Calculate local Earth rotation angle.

Parameters¶

longitude~astropy.units.Quantity, ~astropy.coordinates.EarthLocation, str, or None; optional

The longitude on the Earth at which to compute the Earth rotation angle (taken from a location as needed). If None (default), taken from the location attribute of the Time instance. If the special string ‘tio’, the result will be relative to the Terrestrial Intermediate Origin (TIO) (i.e., the output of ~erfa.era00).

Returns¶

~astropy.coordinates.Longitude

Local Earth rotation angle with units of hourangle.

See Also¶

astropy.time.Time.sidereal_time

References¶

IAU 2006 NFA Glossary (currently located at: https://syrte.obspm.fr/iauWGnfa/NFA_Glossary.html)

Notes¶

The difference between apparent sidereal time and Earth rotation angle is the equation of the origins, which is the angle between the Celestial Intermediate Origin (CIO) and the equinox. Applying apparent sidereal time to the hour angle yields the true apparent Right Ascension with respect to the equinox, while applying the Earth rotation angle yields the intermediate (CIRS) Right Ascension with respect to the CIO.

The result includes the TIO locator (s’), which positions the Terrestrial Intermediate Origin on the equator of the Celestial Intermediate Pole (CIP) and is rigorously corrected for polar motion. (except when longitude='tio').