New measurement of Kerbin's sidereal day (not 6 hours)

[OhioBob]

It has been generally accepted that Kerbin's sidereal rotation period is exactly 6 hours (21,600 seconds), however I'm here to dispel that myth.

I started playing around by observing the time of sunrise each morning. This lead to the discovery that if you observe the sunrise on the same day one year apart, the sun rises 42 seconds earlier on the second year than it did on the first. I then decided to observe the rising times of identifiable stars. In this case I observed that a reference star rises about 37 seconds earlier than it did one year earlier.

To refine these numbers I warped ahead 10 years and found that sunrise differed by 421 seconds, or 42.1 s/yr, and the stars differed by 374 seconds, or 37.4 s/yr. I estimate my accuracy on the accumulative totals to be about ±1 s on both measurements.

From this we can compute the actual length of Kerbin's sidereal rotation period and its solar day:

Sidereal day = (426 * 21600 - 37.4) / 426 = 21599.9122 ±0.00024 seconds

Solar day = (426 * 21600 - 42.1) / 425 = 21650.7245 ±0.00024 seconds

Using these numbers we can compute the length of Kerbin's sidereal year:

Sidereal year = (-21599.9122 * 21650.7245) / (21599.9122 - 21650.7245) = 9,203,554 ±85 seconds

The generally accepted duration of Kerbin's sidereal year is 9,203,545 seconds, which is very close to my computation and clearly within the margin of error. The math matches the observations, so I feel quite confident in my results.

Of course Kerbin's calendar day remains 21,600 seconds regardless.
 

Edited March 3, 2017 by OhioBob

[wossname]

You have earned 1.6 science points.

[Bill Phil]

Are you sure you took into account everything?

Using only sunrise is a bit of a bias, you need to also measure sunset.

[Geschosskopf]

The above numbers are what the wiki says. Not new

[viktor19]

You have earned 1.6 science points.

2 if you transport it to Eve and back

[OhioBob]

The above numbers are what the wiki says. Not new

That's because I edited the Wiki yesterday.

[mikegarrison]

That's because I edited the Wiki yesterday.

LOL. Nice snarky response to a snarky comment.

Edited July 28, 2015 by mikegarrison

[Glaran K'erman]

That's because I edited the Wiki yesterday.

Slaaammmmmm lol

In all seriousness though, interesting to note Bob.

[Dr Farnsworth]

All this math stuff is making my head hurt.

In my aptitude test I scored highest on being a pain in the rear.

[OhioBob]

Bill Phil said:

Are you sure you took into account everything?

Using only sunrise is a bit of a bias, you need to also measure sunset.

I hadn't originally observed the sunsets, but I just checked it out. The sunsets show the same 42-second year-to-year difference as observed with the sunrises.

Because of the internal consistency of the numbers, I feel pretty confident that my observations and computations are correct. For instance, the calculation of the length of the sidereal year from the observations is extremely sensitive to error. You can see that just a ±1" error in the measurements over a ten year period results in a ±85" error in the sidereal year. That fact that my sidereal year calculation is just 9 seconds off from the accepted value I believe is very strong corroboration. I don't think there is any way I could get that close by chance.
 

Edited March 3, 2017 by OhioBob

[Moesly_Armlis]

Quite an observation.

The forty two second difference is way too coincidental. The answer to life the universe and everything.

Congratulations OhioBob, you win a cookie.

[OhioBob]

Glaran K said:

In all seriousness though' date=' interesting to note Bob.

 

My finding is just a minor footnote that really doesn't make a bit of difference to anybody around here. Nonetheless, I found it interesting. It made me feel like I was doing real science.

I find it strange that Squad would intentionally use such an oddball value for Kerbin's sidereal rotation period. I wonder if what I observed is an inaccuracy in the game's internal computations rather than something intentional?
 

Edited March 3, 2017 by OhioBob

[KerikBalm]

so, um, the day length is wrong, and instead of having leapyears where they add a day, we need shortened days every so often?

Basically, Kerbin doesn't have an exact integer value of rotations per revolution?

[Guest]

I find it strange that Squad would intentionally use such an oddball value for Kerbin's sidereal rotation period.

I don't. It's probably completely unintentional.

[hoojiwana]

What does the "Warp to morning" button on the space center do then?

[keeper]

So with the Sun rising slightly earlier, how long would it take for a.. an extra day? Is that right? How much would it cost to organise the oposite of a "leap year?"

But what I wonder more is how did that get your attention and why bother to work all that out? What does knowing this tell us?

[Guest]

What does the "Warp to morning" button on the space center do then?

It calculates directly from the planet's rotation and the sun position relative to the camera, which means rotation could be pretty much anything. I have no need for the button but it probably works great in RSS as well as other "Kerbins".

[Speadge]

Wiki:

[TABLE]

[TR]

[TD=colspan: 2]Sidereal rotation period

5 h 59 m 59.9 s[/TD]

[TD] 21 599,912 s[/TD]

[/TR]

[TR]

[TD][/TD]

[/TR]

[TR]

[TD=colspan: 2]Solar day

1 d 0 h 0 m 50.7 s[/TD]

[TD] 21 650,725 s[/TD]

[/TR]

[TR]

[TD][/TD]

[/TR]

[/TABLE]

Thats not in there since yesterday

and thinking about it- it seems normal:

Kerbin rotates around itself within approx 6h.

If the planet would "stand still" relativ to the sun, the "observer" on Kerbin would have the same angle to the sun every 6h.

Since the planet translates around the sun, the angle changes with every km it moves so that after 6h you dont have the same angle anymore, cause the planet translated another 6h.

Lets say we meassure the time from hour 0, without resetting after a day:

at ~ 12 pm you would ave a 90° angle to the sun an its noon.. eat something

after 213 days (half a year) Kerbin would have rotated 213x around _itself_ every 6 hours. That means when you look up after 1278 hours you would look the same direction in the solar system that you have looked before - but now the Planet has moved around the sun, meaning you are looking in the opposite direction of the sun. so it woul be midnight... you shouldnt eat now, it makes you get fat

if you would have added 50seconds (or 42) after each day, you would need to wait 3 more hours to have noon again (2.9583~) - half a rotation of kerbin to face the sun again.

FACT:

- watching kerbin system itself, without a sun, a full rotation of the planet is done after 6h. you dont even lose 1° of angle between a perfect keostationary satellite in orbit relative to the Point of View

- compared to Kerbin, it changes daily cause the angle to kerbol changes all the time. So if you just want to call a "day" the time between 1sunrise to another, its always more than the time for a whole 360° rotation of a planet.

-> it would be LESS if the planet would turn clockwise around the sun while turning around its own axis counterclockwise (is that even possible?)

Last fact: numbers are rounded and i dont care if its 42.1 seconds or 50

I think thats the easiest explanation i can think of for that.

-single 360° rotation => 6h (sidereal day)

- rotation to the next sunset is > 360° => more than 6h (solar day)

edit:

ok, shoot me.

was explaining "longer days", not longer years.

should get to bed right now. nevermind.

Keep on the good observations

Edited July 28, 2015 by Speadge

[NathanKell]

Nice work!

There was a note in some dev note or changelog that the sidereal period was reduced to try to create a 6h solar day...

[OhioBob]

so, um, the day length is wrong, and instead of having leapyears where they add a day, we need shortened days every so often?

The length of a Kerbin calendar day should really be based on the the solar day rather than the sidereal day (like here on Earth). This would synchronize the motion of the sun with the length of the day so that the sun rises and sets at the same time each day. As it is now, the sun rises about 50.7 seconds later each day. If the sun rises at 0:00 on the first day of the year, by mid-year it is rising at 3:00. That's screwed up and would really mess with schedules, work hours, etc.

The length of Kerbin calendar day should be 21650.7245 seconds, which would make the year 425 days long rather than 426. This would make the calendar year 1987 seconds shorter than the actually sidereal year. Reconciling this would require the addition of 10 leap days every 109 years (there would still have to be some periodic minor adjustments to keep perfectly in sync).

Of course a 21650.7245-second day dosen't work very well with our earthly units of measure. To keep the day 6 hours long, the simple solution is to make a Kerbin second equal to 1.002348356 Earth seconds.

Basically, Kerbin doesn't have an exact integer value of rotations per revolution?

Correct. Kerbin rotates 426.091778 times per one complete revolution.

[Geschosskopf]

That's because I edited the Wiki yesterday.

So tell me, did Kerbin's orbital parameters change when they made the 6-hr day official?

Prior to that change, Kerbin's solar year was 365x 24-hr days. If all that changed was going to 6-hr days, then the solar year should be 1460 days now, not 426. So is Kerbin moving 3.4x faster around the sun than it is now?

[OhioBob]

There was a note in some dev note or changelog that the sidereal period was reduced to try to create a 6h solar day...

That is what they should have done, but it looks like they either didn't do it or they failed to get it right. They should slow Kerbin's sidereal rotation period down to 21549.41452 seconds. This would make the solar day exactly 6 hours long and make the calander year 426 solar days. The sidereal year would still be a little longer than the calendar year, but not enough the worry about. It they wanted to make the sidereal year eactly 426 days, they would just have to decrease Kerbin's semimajor axis by 1916 km.

- - - Updated - - -

So tell me, did Kerbin's orbital parameters change when they made the 6-hr day official?

Prior to that change, Kerbin's solar year was 365x 24-hr days. If all that changed was going to 6-hr days, then the solar year should be 1460 days now, not 426. So is Kerbin moving 3.4x faster around the sun than it is now?

That change occured before my time. I think I started playing soon after the switch to 6-hour Kerbin days, because I recall reading something about that in the version update notes. I briefly played the demo and I vaguely remember that it used 24-hour days, but I have no idea what the orbital parameters where at that time.

EDIT: Clarification ... In the demo version Kerbin had a 6-hour long day, but I believe the game used a 24-hour clock and measured everything in Earth days.

Edited July 28, 2015 by OhioBob

[maltesh]

So tell me, did Kerbin's orbital parameters change when they made the 6-hr day official?

Prior to that change, Kerbin's solar year was 365x 24-hr days. If all that changed was going to 6-hr days, then the solar year should be 1460 days now, not 426. So is Kerbin moving 3.4x faster around the sun than it is now?

Kerbin never had a 24-hour day. When Kerbin began rotating in 0.12, it had a 6-hour sidereal day, and it's year was about 106 Earth days, as it is now.

The MET Clock and other game times did initially count in 24-hour days and 365-day years, however.

[Guest]

Prior to that change, Kerbin's solar year was 365x 24-hr days. If all that changed was going to 6-hr days, then the solar year should be 1460 days now, not 426. So is Kerbin moving 3.4x faster around the sun than it is now?

All that amounted to was how time was accounted for; the change to a 6-hour day was merely in how it was formatted for presentation to the user. Kerbin has always (to my knowledge) had a 6-hour day with a 426-ish day year. The proof is in the delta-V to orbit for all versions prior to 1.0.x and the fact that KSP uses orbital velocity correctly and that Kerbin did not change orbits when the 6-hour day, 426-ish day year was introduced.

E: ninja'd.

[GarrisonChisholm]

So with the Sun rising slightly earlier, how long would it take for a.. an extra day? Is that right? How much would it cost to organise the oposite of a "leap year?"

But what I wonder more is how did that get your attention and why bother to work all that out? What does knowing this tell us?

We have Edimuncated ourselves!! Science is its own reward!