Description
Convert Julian day to modified Julian day while retaining the input time scale.
Geodesy distinguishes a mathematical reference ellipsoid, a realized coordinate datum and the physical gravity field. Geodetic latitude follows the ellipsoid normal; geocentric latitude follows a radius from the center. Ellipsoidal height, orthometric height and geoid undulation are separate signed quantities. Keep the declared ellipsoid, coordinate axes, angular units and time scale with a result.
Convert Julian day to modified Julian day while retaining the input time scale.
A Julian date has a noon-based day origin, so midnight dates end in a half day. This calendar arithmetic must remain separate from the choice or conversion of physical time scale.1
When to use this tool
Use this tool when you need to convert Julian day to modified Julian day while retaining the input time scale.
How to use the tool and how it works
- Enter julianDay using the definitions below.
- Run the operation. Resolve input-format errors before interpreting an output.
- Read result.modifiedJulianDay, using the stated boundary, ordering, sign and absence conventions.
Subtract 2,400,000.5 from the supplied Julian day to obtain modified Julian day.2
| Input field | Meaning and units | Accepted input |
|---|---|---|
| julianDay | Julian day, including fraction, on the caller’s declared time scale in [1721425.5,5373484.5). | Finite numeric value satisfying the stated comparisons. |
| Output field | Interpretation |
|---|---|
| result | Object containing the fields below. |
| result.modifiedJulianDay | Input Julian day minus 2,400,000.5, retaining the same time scale. |
{
"input": {
"julianDay": 2451545
},
"output": {
"result": {
"modifiedJulianDay": 51544.5
}
}
}Limitations and assumptions
- The accepted Julian-day interval is [1721425.5,5373484.5). MJD shares the input time scale; this offset does not turn UTC into UT1, TAI or TT.
- All quantities are finite and retain the units stated for this operation. Ellipsoid-parameter tools use semi-major axis 1–10⁸ m and dimensionless flattening 0–0.02; explicit-axis tools use 0.9a≤b≤a unless a tighter bound is stated. These tools do not fetch datums, geoid grids, satellite observations or Earth-orientation updates.
- Every declared outer field is required and unknown outer fields are rejected. Strings and JSON text have size limits, lists are bounded to 128 entries, and JSON nesting/node counts are bounded. Tighter operation-specific bounds stated above take precedence.
- Numeric results must be finite. Large physical magnitudes are allowed by the output evidence checks, while integer fields and source schemas retain their exactness rules. Source indexes are zero-based. Floating-point residues can remain even where an ideal formula gives an exact zero.
Alternative or Complementary methods
Use a coordinate transform for a change of representation and explicit datum parameters for a change of reference frame. Preserve model and convergence metadata, and use observation and datum information appropriate to a real survey before interpreting a computed tolerance as accuracy.
References
-
Converting between Julian dates and Gregorian calendar dates — U.S. Naval Observatory
-
Tool behavior and example: Modified Julian Date MJD Converter — tool.io
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