Re: Julian Date and Leap Seconds

"JSeb" <JSebBolduc@xxxxxxxxx> wrote in message
news:1176828541.252331.206640@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi all,

Not sure I'm in the right group, as usual... anyway, here goes!

The length of of UTC day is not constant. It generally counts 86400 SI
seconds, except when a leap second is needed. Then a day counts an
extra (so far) second.

On the other hand, it seems that the length of a julian day IS
constant: 86400 SI seconds, no more, no less. If that's the case, I'm
kind of surprised: I thought I could easily convert between julian
days and gregorian dates...

Consider the story at the last second of year 2005, which was a leap
second:

gregorian date and time: december 31, 2005, 23:59:59
Modified julian date: 53735 + 86399/86400

gregorian date and time: december 31, 2005, 23:59:60 -> THAT'S THE
LEAP SECOND
Modified julian date: 53735 + 86400/86400 = 53736

During the leap second, it seems to me MJD and UTC disagree on the
date! MJD is already on january 1st, 2006 (back conversion).

Can anybody shed some light on this? it seems to me UTC and MJD are
drifting appart!

UTC, being tied to the actual rotation of the Earth, is not a
strictly uniform time scale so it's bound to drift away from
time scales that are uniform unless periodic corrections are
made. Julian day number is based upon a uniform scale.

A brief introduction to the relationships amongst the various
time scales may be found here:

http://tycho.usno.navy.mil/systime.html


.

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