May 2025
The Sky Tonight - May 2025
May continues the season of Djeran – the season where the cooler weather begins. Maybe it will get here eventually? …
ExploreFirst Quarter
April 5
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The casual observer
April brings us into the season of Djeran, characterised by cool evenings giving way to cooler mornings, if we’re lucky.
Some of the brighter sections of the Milky Way start to become more prominent in the southeast this month. You can start at the Southern Cross and follow the pointers down to the horizon to trace the outline of the brighter parts of our galaxy.
Image: The Milky Way and emu are getting higher in the evening sky as the year progresses. Credit: Stellarium
The Lyrids meteor shower runs for the second half of the month, and peaks on Apr 22. These meteors are visible low on the northern horizon, not far from the bright star Vega. This star is part of the constellation Lyra, which gives its name to this shower because of its proximity.
The best time to view this shower is in the morning before sunrise. This shower really is best viewed from further north, but in good conditions you might see a few meteors per hour in this part of the sky. Unfortunately, the bright Waning Gibbous Moon will be a thorn in the side of your observations.
Image: Apparent location of Lyrids relative to Vega on Apr 22. Credit: Stellarium
The Full Moon this month occurs when the Moon is at Apogee – its furthest distance from Earth in its orbit. This means the Full Moon will look smaller than average and is sometimes called a Micromoon. In some sense this is the opposite of a Supermoon, but much less reported on.
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 |
| 6 Apr 05:28 AM | 10° above SW | 68° | 10° above NE | -3.5 | 6 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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Lyrids meteor shower peaks
April 22
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Moon, Mercury, Venus, Saturn and Neptune visible in eastern morning sky
April 26
Planets to look for
Jupiter and Mars continue to occupy the northwestern sky this month. Jupiter is approaching the western horizon and appears out of the twilight as the Sun sets. You still have a couple more months to see Jupiter in this part of the sky, but it is gradually moving behind the Sun so sooner is better for any observations.
Image: Jupiter in the western sky during April. Credit: Stellarium
Mars, having hung around Gemini for the last few months, makes a noticeable movement against the background stars. It almost seems to hang there in the northwest while the background stars keep moving by it. That motion of the background stars is actually due to Earth’s motion around the Sun.
Video: Mars (red marker) moves against the background stars during April. Moon moves past in the foreground. Credit: Smith/Scitech, Stellarium
The eastern morning sky is an absolute delight as Mercury, Venus, Saturn and (with a telescope) Neptune are all visible before Sunrise. Venus will dominate the eastern sky as the ‘morning star’ for the next 6 months. If you go out on the morning of Apr 25 or 26 you will see the 4 planets joined by the Moon, all within a few degrees of each other.
Image: Eastern sky on the morning of Apr 26. Credit: Stellarium
Constellation of the month
Cancer – the Crab
Cancer is a medium sized constellation located in the northern sky during April evenings. The name comes from the ancient Latin word for Crab, and so it is depicted as thus.
Image: Cancer, the Crab, in the nothern sky. Credit: Stellarium
The constellation is a zodiacal constellation, meaning that the Sun’s path through the sky over a year passes through this part of the sky. It is however, the faintest of the zodiacal constellations and is somewhat unremarkable to look at.
The brightest star Beta Cancri comes in at only magnitude 3.5 and everything else is fainter. This star system, about 290 lightyears away, contains an orange giant star about 50 times larger than the Sun orbited by a tiny red dwarf companion.
Parhaps most studied star in this constellation is the faint star 55 Cancri, a star slightly smaller than the Sun orbited by a distant red dwarf. This astonishing star system is known to contain at least 5 planets. In order from the primary star they are: a ‘super Earth’ about 8 times heavier than Earth, a Jupiter-sized gas giant, two Neptune-sized planets, and lastly another Jupiter-sized planet.
Image: Relative orbit sizes of the 4 inner planets of the 55 Cnc system to our own Solar System.
Excitingly, the outermost Neptune-sized planet (4th from the star), called 55 Cnc f and also known as ‘Harriot’, is located in the habitable zone of the star. This is the distance from a star where the expected ambient temperature is between 0-100 celcius – the liquid temperatures of water. Being Neptune-sized, astronomers think 55 Cnc f is likely a gas planet, but there is no shortage of speculation that the planet may have moons that could potentially be covered in oceans. More research is needed.
Image: Artists impression of 55 Cnc f.
Object for the small telescope
Messier 44 – the Beehive Cluster
Also known as Praesepe, meaning ‘cot’, this open cluster is located right in the middle of Cancer and consists of about 1000 stars. Located around 600 lightyears away, it fills about 1.5 degrees of the sky and is easily visible in binoculars or smaller telescopes. Numerous O-type blue stars are visible, as well as several red giants, representing the late-stage evolution of stars.
Image: The Beehive Cluster, with the brightest red and blue stars visible. Credit: Fried Lauterbach. CC BY-SA 4.0
Saturn and its moons. And its moons. And its moons. And its moons.
In early March, Saturn’s moon count stomped ahead of the rest of the Solar System with announcements bringing the ringed planet’s total moon count to 274 natural satellites. This is almost twice as many as all the moons around all the other planets added together, totalling ‘only’ 142 moons, of which 95 orbit Jupiter.
Saturn is already well known for its interesting moons. Its largest moon Titan, about 50% larger than Australia, is the only moon in the solar system with an atmosphere, and the only other object in the solar system aside from Earth with lakes and rivers on its surface. Unlike Earth however, these lakes are made of liquid methane – literally rocket fuel, while the atmosphere is made mostly of nitrogen.
Image: Titan, with lakes visible through the hazy atmosphere in the top left. Credit: NASA / JPL-Caltech / Space Science Institute
Titan is (arguably) edged out of the ‘most interesting Saturnian moon’ seat by Enceladus. This icy moon, about as wide side to side as the distance from Perth to Kalgoorlie, is known to have subsurface oceans of liquid water beneath its tortured frozen exterior. The surface of Enceladus bears large cracks, somewhat like the fragmenting of Earth’s surface into tectonic plates. At weak points in the surface, molten ice (aka water) can force through and erupt as ice volcanos.
Image: Ice volcanos on Enceladus, with the cracked icy surface visible. Credit NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill. CC BY 2.0
Titan and Enceladus are part of a group of 24 Saturnian moons classified as regular moons. Most of these moons orbit almost directly above Saturn’s equator and move prograde, in the same direction that the planet rotates. Astronomers generally think the regular moons formed with Saturn billions of years ago as the planet coalesced from the primordial disk that formed our solar system.
Regular moons are so named to distinguish them from remaining 250 moons, classified as irregular moons. The orbits or these irregular moons are much more scattered, often greatly tilted to Saturn’s equator, many of them orbiting retrograde – in the other direction to Saturn’s rotation – and just generally being messy. All 128 of the newly announced moons are irregular moons.
Image: Irregular moon orbits of Saturn. Prograde orbits coloured blue and retrograde orbits coloured red.
Credit: Nrco0e – Own work, CC BY-SA 4.0,
It is worth pointing out that the newly announced moons were actually discovered a while ago. They were picked out of observations mostly performed between 2019 – 2023, but some data goes as far back as 2004. The reason they are in the news now is they have been officially recognised by the International Astronomical Union, the governing world body on astronomical matters.
These new moons are tiny, only a few km across, making them extraordinarily difficult to spot. Astronomers use a technique called ‘shift and stack’, basically a fancy way of saying adding the data together (stack) while accounting for the motion of Saturn (shift). In any one image you won’t see much, but adding lots of observations together allows a weak signal to stand out from ambient noise.
Image: The shift and stack technique allows faint objects to be seen in otherwise difficult to interpret data. Bright streaks are background stars. Credit: Edward Ashton/CFHT/Canadian Astronomy Data Centre, processed by Nrco0e – Own work, CC BY-SA 4.0,
Finding new moons of Saturn isn’t just a mindless intellectual exercise. The point is to look for patterns and interestingly, most of Saturn’s irregular moons have uncorrelated orbits, giving us clues to its history. If the irregular moons were formed by, for example, a single large moon breaking into pieces, then we would still expect them to have similar orbits. While there are some similarities between smaller subgroups of irregular moons, the lack of overall correlation suggests that the irregular moons of Saturn come from possibly many smaller moons involved in breaking apart or violently colliding, or both, in either order. There is also the possibility some of them are captured asteroids. Teasing out the history of the solar system from a few faint pixels in a photograph is some powerful science.
This probably won’t be the last we’ll hear about moons around the outer planets but even based on what we have discovered so far, an interesting pattern emerges. When you plot how many moons of a given size Saturn has compared to Jupiter, Saturn still leads by more than 3 times for the data available, suggesting that Saturn intrinsically has more moons than Jupiter, another interesting clue that might be hinting at something deeper.
Image: Number of moons of Jupiter and Saturn ranked by size shows Saturn intrinsically has more moons. Credit: Ashton et al. 2021
This all does beg the question of what is a moon? And how small is too small? At the moment there is no consensus among astronomers where to draw the line for what is a moon. Most people would agree that the trillions of particles of ice that make up Saturn’s rings probably don’t fit the bill of being called a moon. Somewhere in there is an answer, but more science is needed to really understand what makes moons stand out from the rubble.
Image: The tiny moon Pan orbits inside Saturn’s rings and has an equatorial disk made from accreted ring material. Credit: NASA/JPL-Caltech/Space Science Institute
