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Saturday, May 15, 2010

 

I can see Venus from Here!

Jupiter in the morning as seen from Australia. Can you see Ganymede?

My two recent posts have been on Global Warming silliness and Venus. In the intertubes, there is another confluence of Venus and Global Warming silliness as people try to claim there is no, or negligible, greenhouse contribution to Venus’s atmospheric heat. Now, other people have already done the work of showing why this claim is nonsense (see here, here and here), but I was bemused when some Velikovskian’s turned up to argue that Venus is hot because it was only recently formed.

Now, this was only a minor drop of extreme silliness in a sea of nonsense, but as Velikovskianism made an appearance on the ABC science matters list, and this gives me an opportunity to give people some idea of the scale of the solar system, so I will hare off after the Velikovskian rabbit.

Velikovski, as some may recall, famously suggested that to align established historical chronologies with biblical chronologies you had to insert catastrophes; including ones caused by a close approach of the planet Venus to the Earth. In Velikovski’s vision, Venus was ejected from Jupiter as a comet, which then careened around the solar system, coming close to Earth and stopping its rotation then starting it up again.

The ejection of Venus from Jupiter, according to Velikovsky, was accompanied by tremendous electrical discharges and the dramatic separation produced the legend of Athena springing forth from Zeus’ (Jupiters’) forehead.

There’s one small problem, ancient observes would have never seen it. Well, there’s lots of big problems as well [1], but at the very heart of it, the so-called “dramatic” birth of Venus would have been invisible to an Earth observer.

To get a feel for the explanation, go out this morning around 5:30 am. Look up into the north-eastern sky and you will see Jupiter as the brightest object there. Can you see Ganymede? No? At magnitude 4.6 it is bright enough to be seen with the unaided eye under most suburban skies (and easily seen under dark skies). But it is too close to Jupiter for it to be separated out. Even under conditions where Jupiter is at opposition, and Ganymede is at its maximum separation from Jupiter, where theoretically you could see it, Jupiters’ bright light overwhelms that of Ganymede. It is possible to glimpse Ganymede at Jupiters’ opposition if you know which side of Jupiter Ganymede is on, and block out Jupiters’ light with a wall or some such item when Ganymede is at its furthest from Jupiter.


The evening sky facing west in Australia on Sunday May 16 at 6:00 pm local time showing Venus and the Moon.

But wait, you say, Venus is really bright, surely we would be able to see it easily! Well, no. Venus is only really bright because it is close to us. As you move Venus further away from Earth, it becomes dimmer. At the distance of Jupiter when it is at opposition, Venus would be roughly magnitude 3. Not astoundingly dim, about as bright as Gamma Crucis in the Southern Cross or Metsuba in Gemini. But it would be a fairly inconspicuous object, not the dazzling beacon of the twilight we are used to, and completely overwhelmed by the brightness of Jupiter (magnitude brighter than -2.5). Jupiter is this bright at that distance, even though it is darker than Venus, because it is really, really big, 10 times bigger than Venus.

Jupiter’s brightness also masks the alleged “flashes of lightning” that Velikovsky claimed accompanied Venus’s birth. The brightness of ginormous lightning flashes arcing from Jupiter to a Venus sized object would be astounding. But they are even smaller than Venus, and quite drowned out by Jupiters’ glare. The impact flashes of comet Shoemaker/Levy were brighter than thermonuclear blasts, but no Earth-bound observer saw them with the unaided eye.

So what would an Earth-bound observer see if Velikovskis' ideas were correct. For simplicities sake we will assume that ejection process took place during the opposition of Jupiter, when Jupiter is at its closest to Earth (and that the skies were clear and cloud-free the whole night, with no interfering full Moon to help drown out the light of the Venus-sized object). Also we will assume that this is in midwinter (when Jupiter is at its highest in the sky, and rises clear of the horizon rapidly). We will further assume the Venus sized object will become visible to a keen eyed observer under dark skies when it reaches the furthest extension of the orbit of Ganymede (allowing for the Venus like object to have an albedo of near 1 so being brighter than magnitude 3; and observers with really good eyesight; this is optimistic) and that the object is ejected tangentially to Jupiters’ orbit (that way it takes the shortest time for the Venus-like object to reach the furthest extension of the orbit of Ganymede. We are further going to assume that the Venus-like object is ejected at 60 km/sec, Jupiters’ escape velocity. It can’t be too much higher than this or the object will escape the solar system (and moderately higher velocities won’t change the argument much anyway).


The evening sky at the time of the Opposition of Jupiter, in December 4001 BC (also from Australia, Stellarium messes up the view from Greece because it uses my computers internal clock for setting the time). Can you see Venus?

Now, imagine yourself to be back in time 6000 years. You are a megalithic astronomer, or a guard on a hilltop fort stone fort in the area that would become Athens. Any occupation that means you are going to be hanging around in the night watches, rather than sensibly inside out of the cold, with an eye somewhere around the sky. At 7:00 pm astronomical twilight has just passed, Jupiter is glowing a modest distance above the horizon.

Then something happens. What depends on the mechanism by which Venus is ejected. Some (very unlikely) mechanisms will result in no change, but let’s assume something dramatic happens and Jupiter briefly flashes as bright as Venus (whatever mechanism, the initially brightness change al la Shoemaker Levy impact will be transient). It may even go darker after this (if it was an impact say, and the darker material from Jupiters’ clouds were bought to the surface as in the case of the Shoemaker Levy impact).

Whatever happens, we have now engaged the interest of our watchers, who pay more attention to Jupiter as it climbs in the sky, (in between going to the toilet, trying to keep out of the wind and looking out for any raiders trying to cross the forts walls). The observers continue to watch (the guard, going off duty, tell his replacement about Jupiter’s strange behaviour and the new guard keeps an eye on Jupiter, the astronomer just gets colder). Finally around midnight, an undistinguished star is seen close to Jupiter. To the guard it’s nothing; unlike today’s folks most people then were intimately familiar with the cycle of the Moon, the bright stars and planets. But like todays folks the ordinary stars don’t leave much of an impression.

The astronomer too mostly pays professional attention to the phases of the Moon, the rising and setting of bright stars and the northwards and southwards progression of Moon, Star and Sun rise and set as part of calendrical and ritual observations (as based on the evidence of megalithic observatories). While most non-bright stars are ignored (except where they make interesting asterisms like the Pleiades and Orions’ belt), the astronomer is sufficiently familiar with the sky to realise that something is different there and continues to observe. By the time twilight comes there is small but noticeable gap between the new star and Jupiter. If the Astronomer is lucky, and the weather doesn’t intervene, they will watch the new star slowly creep away from Jupiter. Could this be a new planet? A wanderer far dimmer then even Mars gets in its two yearly courses?

Athena springing from Zeus’s forehead it is not.

The Velikovskian tale appeals to our sense of the dramatic, bright Venus tearing away from Jupiter, with lightning bolts flying everywhere, but space is big, very, very big. You may think it’s a long way to the shop if you want a sausage role, but compared to space…[3]. But space is huge, at 60 Km/se,c very fast by even solar system standards, it would take a Venus sized world 5 hours to get to Ganymede’s orbit. Venus is an astoundingly bright candle in our skies, but at the distance of Jupiter, it is a feeble spark. Velikovskies tale of cosmic thunder is engulfed in the sheer scale of our solar system

[1] Like why Jupiter still has the Galilean moons, why Earth still has its Moon, they should have been ejected by a close encounter with a Venus sized object. Why was the object solid at all, it should have been red hot gravel and vapour. How do you get a rocky planet from Jupiter, a world that is mostly hydrogen with a little bit (comparatively) of water “ice” and some dirt buried below miles of metallic hydrogen? Why is Athena in the Legend, not Venus/Aphrodite? And so on.

[2] You can calculate the brightness of a molten, atmosphere-less Venus at Jupiters’ distance yourself as an exercise. And no, you can’t just move Jupiter closer in so you can see it in, it ejects or sweeps up the asteroids in the asteroid belt, we would have noticed. For Venus to reach magnitude -1, Jupiter would have to be orbiting where Vesta is.

[3] Obligatory Douglas Adams reference.

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