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Full Transcript below
Dr Ian ‘Astroblog’ Musgrave returns with your essential guide to the September night sky — including the spring equinox, an apogee First Quarter Moon for comparison against earlier perigee shots, and Venus at its greatest brilliancy (magnitude -4.5) as it sinks toward evening twilight.
Mercury returns to the evening sky and lines up with Venus and Spica, Saturn climbs ahead of next month’s opposition, and Mars threads through Gemini in the morning sky. Comet 220P/McNaught is still in outburst, T Coronae Borealis is still refusing to go bang, and much of southern Australia gets to watch the Moon occult bright Antares on the 17th–18th.
In the Tangent, Ian traces the tangled, thousand-year argument over why Venus and Mercury never wander far from the Sun — from Ptolemy’s epicycles to Tycho Brahe, Simon Marius, and Galileo’s telescope.
In This Episode:
- Spring Equinox: Earth reaches equinox on September 23 (autumn equinox for Northern Hemisphere listeners)
- Moon Phases: Last Quarter, New Moon, an apogee First Quarter Moon, and Full Moon
- Evening Skies: Mercury returns and lines up with Spica and Venus; Venus sinks toward twilight at its greatest brilliancy; Saturn climbs toward October opposition
- Morning Skies: Mars crosses Gemini; Saturn still well placed pre-dawn; Jupiter re-enters the morning sky
- T Coronae Borealis: still hasn’t gone nova
- Comet 220P/McNaught: a second outburst, still visible in binoculars and small telescopes
- The Occultation of Antares, September 17–18 (Adelaide, Canberra, Hobart, and Melbourne get the disappearance)
- Deep-Sky Spotlight: Sagittarius, and the triangle of globular clusters — M28, Omega Centauri, and 47 Tucanae
- The Andromeda Galaxy, the Magellanic Clouds, the Southern Cross, Triangulum Australe, and Ara
- The Tangent: Ptolemy’s epicycles, Tycho Brahe, Simon Marius, and Galileo’s discovery of the phases of Venus
Full Transcript: Introduction
Welcome to this episode of Astrophiz. My name is Brendan O’Brien and Astrophiz is produced on Yorta Yorta, Pangarang and Kaurna country. We celebrate the first astronomers of this land.
Monthly, we explore the universe with the world’s leading scientists. For transcripts and more, visit Astrophiz.com.
And here is Dr. Ian Musgrave with your September SkyGuide.
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Brendan: Hello, Ian.
Ian: Hello, Brendan.
Brendan: Great to be speaking with you again, Ian. It’s been a fabulous year. Can you tell us what’s up in the sky for September?
Ian: Okay, well, there’s going to be a lot of interesting things in September. Now, the planetary action has returned mostly to the evening skies, although the morning skies are still putting on a bit of a show.
Venus, which has been our companion for many months, is now starting to sink in the late twilight. Then Mercury returns to the evening sky later in the month below Venus, and you will see some interesting interactions between Mercury and Venus.
Saturn is now well visible in the evening sky, but still visible in the morning sky. And Jupiter enters the morning sky below Mars.
So let me talk about it in a bit more detail. First off, the Earth is at spring equinox on the 23rd. For those of you listening to this in the Northern Hemisphere, of course, that’s autumn equinox.
The September Moon Phases
Ian: The Moon is our guide for many of these things. September the 4th is Last Quarter, which is ideal for stargazing in the evening. September the 11th is New Moon, which is also ideal for stargazing. September the 19th is First Quarter. Now, this is an apogee First Quarter Moon. If earlier in the year you took some photographs of the perigee First Quarter Moon when the Moon was closest to Earth, taking some photographs of the apogee First Quarter Moon is going to get you a nice little comparison of the differences in size between the two.
We tend to talk in terms of supermoons and mini-moons, but it’s important to realise that perigee and apogee also occur at all Moon phases. A perigee New Moon you can’t see, not very exciting, but a perigee First Quarter Moon is easy to see early in the evening. And again, you see this difference in size between the two Moons, and it’s very interesting.
And then on September the 27th, there’s a Full Moon, which is a fairly ordinary Full Moon, but still very nice, let me tell you.
The Moon is at perigee on September the 7th, and at apogee on September the 19th.
Evening Skies: Mercury, Venus and Saturn
Ian: Now, let’s have a look at the planets and the skies. As I said, Mercury enters the evening twilight later in the month, and from the 16th, it can be seen low above the western horizon. You’ll probably need a fairly low, level western horizon to see it easily. It forms a line with Spica and Venus, then starts rising rapidly into the evening sky. It will be at its best next month, but this will be a harbinger of what we’ll see next month. And on the 26th, Mercury will be less than a degree from first-magnitude Spica — the brightest star in Virgo — with Venus above it, looking very nice in mid-twilight.
Now, Venus is sinking in the evening sky. It was at its greatest distance from the Sun late last month, but now it’s beginning to sink. It’s still in Virgo and well visible after the sky is fully dark at the beginning of the month, and it starts entering nautical twilight later on in the month. If you’re looking through a telescope, Venus is a distinct crescent, and the crescent becomes more and more obvious as the month goes on, even in small telescopes. Venus is also apparently becoming bigger and bigger as this goes on.
For the first couple of days of September, Venus will be less than two degrees from Spica. And then on the 14th, Venus is two degrees from the four-day-old waxing crescent Moon. If you remember the Venus-crescent Moon conjunction in August, it was a very impressive sight — and also a good time to try to see Venus in the daylight, because on the 19th, Venus is at its greatest brilliance, around magnitude -4.5. Incredibly brilliant, relatively easy to see in daylight — but with an obvious guide like the crescent Moon, it’ll be incredibly easy to find. And don’t forget: make sure you’ve got the Sun blocked out by something like a building or other large, bulky object so you don’t have any chance of damaging your eyesight by looking at the Sun accidentally.
Now, with Venus glowing in the west, Saturn is rising in the east, climbing higher ahead of the opposition next month, when it will be biggest and brightest as seen from Earth. But even now Saturn is an excellent sight in small telescopes. At the beginning of the month, it’s rising about 8:30 p.m. and is highest about 2:30 a.m. Mid-month, it’s rising around astronomical twilight and is highest around 1:30 a.m. And by the end of the month, it’s rising before the Sun sets and is highest around midnight. Then on the 27th, the Moon is only six degrees from Saturn, making it very easy to find if you haven’t been able to work out which bright object above the eastern horizon is Saturn — it’s basically the only really bright object there. Through a small telescope, the rings have been steadily widening from the very thin apparition of last year.
Morning Skies: Mars, Saturn and Jupiter
Ian: Now, let’s go back to the morning sky. Mars is still very obvious above the eastern horizon at astronomical twilight, that’s an hour and a half before sunrise. It’s in Gemini, passing through the body of the twins. On the 7th, it’s about four degrees south of the crescent Moon — this will look very nice in the early morning. It may need an unobstructed horizon to see it at its best, but it’ll be very nice.
Saturn’s still in the morning skies too, and as I said, in the early part of the month it’s highest around about 2 a.m. So for most of this month, Saturn — at least in the very wee hours of the morning — will be nicely visible and probably the best time for telescopic observation, even though it’ll still be very good in the evening. It can be seen sinking lower and lower in the west as the weeks go by.
Now, Jupiter is entering the morning sky and is low above the horizon, although on the 9th the planet will be around two degrees from the 27-day-old waning crescent Moon. It’s very low above the horizon in the twilight, and you’ll need a flat, level horizon without any large trees or obstructions to see it properly. By the end of the month, it’s still low, although higher, and more readily seen.
And those are the planets.
T Coronae Borealis and Comet 220P/McNaught
Ian: Now, T Coronae Borealis: guess what? It still hasn’t gone bang. Oh no!
Brendan: Oh no!
Ian: Yes. Now, in our last podcast, I didn’t talk about Comet 220P/McNaught, because its outburst occurred after the podcast went to air. But Comet 220P has undergone an incredible outburst. It’s zoomed up almost 7,000 times in brightness, from a telescopically-only-visible 14th magnitude to around seventh magnitude — and it’s done it twice. We’re on the second burst now. It’s still relatively bright — I mean, you need strong binoculars or a telescope to see it, but it’s around magnitude eight to nine, maybe even magnitude seven, depending on who’s doing the measuring.
It’s readily visible in the early morning. I put out some spotter’s charts last month in August, which will still be valid because it’s not moving very far. I’ve been getting some nice images from the remote telescope at Siding Spring, but you won’t see it anywhere near as excitingly through your own eyepiece — it’ll be a dim dot — but it’s worthwhile having a look at if it hasn’t faded too much. It might even have another outburst, so keep an eye out. It could be very interesting.
Brendan: Cool.
The Occultation of Antares, September 17–18
Ian: Well, maybe it is cool — it’s looking quite amazing. But the other thing happening this month is an occultation of Antares, on Tuesday, September the 17th to 18th. This is a good occultation because it’s approaching First Quarter, so you’ll be able to see Antares disappear behind the dark limb of the Moon without the Moon’s light getting too much in the way.
It’s a bad occultation because it’ll be quite low to the horizon, so you’ll need a clear and level horizon to see it — but the Moon will be obvious, Antares will be obvious, and if you’re looking a bit before midnight, or half an hour before midnight in Adelaide’s case, you’ll be able to see the dark limb of the Moon, possibly in Earthshine, crossing in front of bright Antares.
Brisbane misses out, Darwin misses out, Sydney and Perth miss out, but Adelaide, Canberra, Hobart, and Melbourne all get to see Antares wink out — and then the Moon sets before you can see it come back again.
Brendan: Nice.
Deep Sky: Sagittarius and the Globular Cluster Triangle
Ian: As you can tell from the occultation story, Scorpius is now heading westward and setting late in the evening, roughly around midnight, but it’s still high in the sky after astronomical twilight. Sagittarius, at the heart of the Milky Way, is at the zenith from about 8 p.m., so it’s a good time to go out with a lawn chair, a pair of binoculars, and search around the teapot of Sagittarius for all the wonderful clusters around there.
The magnificent globular cluster M28 is at its highest at this time, just off the lid of the teapot. But its rival, Omega Centauri, is setting, though it’s still quite high and readily visible. And the other magnificent globular cluster of the southern hemisphere — of the entire sky, really — 47 Tucanae, is rising. These three globular clusters form a broad triangle across the southern sky. They’re quite faint, but if you’re out in the depths of the countryside with nice dark skies, you can see these faint, foggy dots forming a nice triangle across a large chunk of the southern sky.
The Winter Triangle, which I talked about last podcast — Deneb, Vega, and Altair — is very clear in the north. So we’ve got two triangles: the Winter Triangle of bright stars, and this “summer triangle” of faint globular clusters. Both are equally interesting.
By mid-month, the classical Andromeda Galaxy is seen low above the northern horizon. And in the south, the two dwarf galaxies, the Magellanic Clouds, are rising, with the Small Magellanic Cloud almost at its highest. So we’ve not only got the two competing triangles, but competing galaxies too. For a lot of us the Andromeda Galaxy is quite low — you may need binoculars to see it if you’re in the suburbs, just above the horizon — but it’s still interesting in binoculars, and can be seen as a faint, fuzzy patch under dark skies.
Now, our friend the Southern Cross is about at the four o’clock position as it heads southwards. It doesn’t really set from most of Australia, although it does set in Darwin, though not completely. The Pointers are above. The Pointers can be used not only to point to the Southern Cross itself, but if you look a bit south of the Pointers, you’ll see a triangle — the constellation Triangulum Australe. Follow the point of the triangle up and you’ll see a blocky rectangle — the constellation Ara, the Altar. These are unprepossessing, not incredibly bright constellations, but there are some nice little clusters near them, so it’s worth getting out the binoculars and scanning around just to have a look.
And that’s the sky for September. Lots of interesting things to see.
Brendan: Thank you very much, Ian. Now, we’re recording a week earlier than usual because I have to head up to Sydney — my daughter’s running in the Sydney Marathon, so we had to record a bit earlier. I may not have given you enough time to develop a tangent, but Ian, do you have one for us?
Ian: Indeed I do, and it’s a quite interesting tangent.
Brendan: Cool.
The Tangent: Circles, Epicycles, and the Race to Explain Venus and Mercury
Ian: I’m going a bit into history here, but it’s very relevant to what we’re seeing. As I said, Venus is beginning to sink back towards the horizon, becoming bigger and brighter, and more of a crescent. Over a thousand years ago, this was a bit of a puzzle to people: why could all the other classical planets be seen throughout the night, but only Venus and Mercury stayed close to the Sun?
The history of astronomy is often portrayed very simply: Ptolemy and his Almagest with his geocentric system, then along comes Copernicus with the heliocentric system, a bit of argy-bargy with Galileo and Kepler, and then we have the heliocentric system we know today. But it’s never quite as simple as that.
Ptolemy solved the puzzle of why Venus and Mercury don’t get far from the Sun with a series of complicated circular motions — the epicycles — to keep these planets apparently close to the Sun while the rest got very far away. Remember, this is all before telescopes; every observation was done by eye. And when they constructed their cosmology, they brought in this extra piece of baggage: that all orbits had to be circular, because the heavens were perfect, and the most perfect figure was the circle.
Ptolemy synthesised the observations of centuries beforehand into this beautiful system: Earth-centred, because Earth had to be the centre — the heavens were incorruptible, heavenly matter, and Earth was corruptible, and all corruptible things fell towards the Earth, which sat outside the heavens. You had a series of shells: first the Moon, then Mercury, then Venus, then the Sun, then Mars, Jupiter, Saturn. In some versions — because we forget there were lots of other astronomers and groups continually fiddling with Ptolemy’s system — Earth sat at the centre, then Moon, then Mercury and Venus, then the Sun, then the outer planets.
Because we now know planetary orbits are ellipses, not circles, keeping the planets on track so they’d turn up where predicted required all these epicycles — mathematical constructions of circles within circles — plus the deferent and the equant. I won’t go into the details, but it took a lot of mathematical jiggery-pokery to make the planets turn up where you predicted without too much difference between prediction and observation.
Indian astronomers, Muslim astronomers, all had their own versions too. In the Almagest, Earth does not rotate — everything rotates around it. But a number of other astronomers proposed that Earth actually rotates. It’s difficult to explain why they thought Earth was either unmoving or rotating, because they’d already worked out precession of the equinoxes and knew Earth would continually change where it pointed in the sky. That made more sense with a rotating Earth. But with a non-rotating Earth, the outer shells of the solar system could provide the motion for the inner shells — the concept of a primum mobile pushing all the other circles was, to them, an intellectually satisfying answer for why the planets moved at all.
People kept fiddling. A couple of astronomers came up with heliocentric systems, but they weren’t popular. Then in 1614, a man called Simon Marius came up with a somewhat different solar system: Earth, the Moon, the Sun — and Venus and Mercury orbiting the Sun. This wasn’t exactly the first time the idea had come up, but it arrived at a time when Copernicus’s heliocentric system was providing a better explanation for why Venus and Mercury didn’t get far from the Sun, because you had Sun, Mercury, Venus, then Earth — and what we saw with Venus and Mercury was simply a consequence of them orbiting the Sun as seen from Earth.
There was a lot of argy-bargy, but Simon’s system had advantages: the outer planets and the Moon did what they’d always done, Mercury and Venus orbited the Sun, and it solved the puzzle of why Mercury and Venus never got far from the Sun — while still keeping Earth at the centre and preserving most of the existing calculation methods.
Then along comes Tycho Brahe. He puts all the planets around the Sun, and has the Sun and Moon orbiting the Earth. This preserved circular motion — a big deal, because one of the things Copernicus had achieved, besides naturally explaining Venus and Mercury, was getting rid of the equant, which took away from perfect circular motion and made everything more complicated. Copernicus got rid of the equant while keeping epicycles and circular motion — everyone said “fantastic, this is really cool, but we still don’t believe everything goes around the Sun.”
Tycho put everything around the Sun. Fantastic! And he shattered the crystal spheres. The planets were thought to move within spherical shells — often called “crystal spheres,” though not crystal in any mundane sense, but nonetheless tangible material carrying the planets. For Tycho’s system to work, those spheres had to intersect each other, which couldn’t happen if the spheres were sensible, tangible material. Even so, everyone said: this is a really good system — it explains why Venus and Mercury don’t get far from the Sun, it keeps Earth at the centre, and it doesn’t have that weird jump of planet-planet-Sun-planet-planet-around-the-Sun. So this was really good, really cool.
Then along comes Galileo. He puts the Sun at the centre again, refines the calculations a bit beyond the original Copernican system, looks through his telescope — and discovers the phases of Venus and Mercury. You can’t have the full range of phases of Venus and Mercury in the Ptolemaic system. You can get some phases if you place Venus and Mercury between the Sun and the Moon — you can go from gibbous to full — but you can’t get crescent phases. Crescent phases killed Ptolemy’s system dead.
But you can still have crescent phases in both the Tychonic system and Simon Marius’s system, because in both, Venus and Mercury orbit the Sun. So what you needed was a giant intellectual leap: was it the Galilean-Copernican system, with phases of Venus wiping out Ptolemy entirely? Or the Tychonic system — essentially Copernicus’s system inverted, which preserved the appearances but made things more complicated? Distinguishing between them required technical issues that weren’t apparent at the time.
And everyone was ignoring Kepler. Kepler had worked out the ratios of the orbits, why the planets speed up and slow down, and got rid of all the epicycles with elliptical orbits — it just fell out naturally. But it still had to go around the Sun. So everyone was asking: is it Tycho, Simon Marius, or Galileo — which one’s right?
What you needed was parallax — and not just one kind, but two. Stellar parallax was the obvious one, but with the naked eye and the telescopic instruments of Galileo’s time, it was still too small to detect, because the stars were very far away. Copernicus had already anticipated this and said if the stars were far enough away, you simply wouldn’t see the parallax. Tycho countered that if the stars were that far away, they’d have to be more than seven times further out than the sphere of Saturn — and he just couldn’t accept a solar system that big.
So we’ll leave it there — I think we can talk about Kepler and how he approached parallax another time. But I’ll just point out that a number of the things that shaped our view of the solar system depended on people believing circles were the most perfect figure, and believing the stars couldn’t be very far away, because that would mean a ridiculous size for the solar system.
Brendan: Thank you very much, Ian ‘Astroblog’ Musgrave.
Ian: No worries. I found that absolutely fascinating. I love the history of astronomy, and I’ve been having a lot of fun reading about this — I didn’t know of this particular argument before. I knew some people came up with weird ideas for the solar system, but I didn’t know about this particular group, so that was very, very fascinating.
Brendan: Excellent. Well, keep looking up.
Ian: I will indeed, I will indeed. And everyone else out there, keep looking up, because there’s lots of things to see.
Brendan: Clear skies, Ian. Good night.
Ian: Good night, mate. Have a good time at the marathon, and all the best to your daughter.
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