The yearly Wolf sunspot numbers from 1700 to 2011 (red) together with the modified sunspot numbers (blue). Data from the Solar Influences Data Analysis Center, diagram Nick Lomb

Dark spots or sunspots on the surface of the Sun provide an indication of its activity. To make comparisons over long periods researchers usually use the Wolf sunspot numbers. These set of numbers was began in 1849 by Rudolf Wolf, who used earlier records to estimate the numbers prior to the start of his observations.

There is considerable subjectivity in estimating the sunspot numbers, which depend on both the number of spots observed as well as the number of groups in which they are found. Over the years subsequent keepers of the Wolf sunspot numbers took great care to match Wolf’s techniques and procedures, even continuing to use, until recently, the same 8-cm lens telescope that Wolf had used.

At a conference in Mendoza, Argentina in October 2011 Leif Svalgaard of Stanford University, USA suggested that the care to maintain the Wolf sunspot numbers had not been enough. He has found that in 1945 a new director of the Observatory at Zurich, Switzerland that then maintained the Wolf sunspot numbers, changed the way the spots were counted and inflated the subsequent numbers by 20-21%. Svalgaard suggests that to maintain uniformity the currently accepted Wolf sunspot numbers prior to 1945 need to be increased to match the post 1945 inflation of the numbers.

The chart above shows both accepted yearly mean values of the Wolf sunspot numbers from 1700 to 2011 and the same values modified according to the Svalgaard scheme. The main sunspot cycle of about 11-years remains in the modified values, but interestingly the large values from 1950 to 2000 are no longer unique and are matched by similar large sunspot peaks in earlier centuries. The well-known long-term periodicity of about a century, known as the Gleissberg cycle, is more obvious in the modified numbers. Dips can be seen in about 1700, 1800 and 1900, making the low amplitude of the current solar cycle a prediction of this longer-term cycle.

In a paper titled The sunspot cycle revisited, that has just been published as part of the Journal of Physics Conference Series, Volume 440, ‘Eclipse on the Coral Sea: Cycle 24 Ascending (GONG 2012, LWS/SDO-5, and SOHO 27) 12–16 November 2012, Palm Cove, Queensland, Australia’, I examined both the original yearly Wolf sunspot numbers and the numbers with the Svalgaard modification. This is not the first time I had a look at the sunspot number series as I first studied them many years ago, prior to starting work at Sydney Observatory.

The results of the latest study match those of the earlier one in indicating that the Sun has a clock mechanism that maintains its 11-year solar cycle. The peaks of the cycle can sometimes arrive earlier and sometimes later, but they always seem to get back in phase. This finding appears to be in accord with the currently accepted theory of the solar cycle, which suggests that the solar dynamo originates in a conveyor-belt like circulation of magnetic fields. It is this circulation of magnetic fields between the equator and high polar latitudes in both hemispheres that could be the basis of the clock mechanism in the Sun.

Over the last few evenings the brightest of the planets Venus has been rising higher in the western sky and approaching Mercury. This evening the two planets are at their closest with Mercury above Venus and only separated from it by two degrees or four times the width of the full Moon.

Venus and Mercury at 6:00 pm on 20 June 2013 as seen from Sydney. Drawing Nick Lomb

Sunset from Sydney Observatory on 28 July 2006. Photo Nick Lomb

The winter solstice is the day when the Sun reaches its furthest north position in the sky and starts moving back towards the south. It marks one of the main turning points in the year with the others being the equinoxes and the summer solstice in December. This year winter solstice is on Friday 21 June at 3:04 pm AEST. From then on the days start becoming longer and night times shorter.

In ancient Europe the winter solstice (in December in the northern hemisphere) was a time of celebration. The Romans had a week-long celebration called Saturnalia during which all wars had to stop and courts did not try criminals. Later this festival became Dies Natalis of Sol Invicti or the Birthday of the Unconquerable Sun celebrated on 25 December each year.

Changes in the length of daylight in Sydney around the time of the winter solstice. Chart Nick Lomb

As indicated above the length of daylight, that is the time between sunrise and set, starts increasing from the time of the solstice. From then on we can start to look forward to longer days, but only slowly. As can be seen from the chart, there is no noticeable change from day to day on the days immediately before and after the solstice. The name solstice comes from this fact as it means the Sun stands still. Soon though the length of daylight starts to increase.

Just because Friday 21 June is the shortest day of the year does not mean that it is the day of the latest sunrise or the day of the earliest sunset. The latest sunrise will occur at the end of the month while the earliest sunset occurred earlier on about 12 June. This rather complex behaviour is due to the equation of time, which indicates the position of the Sun at 12 noon each day.

In many countries the seasons are fixed in reference to the four astronomical turning points so that we could take the start of winter from the day of the solstice. In Australia, however, the seasons traditionally start on the first of the appropriate month so that winter has already begun on 1 June. This fits in well with our weather for it takes a while for the ground and the oceans to come into equilibrium with the minimum of heat received from the Sun at this time. The coldest days hence tend to be in the middle of July and so in the middle of the three-month winter period.

On Friday 21 June we have reached the shortest day of the year. From now on things are bound to improve. Let’s celebrate!

The ringed planet Saturn is visible each evening in the eastern sky near the bright star Spica. One of the planet’s over 60 moons is Phoebe that not only circles Saturn in a path tilted to its equator, but is going backwards compared to the other moons. Tonight the gibbous Moon is above and to the left or north of the planet.

The Cassini spacecraft imaged Saturn’s moon Phoebe in 2004. Courtesy NASA/JPL/Space Science Institute

The ejection of a filament near the south pole of the Sun on the morning of Sunday 16 June 2013 (Australian time). Sketch and copyright Harry Roberts ©, all rights reserved

As the Sun nears solar maximum, a variety of unusual things are expected; one is the development of so-called polar crown filaments.

As sunspots emerge in the Sun’s lower latitudes, streaks or plumes of magnetic field, the remains of earlier sunspot groups, slowly migrate towards the Sun’s poles, and these ‘streaks’ cause the reversal of polarity of the polar fields. This reversal usually occurs around solar maximum, that is, about now! And when these ‘remnant fields’ encroach on polar fields of opposite ‘sign’, polar filaments can develop. At times, this can result in a succession of filaments that encircle the sun’s north and south poles: termed a polar crown.

Indeed, we may be seeing such a ‘crown’ at the Sun’s south pole right now. For some time H-alpha logs have shown large stable prominences at high southern latitudes, between 60º to 70º south, perhaps a south polar crown. In mid June, tall prominences appeared daily in this latitude range, and on June 15-16, an impressive ejection of the western prominence was seen (Fig above).

Two tall prominences near the south pole of the Sun on the morning of Sunday 16 June 2013 (Australian time). Sketch and copyright Harry Roberts ©, all rights reserved

Peter Meadows © Helio freeware is well suited to finding the latitudes and longitudes of limb features – as it does for sunspots. Although a feature at the limb may be of uncertain longitude, the latitude can be found with accuracy. On June 15-16 two tall prominences flanked the Sun’s south pole (Fig), the western one at the high latitude of 67ºS, with an eastern one at 58ºS. Both had been present at these latitudes for four days, as though they were fixtures, despite solar rotation of ~40º over that period.

Over successive days both prominences showed activity, with material from each apparently ‘flowing’ into space: by the 15th-16th (local day June 16) both had reached great heights, with the easternmost at 110Mm, while the western one reached ~60Mm (Fig). The eastern one seemed the most active – but the western one was, in fact, soon to erupt.

The latter, the fainter of the two, was first recorded at 21:58UT to 22:40, when ‘white light’ viewing began. At 23:40 H-alpha was restarted, when changes were noted in the western prominence – in fact it was in fairly rapid motion, as the figure shows. Over the next 70 minutes the prominence ejected off the disc in a spectacular way. The earlier rounded shape changed into a tall thin plume with an attachment to the disc south of the ejection site. After just ten minutes the prominence had climbed to over 120Mm; by 00:17 (June 16th UT) it was 160Mm above the limb.

The material was faint throughout the ejection with no excitation brightening noted. The logs show the development of a connection point moving southwards (polewards) while the initial ejecta continued to rise almost vertically above the site, and it is assumed that a long polar filament was ‘peeling’ rapidly off the sun. By 00:31 the ejecta was >190Mm above the limb; the length of the rising filament now exceeded 250Mm, almost half a solar radius! By 00:50 the filament had mostly faded to invisibility except for a brighter fragment some 210Mm above the limb, and this faded from view around 01:00.

The eastern prominence remained unaffected by its neighbour’s demise: material continued to circulate up and down within the structure, and it was still present the following day. While the southern polar fields are due to reverse their polarity shortly, there is no sign yet of that happening and more prominences can be expected to reform at these high latitudes.

As Zirin says in his final remarks in Astrophysics of the Sun, “Here before our eyes is a real star (and) we can explore the world of magnetic phenomena and convection in plasmas. I invite the reader to join the fun”.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.

The first American female astronaut Sally Ride flew into space aboard the Space Shuttle Challenger on this day in 1983. During the six-day mission the shuttle crew launched two satellites. Dr Ride flew on another mission in the following year and was later active in encouraging girls and young women to study science. Sadly, she died on 23 July 2012.

Sally Ride in 1978. Courtesy NASA

Astronomers can measure the mass of a star if there is a planet or another star circling around it. They can then use a relationship established by the great German born astronomer Johannes Kepler. Kepler’s third law links the period of the planet or companion star, its distance and the star’s mass.

A possible portrait of Johannes Kepler by the artist Hans von Aachen. Courtesy Wikimedia Commons

These are the 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: Gemini, Cancer, Leo, Virgo, Libra, Scorpius and part of Sagittarius.

The zodiac constellation of Libra the Scales. Image created with Stellarium

About two thirds of the energy in the Universe is made up of mysterious dark energy. Astronomers discovered this force only in the last decade or so when they realised that galaxies are not only moving away from each other but at an increasing rate. It is dark energy that is pushing the galaxies apart.

Distant galaxies imaged by the Hubble Space Telescope. Courtesy NASA, ESA, S. Beckwith (STScI) and the HUDF Team

Surprisingly ordinary matter like the matter in what we see around us such as people and buildings makes up only five per cent of the Universe. According to the results from the Planck space mission released in March 2013, the remainder is made of 27 per cent mysterious dark matter and 68 per cent of even more mysterious dark energy.

Planck’s map of the tiny temperature fluctuations in the Universe. Courtesy ESA and the Planck Collaboration