User blog:Cerne/Rotation velocity & rotation speed

I just thought of a new use for my blog. There are a lot of pieces of information for or about my conworld that would not fit in my actual conworld article(s) very well but that would still be very useful to me. Many of these pieces of information would not be permanently available - like an online bulletin board that prunes its threads after an allotted period of time - or it is taking up too much space where it is right now. I already have many threads and word documents saved up in my email account and I would like to minimalize and/or get rid of them without losing any of the actual information, so instead I would like to try putting that sort of information in this blog. Sort of a "behind the scenes" thing.

I am going to begin with a bunch of information about my planet's rotation velocity and rotation periods. It is a part of my planet's physical statistics and it will go into a template that lists all of the other information that can be conveyed with bare statistical or quantifiable data rather than quality data that can or needs to be conveyed in a paragraph. Most of my statistical data is determined in some way by something else, like gravity does on mass, mass does on volume, volume does on circumference or radii, and so on. I could not find out how to determine my planet's rotational velocity - and consequently my planet's sidereal rotation period - directly using pre-existing data, so I decided to come up with my own method and make an educated guess. I learned that rotation velocity does depend in some way on a planet's density, so I used my planet's density to directly determine what its rotation velocity might be. Since it is not an officially recognized or valid method for determining rotation velocity and rotation periods, there is no way for anyone to really know how I did it.

In short, I want readers to know that the method is not real or scientifically valid (to my knowledge, anyway) but that I didn't just make it up on the spot either. There was som determining involved and I want to show how I did it.

I'll start with part of a post I made on the ZBB:

Earth's density: 5.515 g/cm3 or 5515.3 kg/m^3 -> 5.52 times the density of water due to an Iron core & rock crust.

Planets with lower density: Earth density minus planet density equals percent difference. Planets with higher density: Planet density minus Earth density equals percent difference.

My planet's density: 7452 kg/m^3 -> roughly 1/4 or 25% higher than Earth's density (I rounded down instead of up for an easier number to work with). As stated in Geoff's page, the higher a planet's density is, the faster it will rotate.

[My planet's] rotation velocity: 465 m/s [for Earth] + a 25% higher density = 581 m/s for my planet. 581 m/s - 465 m/s = 116 m/s -> 20% faster than Earth's rotational velocity.

Earth's rotational period: 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day minus a 4 minute difference for a sidereal day. 60*60*24-(60*4)= 86,160 seconds in total.

A sidereal day on my planet: 86,160 seconds minus a 20% faster rotation speed = 68,928 seconds in total, or 19 hours, 8 minutes and 48 seconds. My solar day would be 20 hours long minus 52 seconds and 12 seconds or 3,132 seconds [in] total.

The words in the square brackets represent the changes or additions I made to the original post.

And here are some calculations and jotnotes I made while I was thinking about how to work the whole thing out:


 * 86,160 seconds in Earth's sidereal day -> 1436 minutes.
 * 24 hours in Earth's solar day -> 1440 minutes.
 * 68,928 seconds in my planet's sidereal day.
 * 68928/1148.8=60
 * 68928-68880=48
 * 68880/1148=60
 * 1148-1140=8
 * 1140/19=60
 * My planet's sidereal day has 19 hours, 8 minutes and 48 seconds.
 * My planet's solar day has 20 hours minus 52 minutes and 12 seconds, or 3132 seconds in total.

I tried to find a way to determine rotation velocity and rotation period using existing equations and data but I could not find any, and sufficed to say I think I have asked the folks over at the ZBB enough astronomy and planetology questions to make up for my mathematical inadequacy that they may have gotten tired with me by now. Which is to be expected: sit through the full extent of my questioning on astronomy, planetology and astrophysics AND THEN see how much you like me.

But if in the smallest chance I am right you did find this useful, then I am glad I could do you a favour by providing it for you. If I am wrong, then you could do me a favour by correcting any of the information I have provided. I would appreciate that.

Thanks for reading, Cerne 08:32, August 31, 2010 (UTC)