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.
At 09:11 PM 1/23/2007, Tandy W. Carter Jr. wrote:
> 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]
>----- 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.
THIS MESSAGE IS FROM THE AMATEUR TELESCOPE MAKERS & OBSERVERS
OF THE ALACHUA ASTRONOMY CLUB, INC. MAILING LIST
AAC Home Page: http://www.floridastars.org
ATM-OBSERVERS-L Meetings Page: http://www.floridastars.org/atm/
ATM-OBSERVERS-L Archives: http://www.lists.ufl.edu/archives/atm-observers-l.html
ATM-OBSERVERS-L Subscription Info: http://www.floridastars.org/listserv/atml.html
POSTING MESSAGES TO THE ENTIRE ATM-OBSERVERS-L LIST:
1. Send your message to [log in to unmask]
2. Message will be distributed subject to approval by the sender
3. Please keep messages pertinent to the purpose of this list
TO UNSUBSCRIBE FROM THIS LIST:
1. Send e-mail to [log in to unmask]
2. In body of e-mail put UNSUBSCRIBE ATM-OBSERVERS-L
(You must be logged on under same userid as when you originally subscribed.)