[Kstars-devel] Fwd: Re: error:more info

[David Faure]

Received some feedback about the kstars user manual. Please reply to Stan, not to me ;)

----------  Forwarded Message  ----------

Subject: Re: error:more info
Date: Wednesday 02 March 2016, 07:18:16
From: Stan Wagon <wagon at macalester.edu>
To: David Faure <faure at kde.org>

I was referring to this:

https://docs.kde.org/trunk5/en/kdeedu/kstars/ai-leapyear.html

There I see:

, in the time it takes the Earth to complete one orbital circuit, it completes 365.24219 spin rotations.

Not true. In the time it takes the Earth to complete one orbital circuit around the sum, it completes 366.24219 spin rotations.

To understand this, take one penny and roll it around another. The rolling penny will complete TWO spin rotations as it circles the stationary (sun) penny. 

We “see” only 365 of this 366 because one of the spins gets hidden from our view, since we do not notice our motion around the sun on a daily basis. To be precise, it takes 23 hrs 56 mins for the earth to spin on its axis. But in that time we have moved around a sun 1/365 of the way, and it takes us 4 extra minutes to make that up (on average), hence the 24 hours.  Or look at it this way: as we make one full spin, the sun has moved, and it takes us 4 extra minutes each day to catch up to the moved sun. If we were trying to contact a distant star and wanted to do so when we were closest to it in our spin, we would have 366 chances to do so in a year, not 365.

Google sidereal time….

Stan Wagon
Math Dept
Macalester College



> On Mar 2, 2016, at 2:21 AM, David Faure <faure at kde.org> wrote:
> 
> On Monday 29 February 2016 14:13:00 Stan Wagon wrote:
>> On your web page for leap year I see this:
>> 
>> First, it spins on its rotational axis; a full spin rotation takes one Day to complete. 
>> 
>> This is incorrect. The earth takes 23 hrs 56 minutes to make a “full spin” rotation. The extra 4 mins comes from movement around the sun. Learn about “sidereal time”.
> 
> Hello,
> 
> Thanks for your note, but I lack context.
> 
> Can you be more specific, which web page are you emailing us about ?
> 
> Cheers,
> 
> -- 
> David Faure, faure at kde.org, http://www.davidfaure.fr
> Working on KDE Frameworks 5
> 

-----------------------------------------
-- 
David Faure, faure at kde.org, http://www.davidfaure.fr
Working on KDE Frameworks 5