Description
Calculate approximate Greenwich mean sidereal hours from an explicit UT1 Julian date.
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.
Calculate approximate Greenwich mean sidereal hours from an explicit UT1 Julian date.
Mean and apparent sidereal time differ by the equation of the equinoxes. Earth-rotation timing uses UT1; this tool’s one-input historical mean-time polynomial does not implement the separate apparent-time corrections.1
When to use this tool
Use this tool when you need to calculate approximate Greenwich mean sidereal hours from an explicit UT1 Julian date.
How to use the tool and how it works
- Enter ut1JulianDay using the definitions below.
- Run the operation. Resolve input-format errors before interpreting an output.
- Read result.greenwichMeanSiderealHours, using the stated boundary, ordering, sign and absence conventions.
Set T=(ut1JulianDay−2451545)/36525. Evaluate the supplied historical GMST polynomial in degrees, wrap modulo 360 and divide by fifteen to obtain hours in [0,24).2
| Input field | Meaning and units | Accepted input |
|---|---|---|
| ut1JulianDay | UT1 Julian date in [2415020.5,2488070.5], distinct from an uncorrected UTC date. | Finite numeric value satisfying the stated comparisons. |
| Output field | Interpretation |
|---|---|
| result | Object containing the fields below. |
| result.greenwichMeanSiderealHours | Approximate GMST in hours wrapped to [0,24). |
{
"input": {
"ut1JulianDay": 2451545
},
"output": {
"result": {
"greenwichMeanSiderealHours": 18.697374558
}
}
}Limitations and assumptions
- UT1 Julian day must lie from 2415020.5 through 2488070.5. This is approximate Greenwich mean sidereal time, without nutation or apparent sidereal time; UTC is not silently substituted for UT1.
- 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
Similar or alternative tools
- Julian Date from Calendar Date Standard Converter
Convert a proleptic Gregorian date and uniform-day seconds to fractional Julian day.
- UT1–UTC Interpolator and Delta T Calculator
Interpolate supplied DUT1 observations through UT1−TAI, then recover UT1−UTC and TT−UT1.