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ATM-OBSERVERS-L  January 2007

ATM-OBSERVERS-L January 2007

Subject:

Re: Up-Coming Occultation

From:

"Howard L. Cohen" <[log in to unmask]>

Reply-To:

Howard L. Cohen

Date:

Tue, 23 Jan 2007 22:46:14 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (108 lines)

Tandy,

I guess I must have misinterpreted your statement about planet 
magnitudes.  You originally wrote, "Some of the planets are on the 
order of Magnitude 4."  I did not take this to mean some are 
fainter.  Note that even Uranus is much, much fainter than mag.4 -- 
its magnitude of about +6 is two mag. fainter which means Uranus is 
about 6 times fainter than mag. +4.  Similarly, Neptune is about 40x 
fainter if mag. +8 and Pluto at mag. +14 is 10,000x fainter.

I have not run extensive test on PC clocks but the error is a 
function of what computer tasks are running so the correction can be 
irregular.  I have noticed that after only a few seconds, the clock 
correction is sometimes as large as 0.5 sec.  I suppose it is 
possible that the computer clock would be OK if corrected just before 
and after the event but it may not be possible to know the exact 
correction during the event since the correction is not predictable.

GPS time as kept by the satellite system may be quite accurate and 
the internal GPS clock may also be very accurate but I have read the 
*displayed time* by a typical GPS device is poor due to the display 
driver subroutines having low priority over location.  I have also 
read that Garmin engineers have stated the time displayed on the 
front of the unit should be accurate to within 1 second of UTC 
assuming it corrects for leap seconds.

On the other hand, received WWV signals can be very accurate.  The 
time is kept to 1 nanosecond at the receiver but delays from the 
transmitter to your location increase the error.  Still, the National 
Institute of Standards and Technology claims for most users in the 
United States, the received accuracy should be less than 10 
milliseconds (1/100 of a second).

Therefore, receiving a WWV time signal by radio is far more accurate 
than the displayed time on a typical GPS unit.

I hope we can discuss things further at the next ATM meeting.  I am 
also hoping Dr. John Oliver will be there to help provide further 
advice.  He can probably tell you more about feeding WWV to your 
computer since we used to do this when he and I did occultation work 
some years ago.

Howard

At 09:11 PM 1/23/2007, Tandy W. Carter Jr. wrote:
>Hi Howard,
>
>    No, there aren't any planets that have a Magnitude of exactly 4. 
> However, there are three planets with magnitudes fainter than 4. In 
> TheSky 6 Uranus is listed as having a Magnitude of 5.9. Neptune has 
> a Magnitude of 8.0. Pluto (Yes, I still consider Pluto a planet) 
> has a Magnitude of 14.0.
>
>    You are absolutely correct about the poor time keeping of 
> computer clocks. I don't know how bad they are though. I would 
> think the computer would keep good enough time so that the time is 
> correct if you synchronize a few minutes before the event starts.
>
>    My hand held GPS displays time accurate to the second. GPS works 
> by comparing the time delay between several satellites. Therefore, 
> the time signal sent by the satellites has to be extremely 
> accurate. Also, WWV and the GPS constellation both receive their 
> time signals from the same location, the USNO. So, which is more 
> accurate WWV or GPS?
>
>Tandy W. Carter Jr.
>[log in to unmask]
>http://home.att.net/~tandy.carter/
>----- Original Message ----- From: "Howard L. Cohen" <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Tuesday, January 23, 2007 3:21 PM
>Subject: Re: Up-Coming Occultation
>
>
>>I'm puzzled.  I don't know of any planets that have a mag. of +4.
>>
>>Also, typical Windows computers keep poor time.  Even if you 
>>synchronize with WWV, they can be off after several minutes 
>>pass.  Therefore, you need to continuously feed them WWV signals to 
>>keep the time correct or the time may wander in an unpredictable fashion.
>>
>>Many GPS devices were primarily built for location not for accurate 
>>times -- the displayed time is probably only accurate to a 
>>second.  If the receiver does not correctly account for leap 
>>seconds, it may be many seconds off.
>
>
>!DSPAM:45b6c1ca510005368818224!
>

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