Building an orrery

[madd0ct0r]

Something I've been considering doing since I was about 16.

for those of you who aren't sure what one is, marvel at it here: http://www.my-time-machines.net/astro_orrery_detail.htm

Now, he's got the moons around jupiter working, which is a pretty stunning feat.

I was considering the following elements:

Solar system made of flat rings, one to each planet and a few for things like the Kupier Belt and the one whose name i've forgotten. between mars and jupiter. Pluto and Charon will be with the asteroids.

each ring runs on top of three gears, one of them powered. By making the ring thinner or thicker, and the gear correspondingly larger or smaller I can alter the ratio enough to get the orbital periods.

Data I plan to include is relative distance of the planets and relative size to each other. Not both to the same scale though

Also constellations around the edge? a Year and month tracker, a turtle to base it on.

This leaves a lot of flat space on my rings. Was considering marking up the paths that various famous explorers have taken, so when you set the orrerry to their date of launch all the marked lines on the rings line up.
(i know this ain't exactly accurate, but it's clear to look at. It's not a computer simulation)

could also include discovery date, size, year length, day length ect.

what about the harder things? Planetary spin, moons, eccentricity? What would you like to see in this?

[Molyneux]

Not something for which I possess much technical knowledge, but I wanted to wish you luck! Are you planning on using elliptical rings for the orbits, or are you going to try for circular (and include mechanisms to account for retrograde motion)?

EDIT: Sorry, didn't read the OP quite thoroughly enough, ignore dumbass question.

[madd0ct0r]

no, dumbass question is what i want.

It's a toy, not a scientific calcualtor, but I'd like it to be a sufficiently educational toy.

any problem is solvable given enough thought, and hey, I'm doing this for fun right?

I could do elliptical i think:

have the planet on a little stick that passes through the base ring, and is pivoted about halfway.

other end of stick runs in a machined groove that corresponds to the elliptical eccentricity.
(the drive wheels for the ring have to be moved a bit to the side to give space for this, but that's doable)

as the base ring turns, with the planet on top, the little stick follows the groove, making the planet move towards the sun or away to match the observed ellipse.

A bit of a bugger to do, and I'm not sure how observable the ellipses are; anyone know a good picture?

Pluto/Charon is obviously much more significant, and being at the edge of the system means I have a lot more room for fiddly bits, so I might add it to them only.

---

I keep thinking about the Jovian and Saturn(ian?) moons, becuase that's where I think a lot of the discoveries will be made in the next few decades. Worth doing a separate model for them, using the same principles?

Another traditional form is the sun, earth and moon, for explaining how are base on earth affects what movement we see, tides and ecclipses. it just seems faintly boring. I've never seen one done for the two gas giants.

[Feil]

Mercury and mars have easily noticeable eccentricity. Venus, Earth, and the gas giants are more circular.

If you are keeping the orbital paths to-scale with one another, you might as well have them all circular. The four inner planets are so close to the sun and so closely spaced relative the gas giants that their scale on your model will be small enough such that their excentricities will not be noticeable with the kind of model that you're likely to be able to achieve on an acceptable budget. And the four outer planets have nearly circular orbits.

Including Kuiper belt objects is not worth your time. They all follow highly eccentric - and individual - orbits, they are spread out over a fuzzy doughnut with a thickness the size of Neptune's orbital radius, and they will more than double the radius of your model. The main belt is also a fuzzy doughnut; I would advise against including it.

[madd0ct0r]

well the ateroid belts were only going to be fuzzy rings. I've no intention of modelling them individually, more of a 'here be asteroids' kind of thing.

Doubling the radius is a good point though. might have to sit down and do some calcs including and excluding them.

Initally I'm planning for a diameter of a bit less then a meter.

[madd0ct0r]

Do the mods mind me using this as a project log?

by way of not explaining what mechanism I'm considering:

traditional dual vertical shaft orrery: http://www.youtube.com/watch?v=2oun-m9xj0g

similar, but including more planets: http://www.youtube.com/watch?v=8zkoml-i ... re=related

and very arty version of the above: http://blog.imperialearth.com/engineeri ... own-glass/

Looking at the Jovian Moons: http://en.wikipedia.org/wiki/Moons_of_Jupiter, there's only 4 worth showing.
Might be able to do that within the main system. Orbital periods are small though, ranging from 1.7 to 16 days.
As orbital periods reduce, the height of gearing increases, with the torque reducing. Too small and fast can = stuck mechanism.

[LaCroix]

Ok, now I'm going to believe you.

The first variety isn't doable unless you are a retired mechanic with a full shop and XX decades experience in metalworking.
these three are doable, and should be precise enough to not just be a toy...

I'd go for a wooden one, as it can be cheaply done with plywood and 'normal' tools. If you can copy a printed wheel pattern onto a sheet and use a saw, you're ok. (as long as you don't get fancy with the ecliptic and stuff - but even that is possible if you can improvise a bit.)

You should definitely go with the vertical shaft orrery, as it is an expandable design. You can start out with sun and earth, and then add the other planets slowly by changing the shafts and adding gearboxes.

Are you planning on correct orbits or symbolic? Correct relative sizes? (just kidding)

Edit: using .1 AU = 1 cm to 1 inch or something like that would be practical for inner planets, but outer planets would fill up your room, so I guess you will stay symbolic, at least for them...

[Kodiak]

Chewie (a poster on this board) did the second one you have a youtube link for. I'll point him in this direction and you two can discuss.

[madd0ct0r]

Ok, now I'm going to believe you.

The first variety isn't doable unless you are a retired mechanic with a full shop and XX decades experience in metalworking.
these three are doable, and should be precise enough to not just be a toy...

I'd go for a wooden one, as it can be cheaply done with plywood and 'normal' tools....

You should definitely go with the vertical shaft orrery, as it is an expandable design....

Are you planning on correct orbits or symbolic? Correct relative sizes? (just kidding)

Edit: using .1 AU = 1 cm to 1 inch or something like that would be practical for inner planets, but outer planets would fill up your room, so I guess you will stay symbolic, at least for them...

I'm a soon to be unemployed engineer who has previously worked on a precision sheet metal shop floor, trained before that by a cabinet maker. I was also considering getting the parts laser-cut

The vertical shaft orrery ISN'T expandable in a full version.
It is when your planets are stationary balls on the end of an arm connected to a compound axle pyramid. If you aren't too desperte for perfect accuracy, you can build that with only 2 gears per planet, with 2 compound axles total (like the bottom half of the first link)
If you want planets to spin, it all gets complicated very very quickly. See the top half of the first link. You would need a stationary gear per planet, with no way to hold it. So you'd need to rely on differentials from one planet to a next. A sod to design.


I was thinking about the flat ring version as it's much more tolerant of errors in manufacture (in the simple form). Making it with rotating planets is difficult, but not too bad ( i think.)

Saturn is hopeful - despite the ring system, 96% of orbiting mass is a single moon. So, one moon and that can drive the rings too.


Any good ideas for the comet Hale-Bop?

[CaptainChewbacca]

I'll just say this once: Be very, very careful with your gears. I spent 4 days assembling my orrery, and somewhere there's a 97-toothed gear and a 95-toothed gear that are transposed. I havn't brought myself to take it apart to look for it yet.

[madd0ct0r]

ensure number of teeth, gear radius or whatever is inscribed on both sides of gear. got it.

[LaCroix]

I meant it was easier to expand, if you do it right. A flat 'table' design is more limited since you have to redo the 'table' each time you add a ring. In the axle design, the tower grows upward, and if you have chosen the shafts long enough, you can always add another gear set to the main drive and another hollow axle around the already existing ones with relatively little changes to the rest of the system.

Flat ring design with rotating planets is not hard, you just need a gear below that is linked to a stationary ring below the moving ring, giving you the correct rotation speed.

Are you planing to have it linked with a clock and automatic "real-time" or hand-cranked, so you can speed up things to visualize?
In the last case, synchronising them to a calendar would be a cool thing, so you could see alignments at a certain date and time...

[madd0ct0r]

ohh, I see what you mean.

It's not a matter of adding rings to the outside you're concerned about, it's going back and adding a new ring between planets?

in which case, assuming extendable axles, yes the vertical one is more expandable. I doubt I'll be tweaking it once it's built, I enjoy the planning stage too much anyway. Maybe a wooden one first to test and then a final metal one.