Planet time and calendar?

[Leitz]

Getting into the science of worlds. If a planet is in the solar habitable zone it's "year" may or may not match "standard" year. It's "day" may also not match. Is there something that details how the year and day can be calculated, or is that up to the world definer?

Interestingly, regular outdoor life as we know it requires a planetary temperature between 0-100 C. Extremes exist where a planet has been colonized, but "as we know it" means go outdoors and live a bit. Hadn't thought of it until class mentioned that and the distance from the star being part of the definition of the habitable zone.

 

[robject]

Year length depends on orbital radius, IIRC.

I think a planet's spin is highly random, so I would base it roughly on the Atmosphere in the UWP, plus to some degree Hydrographics. If it looks like a habitable surface, then it's probably got days short enough so that the surface doesn't bake or freeze.

 

[Condottiere]

Your chronometer can be set for any number of different planet or universal times, and like the earlier more fancy contraptions, can show various planetary rotations and calculate out your horoscope.

 

[Vargas]

I've long wanted some way of figuring out what the apparent day/night cycle of a gas giant moon is.

 

[Leitz]

At the moment I'm using standard hours for time, and letting the planets have more or less. Days are numeric, day groupings are based on agriculture since it's an Ag planet, and schools are on four 90 day terms, where each child does at least two terms a year of academics and the other two can be family business, apprenticeships, or more academics for the bookworms.

 

[flg]

I've long wanted some way of figuring out what the apparent day/night cycle of a gas giant moon is.

Simple - it's the same as its orbital period. Gas giant moons (that aren't asteroids) are always tidelocked to their primaries.

 

[flg]

Getting into the science of worlds. If a planet is in the solar habitable zone it's "year" may or may not match "standard" year. It's "day" may also not match. Is there something that details how the year and day can be calculated, or is that up to the world definer?

Orbital period is defined by the orbital distance and mass of the star.

T = 2.pi. SQRT(a³/G.M²)

Where:
T = orbital period in seconds
a = semimajor axis in metres
G = gravitational constant (6.67384e-11)
M = mass of primary (technically it should be M1+M2, the sum of the star and planet, but usually the planet's mass is miniscule compared to the star's so just using the star's mass alone is fine).


The day length depends on a lot of random factors. Broadly speaking, if the planet is in the habitable zone and orbiting a red or late orange dwarf star (K5 V to M9 V) then it's going to be tidelocked to the star. If the planet is a satellite of a gas giant, it'll be tidelocked to the gas giant (note: the gas giant won't be tidelocked to the star, even if it's in the habitable zone). If the planet is the habitable zone of a more massive star (K4 V or brighter) then it's not going to be tidelocked and can have whatever rotation period you want (above say a minimum of 8 hours, due to tidal slowing).

 

[maksimsmelchak]

Many sci-fi stories resolve these kinds of questions by matching up the orbital period with the solar year and filling in the gaps or change with a "dead zone" of those leftover minutes.

For instance, one of the early Battletech novels had a 24-hour period with 38 or so minutes of "dead time" that was floated.

I remember reading other solutions, but that one stood out on the tip of my tongue. I bet Jim remembers other examples readily.

Shalom,
Maksim-Smelchak.

 

[maksimsmelchak]

Getting into the science of worlds. If a planet is in the solar habitable zone it's "year" may or may not match "standard" year. It's "day" may also not match.

Is there something that details how the year and day can be calculated, or is that up to the world definer?

No system in place yet to my knowledge. Want to develop one?

Interestingly, regular outdoor life as we know it requires a planetary temperature between 0-100 C. Extremes exist where a planet has been colonized, but "as we know it" means go outdoors and live a bit.

That's assuming conventional carbon-based "wet" life.

1. Extremophiles ignore many of those rules.
2. The shadow biosphere almost definitely ignores those rules.
3. Unconventional non-carbon-based "dry" life would almost invariably scoff at those rules.
4. Of course, that's all conjecture. Astrobiology is a young and mostly unexplored science.

Hadn't thought of it until class mentioned that and the distance from the star being part of the definition of the habitable zone.

*** What class are you taking? ***

Shalom,
Maksim-Smelchak.

 

[Leitz]

*** What class are you taking? ***

Shalom,
Maksim-Smelchak.

This one. I'm doing the free version for EDU increase, not diploma.

 

[Timerover51]

Getting into the science of worlds. If a planet is in the solar habitable zone it's "year" may or may not match "standard" year. It's "day" may also not match. Is there something that details how the year and day can be calculated, or is that up to the world definer?

Interestingly, regular outdoor life as we know it requires a planetary temperature between 0-100 C. Extremes exist where a planet has been colonized, but "as we know it" means go outdoors and live a bit. Hadn't thought of it until class mentioned that and the distance from the star being part of the definition of the habitable zone.

For an interesting variation, you might want to look at Mars seasons, which vary quite a bit in length between the northern and the southern hemispheres, and use that for a planet in the habitable zone of a star. Mars orbital excentric makes for quite a bit of variation as well.

 

[Vargas]

Simple - it's the same as its orbital period. Gas giant moons (that aren't asteroids) are always tidelocked to their primaries.

It may be simple to you.

To clarify, I can't figure out when it is light and when it's dark at a given time at a given place on the surface. There's tidal locking and the gas giant's shadow to figure in; seems quite complex.

 

[jcrocker]

You could always go with what the plot requires for the start point.

Then pick a time for the moon to orbit the giant. The only ones we know of are the Galilean moons, the orbital periods of the inner 3 are in a 1:2:4 resonance - 1.7 days, 3.5 days, 7.1 days, then the last is 16.7 days.

When it passed behind the giant, that could be the 'short night' of 6 or 10 or whatever hours, then the 'other night' could be longer. And as long as TL is 5 or up, then lots of street lights switch on during whatever night it's in.

 

[G Sanz]

To clarify, I can't figure out when it is light and when it's dark at a given time at a given place on the surface. There's tidal locking and the gas giant's shadow to figure in; seems quite complex.

My initial guess was that tidal locking is in the drivers seat here. My thinking is that our own moon goes through it's phases 29-odd days, whereas lunar eclipses are a bit more infrequent on Earth.

Thinking some more though your mileage may vary when extrapolating that logic out.

I think that how often you get lunar eclipses depends on the orbital inclination of the satellite relative to the orbital plane of the planet.

For an extreme example if the satellite orbits at a right angle to the planet's orbital plane your not going to end up in the planets shadow, whereas if inclination is pretty much 0 and you are orbiting right in line with the plane eclipses happen often.

In the intermediate case you get a situation where at certain times in the year for the planet you can get eclipses since the orbits will line up the full moons with the orbital plane, and at other times of the year you won't get them since when you are at full moon you will be above/below the orbital plane and won't end up in the planet's shadow.

Since distant gas-giants have long years though, you might be stuck in eclipse season for a long or short time.

^do not take any my comments above as expert opinion.

 

[Enoki]

Being on a tidal locked satellite of a world is somewhat different than a tidal locked planet orbiting the star.

With the later, you have one half the planet always bathed in light and heat, the other half in dark and cold. There would be a twilight band between the two that might be the only temperate region on the planet. Worse, without rotation, the world almost certainly would lack a magnetic field.

With a satellite, depending on the size of the planet it orbits, and the period, there could be daylight and night (eclipses amounting to the same thing) over the whole satellite. The moon gets fully exposed to the sun even though it is tidally locked to the Earth.
I'd think a satellite orbiting a gas giant might be a better candidate for being habitable than a tidally locked planet to the star in many cases. Of course, orbital distance, the presence of a planetary magnetic field, along with the magnetic field of the gas giant, would play a role in how habitable a satellite would be.
Too close in and you might have issues with geologic instability like Io. Massive radiation could be a problem. But, at least you'd have a day and night over the whole planet.

 

[flg]

It may be simple to you.

To clarify, I can't figure out when it is light and when it's dark at a given time at a given place on the surface. There's tidal locking and the gas giant's shadow to figure in; seems quite complex.

Day and night are simple - as I said, the satellite will always be tidelocked to the primary, so its effective rotation period is as long as its orbital period. If it takes 50 hours to orbit its primary, a point on the satellite's surface will have 25 hours of daylight and 25 hours of nighttime.

Eclipses are harder to figure out though. It'd depend on the distance to the star and the radius of the primary and how far the satellite is, and then you need to be calculating the orbital velocity of the satellite (among other things):

http://mysite.du.edu/~jcalvert/astro/shadows.htm
http://www.eso.org/public//outreach/eduoff/aol/market/experiments/middle/skills202.html

That can get rather hairy, so it may just be easier to put it into a visualisation program like Celestia or Space Engine and just see how long the eclipse would be directly (though that's a bit of a learning curve in itself).

 

[Spinward Scout]

Something I didn't see brought up is if you're in a binary or trinary star system. What's "noon", "sunset", or "sunrise" when you have two suns in the sky, maybe at the opposite horizons from each other? What's A.M. or P.M.?

I read an article a while back that when a rock band went on tour (might have been the Rolling Stones), a doctor had them eat/sleep/be on their own personal time instead of the time zone of the country or city they were in. I could see starship crews doing this when they would visit a world.

 

[flg]

Something I didn't see brought up is if you're in a binary or trinary star system. What's "noon", "sunset", or "sunrise" when you have two suns in the sky, maybe at the opposite horizons from each other? What's A.M. or P.M.?

Simplest way is to do this using the primary star (the one the planet actually orbits). Noon is when that star is at the highest point in the sky, sunrise/sunset is when that rises and sets. A.M is before noon, P.M. is after noon, as usual. Treat the dimmer star like it was the moon, basically.

The secondary star is either going to be outside the planet's orbit, or orbiting the primary in a close orbit. The effect on the sky is going to depend on how that star's luminosity and far it orbits. A star like Proxima Centauri at 10 AU would be about as bright as the full moon at apparent magnitude -11. (though nowhere near as big, it'd just be a very bright reddish star). A star like Sol at 10 AU would be 100 times less bright than the sun at 1 AU (apparent magnitude -21), but that'd be more than enough to turn night into day while it was up - though it wouldn't provide any noticeable extra heat.

If the secondary is orbiting the primary closely (so both stars are within the planet's orbit) then 'sunrise' and 'sunset' are going to be close together since the companion will never go very far from the primary in the planet's sky (it may not even be visible if it's much dimmer than the primary).

If the secondary is orbiting outside the planet's orbit then depending on how far away it is you may have no truly dark nights for half the planet's year, and then dark nights for the other half - as the planet orbits the primary the secondary will swing from being on the opposite side of the sky to the primary, to being on the same side of the sky as the primary (as viewed from the planet).

I read an article a while back that when a rock band went on tour (might have been the Rolling Stones), a doctor had them eat/sleep/be on their own personal time instead of the time zone of the country or city they were in. I could see starship crews doing this when they would visit a world.

I think they'd pretty much have to, unless there are magic drugs that reset sleep patterns by then.

 

[Enoki]

Something I didn't see brought up is if you're in a binary or trinary star system. What's "noon", "sunset", or "sunrise" when you have two suns in the sky, maybe at the opposite horizons from each other? What's A.M. or P.M.?

I read an article a while back that when a rock band went on tour (might have been the Rolling Stones), a doctor had them eat/sleep/be on their own personal time instead of the time zone of the country or city they were in. I could see starship crews doing this when they would visit a world.

Ship crews, and in particular submarine crews, often do that. There is no particular reason you have to be on local time when you are in essence your own little "universe."

I have brought up something along these lines when starships arrive in a system. By ship's time it's morning. By planetary time where the starport is planet-side it's midnight. Or, it's Imperial calendar wise a weekday, but locally it's the weekend and nobody's open for business.

 

[aramis]

Simplest way is to do this using the primary star (the one the planet actually orbits). Noon is when that star is at the highest point in the sky, sunrise/sunset is when that rises and sets. A.M is before noon, P.M. is after noon, as usual. Treat the dimmer star like it was the moon, basically.

The secondary star is either going to be outside the planet's orbit, or orbiting the primary in a close orbit. The effect on the sky is going to depend on how that star's luminosity and far it orbits. A star like Proxima Centauri at 10 AU would be about as bright as the full moon at apparent magnitude -11. (though nowhere near as big, it'd just be a very bright reddish star). A star like Sol at 10 AU would be 100 times less bright than the sun at 1 AU (apparent magnitude -21), but that'd be more than enough to turn night into day while it was up - though it wouldn't provide any noticeable extra heat.

If the secondary is orbiting the primary closely (so both stars are within the planet's orbit) then 'sunrise' and 'sunset' are going to be close together since the companion will never go very far from the primary in the planet's sky (it may not even be visible if it's much dimmer than the primary).

If the secondary is orbiting outside the planet's orbit then depending on how far away it is you may have no truly dark nights for half the planet's year, and then dark nights for the other half - as the planet orbits the primary the secondary will swing from being on the opposite side of the sky to the primary, to being on the same side of the sky as the primary (as viewed from the planet).

.

Note that the primary star may not hold the mainworld - if using Bk6, it's entirely possible that the mainworld orbits the secondary star.

It's also possible that the planet could be a trojan of the secondary star, in which case, you get even more interesting effects.