During the past, time was measured by observing stars with instruments such as
photographic zenith tubes and
Danjon astrolabes, and the passage of stars across defined lines would be timed with the observatory clock. Then, using the
right ascension of the stars from a star catalog, the time when the star should have passed through the meridian of the observatory was computed, and a correction to the time kept by the observatory clock was computed. Sidereal time was defined such that the March equinox would
transit the meridian of the observatory at 0 hours local sidereal time. Beginning during the 1970s, the
radio astronomy methods
very-long-baseline interferometry (VLBI) and
pulsar timing overtook optical instruments for the most precise
astrometry. This resulted in the determination of
UT1 (mean solar time at 0° longitude) using VLBI, a new measure of the Earth Rotation Angle, and new definitions of sidereal time. These changes became effective 1 January 2003.
Earth rotation angle The
Earth rotation angle (
ERA) measures the rotation of the Earth from an origin on the celestial equator, the
Celestial Intermediate Origin, also termed the
Celestial Ephemeris Origin, that has no instantaneous motion along the equator; it was originally referred to as the
non-rotating origin. This point is very close to the equinox of J2000. ERA, measured in
radians, is related to
UT1 by a simple linear relation:\theta(t_U)=2\pi(0.779\,057\,273\,2640+1.002\,737\,811\,911\,354\,48\cdot t_U)where
tU is the
Julian UT1 date (JD) minus 2451545.0. The linear coefficient represents the
Earth's rotation speed around its own axis. ERA replaces
Greenwich Apparent Sidereal Time (GAST). The origin on the celestial equator for GAST, termed the true
equinox, does move, due to the movement of the equator and the ecliptic. The lack of motion of the origin of ERA is considered a significant advantage. The ERA may be converted to other units; for example, the
Astronomical Almanac for the Year 2017 tabulated it in degrees, minutes, and seconds. As an example, the
Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″. Since
Coordinated Universal Time (UTC) is within a second or two of UT1, this can be used as an anchor to give the ERA approximately for a given civil time and date.
Mean and apparent varieties & Son. It was previously owned by Sir
George Shuckburgh-Evelyn. It is on display in the
Royal Observatory, Greenwich, London. Although ERA is intended to replace sidereal time, there is a need to maintain definitions for sidereal time during the transition, and when working with older data and documents. Similarly to mean solar time, every location on Earth has its own local sidereal time (LST), depending on the longitude of the point. Since it is not feasible to publish tables for every longitude, astronomical tables use Greenwich sidereal time (GST), which is sidereal time on the
IERS Reference Meridian, less precisely termed the Greenwich, or
Prime meridian. There are two varieties,
mean sidereal time if the mean equator and equinox of date are used, and
apparent sidereal time if the apparent equator and equinox of date are used. The former ignores the effect of
astronomical nutation while the latter includes it. When the choice of location is combined with the choice of including astronomical nutation or not, the acronyms GMST, LMST, GAST, and LAST result. The following relationships are true: The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are: \mathrm{GMST}(t_U,t)=\theta(t_U)-E_\mathrm{PREC}(t)\mathrm{GAST}(t_U,t)=\theta(t_U)-E_0(t)such that
θ is the Earth Rotation Angle,
EPREC is the accumulated precession, and
E0 is equation of the origins, which represents accumulated precession and nutation. The calculation of precession and nutation was described in Chapter 6 of Urban & Seidelmann. As an example, the
Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″ (6 h 42 m 28.8291 s). The GAST was 6 h 43 m 20.7109 s. For GMST the hour and minute were the same but the second was 21.1060.
Relationship between solar time and sidereal time intervals has dials showing both sidereal time and
mean solar time. If a certain interval
I is measured in both mean solar time (UT1) and sidereal time, the numerical value will be greater in sidereal time than in UT1, because sidereal days are shorter than UT1 days. The ratio is:\frac{I_\mathrm{mean\,sidereal}}{I_\mathrm{UT1}}=r'=1.002\,737\,379\,093\,507\,95 + 5.9006\times10^{-11}t - 5.9\times10^{-15}t^2such that
t represents the number of Julian centuries elapsed since noon 1 January 2000
Terrestrial Time. ==Sidereal days compared to solar days on other planets==