+++*

Symbolic Forest

A homage to loading screens.

Blog : Posts tagged with ‘Saturn’

The old gods

The astronomy season is starting again

We’re getting to the time of year now when it’s properly dark before a reasonable bedtime; as opposed to a couple of months ago, when it is still twilight in the deepest part of the night, which around here happens at about quarter past one in summer. August, by comparison, is the time of year when I can go outside at 10pm and see if the sky is clear enough to do a small bit of stargazing before bed. It’s too late to wake up The Child Who Likes Space, who nominally owns the telescope, but nevertheless, I rationalise, I can always tell him about it in the morning.

A while ago, I noticed that according to timeanddate.com’s planet apparent size calculator, Jupiter would have a relatively large disc right now. Right now, in fact, it’s receding from us, but it’s still a relatively chunky 49 arcseconds wide. Still a dot to the naked eye—the Moon is about 36 times bigger in apparent diameter if my rough mental calculations are correct—but big for a sky object, and with the best chance we would have of seeing features on it. Over the past few weeks I’ve noticed that Jupiter and Saturn together, although relatively low in the sky, are very definitely the brightest things in the south-eastern sky when I go to bed. So last night, as the sky was almost clear, I decided to get the telescope out.

Last spring I found it rather hard to get the telescope set up in the new garden, due to the street light that shines directly into it over the garden wall. Back then, though, the garden was a rocky, rubble-strewn wasteland, which didn’t help. Now it’s grassed, and at the moment I can tuck the camping table and the telescope into a relatively shady distant corner; from which both planets were shining bright in the sky. It was as easy as any astronomy I could think of: set up the telescope, point the finder on Jupiter, and as soon as I had focused, I had the planet and the four Galilean moons right in the centre of the eyepiece. All four of the moons were on the same side of the planet last night, Io just visible almost touching the planetary disc, the other three clear and sharp and separate spread out to the east of the planet. Jupiter itself was a fairly uniform cream colour, with a thin, darker, more reddish band visible near its equator. It seemed so sharp and clear, much more clear and bright than a photograph.

After Jupiter, I trained the spotter on Saturn, much smaller in the sky. At first it just seemed to be an oval blob, but I’d knocked the focus off slightly. Tweaking it showed the planet, orange in colour, and its rings. We don’t have anywhere near enough magnification to show the ring divisions, and the rings and the planet seemed to have a fully uniform colour. It’s strange to think that when Stegosaurs were alive and tramping the planet, Saturn probably didn’t have any rings at all. I couldn’t make out any of the planet’s moons, but I know they are much fainter than those of Jupiter, my eyes probably hadn’t had time to fully adapt to the dark, and I didn’t know where to be looking in any case. I wonder how different the history of science would have been, if Jupiter didn’t have four clear bright moons for Galileo to spot easily with his early telescopes.

Incidentally, due to geometry, it’s impossible for the planets orbiting outside Earth to have phases like the Moon does: their discs will always appear from our standpoint to be fully illuminated. This coming winter Venus will be the largest planet in the sky—it peaks at just under 63 arcseconds on January 8th—and it will be interesting to see if around then more than a thin crescent is visible. Assuming the skies are clear, of course.

Cloudy skies

And not much we can do about it

Sadly, I didn’t get to see the Great Conjunction of Jupiter and Saturn, at least not at the closest approach that would have been visible. We had heavy rain here this afternoon; and after sunset the sky was a uniform, undifferentiated cloudy mass with not even the moon visible.

Oh well: we had clear skies last night, at least, and I did see them both, maybe only about 10 minutes or so apart in the sky, just over the horizon after sunset. I tried to take a photo on my phone, but although Jupiter was clearly visible on it, Saturn was only really spottable if you already knew it was there. Maybe it’ll be clear skies tomorrow, when they are parting again.

The solstice has passed too, of course. Politically this country might seem be descending into some sort of nightmarish fimbulvetr right now, but at least the heavens don’t know that.

More astronomy news

A significant event is coming up

One last astronomy post for a while, and then I’ll talk about something different, I promise: in a few weeks time, on December 21st Saturn and Jupiter will be at their closest conjunction for several hundred years.

The Plain People Of The Internet: Conwhatnow?

They’ll come close together in the sky. The closest they’ll be for a few hundred years, in fact. They’re already fairly close in the sky right now, but on December 21st they’ll be so close that to some people they’ll look like a single spot of light, although people with 20/20 vision should still be able to see they are two separate dots.

Unfortunately … I’m not sure I’ll be able to see it. They’re both on the far side of the sun from us right now, and we’re basically watching Jupiter passing in front of Saturn from our perspective on the other side of the solar system. Because of this, they’re also fairly close to the sun in the sky, which means you don’t get a very long window of opportunity to see them. They will set together in the south-west sky only about 90 minutes after the sun does, so you have a brief window of time to see them together at dusk. If you’re on a south coast then things are grand; if you’re in a town, they may well be already below your neighbour’s roofline before the sky gets dark enough to see them. Personally, our only chance will be from an upstairs window. Fingers crossed, though, the sky will be clear enough to see something of them both together.

One of our galaxies is missing!

Or, more astronomy for beginners

Yesterday afternoon, sitting at my desk as dusk was falling, the skies were clear and I could clearly see the moon and Mars rising in the sky. As soon as I logged off from work, I scampered downstairs and went outside, and saw Jupiter and Saturn just visible above the rooftops at the back of the house. “Let’s get the telescope!” I said to The Child Who Likes Animals Space. “Before they set!”

“Before what set?” said The Child, but I was already rushing off to get his telescope and set it up in the back garden.

By the time I’d hoicked it out of its box, Jupiter had already gone down below the roofline, but Saturn was still there. Sadly, I could only see it when I was stood up. The telescope, sitting on a camping table, was too low down to spot it. I briefly considered setting the smaller camping table on top of the larger camping table and making some sort of rickety makeshift telescope-tower, but it would probably have ended in some sort of injury to one or both of us. So, like we’d done before, we looked at the moon, we looked at Mars, and we looked at random bright stars. “Point it at that blue thing!” he shouted. “That’s a very hot star!” This time, at least, he was a lot calmer and could stand still looking through the eyepiece without having to break off every couple of seconds to run back and forth with excitement. I experimented with holding my phone camera in front of the eyepiece. Worst. Astrophotography. Ever.

Worst. Astrophotography. Ever.

After The Children decided they’d rather go inside and watch TV I left the telescope set up; and after they’d gone to bed, the skies were still clear. Time for some telescope practice for me, I decided. Using an app on my phone to show me roughly where things were, I tried focusing on key visible stars then swinging the telescope sideways to find a nearby Messier object. The results? Not very successful, other than a possible sighting of M29, the Cooling Tower Cluster.

Getting a rough fix and vaguely hoping to spot the thing clearly wasn’t working. So, I fetched my laptop, and fired up Stellarium in Night Mode. I picked a target—the Triangulum Galaxy—and went looking for it.

Although the skies were clear, the seeing wasn’t great. Even a newbie like me could tell that the seeing wasn’t good. Vega normally stands out to me like a sore thumb, but last night it didn’t really appear any brighter or more significant than the stars of the Northern Cross to its south, which normally are noticably fainter. Nevertheless, I could see Hamal and Sheratan, the brightest stars in Aries, and could spot the telescope nicely onto Hamal and swing it between the two. Zooming in on Hamal in Stellarium, and flicking my head back and forth from the telescope eyepiece to the dim red screen of the computer, I could slowly navigate my way upwards from star to star until I reached Mothallah, which I hadn’t managed to see with the naked eye.

Road map of the stars

From there, I could similarly hop south towards the spot where the Triangulum Galaxy should be, navigating from star to star and matching the scene in the sky to the screen of the computer. But when I reached it: nothing. Just a blank patch of sky. I found Mothallah again, then worked my way across by a different route. Still nothing. The Triangulum Galaxy has been stolen!

Let’s try the Andromeda Galaxy instead, I thought, given that it’s one of the brightest galaxies in the sky. I found Mirach by eye, spotted the telescope on to it, and walked over to where the galaxy should be. Another blank patch of sky, with a faint hazy blob in the middle of it. Hurrah! A faint hazy blob!

I’m almost glad I hadn’t found it the other day when The Child Who Likes Animals had asked me to find it, because I suspect if I had he’d have been awfully disappointed. I was a bit puzzled, though, because in theory the Andromeda Galaxy is of naked-eye magnitude. I should have seen much more than a fuzzy blob, surely?

A quick note about how astronomical “apparent magnitude”, or brightness to you and me, works. It’s a relative scale based loosely on the subjective scales used by ancient astronomers, and as it’s relative it’s written down as a number without units. The higher the number, the fainter the thing is; and a difference of five in the apparent magnitude number means “a hundred times brighter”. If you have a calculator to hand you can work out that a drop in magnitude of 1 therefore means “2.5119 times brighter”.* The star Vega, mentioned earlier, has magnitude 0, so a few things in the sky have negative magnitudes: Sirius is -1.47, Jupiter varies from -1.66 to -2.94, and Venus from -2.98 to -4.92, almost 100 times as bright as Vega.

On a good night, with a clear dark sky, the human eye should in theory be able to see things as faint as magnitude 6 or so. Last night was clearly nowhere near that: I could see Sheratan at 2.655 and Albeiro (in the Northern Cross of Cygnus) at 3, but couldn’t see Mothallah at 3.42. Through the telescope, though, I was happily stepping my way across the sky using stars of 8 to 8.4, roughly speaking, stars around 140 times less bright than the faintest I could see without it.

The Triangulum galaxy, though, is officially** of magnitude 5.72. Andromeda is considerably brighter still, around the same magnitude as Mothallah. So what was going on?

A galaxy isn’t a point of light, like nearly all stars are. Nearly every star in the sky, other than a handful of stars like Betelgeuse when seen through very high magnifications, appears to be just a single point of light to the viewer here on Earth. A galaxy, by comparison, covers a broad chunk of the sky. That small fuzzy blob I could make out in Andromeda was really just the very brightest core of the galaxy; and the rest of that 3.4 magnitude of light is spread out over an area wide enough to fill my entire eyepiece. Through the telescope, it just becomes a vaguely paler area of sky.

Hopefully at some point we will get the telescope out on a night with rather better viewing conditions, and be able to see all of these things properly. Until then, it seems strangely unintuitive to be able to see hundreds of dimmer stars but not a theoretically many-times-brighter galaxy. That, though, is just how the physics works.

* Apparently, the formal definition “five means a factor of 100” was set down by a Victorian astronomer with what I think is a great name, N R Pogson. The relationship 1:2.5119 is therefore known as Pogson’s Ratio.

** I say “officially”. I mean “according to what I’ve just read on Wikipedia”, of course.