April 2024
The Sky Tonight - April 2024
As May brings us into the second half of Djeran the days continue to draw in and sunset is getting earlier every day. Isn’t …
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August 2
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The casual observer
August brings us into the season of Djilba – the First Spring. There will still be plenty of wet weather, but we should start to see more cold, clear nights. Sunrise is getting earlier and sunset is getting later, with Aug 31 having about 27 minutes more daylight than Aug 1.
Sagittarius and Scorpius dominate the night sky this month. Scorpius is easily spotted, with bright red Antares marking the heart of the Scorpion and the distinctive hook shape pattern of stars marking the tail standing out easily from background stars. By comparison, Sagittarius is a complete mess of a constellation, but the teapot asterism is reasonably easy to spot. These constellations straddle the bright centre of the Milky Way galaxy as it looms overhead.
Image: Scorpius and Sagittarius and the bright centre of the Milky Way. The teapot asterism in Sagittarius is shown in green.
Credit: Stellarium
There are two full moons this month, on Aug 2 and Aug 31. The second full moon is called a ‘blue moon’ but of course be aware that this has nothing to do with its actual colour; it will still look like every other full moon you’ve ever seen. Blue moons are quite rare, happening only every 2-3 years, and this exceeding rarity motivates the saying “once in a blue moon” to describe something of great infrequence.
Interestingly, both full moons are ‘supermoons’. A supermoon is when the full moon occurs at or around perigee – the closest point to Earth in the Moon’s orbit. Being at full moon phase at perigee means the supermoon will be larger in the sky by about 5% and thus brighter than an ‘average’ full moon by about 12%. This being said, the actual difference in size is difficult to notice unless you have a before and after.
Image: Supermoon (left) and an average full moon (right)
Credit: Stellarium
A supermoon also means that new moon occurs at apogee – its furthest point from Earth – and so will look smaller than usual, a so called ‘minimoon’.
Mercury is visible in the western sky all month for an hour or so after sunset and is definitely worth a look.
The Perseid meteor shower occurs this month. This is a very reliable and famous shower so you will probably hear about it in your media feeds, but unfortunately it is so far north that it is largely unobservable from Australia. Only those up in the top end have any real chance of seeing this shower as it peaks around Aug 13, however you may see a few meteors peeking out very low above the northern horizon if you are out before sunrise.
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 |
| 9 Aug 05:53 AM | 30° above WSW | 58° | 10° above NNE | -3.6 | 4.5 min |
| 16 Aug 6:25 PM | 10° above NW | 74° | 10° above SE | -3.8 | 6.5 min |
Table: Times and dates to spot the ISS from Perth
Source: Heavens above, Spot the Station
Dates of interest
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First full moon
August 2
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Mercury, Mars and Moon all close in the western sky
August 18
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Saturn at opposition
August 27
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Blue moon
August 31
Planets to look for
Venus is gone is gone from the evening skies after the first week or so of the month. It is currently overtaking Earth in its orbit of the Sun, meaning it will be passing in front of the Sun as seen from Earth. This means it will be out during the day and lost in the glare of sunlight. After a short absence it will appear in the east before sunrise to become the ‘morning star’. And here it will stay for the next 9 months marking the imminent arrival of the Sun.
Mercury is visible in the west after sunset for the whole month, as is Mars. Mercury will be the brighter of the two and moves noticeably from day to day. On Aug 18 they are joined by the waxing Moon in a nice alignment.
Image: Mercury, Mars and the Moon on display on Aug 18
Interestingly, the asteroid 2 Pallas – the third largest asteroid in the solar system – is lurking in the background of this view, not far from the Moon, but you will need a ‘scope and some decent wits to spot it.
Saturn is rising about 6:30pm this month and reaches opposition on Aug 27. Point one hand at the Sun and the other at Saturn and you will be pointing in opposite directions, and on this day, Saturn will rise exactly as the Sun sets. This will also be the closest approach of Earth and Saturn this year, a mere stone’s throw of 1.31 billion km.
Image: Relative position of the planets on Aug 27. Note that the Sun and Saturn are in exact opposite directions seen from Earth.
Credit: NASA, Eyes on the Solar System
Jupiter is rising about midnight so to view it you will need to be up either very late or very early.
Constellation of the month
Sagittarius the Archer
Sagittarius is a large constellation that contains the centre of the Milky Way and is probably the busiest constellation in the sky. Deriving its name from the Latin word for Archer, Sagittarius is depicted as a centaur, a half horse – half human chimera drawing a bow and arrow.
Image: Sagittarius backdropped by the Milky Way
Credit: Stellarium
The orientation of the solar system means that the orbit of the planets and the path of the Sun also pass through Sagittarius making it a zodiacal constellation as well.
Containing the centre of the Milky Way galaxy, Sagittarius is home to an enormous number of stars, the densest regions of which are named the Sagittarius Large and Small Star Clouds. These are not star clusters in the usual sense, and instead are many thousands of lightyears deep and contain millions of stars in the same field of view.
Image: The Large Sagittarius Star Cluster with millions of resolved stars. The dark nebula Barnard 86 is in the centre foreground.
Credit: Panther Observatory
The constellation contains a number of dark nebulae – clouds of dust and gas dense enough to obscure visible light from the stars in the background – such as the Bok Globule Barnard 86 that appears in the foreground of the Large Sagittarius Star Cluster. Bok globules are collapsing clouds of gas and dust where star formation eventually takes place.
More Bok globules are located inside the Lagoon Nebula, also in Sagittarius. The Lagoon nebula is a large emission nebula and a HII region, indicating that active star formation is occurring.
Image: The Lagoon Nebule in Sagittarius
Credit: ESO/VPHAS+ team
Star forming regions often contain many extremely hot O-type stars that shine brightly in UV light, which ionises surrounding gas and causes the nebula to glow, hence the term emission nebula. The atomic transition n=3 → n=2 in hydrogen atoms gives off a photon of wavelength 656nm, placing it firmly in the red part of the spectrum and giving emission nebulas like the Lagoon Nebula a pinkish glow.
Fittingly, the constellation is also home to Sgr A* (pronounced Sagittarius A star), the supermassive black hole at the centre of the Milky Way. The very centre of the galaxy is a complex and chaotic environment, and Sgr A* was first discovered as an intensely bright region in images produced by radio telescopes.
Image: Radio emissions from the centre of the galaxy show complex structures like bubbles and arcs. The supermassive black hole Sgr A* is located inside the bright region in the lower centre.
Credit: I. Heywood, SARAO
Careful studies of stars orbiting Sgr A* eventually concluded that it was a compact object of about 4 million solar masses located in an area no larger than the orbit of Earth. The only plausible object to explain this was concluded to be a supermassive black hole, a discovery that eventually won the Nobel Prize in physics in 2020.
Video: Stellar orbits in the galactic centre imply the existence of a massive compact object.
Credit: UCLA Galactic Center Group
While it is impossible to see the black hole with the eye, you can still determine its location by using the teapot asterism and extending imaginary lines as shown:
Image: Projecting an imaginary line across the teapot by twice its length allows you to locate Sgr A*
In recent years, scientists from around the world formed a collaboration called the Event Horizon Telescope (EHT). The EHT consists of a number of radio telescopes across the globe working in unison to image objects in the sky. The individual telescopes all observe the same target and are then paired up with each other one by one and their data is correlated in a process called ‘Very Long Baseline Interferometry’ to simulate a single telescope as large as the Earth. This effective size gives the EHT the resolution of about 1/100 millionth of a degree, sharp enough to image the centre of the galaxy in detail, and in May 2022 scientists from the Event Horizon Telescope released an image of Sgr A*.
Image: The black hole in the middle of the Milky Way and the bright accretion disk.
Credit: EHT Collaboration
This confirmed beyond any doubt that Sgr A* is indeed a super massive black hole. You can read more about it in a previous edition of The Sky Tonight.
Object for the small telescope
Saturn
As we make our closest approach to this planet, Saturn is best viewed in the late evenings during the middle of the month while the Moon is new. A perpetual favourite for any astronomer, even a modest ‘scope will allow you to see the rings and its largest moon Titan.
Image: Saturn as seen through an amateur telescope
Credit: AstroBackyard
Barnard’s Galaxy
Barnard’s Galaxy is an irregular galaxy located about 1.6 million light years away in the constellation of Sagittarius. A magnitude of 9.3 makes this a faint but worthwhile target.
Image: Barnard’s Galaxy
Credit: ESO
Trifid Nebula
The Trifid Nebula is an unusual emission nebula neighboured by a reflection nebula in the constellation of Sagittarius. The two different nebula classes give it distinct red and blue colours. Foreground dust clouds appear to divide the emission nebula portion into three, hence the name.
Image: The colour dichotomy of the Trifid nebula is striking
Credit: ESO
The incredible star fields of the Milky Way
Just point a telescope anywhere inside Sagittarius and look.
Image: Good golly! It’s completely occupied by stars!
Credie: ESO
Chandrayaan-3 Lunar Mission
On Jul 14 the Indian Space Research Organisation (ISRO) launched the Chandrayaan-3 spacecraft on an ambitious mission to the Moon. The spacecraft consists of a propulsion unit that is guiding the Vikram lander, on board which is the Pragyan rover.
Image: The Integrated Chandrayaan-3 craft
Credit: ISRO
Currently, the spacecraft is undergoing a complex series of orbital manoeuvres to get to the Moon and if all goes well it will enter lunar orbit sometime around Aug 6 and Vikram will attempt a landing around Aug 23. If the landing is successful, the Pragyan rover will then be deployed to explore the landing site.
Image: Chandrayaan-3 flight path.
Credit: ISRO
Chandrayaan-3 follows its predecessors Chandrayaan-1 and 2. Chandrayaan-1 launched in 2008 and consisted of an orbiting vehicle used to construct high resolution maps of the lunar surface, as well as the Moon Impact Probe (MIP) designed to crash into the Moon. Studies of the data from the MIP ultimately determined the presence of water on the Moon, adding to the body of evidence overturning the idea that the lunar soil is completely dry.
Chandrayaan-2 was a more complex mission launched in July 2019 and consisting of an orbiting spacecraft for mapping and communications, as well as a lander and rover, also called Vikram and Pragyan respectively. In September 2019, Vikram attempted a soft landing on the lunar surface but at an altitude of 2km the spacecraft lost control and ended up crashing into the lunar surface at about 200km/h. ISRO put the failure down to a software error but haven’t publicly released the exact details.
Image: Crash site of Chandrayaan-2 Vikram lander.
Credit: NASA/GSFC/Arizona State University
The Chandrayaan-3 Vikram lander has been slightly modified based off the lessons learnt from Chandrayaan-2. Interestingly, the Chandrayaan-2 orbiter is still operational and will be used as a backup communications outlet for Chandrayaan-3.
Chandrayaan-3’s Vikram lander and Pragyan rover each carry a number of scientific instruments. Vikram carries equipment to measure the thermal conductivity and temperature of the surface of the Moon, as well as seismic equipment to measure geological activity around the landing site.
The lander also carries a Langmuir probe to measure the plasma density near the surface of the Moon and its variations. Since the Moon has next to no atmosphere, the surface is constantly bombarded by solar wind, which can ionise what little atmosphere there is and potentially kick up microscopic, charged dust particles from the surface. It is these ionised particles the Langmuir probe will investigate.
Image: Chandrayaan-3 Vikram lander
Credit: ISRO
Hitching a ride on the lander is a Laser Retroreflector Array provided by NASA. Unlike regular mirrors, which reflect light out at an angle equal to the angle of incidence, retroreflectors always reflect light directly back to its source. Scientists on Earth can then aim lasers at the retroreflectors on the Moon and use the reflections to study the Moon’s orbit over time, a practice that started back in 1969 when the Apollo 11 crew left reflectors on the Moon for this purpose.
Image: An example of a retroreflector
Credit: NASA Insight
Meanwhile the Pragyan rover carries the Alpha Particle X Ray Spectrometer and the Laser Induced Breakdown Spectroscope, both of which will be used to determine the composition of the lunar rocks and soil around the landing site.
It is intended that the lander and rover will operate for about 14 days, the length of 1 lunar day, before the intense cold of the lunar night causes them to shut down.
Vikram will be landing at 69.37 degrees south, the equivalent of landing in Antarctica on Earth. The lunar south pole is a place of intense interest in the astronomy community after a convincing body of evidence has built up to show that there is water present around here, both locked up in the soil and exposed on the surface in permanently shadowed craters in the far south.
Image: Ice, highlighted in green, at the Moon’s south pole (L) and north pole (R) detected by NASA’s Moon Mineralogy Mapper (M3). Interestingly, M3 was a payload on the Chandrayaan-1 orbiter.
Credit: NASA
NASA’s Artemis program intends to return people to the Moon at the lunar south pole, and current projections of lunar colonisation all hinge on the abundance of water in this region. Chandrayaan-3 is a tiny step on this journey.
Unrelated, but…
You might have noticed that a few days after Chandrayaan-3 launched, a mysterious piece of debris washed up on the beach in Green Head, 250km north of Perth. Police encouraged people not to speculate what the object is, and this means that everybody immediately did the exact opposite.
Image: Washed up debris
Credit: Kelsey Reid/The West Australian
The general consensus from the internet, which is never wrong, is that the debris appears to be from the booster of an Indian rocket. However, given the weathering of the debris, with extensive barnacle growth covering sections of it, it is not likely to be related to the Chandrayaan-3 launch.
