June 2025
The Sky Tonight - June 2025
June brings us into the season of Makuru, a time of colder weather and rain. Fittingly, it is the beginning of winter. Winter …
ExploreFirst Quarter
July 3
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The casual observer
July continues the season of Makuru and with it continues the fantastic, cool weather. Ironically, this is the best and worst time of the year to see the Milky Way galaxy because Earth’s position around the Sun gives us an excellent view of it at night, if only you can catch a gap between the clouds.
Image: The Milky Way stretches across the sky during July. Credit: Stellarium
As usual, the further you are from city lights, the better your view of the sky will be. The magnificent view of the billions of stars in the Milky Way visible from Australia has been amusingly explored by xkcd as a what-if? “If every country’s airspace extended up forever, which country would own the largest percentage of the galaxy at any given time?”
Australia emerges as a clear winner. You really should click this link to read the details.
Image: The new Australian flag. Credit: xkcd/what-if
Earth reaches aphelion on July 3. This is the point in the orbit where we are furthest from the Sun. A seasonal reminder that this is not why it is winter (remember, it’s summer in the northern hemisphere right now!). It is true that the distance from the Sun does has some effect on the temperature, but most of the temperature difference between summer and winter is caused by the tilt of the Earth. Currently, Earth’s tilt is pointing the southern hemisphere away from the Sun, making it cooler. That this happening at the same time as aphelion is pure coincidence.
Image: The tilt is the reason for the seasons! Credit: © Time and Date
Venus continues to shine brightly in the morning sky before sunrise. It has a close encounter with Uranus on July 4 before threading the gap between Aldebaran and the Pleiades a few days later as it continues to move through Taurus.
Image: Venus moves through Taurus this month. Credit: Stellarium
See the International Space Station 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 |
| 2 Jul 7:12 PM | 10° above SW | 51° | 51° above W | -3.0 | 3.5 min |
| 3 Jul 6:23 PM | 10° above SW | 66° | 10° above NE | -3.8 | 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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Earth at aphelion
July 3
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Venus close to Uranus
July 4
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Venus close to Aldebaran
July 11
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56th anniversary of the Neil and Buzz walking on the Moon.
July 21
Planets to look for
Mercury is visible in the west for an hour or so after sunset for the first few weeks of this month. Mars is also looming in the northwest, and they are joined by the Moon on the first day of the month.
Image: Mercury, Mars and the Moon on July 1. Credit: Stellarium
Saturn rises about midnight this month, moving across the ENE sky before it fades away at sunrise.
Jupiter makes a return to the eastern sky before sunrise, joining Venus in a bright display. This will continue to get more impressive as the month goes on.
Image: Jupiter and Venus in the morning sky during July. Credit: Stellarium
Constellation of the month
Sagittarius – the Archer Centaur
Sagittarius is an enormous constellation, both in size and in scope. Lying across the centre of the Milky Way galaxy, its borders are filled with countless millions of stars and gas clouds. The brightest stars that we can see from Earth trace out the clearly defined shape of a centaur firing a bow and arrow and hahaha no not really, they look nothing like that. They are nevertheless drawn to look like a centaur anyway.
Image: Sagittarius the Archer Centaur, I guess. Credit: Stellarium
More pragmatic observers focus on finding the teapot, discernible in the above image and presented properly below. Easily visible even from city skies, by imagining steam coming from the teapot you can locate the exact centre of the Milky Way galaxy.
Image: The teapot can be used to find the centre of the Milky Way galaxy. Credit: Stellarium, Markup by Smith/Scitech.
At the centre of the Milky Way galaxy is a supermassive black hole called Sagittarius A*. It is pronounced ‘Sagittarius A star’ but it is definitely not a star.
By studying orbits of actual stars at the centre of the galaxy, scientists were able to determine that the only thing that could have enough gravity to dominate stellar orbits like this would be a compact object weighing 4 million times as much as the Sun, squashed into an area smaller than the orbit of Mercury. The only thing that fits this criterion is a super massive black hole. This discovery eventually won the 2020 Nobel Prize in physics.
Video: Stellar orbits around the centre of the galaxy teased out the existence of the supermassive black hole. Credit: ESO.
More recently, the Event Horizon Telescope imaged Sagittarius A*, revealing the ghostly glow of hot plasma standing out from the darkness as it spirals around the black hole, eventually to be consumed.
Image: The black hole at the centre of our galaxy. Credit: EHT
Please Look Up. The universe is amazing!
Object for the small telescope
A Close Encounter of the Uranus and Venus kind
Venus and Uranus will pass within 2.5 degrees of each other in the sky on July 4. Joined by the Pleaides, this will present an exciting challenge to keen observers as the fainter Uranus will be hard to bring into focus in the chilly pre-dawn sky.
Image: Venus and Uranus have a close encounter on July 4 while the Pleiades watch on. Credit: Stellarium
The Vera Rubin Observatory Releases First Images
On June 23, the long-awaited release of the first images from the Vera Rubin Observatory took place. That’s quite a sentence, so let’s unpack it in detail. But first, an image:
Image: The Virgo Cluster of galaxies as seen by the Rubin observatory. Nearly every speck of light in this image is a galaxy. Credit: NSF–DOE Vera C. Rubin Observatory
The Vera Rubin Observatory is a newly constructed telescope facility in the mountains of Chile, where it sits high above the thickest part of the atmosphere, giving it a clearer view of the night sky.
Image: The Vera Rubin observatory is located on the mountain tops in Chile. Credit: Rubin Observatory/NSF/AURA/B. Quint
The telescope is fully automated and has a very specific mission: Map the entire sky every three nights for the next 10 years. Rather than spending hours doing long exposures of extremely faint objects, Rubin will do short exposure snapshots and slowly build up layer after layer of data, allowing even the faintest of objects to shine through.
More importantly, the 3-day turnover means that Rubin returns to the same patch of sky every few nights. This means it will spot things that happen fast. Whether it is an undiscovered asteroid zipping across the sky, or an exploding star suddenly appearing in a far-off galaxy, the ability to rapidly view the whole sky in detail every few days puts Rubin into a very special class of telescope.
Rubin is enormous, both in size and scope (hah!). The heart of the observatory is the 8.4m diameter Simonyi Survey Telescope. The special design of the telescope mirror allows it to see a patch of sky about 50 times larger than the Full Moon, focusing the light onto a digital camera with a staggering 3.2 gigapixels. Each picture it snaps collects about 300 times as much information than a typical smartphone camera. Estimated to take about 200,000 pictures per year, Rubin is expected to generate around 60,000 terabytes of data over the lifetime of the survey.
Image: The Simonyi Survey Telescope contains the largest digital camera ever built. Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA/P. Horálek (Institute of Physics in Opava)
After exposing the camera to the night sky for the first time in October 2024 – ‘first light’ as it is called, the first public release of imagery really is only a snapshot (hah!) of what is to come. You should really click this link to see Rubin’s imagery for yourself.
The observatory is named for the American astronomer Vera Rubin, best known for her groundbreaking work in studying galaxy rotation rates. At the time, it was expected that stars on the outer edge of galaxies would move slower than stars closer towards the middle, much like how the planets in our solar system behave – Neptune moves a lot slower than Mercury!
Rubin’s research uncovered the exact opposite. The ‘galaxy rotation curve’ as it is called, did not drop off, rather it seemed to flatten out, indicating that stars on the outer edge were moving at the same speed as those nearer the middle.
Image: Theoretical and observed rotation curve of galaxy M33. Credit: By Mario De Leo – Own work, CC BY-SA 4.0
The problem this creates is the same as the feeling you get when driving around a roundabout too quickly. The tendency to get ‘flung outwards’ would cause a galaxy rotating at this speed to fling itself to pieces and be torn apart. The fact that galaxies don’t fling themselves to pieces leads astronomers to conclude the gravitational force holding the galaxy together is much stronger than expected. To account for this extra gravity, astronomers were forced to introduce dark matter, an unseen component of galaxies that provides the extra mass to hold the galaxy together. No dark matter = no galaxies. Finding the first solid evidence for the stuff that literally holds galaxies together is a pretty good reason to have an observatory named after you.
