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The casual observer

Djilba is in full swing as warmer days become more common and we enter the season of spring. 

Fittingly, September brings with it the spring equinox, occurring on Sep 23. The equinox marks the point in the Earth’s orbit where the Sun passes directly over the equator. On this day, people living on the equator will see the Sun rise exactly in the east, pass directly overhead at midday and set exactly in the west. This also means that on this day there will be exactly 12 hours of day and night everywhere on Earth. From here on out, Earth’s continued motion along its orbit will make the Sun continue to appear to drift in a southerly direction, moving it higher and higher in the sky, bringing with it the warmer days that lead into summer. 

Image: Earth from space, as seen on the day of the equinox 


Mars is slowly disappearing in the western sky at sunset as it finishes its long, slow trip to the horizon. By the end of the month, it will be barely visible at sunset so now is your last chance to see it. 

Saturn makes for good viewing this month as it is still near its closest point to Earth for this year. 

The full arch of the Milky Way still high up above but makes a noticeable shift to the west this month. The Southern Cross is now visible lower in the southwest evening sky, a sure astronomical reminder that spring is here. 

Image: The Southern Cross is low in the southwest during September evenings.  

Credit: Stellarium 


ISS sightings from Perth 

The International Space Station passes overhead multiple times a day. Most of these passes are too faint to see but a couple of notable sightings are*: 

Date, time  Appears  Max Height  Disappears  Magnitude  Duration 
5 Sep 6:40 PM  10° above SW  85°  10° above NE  -3.7  6.5 min 
19 Sep 5:33 AM  10° above WNW  57°  10° above SE  -3.6  6.5 min 

Table: Times and dates to spot the ISS from Perth 

Source: Heavens above, Spot the Station 

*Note: These predictions are only accurate a few days in advance. Check the sources linked for more precise predictions on the day of your observations. 

Phases of the Moon

Last Quarter

September 7

New Moon

September 15

First Quarter

September 23

Full Moon

September 29

Last Quarter

September 7

Dates of interest

  1. Moon near Jupiter

    September 5

  2. Moon near Antares

    September 21

  3. Spring equinox

    September 23

  4. Moon near Saturn

    September 26

Planets to look for

This is a great time to observe Saturn. It is visible immediately from sunset and appears distinctly yellowish in the eastern sky in the early evenings. If you’re up later you will be able to see Jupiter from about 10pm onwards. The Full Moon on Sep 29 places the Moon almost exactly between these two giants in the sky. Uranus is also viewable not far from Jupiter, but you will need a telescope to see it. 

Image: Full Moon on Sep 29 is located apparently between Saturn and Jupiter.  

Credit: Stellarium 


Mars is still there in the west for an hour or so after sunset. It is soon to be passing behind the Sun, so looks very faint against the twilight. 

Are you looking at a bright object in the eastern sky before sunrise? It’s Venus. Mercury joins party in the latter half of the month, though is lower and fainter than Venus so will be more difficult to observe. 

Constellation of the month

Pavo – The Peacock 

Pavo is a medium sized constellation in the far southern sky. Like the Southern Cross, it is circumpolar from most locations in Australia, meaning that it never goes below the horizon, no matter the time of day or year. 

Pavo was described by Dutch Astronomer Petrus Plancius based on maps of the southern sky produced by Keyser and de Houtman, and is artistically represented as a peacock. The group of constellations consisting of Pavo and nearby Phoenix, Grus and Tucana, is referred to as the Southern Birds. 

Image: Pavo the peacock. 

Credit: Stellarium 

Despite its reasonable size, Pavo is quite faint, with the brightest star Alpha Pavonis having an apparent magnitude of 1.94. This means that the middle of this month is best time to view the constellation to coincide with the New Moon on Sep 15. 

Inside Pavo is located the galaxy NGC 6744. This is a spiral galaxy about 30 million light years away and is thought to be similar in appearance to the Milky Way galaxy, because of the presence of distinct spiral arms and a barred core. 

Image: NGC6744, a Milky Way look alike. 

Credit: ESO 


Pavo is home to the notable star Delta Pavonis, a Sun-like star about 20 light years away. Only marginally larger and older than the Sun, spectroscopic analysis of this star reveals it to be high in metals. Keep in mind that astronomers refer to anything larger than hydrogen or helium as metals. Higher metallicity of stars seems to correlate with the likelihood that it will have planets, suggesting that Delta Pavonis is more likely to have planets than a random baseline sample, and the star remains an interesting topic of research in exoplanetology. 

In the canon of the science fiction saga Dune, Delta Pavonis is orbited by the planet Caladan, the ancestral home of House Atreides. 

Object for the small telescope

NGC 6752 

NGC 6742 is a globular cluster located in the constellation of Pavo. With an apparent magnitude of 5.4, the cluster is technically visible to the naked eye, though you will struggle to see it from light polluted areas. 

Image: The globular cluster NGC 6752 

Credit: ESA/Hubble & NASA 

The cluster is about 13000 light years away and contains about 100000 stars in a region about 100 light years across. Like all globular clusters, NGC 6752 appears to be extremely old, coming in at 11.78 billion years. 

The Aditya-L1 solar observatory 

Far from taking a break after their success with the Chandrayaan-3 mission placing the Vikram Lander and Pragyan rover on the Moon, the Indian Space Research Organisation is launching the Aditya-L1 spacecraft on Sep 2 with the purpose of studying the Sun.  

Image: Artistic representation of Aditya-L1 with solar panels stowed. 

Credit: ISRO 

The spacecraft will study the behaviour of the outer layers of the Sun and their relationships between each other, as well as the effects that these behaviours have at Earth. 

When we look toward the Sun with our eyes (something you shouldn’t do by the way), what we are really seeing is the photosphere. This is the layer of the Sun’s atmosphere from where the light is actually emitted that eventually reaches our eyes. The diameter of the Sun is about 1.4 million km, and the photosphere is a layer of this only about 100km thick.  

Sitting above the photosphere is the chromosphere, a layer of plasma about 2-4000km thick. Ionised hydrogen gives this layer of the solar atmosphere a reddish colour due to hydrogen alpha emission. Normally this layer is drowned out by the bright light of the photosphere, but it is there. Finally at the top is the Sun’s outer atmosphere, the corona. This sparse, tenuous layer is usually only visible during a total solar eclipse when the Moon blocks out the rest of the Sun, or by using a specialised telescopic piece called a coronagraph which does the same thing. 


Image: Ogres have layers, onions have layers, the Sun also has layers. You can draw your own lessons from that. 

Credit: NASA 

Aditya-L1 carries a coronagraph to image the Sun’s corona as well as an ultraviolet imaging telescope that will use a range of filters to study the chromosphere and photosphere. Imaging different layers of the Sun’s atmosphere simultaneously will provide clues to how these layers interact with each other, and how energy is transferred from one layer to another. 

The complexity of the Sun’s behaviour comes from the merging of fluid mechanics and electrodynamics. The gases in the Sun’s atmospheric layers behave like fluids, whose behaviour is described by the Navier-Stokes equations of fluid dynamics. However, in the case of the Sun, these fluids consist of charged particles, and the behaviour of charged particles is described by Maxwell’s equations of electromagnetism. Crucially, electromagnetic effects between the particles feed back into their fluidic behaviour, which then feeds back into their electromagnetic interactions and so on in a highly nonlinear way. The complete mess of analysis is called magnetohydrodynamics and that’s all we’re going to say about it here. 

 An active area of solar physics research concerns the temperature of the Sun’s corona. Curiously, while the photosphere has a temperature of about 5700K – the generally accepted ‘temperature of the Sun’s surface’ – the corona reaches temperatures of several million Kelvin. Why the upper atmosphere is several orders of magnitude hotter than the photosphere is still not fully understood, but understanding the relationship between the corona and the photosphere and chromosphere may shed light (ha!) on the situation. 

Image: The Sun’s corona as seen in the April 2023 Ningaloo Total Solar Eclipse 

Credit: Dr John Chappell/Scitech 

Aditya-L1 also carries two X-ray instruments for studying high energy radiation. Solar flares – intense localised bursts of radiation- and coronal mass ejections – large eruptions of plasma ejected away from the Sun – are both strong sources of solar X-rays and their activity may be related to the corona temperature problem. 

NASA | Magnificent Eruption in Full HD 

Video: Timelapse footage of a coronal mass ejection. 

Credit: NASA Goddard 

The spacecraft will enter a halo orbit around the first Lagrange point L1, hence its name. This is a point of gravitational stability between Earth and the Sun about 1.5 million km in the direction of the Sun. This has the advantage of always giving an unobstructed view of the Sun as well as placing it well outside of Earth’s magnetosphere.  

To this end, the craft also carries a magnetometer to measure the interplanetary magnetic field, as well as two particle detectors to study the solar wind. The solar wind is the term given to the constant stream of high energy charged particles that are ejected from the Sun. It consists mainly of protons, electrons and alpha particles, as well as a few heavy ions. The instruments will study how the solar wind varies over time, with a particular emphasis on when coronal mass ejections are observed and correlating that to effects on the magnetic environment near Earth. 

Image: Artistic representation of the solar wind interacting with a planet.  

Credit: NASA 

Since the solar wind consists of charged particles, it itself interacts with and carries the interplanetary magnetic field. The variation of the solar wind and interplanetary magnetic field, along with its interaction with planets, especially ones with their own magnetic field like Earth, is collectively referred to as space weather. Understanding the impact of space weather on Earth’s weather and climate is an important area of research. 

Ultimately Aditya-L1 will provide valuable information about how the interactions between, and the processes that occur within, the different layers of the Sun’s atmosphere have an effect here at Earth. Studying the Sun is complicated. One might say that it’s easier to land on the Moon instead! 


Other space news 

Chandrayaan 3 lands on the Moon! 

Russia Luna 25 mission fails to land on the Moon 

First images from the Euclid spacecraft are released 

SpaceX Starship Booster 9 completes a static fire 

Crew 7 mission launched to the International Space Station 

Array ( )

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