# dateTimeToMJD: Calculate Modified Julian Date for a given date and time In asteRisk: Computation of Satellite Position

 dateTimeToMJD R Documentation

## 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")
```

### 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).

### Value

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

### 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
}
```

asteRisk documentation built on Jan. 14, 2023, 5:07 p.m.