zompist bboard

For my conworld, I want to work out some basic facts about the other planets orbiting the same sun. Not technical data like how long their days and years are, but stuff that is visible from the conworld before the invention of the telescope. Can someone point me towards a resource for coming up with a realistic description of a planets' path across the sky?

sam wrote:For my conworld, I want to work out some basic facts about the other planets orbiting the same sun. Not technical data like how long their days and years are, but stuff that is visible from the conworld before the invention of the telescope. Can someone point me towards a resource for coming up with a realistic description of a planets' path across the sky?

You could certainly start with the Font of All Knowledge and work from there.

Since you're examining naked eye astronomy, things are a little simpler. For the most part, and assuming a night sky very similar in appearance to that of Earth, astronomy pretty much comes down a bunch of wee white dots in the sky. Some of them seem to rotate around an axis over the course of the night (those would be the stars); others seem to "wander" on their own (those would be the planets). A few objects seem to burn a long streak across the sky (those would be the comets).

Patterns repeat. Certain stars appear in the sky for a while only to fade and be replaced by others --- these would be your "summer stars" versus your "winter stars", where what you can see at night depends upon your planet's position with respect to its star at a given time of year. Thus the concept of relating seasons on the planet (if your planet has them) to the parade of stars across the sky.

Just like *here* on Earth, *there*, on your Otherworld, stars and planets will rise and set. The direction of their movement will be dictated by the direction of the planet's orbit and its own rotational direction.

The wonder of a solar eclipse is that, the night before when the sky was dark, you were seeing for example, the summer stars (because at that point, you were facing away from the Sun and looking at the stars for example to the galactic west of the Sun. A few hours later, as the planet rotates and the Moon crosses the Sun's face and blocks it out, you'll be facing the Sun and now looking towards galactic east, and thus will be seeing the winter stars for a few minutes as the sky goes dark during the eclipse. Then again that night, you'll be facing galactic west again and will see the summer stars once again.

Also consider what's in the neighborhood that might be visible to the naked eye but that we don't have around here. Nebulas, double planets, a galaxy seen face on as your own careens into it edge on, etc.

There is an interesting twist you could add to your planetary system: moons of another planet visible to the naked eye. In our system, the Galilean moons of Jupiter fail to be that just barely. If you have a gas giant that's a bit closer than Jupiter (say, at 3 AU), and its moons are a bit bigger (say, Mars-sized) and perhaps with a bit greater orbital radii, people may notice that this planet is accompanied by a few little starlets in their equivalent of antiquity.

elemtilas wrote:and thus will be seeing the winter stars for a few minutes as the sky goes dark during the eclipse

That's so cool. It makes perfect sense but I never thought about it. I like all the ideas of other things visible in the night sky.

WeepingElf wrote:There is an interesting twist you could add to your planetary system: moons of another planet visible to the naked eye.

This is a really neat idea, that I think I'll use. I'm imagining stories of a particular planet's "attendants".



I guess what I really want to make sure I do "right", is tracing the actual paths of these planets orbits. I guess the winter and summer constellations just go around the Earth in a (mostly) straight path over the course of a year. But the paths of the planets could be interesting for history. For example, I've read that Venus traces a five-pointed star in the sky every four years. This is the reason the Olympics are held every four years and the reason why five-pointed stars are associated with devil worship among Christians. I'm looking to make sure I know what kinds of paths planets closer to and farther from the sun could follow, based on their distance.

WeepingElf wrote:There is an interesting twist you could add to your planetary system: moons of another planet visible to the naked eye. In our system, the Galilean moons of Jupiter fail to be that just barely. If you have a gas giant that's a bit closer than Jupiter (say, at 3 AU), and its moons are a bit bigger (say, Mars-sized) and perhaps with a bit greater orbital radii, people may notice that this planet is accompanied by a few little starlets in their equivalent of antiquity.

Actually, the Galilean moons are bright enough to be visible to the naked eye; they're just lost in Jupiter's glare. (There's an unconfirmed theory that some Chinese astronomer spotted one of them, but most astronomers agree that that's highly unlikely.) On the other hand, Earth and the Moon are able to be discerned as discrete objects from Mars.

sam wrote:the reason why five-pointed stars are associated with devil worship among Christians.

Actually, in Christianity the pentagram is associated with the Five Wounds of Christ (viz., Sir Gawain and the Green Knight). It only attained its association with magic and the occult in the Renaissance, and then only when facing down.

Might be worth getting a copy of Universe Sandbox 2 on Steam and building a model of your system in it. Universe Sandbox has a command (I think it's the 'C' button) that lets you stand on the surface of a planet and view the rest of the system in its sky, watching the sun rise and fall and the planets move about in turn. If you switch off trails and projected paths, you can just watch the labeled dots like an ancient astronomer would; they'll show retrograde motion.

dhok wrote:Might be worth getting a copy of Universe Sandbox 2 on Steam and building a model of your system in it.

Woah, I never heard of that, and it's pretty much exactly what I was looking for. Thanks!

dhok wrote:Might be worth getting a copy of Universe Sandbox 2 on Steam and building a model of your system in it. Universe Sandbox has a command (I think it's the 'C' button) that lets you stand on the surface of a planet and view the rest of the system in its sky, watching the sun rise and fall and the planets move about in turn. If you switch off trails and projected paths, you can just watch the labeled dots like an ancient astronomer would; they'll show retrograde motion.

It's been on my wishlist a long time, but it just got bumped up a little higher now that I know it can do that. Shame it never gets any really good sales (not that that's unusual for Early Access games).

Zaarin wrote:Actually, the Galilean moons are bright enough to be visible to the naked eye; they're just lost in Jupiter's glare. (There's an unconfirmed theory that some Chinese astronomer spotted one of them, but most astronomers agree that that's highly unlikely.)

It's actually quite easy to see Jupiter as an elongated object by naked eye. The catch is that, when you pay attention to it, you can see it being elongated into whatever direction you want. That's a pretty good argument that this sort of observations are simply optical errors in your eye.

You don't need much magnification, though, to resolve the Galilean moons from Jupiter. Pretty much any small binocular will let you see them, provided that you have a mount or a steady hand. Make their orbital radii a couple of times larger and you'll soon start to make them naked eye objects.

Annoyingly, there's a similar sort of situation with Venus. When it's closest to us, it's just a bit too small to be seen as a crescent by naked eye.

elemtilas wrote:The wonder of a solar eclipse is that, the night before when the sky was dark, you were seeing for example, the summer stars (because at that point, you were facing away from the Sun and looking at the stars for example to the galactic west of the Sun. A few hours later, as the planet rotates and the Moon crosses the Sun's face and blocks it out, you'll be facing the Sun and now looking towards galactic east, and thus will be seeing the winter stars for a few minutes as the sky goes dark during the eclipse. Then again that night, you'll be facing galactic west again and will see the summer stars once again.

Having solar eclipses depends on having a large moon.

Being a broke bastard, I haven't gotten a hold of Universe Sandbox, so instead I've used various calculators to do astronomical stuff like figuring out the apparent size objects in the sky.

dhok wrote:Might be worth getting a copy of Universe Sandbox 2 on Steam and building a model of your system in it. Universe Sandbox has a command (I think it's the 'C' button) that lets you stand on the surface of a planet and view the rest of the system in its sky, watching the sun rise and fall and the planets move about in turn. If you switch off trails and projected paths, you can just watch the labeled dots like an ancient astronomer would; they'll show retrograde motion.

How is it compared to Space Engine?

If you're programming-inclined, you can also check out the Rebound library for Python. In some ways, it will give you better ideas of things than Universe Sandbox.

A lot of the rest is a bit of geometry and algebra.

sam wrote:For example, I've read that Venus traces a five-pointed star in the sky every four years. This is the reason the Olympics are held every four years and the reason why five-pointed stars are associated with devil worship among Christians.

This smells like massive bullshit to me.

Axiem wrote:

sam wrote:For example, I've read that Venus traces a five-pointed star in the sky every four years. This is the reason the Olympics are held every four years and the reason why five-pointed stars are associated with devil worship among Christians.

This smells like massive bullshit to me.

Well, Venus does indeed trace a pentagram, but considering the antiquity of the Olympic traditions I highly doubt that that influenced the four-year cycle. As for the association with "devil worship" in Christianity, that's patently wrong--the pentangle was traditionally associated with Solomon (viz., the Star of David) and with the Five Wounds of Christ; in the Renaissance, an upside down pentangle became associated with magic, but it would still be some time before magic became associated with Satan.

Zaarin wrote:

Axiem wrote:

sam wrote:For example, I've read that Venus traces a five-pointed star in the sky every four years. This is the reason the Olympics are held every four years and the reason why five-pointed stars are associated with devil worship among Christians.

This smells like massive bullshit to me.

As for the association with "devil worship" in Christianity, that's patently wrong--the pentangle was traditionally associated with Solomon (viz., the Star of David) and with the Five Wounds of Christ; in the Renaissance, an upside down pentangle became associated with magic, but it would still be some time before magic became associated with Satan.

Yeah, I just looked it up and was about to post the same correction. When I think back, I realize I got that fact by way of The Da Vinci Code, so it figures it's not quite right.

Richard W wrote:

elemtilas wrote:The wonder of a solar eclipse is that, the night before when the sky was dark, you were seeing for example, the summer stars (because at that point, you were facing away from the Sun and looking at the stars for example to the galactic west of the Sun. A few hours later, as the planet rotates and the Moon crosses the Sun's face and blocks it out, you'll be facing the Sun and now looking towards galactic east, and thus will be seeing the winter stars for a few minutes as the sky goes dark during the eclipse. Then again that night, you'll be facing galactic west again and will see the summer stars once again.

Having solar eclipses depends on having a large moon.

Having solar eclipses with corona (like ours) depends on having a moon with an angular diameter just slightly less than the sun's when viewed from your planet. It is quite a coincidence that we have this. Having a moon that occasionally comes between you and the sun is less of a stretch.

Echobeats wrote:

Richard W wrote:

elemtilas wrote:The wonder of a solar eclipse is that, the night before when the sky was dark, you were seeing for example, the summer stars (because at that point, you were facing away from the Sun and looking at the stars for example to the galactic west of the Sun. A few hours later, as the planet rotates and the Moon crosses the Sun's face and blocks it out, you'll be facing the Sun and now looking towards galactic east, and thus will be seeing the winter stars for a few minutes as the sky goes dark during the eclipse. Then again that night, you'll be facing galactic west again and will see the summer stars once again.

Having solar eclipses depends on having a large moon.

Having solar eclipses with corona (like ours) depends on having a moon with an angular diameter just slightly less than the sun's when viewed from your planet. It is quite a coincidence that we have this. Having a moon that occasionally comes between you and the sun is less of a stretch.

...Yes, but then it's an occultation, not an eclipse. Venus and Mercury both occult the Sun from our perspective, but they don't eclipse it.