The Sky Tonight is a monthly update of the amazing things you can find when looking up from here in Western Australia. This section is written by Jacquie Milner, resident astronomer at the Scitech Planetarium.
The casual observer
The Equinox occurs on the 23rd of September this year, when the Sun is directly over the equator. After this day southern hemisphere observers will have more daylight than night time for the next six months as the Sun appears to move southward. Summer is on the way!
There will be a daytime occultation of Venus during the 18th of September that will be visible from all over Australia and New Zealand. What will be tricky, however, is that the Moon and Venus will be close to the Sun, with the Moon a thin crescent only two days from being new. This is usually a good time to see Venus during the day, as the Moon makes it easier to focus on the normally depthless sky, but their close proximity to the Sun will make it challenging. To be safe and to make it easier, block the Sun out by standing next to a wall.
Below are the times of disappearance of Venus on the bright side of the Moon and then when Venus will reappear on the dark limb of the Moon (away from the Sun) for the capital cities around Australia:
|Perth||7.41 am||8.33 am|
|Darwin||8.58 am||10.41 am|
|Adelaide||9.58 am||11.15 am|
|Melbourne||10.44 am||12.00 pm|
|Brisbane||10.47 am||12.20 pm|
|Sydney||10.49 am||12.17 pm|
|Hobart||10.57 am||11.58 am|
The region of visibility of the daytime occultation of Venus (between the dashed red lines).
Dates of interest
18th – Moon next to Venus, dawn sky
18th – Daytime occultation of Venus by the Moon (see The Casual Observer above)
19th – Thin crescent Moon next to Mars and Mercury, low in dawn twilight
22nd – Thin crescent Moon to the right of Jupiter (brightest) and Spica, evening sky
27th – Moon above Saturn, evening sky
Planets to look for
Jupiter is starting to get lower in the evening sky now, so September will be the last month this year to see it at a decent hour. By the end of the month it will be setting with the end of twilight, and will quickly sink out of reach during October. It passes by the bright star Spica during the second week of
September, and the new Moon joins them for a nice photo opportunity on the 22nd.
Saturn is high overhead as darkness falls. It will be the focus of attention mid-month as the Cassini mission comes to an end.
The other three planets are low in the morning twilight, which will be a challenge even to the dedicated observer. Venus continues to sink down towards the Sun. A 26 day-old Moon will hover close to it on the morning of the 18th, a few hours later travelling over Venus in a daytime occultation (see The Casual Observer above).
The next morning the Moon will be even lower in the morning twilight meeting up with Mars and Mercury. You may have seen Mercury earlier in the month below Venus, as it makes a poor morning apparition.
As for Mars, it is slowly climbing out of the Sun’s glare now. It won’t be much to look at this month, or the next, either. Patience is needed until next year for the red planet.
Constellation of the month
Capricornus the Sea Goat
Capricornus is one of the constellations of the Zodiac and most people would know it as just 'Capricorn'. Its name means “the Horned Goat” and it is a very old constellation. It is usually depicted as a goat with a fish's tail for his hindquarters, an odd combination at any time. So why would such a fantastic creature be created? Capricornus seems to date from Sumerian (pre Babylonian) times and is connected with the god Ea or Oannes. Ea had many names, and one of them was "the Antelope of the Subterranean Ocean," so this seems the most likely explanation.
About two thousand years ago the Sun reached the southern-most point of its annual journey amongst the stars of this constellation, hence the name of the Tropic of Capricorn. (Due to Precession, caused by the wobble in the Earth's axis, this point now occurs in neighbouring Sagittarius.)
Objects for the small telescope
Neptune, the 8th Planet
The two outermost planets, Uranus and Neptune, are little fainter than the eye can see, but if you know where to look even a small telescope will be able to pick them out. Neptune has been moving slowly through Aquarius in recent years, and this September it will be near a decent star to make it easy to find. You will have to do a little bit of work but the patience should pay off.
Aquarius is in an area of the sky where we look out and away from the galaxy, so there aren’t many bright stars to grab our attention – or distract us, either. We can use the constellation of the month, Capricornus, as a guide, and the bright star Fomalhaut, which marks the mouth of Pisces Austrinus, the Southern Fish. Further eastwards in the sky is the line of stars that represents the stream of water that the Water Bearer, Aquarius, is pouring from his jug. Lambda Aquarii is directly below the tail of Capricornus.
Once you have centred lambda Aquarii in you telescope use the second chart to inspect the nearby star field. You should be able to notice a small, blue star that’s out of place – Neptune! If you are able to go back and visit this same area at different times over the month you will also get to see that it has moved. Why not keep a journal of these observations? Do a quick sketch of the stars you can see in your telescope, then when you come back, make a new mark where Neptune has moved to. Then you’ll both proof and a nice record that you’ve observed the distant ice giant of our solar system.
Neptune is not far from the star lambda Aquarii during September.
The movement of Neptune during September 2017, with lambda Aquarii for reference.
The Finale of the Cassini Mission
After 13 years in orbit around Saturn the Cassini mission is coming to end on September 15th. It will be a sad but triumphant moment for many astronomers and scientists around the world, and for the many fans of this mission that have been awed by the beauty of this unique planet and its surprising moons. The spacecraft will soon runout of fuel for correcting its direction, and rather than have it accidentally collide with two of the most interesting moons in the solar system, Titan and Enceladus, mission control will direct Cassini to plunge into the clouds of Saturn itself and hopefully reveal a bit more about what lies beneath the swirling yellow atmosphere we see from space.
So many discoveries and stunning images were returned during the mission that it’s hard to pick just a few, but here are some of the more important results from the mission.
One of the first jobs Cassini had was to release the Huygens probe down onto the surface of Titan. Titan is covered in a thick layer of methane – the sky is yellow as a result, and it rains methane. There are methane lakes and rivers and they change over time. The left hand image below is from Huygens on the surface – that is what it looks like to stand on another world! The right hand image is of a complex river system running into a lake – it looks just like river systems here on Earth!
Credit1&2: ESA/NASA/JPL/University of Arizona;
Cassini took this image of the rings of Saturn from directly behind the planet, and in doing so discovered they extended even further into space than thought.
Credit: NASA/JPL/Space Science Institute
In the image below, we can see huge spikes along the edge of the main rings, probably pushed up by one of the moons, and they are casting long shadows on the rings behind them. On the average the rings are only 20 metres thick, and are extraordinarily and uniformly flat, so deformities like this are very noticeable.
Lastly, the biggest surprise at Saturn was the salty water jets from the moon Enceladus, indicating that there is not only an ocean of water under its icy crust, but a heat source pushing the jets of water out. This place is a very real possibility for finding life in the solar system.
Credit: NASA/JPL/Space Science Institute
The last images from Cassini will be sent down the day before atmospheric entry, on the 14th September, before the cameras are turned off. If you follow along on the evening of the 15th, remember that it will take 82 minutes for the signal to travel from Saturn back to Earth.
Head over to the mission website at saturn.jpl.nasa.gov for more information and perhaps to find your highlights of this stunning mission that we’re all going to miss once it ends.