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

The winter constellation of Scorpius the Scorpion is prominent in the eastern sky. In the early evening it is lying on its side with its tail or sting curving down to the right or south and its claws to the left or north.

The constellation of Scorpius. Photo Nick Lomb

Chart showing daily maximum temperatures for 2012, 1992, 1972 and 1952 at Observatory Hill in Sydney during the winter months. Data courtesy of the Bureau of Meteorology, chart Nick Lomb

In Australia seasons begin on the first of the appropriate month so that winter begins on the first of June. The origins of this tradition are said to go back to the early days of the Colony in Sydney when the New South Wales Corps also known as the Rum Corps changed to their winter uniforms at the beginning of the month.

In some overseas countries the seasons begin on the astronomical turning points of the year, the solstices and the equinoxes. Some people think that there is a scientific or logical reason for these starting dates. There is not. These are just conventions and there is no fundamental rule to decide when seasons should start. Quite properly it is up to each individual country to decide how to match the seasons to their local climate.

At first thought the coldest day of the year should be at the time of the winter solstice, when the tilt of the Earth is such that the minimum energy (heat and light) for the year reaches the southern hemisphere. That would suggest that winter should start six weeks earlier, that is, on about the May 10, and end six weeks later, that is, about August 10, so that the winter solstice is in the middle of the winter season. However, that scheme does not match experience.

The reason why the coldest temperatures occur later than the winter solstice is that the atmosphere, the land and the oceans all retain some of the heat that they receive from the Sun earlier in the year and they take some time to come into balance with the reduced heat that they are receiving. As the land, the atmosphere and the oceans retain different amounts of heat and cool by radiating it away at different rates, calculations would be difficult and it is best to actually observe what happens at each location.

Starting winter on the solstice, which is on about 21 June each year is tantamount to stating that there is a six week delay for the coldest temperatures to be reached. Starting at the beginning of June estimates the delay at just three weeks. Which assumption is correct?

Let us look at the data for winter temperatures in Sydney. According to climate data from the Bureau of Meteorology, the average mean temperature at Observatory Hill in June is 16.9°, in July it is 16.3° and in August it is 17.8°. This clearly indicates July as the coldest month.

To look more carefully at the day to day temperatures, the chart at the top shows, with data again taken from the Bureau of Meteorology, the daily maximum temperatures for June, July and August for four arbitrarily chosen years: 2012, 1992, 1972 and 1952. There is a lot of variation for each year so the underlying pattern is not easily obvious, but lower temperatures seem more common in mid July matching the indication from the June 1 starting date for winter and not at the end of July/early August as indicated by the solstice starting date.

Whatever the origins of the Australian tradition of beginning winter at the beginning of June, we are well served by it as it fits the winter season in Sydney and elsewhere in the country.