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Aotrs Commander
2012-06-18, 08:09 AM
I am a terribly picky world-builder. I like to get things right, sometimes - like now - to an absurd degree...

I am currently wrestling with the relationship between the phases of the moon, and the equinoxes/solstices - specifically, as to whether there is (or necessarily is) one. Wiki, for once, hasn't proven to be especially helpful in this regard, and I'm a necromancer, and not an astrophysist...

I have the year length, I know when the equinixes and soltices occur in the world calendar. I have even got a table for (nominal) the length of the luna cycle, but I haven't yet correlated that to the calendar.

And therein lies my query. (And it may even be an idiot question, the glaringly obvious answer to which that I'm just not seeing...!)

Does anyone know whether there is a link (i.e. talking astrophysics, not astrology here, i.e. the physical movement of celestial bodies) between the moon phaes and the soltice/equinox cycle, or are they more-or-less unrelated?

I know they don't fall on exactly the same cycle, but what I'm not sure is whether they are on their own cycle completely (as and such, I only need to say "okay, the 15th of Primus 2764 is a new moon" and be able to calculate what the moon is doing at any given point in world history) or whether there's a stronger physical link between the two that I'd have to calibrate the moon phases to.

I'm prepared to have a fairly wide margin of error (given the moon may not be the same size, orbit-shape (and isn't, currently) the same orbital speed as Earth's moon), but I mostly want to do a ball-park check so that I can't say, I dunno, the summer solstice and the new moon happen at the same time if that would be physically impossible.

My wiki-ing seems to suggest that the cycles are more or less independant, but it would be nice is someone else knows (or can point to a Person What Does Know.)

hamlet
2012-06-18, 08:36 AM
OK, first, a really pedantic nitpick: it's "astronomy" no "astrophysics." They're actually two different fields of study.



Ok, what you ask really depends on the orbital periods of the celestial bodies in question. In real life, our world that is, as far as I know, there is no link between moon phases and equinoxes and solstices. Such things are actually linked to the solar rather than lunar calendar, and in our world, those two things don't really link up well for a whole host of reasons, some of them more stupid than others.

If, however, the orbital periods in your proposed world are more regularly linked, then yes, there could be a strong link between phases of various celestial bodies and the equinoxes etc. A 28 day lunar cycle with a regular and reliable 28 day month means that the phase of the moon is the same for each day of every month. And if you figure it right (lots of math and/or charting) you can set it up so that the equinoxes fall on exactly the same phase of moon every year.

Ashtagon
2012-06-18, 08:45 AM
There is no link between lunar phases and solar equinoxes.

In a variant solar system, they could have a relationship, but it would be an entirely coincidental relationship, not a relationship where one causes the other.

Frenth Alunril
2012-06-18, 08:48 AM
I made a calendar for my game that was so specific it showed inconsistency in the faerun campaign guide. I built my equations on a model that is accurate for our own Moon and then adjusted the values to fit the material.

If you would like, I could share my calendar with you.

Aotrs Commander
2012-06-18, 09:35 AM
OK, first, a really pedantic nitpick: it's "astronomy" no "astrophysics." They're actually two different fields of study.

*skullpalm*

Bad Lich, really not being on the ball today!

I wouldn't mind, but I even got it right in the thread title...!

*slaps wrist*


Ok, what you ask really depends on the orbital periods of the celestial bodies in question. In real life, our world that is, as far as I know, there is no link between moon phases and equinoxes and solstices. Such things are actually linked to the solar rather than lunar calendar, and in our world, those two things don't really link up well for a whole host of reasons, some of them more stupid than others.

If, however, the orbital periods in your proposed world are more regularly linked, then yes, there could be a strong link between phases of various celestial bodies and the equinoxes etc. A 28 day lunar cycle with a regular and reliable 28 day month means that the phase of the moon is the same for each day of every month. And if you figure it right (lots of math and/or charting) you can set it up so that the equinoxes fall on exactly the same phase of moon every year.


There is no link between lunar phases and solar equinoxes.

In a variant solar system, they could have a relationship, but it would be an entirely coincidental relationship, not a relationship where one causes the other.

Righto. Excellent. That makes life so much easier, as it means I can pick any arbitary point to stick the luna cycle onto the calendar. Well, I say that, what I'll actually do is define it first by picking an equinox or solstice date and saying that that is the full or new moon (as like a quest plot point, since on that day the magical powers would be in convergeance (not unlike the convergeance of tides for maximum or minimum spring/neeps), suitable for Evil Villains to enact their Evil Plans. And from that, it'll be quite easy to back work (since the lunar cycle is (near as dammit as the reckoning matters) forty days on a 384-day year (divided into twelve, regular 32-day months).

And accounting for the leap-years that happen every 25 years except years divisible by 200, because I wasn't going to make it completely too unrealistically easy for myself perfect...!

Thanks, folks!



Edit:
*skullpalm*

And it's just this second occurred to me that changing the lunar month is going to change how the tides work, isn't it?

Ah well, it's not like there's much naval stuff in this campaign world anyway, so I don't need to immediately work that one out...!

Editedit:
And on looking, I think all I really ought to take away is that, given that this moon orbits at 75% of the speed of Luna, (i.e. 40 days instead of 29), the tides should be 133% of the length of Earth-tides (very very roughly), such that a tide should take about sixteen hours, not twelve should that become important, and just whistle for the rest! (And I'm probably already waay over-complicating things compared to most world-building folk...!)

JoshuaZ
2012-06-18, 10:04 AM
Tides are really complicated and weren't even really well understood until the 19th century, although the link between tides and the moon was understood as an empirical matter about 2000 years ago. The basic mechanism of why tides and the moon are linked was worked out by Newton but the basic understanding didn't help predictions much until a lot more work was done. Empirically derived tide tables date at least to Middle Ages. Tidal prediction was one of the major cause of the development of mechanical calculators in the 19th century, where they used special components to do the calculations.

Most of the world should have two high tides a day regardless of lunar period. Tides should be larger near the full moon and and new moon but unless you are in an area that is extreme this shouldn't be a change of more than a few feet, so won't matter much. Similarly, in the quarter points of the lunar month tides should be less extreme.

There are occasional issues that can occur that make things more complicated. In some areas you can get four daily high tides or a single daily high tide. This related to issues like current, and the nature of the sea floor, but these are extremely rare.

erikun
2012-06-18, 11:17 AM
And on looking, I think all I really ought to take away is that, given that this moon orbits at 75% of the speed of Luna, (i.e. 40 days instead of 29), the tides should be 133% of the length of Earth-tides (very very roughly), such that a tide should take about sixteen hours, not twelve should that become important, and just whistle for the rest! (And I'm probably already waay over-complicating things compared to most world-building folk...!)
Tides are, as far as I am aware, based on the moon above the planet at that particular point. Since the planet still rotates at the same speed (24 hours per day, I'd assume) then the tides would still happen at the same time.

The tides wouldn't get any stronger unless the moon was closer and/or more massive. It is the gravitational pull of the moon on the water at that point in the day, which would depend on how quickly the moon passes over the land (planet rotation) and how massive the gravitational pull is (lunar mass).


Tides would last longer if the moon hung in the sky longer, but the moon would display a different pattern in that case - it would stay in the sky longer, being visible throughout the night and day, and then disappearing for a night and day (for instance).

Aotrs Commander
2012-06-18, 11:36 AM
Tides are, as far as I am aware, based on the moon above the planet at that particular point. Since the planet still rotates at the same speed (24 hours per day, I'd assume) then the tides would still happen at the same time.

Ah.

I knew I was missing something while I was running it through my head... Yeah, I'm confusing the rotation of the Earth relative to the moon with the orbit of the moon around the Earth. Yeah, you're right, with 24 hour rotation, the tides won't change (much!)... but, like I even said earlier myself (*skullpalm*) the time between spring and neeps tides will be 33% longer. (So really, as I don't think even I will need to worry about spring and neeps tides in the context of advntures, there's cosmically speaking (ahaha) not much difference in having a longer luna month, at least not within the constraints of "ball-park figure close enough.")

Guh. Overthinking things again...

Thanks, erikun, JoshuaZ, my brain just ain't working properly today...!

SiuiS
2012-06-19, 07:19 AM
There is no link between lunar phases and solar equinoxes.

In a variant solar system, they could have a relationship, but it would be an entirely coincidental relationship, not a relationship where one causes the other.

This is the right stuff.
Also, this thread wil be fun to come back to when I've reallocated my mind to not being in a sleep fog :)

Lessee. Moon phase is based on time of moonrise relative to sunrise. Or if that sounds dumb, that every full moon rises at dusk, and that every waning gibbous moon rises at around the same physical time, etc.

Hoo boy, that doesn't even make sense to me. Time to nod off.

Ashtagon
2012-06-19, 07:51 AM
re: tides

http://en.wikipedia.org/wiki/Earth_tide#Tidal_constituents

It's complicated.

Vladislav
2012-06-22, 06:28 PM
Executive summary, high tides happen either when the moon is directly above your head, or when diametrically opposite (on the other side of the earth, below your feet).

That makes one high tide approximately every 13 hours (remember, the moon also moves around the earth while the earth turns, which is why it's not exactly 12 hours).

Particularly high tides occurs when both the sun and the moon are aligned (either both directly overhead, or both on the other side of the earth), approximately twice a month.