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The casual observer

The coming of September welcomes the beginning of Spring so it is warmer days from here on out. Fittingly, September brings with it the equinox, occurring on Sep 23. The September equinox marks the point in the Earth’s orbit where the Sun’s southerly motion carries it directly over the equator. People living on the equator will see the Sun pass exactly overhead on this day. This also means there will be exactly 12 hours of day and night everywhere on Earth on this day.

Image: The Sun crosses the celestial equator (0°) on the day of the equinox. The horizontal lines represent the Earth’s lines of latitude projected onto the sky. The Sun’s motion from positive latitudes to negative (north to south of equator) is clear.

If you haven’t yet gotten out of the city to see the heart of the Milky Way in all its glory, now is your final boarding call. As the year carries out, Earth will move into position such that the central regions of the milky way will soon be behind the Sun. Observationally, this manifests as the Milky Way looms closer towards the western sky in the evenings throughout this month. Take a look each night at Scorpius and Sagittarius, which lie across the very centre of the Milky Way, and you’ll see.

From here onwards, Earth’s path along its orbit will continue to carry the Sun south in the sky, bringing with it the warmer days.

Saturn and Jupiter are easy to see in the evenings in the eastern sky. Any time after 9pm and Jupiter should be up high enough above the horizon to see.


ISS sightings from Perth 

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

Date, time  Appears  Max Height  Disappears  Magnitude  Duration 
4 Sep, 7:23 PM  10° above SW  62°  62° above SE  -3.8  3.5 min 
22 Sep 4:46 AM  54° above WNW  77°  10° above SE  -3.9  4 min 

Table: Times and dates to spot the ISS from Perth 

Source: Heavens above, Spot the Station 

Phases of the Moon

First Quarter

September 4

Full Moon

September 10

Last Quarter

September 18

New Moon

September 26

First Quarter

September 4

Dates of interest

  1. Possible second attempt at Artemis 1 launch

    September 2

  2. Moon near Saturn

    September 8

  3. Moon near Jupiter

    September 10

  4. Moon near Mars

    September 17

  5. 100th anniversary of 1922 solar eclipse and the Wallal expedition

    September 21

  6. Spring Equinox

    September 23

Planets to look for

Jupiter and Saturn continue to impress as they rise earlier in the East every night. Saturn is out before the Sun sets and is joined by Jupiter at about 8pm. They make a nice triangle with Fomalhaut – a bright and obvious star in and otherwise unremarkable part of the sky. 

Mercury is still visible in the West just after sunset very low on the horizon. It will be gone by the middle of the month so be sure to catch a glimpse. 

Mars is visible in the east before sunrise. It is close to the star Aldebaran this month, and with Mars being the slightly brighter of the two, it will move noticeably eastwards relative to this star as the month progresses.  


Constellation of the month

Crux – The Southern Cross 

It’s simplicity is the reason for it’s ubiquity. Four stars in the shape of a cross and a fainter off-axis fifth star. Easily visible from all southern latitudes, the pattern of stars appears on the flags of Australia, New Zealand, Papua New Guinea, Samoa and Brazil as well as many states and territories of these countries.

Combined with the pointer stars, Alpha and Beta Centauri, the Southern Cross can be used to find South, since the lack of a north-star-equivalent can make navigating in southern latitudes challenging. 

Image: Lines drawn from the Southern Cross and between the pointers will meet above direct South. 

Most of the stars in the Southern Cross are part of a larger group called the Scorpius-Centaurus Association. Stars are formed in large numbers all at once inside huge gas clouds called star forming regions. As the stars form, they gradually migrate out of the parent cloud as it evaporates and move independently across the galaxy.  

By tracking the motion of many stars in the Southern Cross, Scorpius, Centaurus and other nearby constellations, astronomers can work backwards to determine that many of them once occupied a similar region of the Milky Way about 15 million years ago, their birthplace. Stars that were formed as part of this group make up the Scorpius-Centaurus association. 

A cultural icon, a useful navigational tool, and members of a stellar family. All represented by two crossed lines in our imaginations. 

Object for the small telescope

Coalsack Nebula 

The Coalsack Nebula is a dark nebula that appears just below the Southern Cross. It is really defined by its absence of light, as the nebula obscures the starlight from more distant stars making it appear as a dark blotch against the bright Milky Way. 

Credit: ESO 

Dark nebulas are composed of cosmic dust, microscopic particles often with organic molecules that in large enough numbers can absorb enough visible light to obscure the view. Observationally, the Coalsack Nebula forms the head of the emu in the sky that was mentioned in the Sky Tonight August


Jewel Box cluster 

The jewel box cluster is an open cluster of stars inside the Southern Cross. The cluster is so named because when first viewed through a telescope it was described as “a superb piece of fancy jewellery”. Interestingly, though it is inside the borders of the Southern Cross, it is not a part of the Scorpius-Centaurs association. It’s distance of about 6000 light years puts it significantly further from us than the stars of that association. 

Credit: ESO 

However, like the Scorpius-Centaurus association, the Jewel Box consists of many young stars, only about 14 million years old and all of them siblings. Despite the young age, DU Crucis has already reached late-stage stellar evolution and has swelled into a red supergiant, standing out prominently against the rest of the stars in the cluster. 

100 Year Anniversary of the Wallal Expedition 

On September 21 1922 a solar eclipse occurred, it’s path of totality moving right across Australia. This was particularly interesting because in 1915 a plucky young theoretical physicist called Albert Einstein had published the general theory of relativity, predicting (amongst other things) that gravity should bend starlight.  

Image: Gravitational lensing by stars or galaxies in the foreground can warp the appearance of objects in the background. 

Credit: ESO 

Because gravity is so weak, a hefty object like the sun is needed to cause a measurable bending of light. So, the experiment is simple. Observe some distant stars one night. Then, observe the same stars when the Sun appears close to them in the sky. The problem is of course, and stop me if we’re going to quickly here, the Sun is very bright, making this essentially impossible. However, a solution presents itself in a solar eclipse. When the Moon passes in front of the Sun it temporarily blocks out the Sun’s light, making it possible to see stars that are otherwise hidden by the Sun’s glare. 

Observers from Australia and around the world travelled to several locations, including Wallal in the north of Western Australia, to observe the eclipse. Or really, they were there to observe the stars behind the Sun that temporarily became visible during the eclipse. 

Image: Don’t worry, nobody else has heard of it either. 

The purpose was to compare the apparent location of stars before and during the eclipse to see if the presence of the Sun in the foreground had any effect on the light coming from the stars in the background. And when all was said and done, Einstein was correct! The starlight had been bent by the Sun’s gravity. Observations made during this eclipse were some of the first in a long line of extensive tests of Einstein’s theory of general relativity, all of which it has passed with great success. And yes this actually poses a bit of a problem for quantum theorists but that is a story for another day. 

Scitech, along with UWA, Perth Observatory and the Gravity Discovery Centre will be holding various events in the celebration of the centenary of this event. The Scitech Planetarium will be including a simulation of the eclipse and a discussion of the relevance of Einstein’s science during the school holidays from 24 Sep – 9 Oct. 


Other space news 

Engine troubles cause the abort of the first launch attempt of Artemis 1. 


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