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Winter solstice

by Martin Bush
Publish date
21 June 2012
Comments (3)

Martin is the programmer at the Planetarium at Scienceworks.

Today is the shortest day of the year, also known as the winter solstice. More correctly, it's the day on which the solstice fell, at 9:09am AEST.

Solstice means 'the Sun stands still'. Although we never see the Sun stop moving across our sky from east to west, it does stop moving in a south-north direction. Our winter solstice is the precise moment at which the Sun reaches its northernmost point in the sky, stops, then starts moving south again.

After the solstice, the Sun starts rising higher in the sky, and the points on the horizon where the Sun rises or sets start moving south.

Analemma Image of an analemma taken over the course of a year by Robert Price in Bethanga, Victoria, consisting of 48 images of the Sun superimposed on a single background image. The winter solstice occurs when the Sun is at the lowest point in this image.
Image: Robert T. Price
Source: Robert T. Price

Of course the Sun is not really moving south and north. Its apparent movement is a result of the Earth’s tilted axis moving around the Sun. On the winter solstice the axis is tilted away from the Sun, the Sun rises lowest in the sky and the sunlight's energy is the most diluted across the ground. You can learn more about the cause of the seasons in the Melbourne Planetarium show Tilt!

The day on which the solstice falls is the shortest day of the year, but not the day with the latest sunrise or the earliest sunset. This is because Sun time is not exactly the same as clock time.

At the winter solstice, Sun time is drifting later relative to clock time because the solar day is a little bit longer than 24 hours. For a few days after the solstice the small increase in the length of a day is not enough to overcome this drift, so the time of sunrise as measured by our clocks keeps getting later. Similarly the earliest sunset was a few days before the solstice.

Nor is the solstice the coldest day of the year. This is because of what is known as thermal inertia. It takes a lot of energy to heat up the ground and the oceans. At the moment the ground is still too warm to be heated by the amount of sunlight we are receiving, so it is continuing to cool. In around a month the balance will change. The ground will be a bit cooler and the sunlight a bit stronger, and the earth will start warming up again.


Infosheet: The Sun and the Seasons

Infosheet: The path of the Sun

Mesopotamian lunar table

by Martin Bush
Publish date
25 May 2012
Comments (0)

Martin is the programmer at the Planetarium at Scienceworks.

A personal highlight for me in The Wonders of Ancient Mesopotamia exhibition is the Babylonian lunar table. I love this artefact not just because of the antiquity of its writing or how long it lay preserved in the ground, and certainly not just because of the skill needed to make the rows of tiny cuneiform script. (How did they do it? I could never have managed.)

Babylonian lunar table Lunar table K.90 from the British Museum on display in The Wonders of Ancient Mesopotamia.
Source: © The Trustees of the British Museum

This tablet is exciting because shows just how seriously Mesopotamian cultures took astronomy. Observers recorded the appearance of the Moon – and also the stars and planets – every single night of the year. (Ok, unless it was cloudy.) They sent reports of these observations to the king. Babylonian astronomers had centuries of astronomical observations to work with. Unfortunately we don’t as not so many of these reports have survived.

Some concerns of these ancient astronomers – like making horoscopes to advise the king – are no longer of much interest to modern astronomers. But many ancient achievements live on to this day. Astronomers still number lunar eclipse using a system known as the Saros Cycle. This cycle was discovered by Babylonian astronomers around the 5th century BCE.

The work of these Babylonian astronomers can also be seen in the Jewish calendar. Sometime around the 4th Century BCE Mesopotamian astronomers calculated the average length of the lunar month. The extensive observations they had to work with meant that they came up with a remarkably accurate figure, different to the modern value by only a fraction of a second. This value was taken up by Greek astronomers such as Ptolemy and from there it was incorporated into the Jewish calendar when it was codified in the first millennium CE. The value determined by Babylonian astronomers is still used today to determine the date of the Jewish New Year.

This lunar table survived for centuries in the ground while the influence of Mesopotamian astronomy on our study of the skies has lasted even longer.

Lost in Space

by Martin Bush
Publish date
22 July 2011
Comments (0)

Objects in our collection don’t just go on display at our own museums. It’s also exciting to see them help other people’s exhibitions come to life. I’m particularly happy about seeing items from the space and astronomy collections being prepared for a new exhibition at ACMI called Star Voyager that will run from September this year through to January 2012. The objects being loaned include rare 19th century astronomical lantern slides, a historic surveying telescope and the gloves of a Soviet cosmonaut.

The cosmonaut glove was used by Vladimir Georgiyevich Titov on the Mir space station. Titov left Earth on Soyuz TM-4 on December 21 1987 and returned on Soyuz TM-6 on December 21 1988. He and fellow cosmonaut Musa Manarov had spent just over a whole year in space – a new record at the time. Titov, who had also been on one previous Soyuz mission, would go on to have two further trips to space on the Space Shuttle.

Photo of cosmonaut glove Photgraph of Sokol glove worn by cosmonaut Vladimir Titov.
Image: Marion Parker
Source: Museum Victoria

The glove is part of a Sokol KV-2 space suit. Each suit was custom made for a single cosmonaut, including individual moulding of the rubber part of the glove, shaped to the cosmonaut’s fingers. The Sokol suits were pressurised, and the gloves attached to the suits with an aluminium clip.

A lot of work went into making these gloves and there is also a lot of work involved in getting objects ready for display. Unfortunately, historic items like this aren’t always built to last. Museum conservator Marion Parker explains: “Modern materials like this will slowly degrade and we can't do much to stop this. What we can do is to control the conditions the objects are stored and displayed in to slow down these reactions.”

One of the nice things about getting objects out of the collection to show other people is that you get the chance to see them through new eyes and remember how exciting they can be. According to Sarah Tutton, curator at ACMI: “The opportunity to delve into the collection at Scienceworks has been invaluable and has led to some interesting tangents and avenues for exploration.”

I know what she means – it’s easy to get lost in the space collection!

About this blog

Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.