Iau1976 Epoch Precession

Precess a mean equatorial direction with the bounded IAU 1976 approximation; proper motion, nutation and modern frame bias are not included.

Description

Precession changes the mean equatorial coordinate axes between epochs.

Precession changes the mean equatorial coordinate axes between epochs. It is separate from a star's proper motion and from nutation, so those effects should not be silently included in a mean-direction rotation.^[1]

Precess a mean equatorial direction with the bounded IAU 1976 approximation; proper motion, nutation and modern frame bias are not included.

When to use Iau1976 Epoch Precession

Use this tool to precess a mean equatorial direction with the bounded IAU 1976 approximation; proper motion, nutation and modern frame bias are not included.

Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.

How to use Iau1976 Epoch Precession and understand its results

Prepare the source, detector or coordinate measurements. Confirm frame, epoch, angular units, axis directions and the stated RA proper-motion convention before entering values.
Enter raDegrees, decDegrees, startJulianEpoch, endJulianEpoch using the field table and calculation rules, then run the tool.
Read precessedRaDegrees, precessedDecDegrees. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
raDegrees	Ra degrees	number	Required
decDegrees	Dec degrees	number	Required
startJulianEpoch	Start julian epoch	number	Required
endJulianEpoch	End julian epoch	number	Required

Enter RA/Dec degrees and start/end Julian epochs each within 1800–2200. The tool evaluates the three supplied IAU 1976 cubic-angle expressions, rotates the mean direction and returns wrapped RA and declination in degrees. Identical epochs give the same direction within numeric roundoff.

T=(startEpoch−2000)/100; t=(endEpoch−startEpoch)/100; IAU 1976 angles ζ,z,θ rotate the direction

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
precessedRaDegrees	Precessed ra degrees	number
precessedDecDegrees	Precessed dec degrees	number

json

{
  "input": {
    "raDegrees": 0,
    "decDegrees": 0,
    "startJulianEpoch": 2000,
    "endJulianEpoch": 2050
  },
  "output": {
    "precessedRaDegrees": 0.6407181591449103,
    "precessedDecDegrees": 0.27834108271999486
  }
}

Limitations and assumptions

This bounded low-precision approximation omits proper motion, nutation and modern frame bias. It does not fetch an ephemeris or convert apparent positions into mean ones.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Rectilinear Astrometric Epoch Propagation: Propagate a Cartesian constant-space-velocity astrometric state using RA* proper motion and Julian years; light-time, gravity and reference-frame precession are excluded. For a complementary task, use Four Term Nutation Components: Evaluate a four-term low-precision nutation approximation from supplied mean angles; not an IAU 2000/2006 implementation.

References

Iau1976 Epoch Precession — tool.io

Similar or alternative tools

Rectilinear Astrometric Epoch Propagation
Propagate a Cartesian constant-space-velocity astrometric state using RA* proper motion and Julian years; light-time, gravity and reference-frame precession are excluded.
Four Term Nutation Components
Evaluate a four-term low-precision nutation approximation from supplied mean angles; not an IAU 2000/2006 implementation.