# [time-nuts] PTTI

Titan Corp titancorp at direcway.com
Fri Aug 12 17:08:12 EDT 2005

```TVB,
I want to thank you for the use of your website. I've been using it for the
last two years in my GPS/Cesium classes I've been teaching to the military.
Instead of doing the calculations for MJD setting I just have been directing
them to your web site. Thanks! I'm also in the process of updating the
military Technical manual(TM) / Technical Order (T.O.) for the 5071A and
including in the new documentation a section on using your website for MJD
info.

Below is what I've been told is the origin of the starting date for MJD. Got

November 17, 1858 is the base of the Modified Julian Day system.

The original Julian Day (JD) is used by astronomers and expressed in days
since noon January 1, 4713 B.C.  This measure of time was introduced by
Joseph Scaliger in the 16th century.  It is named in honor of his father,
Julius Caesar Scaliger (note that this Julian Day is different from the
Julian calendar named for the Roman Emperor Julius Caesar!).

Why 4713 BC?  Scaliger traced three time cycles and found that they were all
in the first year of their cycle in 4713 B.C.  The three cycles are 15, 19,
and 28 years long.  By multiplying these three numbers (15 * 19 * 28 =
7980), he was able to represent any date from 4713 B.C. through 3267 A.D.
The starting year was before any historical event known to him.  In fact,
the Jewish calendar marks the start of the world as 3761 B.C.  Today his
numbering scheme is still used by astronomers to avoid the difficulties of
converting the months of different calendars in use during different eras.

So why 1858?  The Julian Day 2,400,000 just happens to be November 17, 1858.
The Modified Julian Day uses the following formula:

MJD = JD - 2,400,000.5

The .5 changed when the day starts.  Astronomers had considered it more
convenient to have their day start at noon so that nighttime observation
times fall in the middle.  But they changed to conform to the commercial
day.

The Modified Julian Day was adopted by the Smithsonian Astrophysical
Observatory (SAO) in 1957 for satellite tracking.  SAO started tracking
satellites with an 8K (non-virtual) 36-bit IBM 704 computer in 1957, when
Sputnik was launched.  The Julian day was 2,435,839 on January 1, 1957.
This is 11,225,377 in octal notation, which was too big to fit into an
18-bit field (half of its standard 36-bit word).  And, with only 8K of
memory, no one wanted to waste the 14 bits left over by keeping the Julian
Day in its own 36-bit word.  However, they also needed to track hours and
minutes, for which 18 bits gave enough accuracy.  So, they decided to keep
the number of days in the left 18 bits and the hours and minutes in the
right 18 bits of a word.

Eighteen bits would allow the Modified Julian Day (the SAO day) to grow as
large as 262,143 ((2 ** 18) - 1).  From Nov. 17, 1858, this allowed for
seven centuries.  Using only 17 bits, the date could possibly grow only as
large as 131,071, but this still covers 3 centuries, as well as leaving the
possibility of representing negative time.  The year 1858 preceded the
oldest star catalog in use at SAO, which also avoided having to use negative
time in any of the satellite tracking calculations.

This base time of Nov. 17, 1858 has since been used by TOPS-10, TOPS-20, and
VAX/VMS.  Given this base date, the 100 nanosecond granularity implemented
within VAX/VMS, and the 63-bit absolute time representation (the sign bit
must be clear), VMS should have no trouble with time until:

31-JUL-31086 02:48:05.47

At this time, all clocks and time-keeping operations within VMS will
suddenly stop, as system time values go negative.

Note that all time display and manipulation routines within VMS allow for
only 4 digits within the 'YEAR' field.  We expect this to be corrected in a
future release of VAX/VMS sometime prior to 31-DEC-9999.
Ken Worsham
SAS Institute Inc
worsham at vms.sas.com

Raimond Melkers
AFSPC SMC MCL //L-3 Comm Titan Corporation
719-930-8011
RMelkers at Titan.com
L-3 Titan, its subsidiaries or affiliates.

-----Original Message-----
From: Tom Van Baak [mailto:tvb at leapsecond.com]
Sent: Friday, August 12, 2005 12:29
To: RMelkers at Titan.com
Subject: Re: [time-nuts] PTTI

Raimond,

I'll be there (it's very close to Seattle).
Doug Hogarth also.

/tvb
http://www.leapsecond.com/time-nuts.htm

----- Original Message -----
From: "Titan Corp" <titancorp at direcway.com>
To: <time-nuts at febo.com>
Sent: Tuesday, August 09, 2005 11:04
Subject: [time-nuts] PTTI

> Just started subscribing to this list, It's great!
> Who will be attending this years PTTI?
> Glad I found this group.
>
>
> Raimond Melkers
> AFSPC SMC MCL file://L-3 Titan
> SCTS/AFSATCOM Program Manager support
> DAYMARK/GPS timing/Ce(a)sium support
> 719-930-8011
> RMelkers at Titan.com
> TitanCorp at DirecWay.com
> ICQ 26419445
> Any comments or statements made are not necessarily those of L-3
> Titan,
its
> subsidiaries or affiliates.
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

```