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rms-julian

PDS Ring-Moon Systems Node, SETI Institute

Supported versions: Python >= 3.7

This is a set of routines for handing date and time conversions. It handles these time systems:

• UTC = Universal Coordinates Time, similar to Grenwich Mean Time, expressed by integer days since January 1, 2000 plus floating-point seconds since beginning of day. UTC can also be represented in various standard formats for a calendar date plus an optional time.
• TAI = International Atomic Time, which is number of actual elapsed seconds since December 31, 1999 at 23:59:28. This running tally accounts for all leap seconds.
• TDB = Terrestrial Barycentric Time, which is the number of elapsed seconds since noon (not midnight!) on January 1, 2000, and adjusted for relativistic effects that cause a clock on the Earth to vary in speed relative to one at the solar system barycenter. This quantity is equivalent to "ephemeris time" in the SPICE time system, although differences at the level of milliseconds can occur.
• TDT = Terrestrial Dynamical Time, which is the preferred time system for Earth-centered orbits. This is also defined in a manner consistent with that in the SPICE toolkit.
• JD = Julian Date as a number of elapsed days since noon (not midnight!) on Monday, January 1, 4713 BCE. Each period from one noon to the next counts as one day, regardless of whether that day contains leap seconds. As a result, some days are longer than others. (Note that leap seconds always occur at midnight, and therefore at the middle of a Julian day.)
• MJD = Modified Julian Date, defined to be JD minus 2400000.5.
• JED = Julian Ephmeris Date, defined to be TDB/86400 + 2451545. It is compatible with SPICE ephemeris time but in units of days rather than seconds.
• MJED = Modified Julian Ephmeris Date, defined as JED minus 2400000.5.

Throughout the library, TAI is the intermediary time relative to which all others are defined. Note: The term "TAI" is also used infrequently in the SPICE Toolkit, but the SPICE value is smaller by exactly 43200 seconds. All other terms used here are essentially identical in meaning to their SPICE Toolkit equivalents.

If the environment variable SPICE_LSK_FILEPATH is defined, then this SPICE leapseconds kernel is read at startup. Otherwise, leap seconds through 2020 are always included, as defined in NAIF00012.TLS.

The library also handles calendar conversions and both parses and formats strings that express time in UTC.

This library duplicates much of the functionality of Python's built-in datetime library, but is separate from them because the datetime library cannot handle leap seconds.

This library duplicates some of the SPICE toolkit, but has the advantage of supporting array-based time operations, which can be much faster when processing large amounts of data. It is also pure Python, and so does not need to be linked with C or FORTRAN libraries.

Aside from the I/O routines, every argument to every function can be either a scalar or something array-like, i.e, a NumPy array, a tuple or a list. Arguments other than scalars are converted to NumPy arrays, the arrays are broadcasted to the same shape if necessary, and the complete array(s) of results are returned.