The Barycentric Julian Date (BJD) is the
Julian Date
The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).
...
(JD) corrected for differences in the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's position with respect to the
barycentre
In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important co ...
of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
. Due to the finite
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, the time an astronomical event is observed depends on the changing position of the observer in the Solar System. Before multiple observations can be combined, they must be reduced to a common, fixed, reference location. This correction also depends on the direction to the object or event being timed.
In 1991, the BJD replaced the
Heliocentric Julian Date
The Heliocentric Julian Date (HJD) is the Julian Date (JD) corrected for differences in the Earth's position with respect to the Sun. When timing events that occur beyond the Solar System, due to the finite speed of light, the time the event is obs ...
(HJD), which reduced times to the centre of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, which itself orbits the barycentre. The difference between HJD and BJD is up to ±4 s.
Magnitude of the correction
The correction is small for objects at the poles of the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
. Elsewhere, it is approximately an annual sine curve, and the highest amplitude occurs on the ecliptic. The maximum correction corresponds to the time in which light travels the distance from the barycentre to the Earth, i.e. ±8.3 min (500 s, 0.0058 days).
Time Standard
JD and BJD are defined independent of the
time standard
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters ...
. JD can be expressed as e.g.
UTC,
TT,
TAI
Tai or TAI may refer to:
Arts and entertainment
*Tai (comics) a fictional Marvel Comics supervillain
*Tai Fraiser, a fictional character in the 1995 film ''Clueless''
*Tai Kamiya, a fictional character in ''Digimon''
Businesses and organisations ...
,
TDB, etc. The differences between these time standards are of the order of a minute, so for better than one-minute accuracy, the time standard must be stated. While many quote the BJD in UTC, UTC is discontinuous and drifts with the addition of each
leap second
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observe ...
, and should therefore only be used for relative timing over a short time span (~1 year). For high-precision, absolute timing, TDB should be used. However, applications for which ±1.7 ms precision is sufficient may use TT to approximate TDB, which is much easier to calculate.
Calculation
Exact expression
Neglecting effects of
special
Special or specials may refer to:
Policing
* Specials, Ulster Special Constabulary, the Northern Ireland police force
* Specials, Special Constable, an auxiliary, volunteer, or temporary; police worker or police officer
Literature
* ''Specia ...
and
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
, the correction of
Terrestrial Time
Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.
For example, the Astronomical Almanac uses T ...
(TT) is
where
is the vector from the barycentre to the observer,
is the unit vector from the observer to the object or event,
is the distance from the observer to the observed object or event, and
is the speed of light.
This expression should be used for objects within the Solar System.
Expression for infinite distance
In the limit of infinite distance to the object, the exact expression becomes
This expression should be used for objects beyond the Solar System. The error is at the level of 100 s for objects in the
main asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
, 5 s for
Edgeworth-Kuiper Belt
The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
objects. At the distance of
Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
the accuracy is 1 ms.
Approximation for large distance
Due to the limited precision with which
floating point
In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be ...
numbers are stored in
computer
A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
s, the exact expression is in practice not accurate for large distances. The approximation
is accurate for large distances. It should be used if the object is beyond the Solar System and if also millisecond accuracy is required.
See also
*
Heliocentric Julian Date
The Heliocentric Julian Date (HJD) is the Julian Date (JD) corrected for differences in the Earth's position with respect to the Sun. When timing events that occur beyond the Solar System, due to the finite speed of light, the time the event is obs ...
*
Time standard
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters ...
References
* J. Eastman, R. Siverd, B. Scott Gaudi (2010). "Achieving better than one-minute accuracy in the Heliocentric and Barycentric Julian Dates". ''Publications of the Astronomical Society of the Pacific'', submitted. Online a
https://arxiv.org/abs/1005.4415 retrieved 2010-05-27.
External links
http://astroutils.astronomy.ohio-state.edu/time/ Online converter from UTC to BJD
TDB, BJD
TDB to UTC, or HJD (UTC or TT) to BJD
TDB.
Time scales
Time in astronomy