Planet time and calendar?

[G. Kashkanun Anderson]

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.?

That should only rarely be an issue. The overwhelming majority of worlds orbiting a single star of a multiple star system will have an obviously dominant one. Even if Jupiter were a Sol-sized star, for example, it would appear to be only about 1/15th as bright and about 1/4 the size as viewed from Terra. A much more likely scenario would be a middling red dwarf in Jupiter's position, resulting in an object about 1/1000th as bright as the Sun (or about 150 times as bright as a full moon, assuming a higher percentage of the light being infrared) and 1/10th the apparent diameter from our point of view, if my off-the-cuff calculations are correct.

And most multiple star systems are far wider than a mere few AUs like the Jupiter-Sol distance; most are are about Neptune-Sol distance from each other or farther. At that range a typical red dwarf would be a pinpoint of red light 10-15 times the brightness of the full moon (or less than 1/40,000th the brightness of the Sun), depending on its output spectrum. Go even further out, like the 15,000 AU distance between Proxima Centauri and Alpha Centauri, and the companion star probably wouldn't even be notable to a casual observer.

 

[flg]

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

You'll note that I explicitly said "(the star that the world orbits)" after I said "primary star". It doesn't matter whether that star is the biggest star in the system or not.

 

[flg]

That should only rarely be an issue. The overwhelming majority of worlds orbiting a single star of a multiple star system will have an obviously dominant one.

Yes, but I don't know if you can really say it'd "only rarely be an issue". Near Companions (with companion star within the planetary system proper, rather than far beyond it) come up quite often in Traveller.

A companion star outside the planet's orbit in those situations will still end up being quite bright in its sky, even if it's a dim red dwarf like Proxima. Even if it was at Pluto's distance it'd be about 1/50th as bright as the full moon (or about 30-40 times brighter than Venus). And that's assuming a companion star that is among the dimmest possible, so this is the lowest end of the scale.

From CT book 6, it looks like this situation could crop up in about 20% of systems (40% are multiple systems, about half of those would have stars in the Jupiter-Pluto range of distances in the outer system),so it could be more common than you give it credit for.

 

[aramis]

You'll note that I explicitly said "(the star that the world orbits)" after I said "primary star". It doesn't matter whether that star is the biggest star in the system or not.

It can matter for climate.

Let's go with a hypothetical...

G0 V & and M0 V pair...
G0 V is L=1.21
M0 in orbit 4 (≅ Mars), 1.6 AU, L=0.04
World orbiting M0V in orbit 1 (.4 AU), tidelocked to it's large moon (say, Size 9 and size 6).

The M0 is effectively 0.25 × sol@earth (Illumination = Luminousity/(distance²)=0.04/0.4²)
The G0 varies from 0.3025× to 0.84× sol@earth
Combined insolation is thus about 0.5× to 1.09× earth. "Summer" is when it's towards the primary, winter when it's away from system primary. The spring/fall will be roughly 0.72× Sol@earth.

Winters will be BITTERLY cold. Summers a little hot.
And primary eclipses by the secondary in winter will be DEVASTATINGLY cold.

Local apparent year will be most affected by the orbit relative to the line between stars - this will actually be longer than the actual orbital period, due to the advancing of the orbit. It won't match the Zodiacal year.

Temperature is a function of illumination, albedo, and angle of incidence, as well as blackbody radiation out...

Visual day will be almost constant in the summer, and dim but half the rotation period in the winter.

Spring & fall, brightest will be roughly halway between Primary noon and secondary noon, dimmest will be directly opposite (shaded from both stars)

Only reason for the large moon is to prevent tidelock to the M0 star instead...

 

[flg]

The planet won't have a moon, it'd be tidelocked to its star if it's orbiting an M0 V at 0.4 AU. You can't "prevent a tidelock" with a moon - the stellar tidelock takes precedence, it removes angular momentum from the planet-moon system.

Also, we're not considering climate here. If you want to get into that then you're opening a much bigger, more complex can of worms (which Traveller rules have never handled properly), probably best left for another thread.

And CT vastly overstates the luminosities of M V stars, those values are very wrong. M0 V stars for example have visual luminosity closer to 0.025 of Sol (see e.g. https://en.wikipedia.org/wiki/Lacaille_8760 ).

For a planet orbiting an M0 V (with 0.025 Luminosity) at 0.4 AU, the M0 V would be magnitude -25. The G0 V (with 1.21 luminosity) at 1.6 AU would be magnitude -26. To the human eye both stars would be similar brightness to the sun (actually they're slightly dimmer than the sun from Earth but we probably wouldn't be able to recognise that), so you may as well still pick the M0 V star that the planet orbits for timekeeping purposes in that case.

It is potentially possible for the star that the planet orbits to be dimmer in the sky than the companion, but that would be an odd case for a habitable planet - usually its primary would be brighter or as bright in the sky as the companion.

 

[Enoki]

Of course, you can get other weird here with binary systems. Imagine (refer to the graphic below) that the Earth is the second star. L3, 4, 5 are planets in the same orbit or L1 and 2 are planets. Lagrange points could make things weird for a calendar.

Or, you could have a very high eccentricity orbit using them like this...

http://piecubed.co.uk/wp-content/uploads/2015/01/Precessing_Kepler_orbit_

Now the length of days is variable, it might cause planetary rotation to slow or speed up over time too.

Or apply the same thing to the stars themselves...

Have fun with that calendar...

Or, two stars doing this with your world circling one or the other, or worse, doing a figure eight with them...

 

[flg]

Lagrange points are very unlikely places for worlds to form. After all, we've had four billion years for the trojan asteroids on Jupiter's orbit to coalesce into a planet, and it's not happened - I think that while the L4/L5 points are "stable", there's something about them that doesn't allow planetary formation to occur there (possibly because anything there would be too easily influenced by gravitational effects of other planets. Or perhaps if enough material accumulates to form a large mass there then it becomes unstable). L1, L2, L3 definitely won't have planets - they're not stable at all.

This is just an eccentric orbit (this is the second diagram that didn't show properly in your post). The day length wouldn't be variable here (the tidal forces would be, but that just affects how quickly the planet's rotation slows down, it doesn't make it increase or decrease over a short time scale).


The other diagrams you showed are actually how stars orbit eachother. They don't "orbit the primary" as described in Traveller (unless one star is hundreds of times more massive than the other) - they orbit a common barycentre between the two, and they often have eccentric orbits. Again, Traveller doesn't handle this correctly.

 

[aramis]

A world would not FORM in a lagrangian point, but it may MIGRATE to one.

 

[flg]

A world would not FORM in a lagrangian point, but it may MIGRATE to one.

Exceedingly unlikely. You'd need planets to be migrating at different rates, crossing orbits, and just happening to settle into an L4/L5 point (of another object of the right mass that can call allow it to be stable there) and stay there without being affected by other worlds. All without being disrupted or ejected from the system. And it's even less likely that the world would be habitable.

I don't think it's worthwhile to consider such extremely unlikely scenarios here (if it's even possible at all).

 

[Enoki]

I'd think it's possible. If models of Jupiter and Saturn migrating orbits, etc., are possible it is at least possible a planet or a satellite of a gas giant might migrate to such a point over millions or billions of years.

Weird seems to be common place in the universe...

 

[flg]

I'd think it's possible. If models of Jupiter and Saturn migrating orbits, etc., are possible it is at least possible a planet or a satellite of a gas giant might migrate to such a point over millions or billions of years.

A lot of people seem to confuse "possible" and "likely to happen".

It's possible for the molecules in my coffee to spontaneously separate out into separate coffee, milk, and water layers due to random particle motion - but the probability of it happening is so small that it won't happen in the lifetime of the universe. Therefore we can say that it's so unlikely to happen that we don't need to worry about it.

Planets migrating inwards or outwards in a system is possible (and they can cross orbits too). But migrating and settling into the L4 or L5 point of another object and staying there for millions or billions of years without being destroyed, ejected or colliding with that object is phenomenally unlikely (one idea is that the object that collided with Earth to form the moon started out at the Earth-Sun L4 or L5 point - but it got too massive, started to veer closer to Earth as a result, and then ultimately collided with it). And the chance that this phenomenally unlikely event would happen in the tiny area of the 3I (compared to the rest of the universe) is even smaller.

And planets don't really migrate after they form, because there's no gas drag to change their orbits (that all happens over the few million years while they're forming, when they're surrounded by gas and dust), so it won't happen over "billions of years" either.

 

[Enoki]

The problem with that POV is we know so little about other planetary systems other than they exist not only around single stars but binary, trinary, and even in odd places they "shouldn't" exist at all.
Right now we really don't know how likely such an occurrence is.

 

[whartung]

Exceedingly unlikely.

Nonsense. The Puppeteers would would just move the planets there.

 

[flg]

The problem with that POV is we know so little about other planetary systems other than they exist not only around single stars but binary, trinary, and even in odd places they "shouldn't" exist at all.

Nothing precludes planets from forming around any star (though there are many circumstances that prevent them from remaining there for long - tidal disruption, migration, ejection from the system, etc) - but if the circumstances are right you can find planets around binaries, or individual stars in such systems. The only "odd place" we found them were close to pulsars (because we didn't think they could form from post-supernova material).

Right now we really don't know how likely such an occurrence is.

I would say we know enough. We know the likelihood of the events that need to happen to make it possible, and we know that having them all happen together is very unlikely. Maybe it's possible for a planet to get there somehow and remain on a short timescale (if the various mass ratios involved allow it), but it's unlikely to be stable and you're not going to have a habitable planet in such a scenario anyway.

Seriously, there are enough interesting scenarios to consider already. Planets around gas giants and brown dwarfs, planets around binaries, planets between stars in binaries, tidelocked planets, various combinations of the above, etc.

Obviously we can consider really extreme or unlikely cases as a thought exercise, but I'm just saying that this doesn't mean that they're going to actually show up in reality or in a setting (though if a race can move planets around then obviously it can show up artificially)