The Southern Cross gives us an easy means of finding south on any clear night. Extend its long axis by four and a half times its length to reach a point known as the South Celestial Pole. South is the point on the horizon directly below the pole.
How to find south using the Southern Cross. Chart Nick Lomb
The stars of the Cross all appear to be at the same distance but that is not the case. Like the stars of all constellations, they are physically far apart: the closest star to us in the Cross is Gamma at a distance of 88 light years, while the furthest star Delta is at 364 light years distance.
The Southern Cross. Photo Nick Lomb
Today is the anniversary of the birth of the first Astronomer Royal John Flamsteed in 1646 in Denby (near Derby), Derbyshire, England. Flamsteed took charge of the newly built Greenwich Observatory in 1675 and began plotting star positions. His catalogue of 3000 stars was published after his death.
Flamsteed House at Greenwich. Photo Nick Lomb
Below Spica, the brightest star of Virgo the Maiden, is the star Porrima, the name of which means the goddess of prophecy. Porrima, also in Virgo, is a double star 38 light years away. The two components circle each other every 170 years.
The constellation of Virgo with the position of Porrima indicated. Chart drawn with Stellarium
If the sky is clear it maybe worthwhile rising early tomorrow morning as the two brightest objects in the night sky apart from the Moon, Venus and Jupiter will be close to each other. Separated by less than a moon-width, the two planets will be low in the east in the brightening dawn sky.
Venus and Jupiter on the morning of 18 August 2014. Chart Nick Lomb
These are the 13 constellations that the Sun passes through during the course of a year. Looking from west to east in the early evening the following zodiac constellations are visible: Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus and Aquarius.
The zodiacal constellation of Libra the Scales. Chart made with the help of Stellarium
Spica, the brightest star in the constellation of Virgo the Maiden, is high in the west. Astronomers have found that it is made up of a pair of stars whirling around each other every four days.
An artist’s impression of the two stars of Spica. Courtesy Manuel Perez de Lema Lopez
The constellation of Virgo the Maiden is high in the western sky in the early evening. According to one legend the constellation represents the daughter of the harvest goddess Ceres. Thus Virgo is usually pictured with a palm branch in her right hand and an ear of wheat in her left.
Virgo in the August evening sky. Chart made Stellarium
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