where he used a large 26-inch (66-cm) lens telescope for his observations.
Deimos. Courtesy NASA/JPL-Caltech/University of Arizona
The Moon is full tomorrow morning and it will appear larger than usual. This is because the instant of full Moon at 4:09 am AEST almost coincides with the instant that the Moon is at its closest to Earth for the month at 3:43 am. The Moon will look biggest at moonset as its apparent size is augmented by the Moon illusion.
The full Moon in May 2014. Photo Nick Lomb
Prominent Australian radio astronomer Wilbur Norman Christiansen was born on this day in 1913 in Elsternwick, Melbourne. As Professor of Electrical Engineering at the University of Sydney he was responsible for the building of the Fleurs Synthesis Telescope, at the time the highest-resolution radio telescope in the southern hemisphere.
The Fleurs Synthesis Telescope. Photo Nick Lomb
Bernard Yarnton Mills was an early pioneer in radio astronomy. Born on this day in 1920 at the Sydney suburb of Manly, he invented a new cross-shaped form of radio telescope while working with the CSIRO. Later, as a professor at Sydney University he was the driving force behind the construction of the large Molonglo Cross telescope.
Bernie Mills (right) receiving the Grote Reber Award in 2006 from Martin George of the Queen Victoria Museum, Launceston. Photo Nick Lomb
The largest optical telescope in Australia is the Anglo-Australian Telescope at Siding Spring in NSW with an aperture of just under 4-metres. However, it is dwarfed by recent telescopes such as the two Gemini telescopes in Chile and Hawaii that are each double its size.
The Anglo-Australian Telescope. Photo Nick Lomb
After a decade long journey the European Space Agency’s Rosetta spacecraft is expected to reach comet 67P/Churyumov–Gerasimenko this evening Australian time. The spacecraft will continue to stay with the comet as it approaches its closest point to the Sun in a year’s time.
Rosetta imaged Comet 67P/Churyumov-Gerasimenko from a distance of approximately 12 000 km on 14 July 2014. Courtesy ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The first quarter Moon on the evening of 4 August 2014 slipped in front of the ringed planet Saturn. Photo Nick Lomb
On the evening of 4 August 2014 as seen from Australia the first quarter Moon moved in front of the the ringed planet Saturn. This occultation of Saturn was the third and last for this year. Sydney Observatory provided an excellent live webcast of the event so that people without telescopes or those who did not want to brave the cold winter evening could watch the event in indoor comfort.
In the spirit of this morning’s Daily cosmobite on small telescopes, here is a brief sequence of three images made with a small (70 mm aperture) telescope of the re-emergence of Saturn from behind the Moon. Note that as the lit edge of the Moon is much brighter than Saturn, it appears badly overexposed on the images below. A properly exposed image of the whole first quarter Moon is above.
Three images showing Saturn emerging from behind the Moon on the evening of 4 August 2014. In the first image Saturn is still partially behind the Moon, in the second it is still very much in the glare of the bright Moon and it can be seen most clearly in the last image. In all images the edge of the Moon is overexposed so that Saturn can be seen. Photos Nick Lomb
If you are buying a small telescope for yourself, or as a present, remember that the most important size in a telescope is the width of the front lens or main mirror. Generally, the bigger it is the better. The magnification is irrelevant as that can be varied by using different eyepieces.
A small telescope at Sydney Observatory. Photo Nick Lomb
The occultation is now over. But you can watch our recorded stream above. If you can’t wait to see the exciting moments: Saturn begins to disappear behind the dark limb of the Moon near 41 minutes into the stream and reappears just before 1 hour 23 minutes into the stream.
On Monday August 4 Australia gets its last chance this year to view the Moon pass in front of the beautiful planet Saturn and hide (or occult) it. Sydney Observatory will live stream this occultation of Saturn. You can watch it here or on YouTube.
For Sydney the event lasts almost an hour . It begins at 9:22pm AEST and ends at 10:13pm AEST. It is visible from all of mainland Australia with only Tasmania missing out.
This event is clearly visible, and completely safe to view, by eye without any special equipment such as binoculars or telescopes. However, binoculars or telescopes will improve the view and Sydney Observatory will live stream the event using one of our telescopes.
We plan to live stream from 8:45pm AEST on Monday 4 August 2014. Our telescope will be following Saturn. The Moon will move into view and at 9:22pm AEST slide over Saturn, occulting it. Of course, the Moon is relatively close at a distance of almost 38o,ooo-km, while Saturn is around 1.3-billion km further away. Our telescope will continue to follow Saturn’s position and we will see the planet reappear on the other side of the Moon at 10:13pm AEST.
The August 4, 2014 occultation of Saturn is visible from mainland Australia and the south-west Pacific region. If you live between the white lines you can see it at night. If you live between the red lines it occurs during daylight. Map courtesy of Occult4 by Dave Herald.
This occultation will be visible from all of mainland Australia. From locations other than Sydney the times of disappearance and reappearance will be different. ABC Science has an excellent post in which Ian Musgrave has calculated times for other cities in Australia, and Fred Watson explains how often these occultations occur.