Index: trunk/doc/pslib/psLibADD.tex =================================================================== --- trunk/doc/pslib/psLibADD.tex (revision 3213) +++ trunk/doc/pslib/psLibADD.tex (revision 3428) @@ -1,3 +1,3 @@ -%%% $Id: psLibADD.tex,v 1.63 2005-02-14 21:06:55 eugene Exp $ +%%% $Id: psLibADD.tex,v 1.64 2005-03-16 00:29:31 jhoblitt Exp $ \documentclass[panstarrs]{panstarrs} @@ -783,17 +783,16 @@ \subsubsection{Time} -Correct time representation is critical in astronomical software. -PSLib uses the \code{psTime} structure to represent time values. This -structure represents a time which is consists of seconds and fractions -of seconds in a time system defined by the \code{psTimeType} element -\code{type}. Two possible time systems are currently available: TAI -and UTC. Both are defined in terms of the reference epoch -1970-01-01T00:00:00Z, but with minor modifications for leap seconds as -needed. The first represenatation, TAI (International Atomic Time), -has seconds of uniform length and no leap seconds. The exact zero -reference is 1970/01/01,00:00:10 UTC. The second representation is -UTC, which has seconds of uniform length and leap seconds as needed to -adjust it to remain within 0.9 seconds of the Earth's rotation. It -has a zero-point of exactly 1970/01/01,00:00:00 UTC. +Correct time representation is \emph{critical} in astronomical software. PSLib +uses the \code{psTime} structure to represent time values. This structure +represents a time which is consists of seconds and fractions of seconds in a +time system defined by the \code{psTimeType} element \code{type}. Two possible +time systems are currently available: TAI and UTC. Both are defined in terms +of the reference epoch ``1970-01-01T00:00:00Z'', but with minor modifications +for leap seconds as needed. The first represenatation, TAI (International +Atomic Time), has seconds of uniform length (SI seconds) and no leap seconds. +The exact zero reference is ``1970-01-01T00:00:10Z'' UTC. The second +representation is UTC, which has seconds of uniform length and leap seconds as +needed to adjust it to remain within 0.9 seconds of the Earth's rotation. It +has a zero-point of exactly ``1970-01-01T00:00:00Z'' UTC. \paragraph{Coordinated Universal Time (UTC)} @@ -803,5 +802,5 @@ insertion of leap second whenever UTC-UT1 $\ge$ 0.9s. By definition UTC-TAI is an integer number of seconds. UTC went into effect on -"1972-01-01T00:00:00" and is defined as being TAI-UTC = 10s on that +``1972-01-01T00:00:00Z'' and is defined as being TAI-UTC = 10s on that date. For dates prior to 1972-01-01 a fixed offset of 10s relative to TAI will be assumed. @@ -811,14 +810,14 @@ \end{equation} -Leapseconds are declared by the International Earth Rotation and -Reference Systems Service (IERS). Leapseconds only occur in the UTC -time system and cannot be accurately predicted due to variations in -the Earth's rotational period. To determine the number of leapsecond -in a given UTC date a table of leapseconds as annouced by the IERS -must be consulted. This table will have to be updated each time a new -leapsecond occurs. - -For ease of conversion, UTC should be represented as the number of -seconds since the UNIX epoch of "1970-01-01T00:00:00". +Leapseconds are declared by the International Earth Rotation and Reference +Systems Service (IERS). Leapseconds only occur in the UTC time system and +cannot be accurately predicted due to variations in the Earth's rotational +period. To determine the number of leapsecond in a given UTC date a table of +leapseconds as annouced by the IERS must be consulted. This table will have to +be updated each time a new leapsecond occurs. + +For ease of conversion, UTC should be represented as the number of seconds +since the UNIX epoch of ``1970-01-01T00:00:00Z''. \emph{Times will always be +expressed in the 'UTC' timezone. Use of the local timezone is forbidden.} \paragraph{International Atomic Time (TAI)} @@ -982,5 +981,5 @@ \end{verbatim} -$2451545.0$ JD $= 51544.5$ MJD is equivalent to "2000-01-01T00:00:00". +$2451545.0$ JD $= 51544.5$ MJD is equivalent to ``2000-01-01T00:00:00Z''. \begin{equation} @@ -988,27 +987,24 @@ \end{equation} -\paragraph{Terrestrial Dynamical Time (TDT)} - -Terrestrial Dynamical Time (TDT) is defined as a fixed offset from -TAI. Its only purpose as far as we are concerned is for its utility -in obtaining the GMST. - -\begin{equation} -{\rm TDT} = {\rm TAI} + 32.184{\rm s} -\end{equation} - -\paragraph{TDT as Julian Centuries since J2000.0} - -The algorithm for calulating GMST requires TDT formated in Julian centruies +\paragraph{Terrestrial Time (TT)} + +Terrestrial Time (TT) is defined as a fixed offset from TAI. + +\begin{equation} +{\rm TT} = {\rm TAI} + 32.184{\rm s} +\end{equation} + +\paragraph{TT as Julian Centuries since J2000.0} + +The algorithm for calulating GMST requires TT formated in Julian centruies since the J2000.0 epoch. -\begin{equation} -t_u = \frac{{\rm JD}_{\rm TDT} - 2451545.0}{36525} +\begin{equation} t_u = \frac{{\rm JD}_{\rm TT} - 2451545.0}{36525} \end{equation} \paragraph{UT1 as Julian Centuries since J2000.0} -The algorithm for calulating GMST requires UT1 be formated in Julian -centuries since the J2000.0 epoch. +The algorithm for calulating GMST requires UT1 be formated in Julian centuries +since the J2000.0 epoch. \begin{equation} @@ -1018,10 +1014,11 @@ \paragraph{Greenwich Mean Sidereal Time (GMST)} -Greenwich Mean Sidereal Time (GMST) is caclulated from UT1 and TDT. -This algorithm requires that both time inputs are expressed as Julian -centuries since J2000.0. +Greenwich Mean Sidereal Time (GMST) is caclulated from UT1 and TT. This +algorithm requires that both time inputs are expressed as Julian centuries +since J2000.0 \footnote{Expressions to implement the IAU 2000 definition of UT1 +- http://www.edpsciences.org/articles//aa/abs/2003/30/aa3487/aa3487.html}. Here $t_u$ is UT1 expressed in Julian centuries since J2000.0, and $t$ -is TDT expressed in Julian centuries since J2000.0. +is TT expressed in Julian centuries since J2000.0. \begin{eqnarray} @@ -1034,4 +1031,5 @@ Gives $GMST00$ in seconds. + \paragraph{Longitude} @@ -1045,8 +1043,8 @@ \paragraph{Local Mean Sidereal Time (LMST)} -Local Mean Sidereal Time (LMST) is Greenwich Mean Sideral Time (GMST) -plus the observer's location in East longitude. Calculating LMST -requires the input of Universal Time (UT1), Terrestrial Dynamical Time -(TDT) and a longitude (measured East of Greenwich). +Local Mean Sidereal Time (LMST) is Greenwich Mean Sideral Time (GMST) plus the +observer's location in East longitude. Calculating LMST requires the input of +Universal Time (UT1), Terrestrial Dynamical Time (TT) and a longitude (measured +East of Greenwich). \begin{equation}