asteRisk (version 1.4.3)

dateTimeToMJD: Calculate Modified Julian Date for a given date and time

Description

The Julian Date (JD) of a given date and time is the number of days since noon of Monday 1st of January 4713 BC, including a fractional part. Modified Julian Date (MJD) are instead the number of days since 00:00 of November 17th, 1858. The difference JD and MJD for a given instant is always 2400000.5, which is the JD of the reference time for MJD.

This function calculates the MJD of a date and time, provided as a date-time character string in UTC time. The output refers by default to the MJD in UTC, but different time systems can be chosen: UTC (Coordinated Universal Time), UT1 (Universal Time), TT (Terrestrial Time) and TDB (Barycentric Dynamical Time).

Usage

dateTimeToMJD(dateTime, timeSystem="UTC")

Value

The MJD for the specified date and time in the chosen time system.

Arguments

dateTime

Date-time string with the date and time in UTC corresponding to the provided geodetic coordinates.

timeSystem

Time system into which the MJD should be calculated. Should be one from "UTC" (Coordinated Universal Time; default), "UT1" (Universal Time), "TT" (Terrestrial Time) and "TDB" (Barycentric Dynamical Time).

References

https://gssc.esa.int/navipedia/index.php/Julian_Date https://gssc.esa.int/navipedia/index.php/Transformations_between_Time_Systems

Examples

if(requireNamespace("asteRiskData", quietly = TRUE)) {
# Let's calculate the MJD of the 12th of June, 2000 at 10:00:00 UTC time, in UTC

MJD_UTC <- dateTimeToMJD("2000-06-12 10:00:00", timeSystem = "UTC")

# Let's now calculate the MJD for the same instant in TDB:

MJD_TDB <- dateTimeToMJD("2000-06-12 10:00:00", timeSystem = "TDB")

# We can now calculate the difference in seconds, which matches the difference 
# between UTC and TDB for that day:

(MJD_UTC - MJD_TDB) * 86400
}