The Astro-Physics of Discworld

[poohbcarrot]

I know almost as little about astro-physics as I do about John Dunne.

Now the Discworld is a planet. There is a moon which orbits the Discworld. This I understand.

Our sun is a star. As far as I know stars do not orbit planets, but vica versa. Therefore the Discworld orbits its sun. Which means in order to have day and night, the Discworld must also be rotating as it orbits. Also the stars seen from Discworld move during the night, further proving the Discworld is rotating.

Every now and then Great Atuin snaps at a passing comet and does a 360 degree roll. Wouldn't that result in an extremely short day? And after the roll, does Great Atuin manage to get back on to the exact same orbit round its sun as before?

I'm sure some clever bod somewhere has worked all this out, does anyone know where I can find the information?

(It's all probably quantum anyway! )

[Tonyblack]

I'm pretty sure the sun (a very small one) orbits the Disc and the elephants have to sometimes lift a leg to let it past.

The Discworld is moving through space and therefore the constellations change over the years. You have to suspend convetional Real World astrophysics when it comes to Discworld. However, I suspect the ideas put forward in the books have been the belief of past civilisations here on Earth.

[Dotsie]

I can see how the rimwards countries have seasons, but it makes my head steam a bit thinking about how they get seasons in the Ramtops. But then I remember that it is, in fact, all probably just quantum.

[Bickaxe]

Like Tony, I think the sun and moon orbit The Disc and elephants etc...

Really confused about how seasons work if it spins rather than rotates but then I don't look for all these apparent hidden meanings in books, I just enjoy them.

I think Terry just writes them rather than trying to be a clever bugger and insert things for people to think about.

[Trish]

There's a book, "How the World Works," by a fella named Boyce Rensenberger.

The entire book is physics made understandable.
It's also easy to find because who can forget a name like 'Rensenberger.'

After you read it, apply what you have read sideways. :0)
He even explains quantum. Just don't ask me to.

[chris.ph]

i will go along with tony i vaguley remember in one of the intros mentioning an elephant lifting a leg to let the sun pass. also in pyramids a beetle pushed the sun around the cosmos and one of the other deities trying to nick it wasnt the orbit of the sun mentioned in last heroes as well when leonardo went over the rim in the osprey spaceship

[Tonyblack]

An extract from the New Discworld Companion:

Its tiny orbiting sunlet, with prominences no bigger than croquet hoops, maintains a fixed elliptical orbit, while the Disc revolves beneath it, taking about 800 days to complete the full circle. The little moon shines by it's own light, owing to cramped and rather inefficient astronomical arrangements.

Presumably, when people thought the Earth was flat and at the centre of the Universe, they must have thought similar things. Thinking anything else in those days meant you had a somewhat short life expectancy.

[kakaze]

The shape and cosmology of the disc system are perhaps worthy of note at this point. There are, of course, two major directions on the disc: Hubward and Rimward. But since the disc itself revolves at the rate of once every eight hundred days (in order to distribute the weight fairly upon its supportive pachyderms, according to Reforgule of Krull) there are also two lesser directions, which are Turnwise and Widdershins. Since the disc’s tiny orbiting sunlet maintains a fixed orbit while the majestic disc turns slowly beneath it, it will be readily deduced that a disc year consists of not four but eight seasons. The summers are those times when the sun rises or sets at the nearest point on the Rim, the winters those occasions when it rises or sets at a point around ninety degrees along the circumference. Thus, in the lands around the Circle Sea, the year begins on Hogs’ Watch Night, progresses through a Spring Prime to its first midsummer (Small Gods’ Eve) which is followed by Autumn Prime and, straddling the half-year point of Crueltide, Winter Secundus (also known as the Spindlewinter, since at this time the sun rises in the direction of spin). Then comes Secundus Spring with Summer Two on its heels, the three quarter mark of the year being the night of Alls Fallow—the one night of the year, according to legend, when witches and warlocks stay in bed. Then drifting leaves and frosty nights drag on towards Backspindlewinter and a new Hogs’ Watch Night nestling like a frozen jewel at its heart.
Since the Hub is never closely warmed by the weak sun the lands there are locked in permafrost. The Rim, on the other hand, is a region of sunny islands and balmy days. There are, of course, eight days in a disc week and eight colours in its light spectrum.

The Discworld offers sights far more impressive than those found in universes built by Creators with less imagination but more mechanical aptitude. Although the disc’s sun is but an orbiting moonlet, its prominences hardly bigger than croquet hoops, this slight drawback must be set against the tremendous sight of Great A’Tuin the Turtle, upon Whose ancient and meteor-riddled shell the disc ultimately rests. Sometimes, in His slow journey across the shores of infinity, He moves His countrysized head to snap at a passing comet.

However...

Through the fathomless deeps of space swims the star turtle Great A'Tuin, bearing on its back the four giant elephants who carry on their shoulders the mass of the Discworld. A tiny sun and moon spin around them, on a complicated orbit to induce seasons, so probably nowhere else in the multiverse is it sometimes necessary for an elephant to cock a leg to allow the sun to go past.

So it seems that, whether or not the orbit is stable, the sun is definitely a small "sunlet" that orbit's the disc (passing between the disc and Great A'tuin at night).

[Dotsie]

All that complicated stuff about seasons from CoM was quite rightly abandoned though. Who can remember all that?

[Tonyblack]

There are a few things that were abandoned I think. Like the 800 day year - that would mean only one Hogswatch every 800 days.

[Trish]

Eight's the number.
8-day week, 800-day year. 8 colors of magic from the Disc's high-magic field.
Hogswatch is when it is, every 800 days.

[kakaze]

There are a few things that were abandoned I think. Like the 800 day year - that would mean only one Hogswatch every 800 days.

That wouldn't be too unusual. After all, it takes Mars about 687 Earth days to have a full year.

Venus is the really odd planet. It spins backwards, so the sun rises in the West and sets in the East. A year on Venus only lasts 225 (Earth) days but a Venus day lasts 243 Earth days (18 Earth days longer than it's year)!

Phobos, Mars's larger moon, only takes 7.3 hours to go around Mars. A day on mars is 24.5 hours. This means the moon rises and sets three and a half times during a Martian day. The smaller moon, Deimos, takes 30.3 hours to orbit Mars. Since Phobos orbits Mars fast than Mars spins, and Deimos orbits slower than Mars spins, from the surface of Mars they appear to be traveling in opposite directions.

In fact, Uranus takes about 84 (Earth) years to go around the sun. Compared to these real-life examples, an 800-day year doesn't seem so strange.

[Tonyblack]

I was referring more to the fact that as the people on the Disc are so much like us it means they only get to celebrate Hogswatch every two Earth years or so.

[Cheery]

He actually never talks about 800 day years or eight seasons after CoM. There is, however the month Grune, which shows up every now and then. And what tha eighth day of the week is called, is never mentioned either. I think he dropped it after all. I've read that passage thousands of times and still don't fully understand it, so I'm glad he did.

[Sjoerd3000]

According to wikipedia(so it must be accurate ) the eight day is called Octeday I had to look it up can´t remember if it´s mentioned somewhere in the books. http://en.wikipedia.org/wiki/Discworld_(world)#Calendar