JULIAN DATES IN THE DEEP IMPACT AND EPOXI ARCHIVES AND THEIR INTERACTION WITH LEAP SECONDS History ======= 2010-09-13 B.T.Carcich Initial version Table of Contents ================= References Prerequisites Summary Julian dates and leap seconds in data sets from the Deep Impact Instruments Overview Details Julian dates in the DI data sets JD UTC values in the DI data sets JD TDB values in the DI data sets Julian dates in the PDS DI FITS headers Julian dates in the PDS DI data labels Cognizant persons References ========== [1] NAIF/SPICE TIME required reading document, available from http://naif.jpl.nasa.gov/. [2] Astronomical Almanac Online. (2010). U.S. Nautical Almanac Office and Her Majesty's Nautical Almanac Office. http://asa.usno.navy.mil/ and http://asa.hmnao.com/. [3] Planetary Data System Standards Reference, Version 3.9, JPL D-7669 Part 2, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 2009. [4] Pence, W.; Chiappetti, L.; Page, C.; Shaw, R.; Stobie, E.; (IAUFWG Technical Panel), Definition of the Flexible Image Transport System (FITS). FITS Standard version 3.0 Internat. Astronomical Union Commiss. 5: Documentation and Astronomical Data. FITS Working Group (2008) http://fits.gsfc.nasa.gov/standard30/fits_standard30.pdf Prerequisites ============= ===================================================================== == N.B. it is assumed that the reader is familiar with the time == == required reading material from the NAIF/SPICE toolkit == == documentation [1], specifically with terms and concepts == == such as Julian Date, UTC, TDB, leapseconds, &c. Refer to == == the references above for details. == ===================================================================== Summary ======== The Deep Impact (DI) and EPOXI data sets to-date span two leap seconds: 2005-DEC-31T23:59:60 and 2008-DEC-31T23:59:60. Therefore, any differences calculated between DI UTC JDs which span one or both of those leap seconds must add one, or two seconds to the calculated result. With two exceptions, all keywords using Julian dates (JDs) in the FITS headers and PDS data file labels of the Deep Impact (DI) data sets are UTC JDs. This means that any two JDs from these data sets may not be arbitrarily associated by simple differencing of the JD values. The two exceptions are FITS header keywords OBSMIDJT and KPKSSBJT which are JD TDB values, and are plainly labelled as such in the header keyword comments. Julian dates and leap seconds in data sets from the Deep Impact Instruments =========================================================================== Overview -------- The purpose of this document is to alert the data set user to pitfalls when calculating differences between Julian Dates within the data sets and between Julian Dates from the data sets with other time systems. Almost all JD time representations in the DI data sets are UTC JDs and therefore require that leap seconds be taken into account when calculating time differentials between UTC JDs. Details ------- There are several representations of time used in the DI data sets; one of them is Julian Date. The Julian Date system uses a number which represents the time since an epoch, specifically Julian date (JD): the interval of time in days and fractions of a day, since 4713 B.C. January 1, Greenwich noon, Julian proleptic calendar [2]. That definition, from the Glossary of reference [2], goes on to prescribe the following: In precise work, the timescale, e.g., Terrestrial Time (TT) or Universal Time (UT), should be specified [2]. This intent of this document is to fulfill that prescription. The issue is that calculating time differences between Julian dates involves more than subtracting two floating point numbers. The specific pitfall is that Julian dates look like real numbers, e.g. 2452545.000000, which appear to represent a time using two parts: an integral part which corresponds to a number of whole days; a fractional change part which corresponds to a fractional day. However, in some contexts, one day in the integral part may not represent a change of 86,400 seconds in time. This is due to leap seconds, an explanation of which is outside the scope of this document. The reader is referred to the references above plus available literature, but suffice to say that since the January 2005 launch of the DI spacecraft, two leap seconds have occurred: 2005-DEC-31T23:59:60; 2008-DEC-31T23:59:60. Julian dates in the DI data sets -------------------------------- There are two kinds of Julian dates used in the FITS headers and in the PDS data labels of the DI data. The most used kind is JD UTC, aka UT JD and UTC JD. The second kind is JD TDB. JD UTC values in the DI data sets --------------------------------- Due to leap seconds, and like UTC calendar times, JD UTC values are not continuous with respect to continuous time systems such as TT or TDB, and should therefore not be treated as real numbers subject to arithmetic operations such as addition and subtraction. JD UTC values should instead be thought of as symbols representing specific times; in that sense JD UTCs are analogous to calendar strings such e.g. 2010-09-13T01:23:45.6789 UTC N.B. The JD UTC time system is discontinuous across leap seconds and therefore has no way to represent time during a leap second. However, there have been no DI data taken during any leap second that would require such a problem to be resolved. JD TDB values in the DI data sets --------------------------------- JD TDB values are continuous and linear with respect to the continuous TDB time system, and typically differed from JD UTC values by about 0.000075d (~65s) during the DI mission. For details regarding time conversions between UTC times and other time systems the reader is referred to the NAIF/SPICE toolkit library and its attendant documentation, specifically the TIME required reading document [1]. Julian dates in the PDS DI data labels -------------------------------------- In the PDS labels, the relevant PDS keywords using Julian Dates are START_JULIAN_DATE_VALUE MID_JULIAN_DATE_VALUE STOP_JULIAN_DATE_VALUE Although not defined by the PDS standard [3], these keywords' floating-point values have been written as UT JDs at the direction of the Science Team. Julian dates in the PDS DI FITS headers --------------------------------------- In the FITS headers of the DI data, all JDs, except two, are UT JDs. The two JDs in the FITS headers are comments defining them as TDB JDs: OBSMIDJT= ... / [JD TDB] Barycentric time at mid-obs at s/c KPKSSBJT= ... / [JD TDB] When light reaches Solar Barycenter The reader is directed to the FITS standard [4] for the format of keyword comments in FITS headers. Again, for details regarding time conversions between UTC times and other time systems the reader is directed to the NAIF/SPICE toolkit library and its attendant documentation, specifically the TIME required reading document [1]. Cognizant persons ================= This document was written by Brian Carcich, Cornell University, Ithaca, NY, and reviewed by members of the EPOXI Science Team.