Re: Julian Date and Leap Seconds

In article <46250a92$0$20142$9a6e19ea@xxxxxxxxxxxxxxxxxxxx>,
Greg Neill <gneillREM@xxxxxxxxxxxxxx> wrote:

"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.

You meant UT here, not UTC - didn't you?

UTC is as uniform as TAI, but with discontinuities introduced on purpose
to keep UTC from drifting too far away from the slightly irregular UT
time scale.

Julian day number is based upon a uniform scale.

Not necessarily - the web page you refer to below says:

# Julian Day Number is a count of days elapsed since Greenwich mean noon
# on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is
# the Julian day number followed by the fraction of the day elapsed
# since the preceding noon.

There's no requirement of a uniform time scale here. As a matter of fact,
if you just say "JD" without any further qualification, you usually mean
a JD time scale based on the slightly irregular UT time scale.

Trying to base JS in the UTC time scale is a bit tricky if you want an
accuracy of one second or better, due to the discontinuities (aka leap
seconds) in UTC.

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

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

This web page also says:

# We frequently make use of the Modified Julian Date (MJD), which is
# defined as MJD = JD - 2400000.5. An MJD day thus begins at midnight,
# civil date. Julian dates can be expressed in UT, TAI, TDT, etc. and
# so for precise applications the timescale should be specified, e.g.
# MJD 49135.3824 TAI.

And of course JD based on UT is slowly drifting away from JD based on
TDT or on TAI, because UT is slowly drifting away from TDT and from TAI.

Before TDT was introduced, astronomers used either UT or ET (Ephemeris
Time, which can be considered equal to TDT). JD based on ET rather than
UT was then usually written as JED.


My own time scales web page can be found here:
http://stjarnhimlen.se/comp/time.html

--
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e-mail: pausch at stockholm dot bostream dot se
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