August 2025
The Sky Tonight - August 2025
August brings us into the season of Djilba. This is a transitional period where winter continues but eventually gives way to …
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September 8
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The casual observer
September continues the season of Djilba. This is a time of wet weather but a slow change into warmer days. Fittingly, the Spring Equinox is on September 23, marking the beginning of astronomical spring. From here on out, it should get warmer as the days get have increasing amounts of daylight.
The Milky Way still stretches across the sky and makes for good viewing in the evenings. The Southern Birds group, made up of the constellations Pavo, Phoenix, Grus and Tucana makes for excellent viewing in the southeastern sky all month.
Image: The Southern Birds and the Milky Way make for great viewing during September evenings, with Southern Cross for reference. Credit: Stellarium
There is a total lunar eclipse – a ‘Blood Moon’ this month, though it’s not for the early sleepers. Beginning at 11:30pm on September 7, the Moon will be fully in Earth’s shadow by 1:30am on September 8 and will be the distinctive dark red colour that gives these eclipses their name. What’s happening is that faint sunlight scattered through Earth’s atmosphere is still able to reach the Moon, even though it is behind the Earth. Due to the way light scatters through Earth’s atmosphere, only the red light makes it all the way to the Moon, resulting in the stunning colour change. The Moon will have completely passed through earth’s shadow by 4:30am on September 8 and the eclipse is over.
Image: A red Moon will be visible to those willing to stay up to see it. Credit: Public Domain
Correspondingly, there is a sister partial solar eclipse two weeks later on September 22 over the South Pacific Ocean, New Zealand and parts of Antarctica. We won’t be able to see this from Perth.
Saturn is at o on September 21. This means it is exactly opposite the Sun in the sky. If you point one hand at the Sun and the other toward Saturn, you will be pointing in exactly opposite directions. The observational consequence of this is that Saturn rises as the Sun sets and will be up all night. This is the best time of year to see the ringed planet in all its glory as it also means we will see the planet face on.
Image: Saturn is visible in the eastern sky 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 |
| 1 Sep 6:46 PM | 10° above SW | 86° | 10° above NE | -3.8 | 6.5 min |
| 15 Sep 05:39 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.
Dates of interest
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Total Lunar Eclipse begins late evening and continues into morning of September 8
September 7
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10th anniversary of first detection of gravitational waves
September 14
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Mars close to Spica
September 14
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Saturn at opposition
September 21
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Spring equinox
September 23
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Moon next to Mars
September 24
Planets to look for
Venus and Jupiter still dominate the eastern morning sky. Jupiter moves higher to the northeast, while Venus gets lower in the sky, sinking into the morning twilight by the end of the month. They are joined by the Moon on September 18 for a nice viewing.
Image: Venus and Jupiter in the eastern morning sky on September 18. Credit: Stellarium
Saturn makes for excellent viewing all month as its location at opposition makes this the best time of year to see it.
Mars hangs in the west during the evenings on its interminable march across the sky. It’s quite faint now at about magnitude 1.6. It has a close encounter with the brighter star Spica on September 14, passing about 2 degrees from it.
Image: Mars is close to Spica on September 14. Credit: Stellarium
Mercury is lost in the glare of the Sun this month.
Constellation of the month
Scutum the Shield
Scutum is a tiny constellation – the fifth smallest of them all – located just north of Sagittarius, placing it almost directly overhead during September evenings. Astronomer Johannes Hevelius originally named it Scutum Sobiescianum after Polish King John III Sobieski (‘The Sheild of Sobieski’) but these days we just remember the first part of it.
Image: Scutum is next to Sagittarius in the sky. Credit: Stellarium.
Scutum is unusual in that it straddles part of the Milky Way but is still quite faint, with the brightest star in its boundaries Alpha Scuti being a miniscule magnitude 3.8. Despite this, in fact, because it straddles the Milky Way, it presents no shortage of astronomical wonders.
Scutum contains the fascinating but sadly uninspiringly named planetary nebula IC 1295. Despite their name, planetary nebula have nothing to do with planets. Rather, when a Sun-like star dies of old age it throws off its outer layers of gas, exposing its hot core. The radiation from the hot core of the now dead star ionises the expelled gas, causing it to glow in all sorts of pretty colours, which gives us the beautiful images in our telescopes that we call planetary nebulae. The name comes from the days of ye olde – planetary nebulae tend to look round in shape through a telescope, just like planets – so early astronomers named them thus. We now know they have nothing to do with planets.
Image: Planetary nebula IC 1295 glows green from ionised oxygen. Credit: ESO
Scutum is also home to the star UY Scuti. This star is one of the largest known stars in the galaxy, an 900 times wider than the Sun, at minimum. Coming in at 125,000 times brighter than the Sun, its surface would probably extend past Jupiter if we placed it in our Solar System.
Image: UY Scuti compared to the Solar System. Credit: 21.Andromedae. CC BY 4.0
You should really watch this video.
Object for the small telescope
Saturn – The jewel of the Solar System
The ringed planet is a must see this month. Rising in the early evening as the Sun sets, it is visible to the naked eye as a yellowish blob. A small telescope quickly reveals the rings and the noticeable flattening of Saturn at its poles as the planet’s rapid rotation squashes it out. You might also see a couple of its myriad moons. The latter half of the month is the best time for viewing, as Earth’s pesky Moon will be out of the way.
Image: Saturn as it appears during September. Credit: Stellarium
10 years of Gravitational Waves
On 14 September 2015, the LIGO facility made the first direct detection of gravitational waves. A wise man later said: “LIGO feels when space is rippling through with a wave of gravitation.”
Video: LIGO feels when space is rippling due to a tensor perturbation. Credit: acapellascience
That’s quite a lot to unpick so let’s begin at the beginning, as the King said. In the beginning there was a smart guy named Albert Einstein. He realised that the space and time of our Universe could be smooshed together mathematically and that the force we call ‘gravity’ could be understood as the shape of this space-time description of the Universe. Don’t ask for details, you’ll only regret it.
In the same way that as you jump into a pool of water and the wave of your splash travels out across the vicinity, Einstein realised that if you smash two things together hard enough then the energy of their collision would change the shape of space-time in a ripple wobbling away from the collision and out into the universe. We call this wobbling shape of the Universe a gravitational wave.
In order to be directly detected by our current technology, these collisions have to be large, combining objects many times heavier than the Sun packed into a small space. Can you think of something that is many times heavier than the Sun packed into a small space? You bet it is black holes! When black holes collide, they spiral into each other, moving faster and faster as they get closer and closer together. What starts as a gentle spiralling infall turns into a furious pirouetting pas de deux. In this process, copious amounts of energy are released as gravitational waves, usually on the order of several solar masses. That is, take E=mc^2, plug in the mass of the Sun for m and pick up your eyeballs off the floor when your calculator tells you just how much energy is released during the collision.
Video: Simulation of two black holes colliding, creating gravitational waves in the process. Credit: LIGO Lab Caltech: MIT
As a gravitational wave propagates across the Universe it wobbles the shape of anything it passes through as it goes. In practice this means that if you point your left arm at a pair of colliding black holes and your right arm out to the side, then a gravitational wave hitting you face-on will rhythmically make your left arm a fraction shorter-longer-shorter-longer-shorter-longer as it ripples through. Your right arm will stay the same length because the gravitational wave is passing through it side-on. So, to detect gravitational waves, all you need to do is measure the length of your arms. It’s as “easy” as that.
Enter LIGO – The Laser Interferometer Gravitational-Wave Observatory. LIGO consists of two facilities, one in Hanford, Washington and the other in Livingston Louisiana. Each LIGO facility consists of two arms, both 4km long, with exquisitely tuned optical systems that constantly measure the length of each arm. By shining a laser down each arm and reflecting it back off a mirror at the far end, scientists combine the reflected laser light back together. If the arms are the same length, you get a nice bright spot on your detector. If the arms are different lengths, you get an interference pattern on your detector.
Video: Animation of how the LIGO interferometers work. Credit: INFN
Gravitational waves from colliding black holes leave a distinctive interference pattern called a ‘chirp’ followed by a ‘ringdown’. Physically, the chirp represents the final furious inspiral of the two merging black holes, and the ringdown is the “surface” of the newly merged black hole radiating excess energy as it becomes perfectly symmetric. More pragmatically, when you play this signal back through a speaker, it sounds like a chirp.
Video: The chirp signal of the first gravitational wave ever detected. Credit: LIGO Lab Caltech: MIT
By September 2015, LIGO had been upgraded to the point where scientists were confident it would detect gravitational waves, if they even existed at all. On September 12, LIGO went ‘live’, collecting data. Two days later it got its first hit. Scientists were able to determine that this signal was caused by the merger of two black holes each about 30 times heavier than the Sun. In the decade since this discovery, LIGO has been joined by the similarly designed Virgo facility in Italy and the KAGRA facility in Japan. Together, these four observatories have recorded almost 300 direct detections of gravitational waves.
Did you know there is a gravitational wave research facility right here in Perth? OzGrav, as it is called, operates the High Optical Power Facility in Gingin, north of Perth. This mini detector consists of two 80m long arms in the same configuration as LIGO. While not sensitive enough to directly detect gravitational waves, research performed at this facility has made vital steps contributing to the international effort to detect these elusive wobbles.
