$\boldmath{\Delta {\rm T}}$:
$\boldmath{\Delta {\rm UT1}}$ (or $\boldmath{\Delta {\rm UT}}$):
aberration (of light):
the relativistic apparent angular displacement of the observed position of a celestial object from its
geometric position
, caused by the motion of the observer in the reference system in which the trajectories of the observed object and the observer are described. (See
aberration, planetary
.)
aberration, annual:
aberration, diurnal:
aberration, E-terms of:
aberration, elliptic:
aberration, planetary:
the apparent angular displacement of the observed position of a solar system body from its instantaneous geometric direction as would be seen by an observer at the geocenter. This displacement is produced by the combination of
aberration of light
and
light-time displacement
.
aberration, secular:
aberration, stellar:
altitude:
the angular distance of a celestial body above or below the
horizon
, measured along the great circle passing through the body and the
zenith
. Altitude is 90° minus the
zenith distance
.
annual parallax:
anomaly:
anomaly, eccentric:
in undisturbed elliptic motion, the angle measured at the center of the
orbit
ellipse from
pericenter
to the point on the circumscribing auxiliary circle from which a perpendicular to the major axis would intersect the orbiting body. (See
anomaly, mean
;
anomaly, true
.)
anomaly, mean:
the product of the
mean motion
of an orbiting body and the interval of time since the body passed the
pericenter
. Thus, the mean
anomaly
is the angle from the pericenter of a hypothetical body moving with a constant angular speed that is equal to the mean motion. In realistic computations, with disturbances taken into account, the mean anomaly is equal to its initial value at an
epoch
plus an integral of the mean motion over the time elapsed since the epoch. (See
anomaly, eccentric
;
anomaly, mean at epoch
;
anomaly, true
.)
anomaly, mean at epoch:
anomaly, true:
aphelion:
the point in an
orbit
that is the most distant from the Sun.
apocenter:
the point in an
orbit
that is farthest from the origin of the reference system. (See
aphelion
;
apogee
.)
apogee:
the point in an
orbit
that is the most distant from the Earth. Apogee is sometimes used with reference to the apparent orbit of the Sun around the Earth.
apparent place (or position):
apparent solar time:
appulse:
the least apparent distance between one celestial object and another, as viewed from a third body. For objects moving along the
ecliptic
and viewed from the Earth, the time of appulse is close to that of
conjunction
in
ecliptic longitude
.
Aries, First point of:
aspect:
the position of any of the
planets
or the Moon relative to the Sun, as seen from the Earth.
asteroid:
a loosely defined term generally meaning a small solar system body that is orbiting the Sun, does not show a comet-like appearance, and is not massive enough to be a
dwarf planet
. The term is usually restricted to bodies with
orbits
interior or similar to Jupiter's. "Asteroid" is often used interchangeably with
"minor planet"
, although there is no implicit contraint that a minor
planet
be interior to Jupiter's orbit.
astrometric ephemeris:
astrometric place (or position):
direction of a solar system body formed by applying the correction for
light-time displacement
to the
geometric position
. Such a position is directly comparable with the catalog positions of background stars in the same area of the sky, after the star positions have been updated for
proper motion
and
parallax
. There is no correction for
aberration or
deflection of light since it is assumed that these are almost identical for the solar system body and background stars. An astrometric place is expressed in the reference system of a star catalog; in
The Astronomical Almanac
, the reference system is the
International Celestial Reference System (ICRS)
.
astronomical coordinates:
astronomical refraction:
astronomical unit (au):
a conventional unit of length equal to 149$\,$597$\,$870$\,$700$~$m exactly. Prior to 2012, it was defined as the radius of a circular
orbit
in which a body of negligible mass, and free of
perturbations
, would revolve around the Sun in 2$\pi/$k
days
, k being the
Gaussian gravitational constant
. This is slightly less than the
semimajor axis
of the Earth's orbit.
astronomical zenith:
atomic second:
augmentation:
the amount by which the apparent
semidiameter
of a celestial body, as observed from the surface of the Earth, is greater than the semidiameter that would be observed from the center of the Earth.
autumnal equinox:
azimuth:
the angular distance measured eastward along the
horizon
from a specified reference point (usually north). Azimuth is measured to the point where the great circle determining the
altitude
of an object meets the horizon.
barycenter:
the center of mass of a system of bodies;
e.g.
, the center of mass of the solar system or the Earth-Moon system.
barycentric:
with reference to, or pertaining to, the
barycenter
(usually of the solar system).
Barycentric Celestial Reference System (BCRS):
a system of
barycentric
space-time coordinates for the solar system within the framework of General Relativity. The metric tensor to be used in the system is specified by the
IAU
2000 resolution B1.3. For all practical applications, unless otherwise stated, the BCRS is assumed to be oriented according to the
ICRS
axes. (See
Barycentric Coordinate Time (TCB)
.)
Barycentric Coordinate Time (TCB):
Barycentric Dynamical Time (TDB):
A time scale defined by the
IAU
(originally in 1976; named in 1979; revised in 2006) for use as an independent argument of
barycentric
ephemerides
and equations of motion. TDB is a linear function of
Barycentric Coordinate Time (TCB)
that on average tracks
TT
over long
periods
of time; differences between TDB and TT evaluated at the Earth's surface remain under 2 ms for several thousand
years
around the current
epoch
. TDB is functionally equivalent to T$_{\rm eph}$, the independent argument of the JPL planetary and lunar ephemerides DE405/LE405. (See
second, Systeme International (SI)
.)
Besselian elements:
quantities tabulated for the calculation of accurate predictions of an
eclipse or
occultation for any point on or above the surface of the Earth.
calendar:
a system of reckoning time in units of solar
days
. The days are enumerated according to their position in cyclic patterns usually involving the motions of the Sun and/or the Moon.
calendar, Gregorian:
The
calendar
introduced by Pope Gregory XIII in 1582 to replace the
Julian calendar
. This calendar is now used as the civil calendar in most countries. In the Gregorian calendar, every
year
that is exactly divisible by four is a leap year, except for centurial years, which must be exactly divisible by 400 to be leap years. Thus 2000 was a leap year, but 1900 and 2100 are not leap years.
calendar, Julian:
the
calendar
introduced by Julius Caesar in 46
B.C.
to replace the Roman calendar. In the Julian calendar a common
year
is defined to comprise 365
days
, and every fourth year is a leap year comprising 366 days. The Julian calendar was superseded by the
Gregorian calendar
.
calendar, proleptic:
the extrapolation of a
calendar
prior to its date of introduction.
catalog equinox:
Celestial Ephemeris Origin (CEO):
celestial equator:
Celestial Intermediate Origin (CIO):
Celestial Intermediate Origin Locator (CIO Locator):
Celestial Intermediate Pole (CIP):
Celestial Intermediate Reference System:
celestial pole:
celestial sphere:
an imaginary sphere of arbitrary radius upon which celestial bodies may be considered to be located. As circumstances require, the celestial sphere may be centered at the observer, at the Earth's center, or at any other location.
center of figure:
that point so situated relative to the apparent figure of a body that any line drawn through it divides the figure into two parts having equal apparent areas. If the body is oddly shaped, the center of figure may lie outside the figure itself.
center of light:
conjunction:
the phenomenon in which two bodies have the same apparent
ecliptic longitude
or
right ascension
as viewed from a third body. Conjunctions are usually tabulated as
geocentric
phenomena. For Mercury and Venus, geocentric inferior conjunctions occur when the
planet
is between the Earth and Sun, and superior conjunctions occur when the Sun is between the planet and Earth. (See
longitude, ecliptic
.)
constellation:
1.
A grouping of stars, usually with pictorial or mythical associations, that serves to identify an area of the
celestial sphere
.
2.
One of the precisely defined areas of the celestial sphere, associated with a grouping of stars, that the
International Astronomical Union (IAU)
has designated as a constellation.
Coordinated Universal Time (UTC):
culmination:
the passage of a celestial object across the observer's
meridian
; also called "meridian passage".
culmination, lower:
(also called
"culmination
below pole" for circumpolar stars and the Moon) is the crossing farther from the observer's
zenith
.
culmination, upper:
(also called
"culmination
above pole" for circumpolar stars and the Moon) or
transit
is the crossing closer to the observer's
zenith
.
day:
declination:
defect of illumination:
(sometimes, greatest defect of illumination): the maximum angular width of the unilluminated portion of the apparent disk of a solar system body measured along a radius.
deflection of light:
the angle by which the direction of a light ray is altered from a straight line by the gravitational field of the Sun or other massive object. As seen from the Earth, objects appear to be deflected radially away from the Sun by up to 1$\farcs$75 at the Sun's
limb
. Correction for this effect, which is independent of wavelength, is included in the transformation from
mean place
to
apparent place
.
deflection of the vertical:
delta T:
delta UT1:
direct motion:
for orbital motion in the solar system, motion that is counterclockwise in the
orbit
as seen from the north pole of the
ecliptic
; for an object observed on the
celestial sphere
, motion that is from west to east, resulting from the relative motion of the object and the Earth.
diurnal motion:
the apparent daily motion, caused by the Earth's rotation, of celestial bodies across the sky from east to west.
diurnal parallax:
dwarf planet:
a celestial body that is in
orbit
around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared the neighbourhood around its orbit, and is not a satellite. (See
planet
.)
dynamical equinox:
dynamical time:
Earth Rotation Angle (ERA):
eccentricity:
1.
A parameter that specifies the shape of a conic secton.
2.
One of the standard
elements
used to describe an elliptic or
hyperbolic orbit
. For an
elliptical orbit
, the quantity $e = \sqrt{1 - (b^2/a^2)}$, where $a$ and $b$ are the lengths of the semimajor and semiminor axes, respectively; for a hyperbolic
orbit
, the quantity $e = \sqrt{1 + (b^2/a^2)}$. (See
orbital elements
.)
eclipse:
the obscuration of a celestial body caused by its passage through the shadow cast by another body.
eclipse, annular:
a
solar eclipse
in which the solar disk is not completely covered but is seen as an annulus or ring at maximum
eclipse
. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun. (See
eclipse, solar
.)
eclipse, lunar:
an
eclipse
in which the Moon passes through the shadow cast by the Earth. The eclipse may be total (the Moon passing completely through the Earth's
umbra
), partial (the Moon passing partially through the Earth's umbra at maximum eclipse), or penumbral (the Moon passing only through the Earth's
penumbra
).
eclipse, solar:
actually an
occultation
of the Sun by the Moon in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's
umbra
), partial (observer in the Moon's
penumbra
), annular, or annular-total. (See
eclipse, annular
.)
ecliptic:
1.
The mean plane of the
orbit
of the Earth-Moon
barycenter
around the solar system barycenter.
2.
The apparent path of the Sun around the
celestial sphere
.
ecliptic latitude:
ecliptic longitude:
elements:
a set of parameters used to describe the position and/or motion of an astronomical object.
elements, Besselian:
elements, Keplerian:
elements, mean:
elements, orbital:
elements, osculating:
elements, rotational:
elongation:
the
geocentric
angle between two celestial objects.
elongation, greatest:
elongation, planetary:
elongation, satellite:
epact:
1.
The age of the Moon.
2.
The number of
days
since new moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (See
calendar, Gregorian
;
lunar phases
.)
ephemeris:
a tabulation of the positions of a celestial object in an orderly sequence for a number of dates.
ephemeris hour angle:
ephemeris longitude:
ephemeris meridian:
ephemeris time (ET):
the time scale used prior to 1984 as the independent variable in gravitational theories of the solar system. In 1984, ET was replaced by
dynamical time
.
ephemeris transit:
epoch:
an arbitrary fixed instant of time or date used as a chronological reference datum for
calendars
, celestial reference systems, star catalogs, or orbital motions. (See
calendar
;
orbit
.)
equation of the equinoxes:
equation of the origins:
equation of time:
equator:
the great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (See
celestial equator
.)
equinox:
equinox, autumnal:
equinox, catalog:
equinox, dynamical:
equinox, vernal:
era:
a system of chronological notation reckoned from a specific event.
ERA:
flattening:
a parameter that specifies the degree by which a
planet
's figure differs from that of a sphere; the ratio $f = (a-b)/a$, where $a$ is the equatorial radius and $b$ is the polar radius.
frame bias:
frequency:
frequency standard:
a generator whose output is used as a precise
frequency
reference; a primary frequency standard is one whose frequency corresponds to the adopted definition of the
second
, with its specified accuracy achieved without calibration of the device. (See
second, Systeme International (SI)
.)
GAST:
Gaussian gravitational constant:
(k = 0.017$\ $202$\ $098$\ $95). The constant defining the astronomical system of units of length (
astronomical unit (au)
), mass (solar mass) and time (
day
), by means of Kepler's third law. The dimensions of ${\rm k^{2}}$ are those of Newton's constant of gravitation: ${\rm L^{3}M^{-1}T^{-2}}$.
geocentric:
with reference to, or pertaining to, the center of the Earth.
Geocentric Celestial Reference System (GCRS):
Geocentric Coordinate Time (TCG):
geocentric coordinates:
geocentric zenith:
geodetic coordinates:
geodetic zenith:
geoid:
an equipotential surface that coincides with mean sea level in the open ocean. On land it is the level surface that would be assumed by water in an imaginary network of frictionless channels connected to the ocean.
geometric position:
the position of an object defined by a straight line (vector) between the center of the Earth (or the observer) and the object at a given time, without any corrections for
light-time
,
aberration
, etc.
GHA:
GMST:
greatest defect of illumination:
Greenwich Apparent Sidereal Time (GAST):
Greenwich Hour Angle (GHA):
Greenwich Mean Sidereal Time (GMST):
Greenwich meridian:
Greenwich sidereal date (GSD):
Greenwich sidereal day number:
Gregorian calendar:
height:
the distance above or below a reference surface such as mean sea level on the Earth or a planetographic reference surface on another solar system
planet
.
heliocentric:
with reference to, or pertaining to, the center of the Sun.
heliocentric parallax:
horizon:
1.
A plane perpendicular to the line from an observer through the
zenith
.
2.
The observed border between Earth and the sky.
horizon, astronomical:
the plane perpendicular to the line from an observer to the
astronomical zenith
that passes through the point of observation.
horizon, geocentric:
the plane perpendicular to the line from an observer to the
geocentric zenith
that passes through the center of the Earth.
horizon, natural:
the border between the sky and the Earth as seen from an observation point.
horizontal parallax:
horizontal refraction:
hour angle:
hour circle:
IAU:
illuminated extent:
the illuminated area of an apparent planetary disk, expressed as a solid angle.
inclination:
1.
The angle between two planes or their poles.
2.
Usually, the angle between an orbital plane and a reference plane.
3.
One of the standard
orbital elements
that specifies the orientation of the
orbit
. (See
orbital elements
.)
instantaneous orbit:
intercalate:
to insert an interval of time (e.g., a
day
or a
month
) within a
calendar
, usually so that it is synchronized with some natural phenomenon such as the seasons or
lunar phases
.
intermediate place (or position):
International Astronomical Union (IAU):
an international non-governmental organization that promotes the science of astronomy. The IAU is composed of both national and individual members. In the field of positional astronomy, the IAU, among other activities, recommends standards for data analysis and modeling, usually in the form of resolutions passed at General Assemblies held every three
years
.
International Atomic Time (TAI):
the continuous time scale resulting from analysis by the Bureau International des Poids
et
Mesures of atomic time standards in many countries. The fundamental unit of TAI is the
SI second
on the
geoid
, and the
epoch
is 1958 January 1. (See
second, Systeme International (SI)
.)
International Celestial Reference Frame (ICRF):
1.
A set of extragalactic objects whose adopted positions and uncertainties realize the
International Celestial Reference System (ICRS)
axes and give the uncertainties of those axes.
2.
The name of the radio catalog whose defining sources serve as fiducial points to fix the axes of the ICRS, recommended by the
International Astronomical Union (IAU)
. The first such catalog was adopted for use beginning in 1997. The |second catalog, termed ICRF2, was adopted for use beginning in 2010.
International Celestial Reference System (ICRS):
international meridian:
International Terrestrial Reference Frame (ITRF):
International Terrestrial Reference System (ITRS):
a time-dependent, non-inertial reference system co-moving with the geocenter and rotating with the Earth. The ITRS is the recommended system in which to express positions on the Earth.
invariable plane:
the plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system.
irradiation:
an optical effect of contrast that makes bright objects viewed against a dark background appear to be larger than they really are.
Julian calendar:
Julian date (JD):
Julian date, modified (MJD):
Julian day number:
Julian year:
Keplerian elements:
Laplacian plane:
1.
For
planets
see
invariable plane
.
2.
For a system of satellites, the fixed plane relative to which the vector sum of the disturbing forces has no orthogonal component.
latitude, celestial:
latitude, ecliptic:
angular distance on the
celestial sphere
measured north or south of the
ecliptic
along the great circle passing through the poles of the ecliptic and the celestial object. Also referred to as
celestial latitude
.
latitude, terrestrial:
angular distance on the Earth measured north or south of the
equator
along the
meridian
of a geographic location.
leap second:
librations:
the real or apparent oscillations of a body around a reference point. When referring to the Moon, librations are variations in the orientation of the Moon's surface with respect to an observer on the Earth. Physical librations are due to variations in the orientation of the Moon's rotational axis in inertial space. The much larger optical librations are due to variations in the rate of the Moon's orbital motion, the
obliquity
of the Moon's
equator
to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface.
light, deflection of:
light-time:
the interval of time required for light to travel from a celestial body to the Earth.
light-time displacement:
the difference between the geometric and
astrometric place
of a solar system body. It is caused by the motion of the body during the interval it takes light to travel from the body to Earth.
light-year:
the distance that light traverses in a vacuum during one
year
. Since there are various ways to define a year, there is an ambiguity in the exact distance; the
IAU
recommends using the
Julian year
as the time basis. A light-year is approximately 9.46$\times$10$~^{12}$ km, 5.88$\times$10$~^{12}$ statute miles, 6.32$\times$10$~^{4}$
au
, and 3.07$\times$10$~^{-1}$
parsecs
. Often distances beyond the solar system are given in parsecs. (See
parsec (pc)
.)
limb:
the apparent edge of the Sun, Moon, or a
planet
or any other celestial body with a detectable disk.
limb correction:
generally, a small angle (positive or negative) that is added to the tabulated apparent
semidiameter
of a body to compensate for local topography at a specific point along the
limb
. Specifically for the Moon, the angle taken from the Watts lunar limb data (Watts, C. B., APAE XVII, 1963) that is used to correct the semidiameter of the Watts mean limb. The correction is a function of position along the limb and the apparent
librations
. The Watts mean limb is a circle whose center is offset by about 0$\farcs$6 from the direction of the Moon's center of mass and whose radius is about 0$\farcs$4 greater than the semidiameter of the Moon that is computed based on its
IAU
adopted radius in kilometers.
local place:
local sidereal time:
longitude of the ascending node:
given an
orbit
and a reference plane through the primary body (or center of mass): the angle, $\Omega$, at the primary, between a fiducial direction in the reference plane and the point at which the orbit crosses the reference plane from south to north. Equivalently, $\Omega$ is one of the angles in the reference plane between the fiducial direction and the line of
nodes
. It is one of the six
Keplerian elements
that specify an orbit. For planetary orbits, the primary is the Sun, the reference plane is usually the
ecliptic
, and the fiducial direction is usually toward the
equinox
. (See
node
;
orbital elements
.)
longitude, celestial:
longitude, ecliptic:
longitude, terrestrial:
luminosity class:
distinctions in intrinsic brightness among stars of the same
spectral type
, typically given as a Roman numeral. It denotes if a star is a supergiant (Ia or Ib), giant (II or III), subgiant (IV), or main sequence -- also called dwarf (V). Sometimes subdwarfs (VI) and white dwarfs (VII) are regarded as luminosity classes. (See
spectral types or classes
.)
lunar phases:
cyclically recurring apparent forms of the Moon. New moon, first quarter, full moon and last quarter are defined as the times at which the excess of the apparent
ecliptic longitude
of the Moon over that of the Sun is 0°, 90°, 180° and 270°, respectively. (See
longitude, ecliptic
.)
lunation:
the
period
of time between two consecutive new moons.
magnitude of a lunar eclipse:
the fraction of the lunar diameter obscured by the shadow of the Earth at the greatest
phase
of a
lunar eclipse
, measured along the common diameter. (See
eclipse, lunar
.)
magnitude of a solar eclipse:
magnitude, stellar:
a measure on a logarithmic scale of the brightness of a celestial object. Since brightness varies with wavelength, often a wavelength band is specified. A factor of 100 in brightness is equivalent to a change of 5 in stellar magnitude, and brighter sources have lower magnitudes. For example, the bright star Sirius has a visual-band magnitude of -1.46 whereas the faintest stars detectable with an unaided eye under ideal conditions have visual-band magnitudes of about 6.0.
mean distance:
an average distance between the primary and the secondary gravitating body. The meaning of the mean distance depends upon the chosen method of averaging (i.e., averaging over the time, or over the
true anomaly
, or the
mean anomaly
. It is also important what power of the distance is subject to averaging.) In this volume the mean distance is defined as the inverse of the time-averaged reciprocal distance: $(\int r^{-1}\, \mathrm{d}t)^{-1}$. In the two body setting, when the disturbances are neglected and the
orbit
is elliptic, this formula yields the
semimajor axis
, $a$, which plays the role of mean distance.
mean elements:
average values of the
orbital elements
over some section of the
orbit
or over some interval of time. They are interpreted as the
elements
of some reference (mean) orbit that approximates the actual one and, thus, may serve as the basis for calculating orbit
perturbations
. The values of mean elements depend upon the chosen method of averaging and upon the length of time over which the averaging is made.
mean equator and equinox:
mean motion:
defined for bound
orbits
only.
1.
The rate of change of the
mean anomaly
.
2.
The value $\sqrt{Gm/a^3}$, where $G$ is Newton's gravitational constant, $m$ is the sum of the masses of the primary and secondary bodies, and $a$ is the
semimajor axis
of the relative orbit. For unperturbed elliptic or circular orbits, these definitions are equivalent; the mean motion is related to the
period
through $nT=$2$\pi$ where $n$ is the mean motion and $T$ is the period. For perturbed bound orbits, the two definitions yield, in general, different values of $n$, both of which are time dependent.
mean place:
mean solar time:
meridian:
a great circle passing through the
celestial poles
and through the
zenith
of any location on Earth. For planetary observations a meridian is half the great circle passing through the
planet
's poles and through any location on the planet.
meridian, ephemeris:
meridian, Greenwich:
meridian, international:
meridian, prime:
minor planet:
a loosely defined term generally meaning a small solar system body that is orbiting the Sun, does not show a comet-like appearance, and is not massive enough to be a
dwarf planet
. The term is often used interchangeably with
"asteroid"
, although there is no implicit constraint that a minor
planet
be interior to Jupiter's
orbit
.
month:
a calendrical unit that approximates the
period
of revolution of the Moon. Also, the period of time between the same dates in successive
calendar
months.
month, sidereal:
the
period
of revolution of the Moon about the Earth (or Earth-Moon
barycenter
) in a fixed reference frame. It is the mean period of revolution with respect to the background stars. The mean length of the sidereal
month
is approximately 27.322
days
.
month, synodic:
the
period
between successive new moons (as seen from the geocenter). The mean length of the synodic
month
is approximately 29.531
days
.
moonrise, moonset:
nadir:
node:
nutation:
oscillations in the motion of the rotation pole of a freely rotating body that is undergoing torque from external gravitational forces. Nutation of the Earth's pole is specified in terms of components in
obliquity
and longitude.
obliquity:
in general, the angle between the equatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of the
ecliptic
is the angle between the planes of the
equator
and the ecliptic; its value is approximately 23°.44.
occultation:
the obscuration of one celestial body by another of greater apparent diameter; especially the passage of the Moon in front of a star or
planet
, or the disappearance of a satellite behind the disk of its primary. If the primary source of illumination of a reflecting body is cut off by the occultation, the phenomenon is also called an
eclipse
. The occultation of the Sun by the Moon is a
solar eclipse
. (See
eclipse, solar
.)
opposition:
orbit:
the path in space followed by a celestial body as a function of time. (See
orbital elements
.)
orbit, elliptical:
orbit, hyperbolic:
orbit, instantaneous:
the unperturbed two-body
orbit
that a body would follow if
perturbations
were to cease instantaneously. Each orbit in the solar system (and, more generally, in any perturbed two-body setting) can be represented as a sequence of instantaneous ellipses or hyperbolae whose parameters are called
orbital elements
. If these
elements
are chosen to be osculating, each instantaneous orbit is tangential to the physical orbit. (See
orbital elements
;
osculating elements
.)
orbit, parabolic:
orbital elements:
a set of six independent parameters that specifies an
instantaneous orbit
. Every real
orbit
can be represented as a sequence of instantaneous ellipses or hyperbolae sharing one of their foci. At each instant of time, the position and velocity of the body is characterised by its place on one such instantaneous curve. The evolution of this representation is mathematically described by evolution of the values of orbital
elements
. Different sets of geometric parameters may be chosen to play the role of orbital elements. The set of
Keplerian elements
is one of many such sets. When the Lagrange constraint (the requirement that the instantaneous orbit is tangential to the actual orbit) is imposed upon the orbital elements, they are called
osculating elements
.
osculating elements:
parallax:
the difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points.
parallax, annual:
parallax, diurnal:
parallax, geocentric:
parallax, heliocentric:
the angular difference between the
geocentric and
heliocentric directions toward an object; it is the angle subtended at the observed object. Also called
annual parallax
.
parallax, horizontal:
parallax, solar:
the angular width subtended by the Earth's equatorial radius when the Earth is at a distance of 1
astronomical unit (au)
. The value for the solar
parallax
is 8.794143 arcseconds.
parallax in altitude:
parsec (pc):
the distance at which one
astronomical unit (au)
subtends an angle of one arcsecond; equivalently the distance to an object having an
annual parallax
of one arcsecond. One parsec is 1/sin(1$^{\prime\prime}$) = 206264.806 au, or about 3.26
light-years
.
penumbra:
1.
The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body.
2.
The area of partial shadow surrounding the
umbra
.
pericenter:
pericenter, argument of:
perigee:
the point in an
orbit
that is nearest to the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth.
perihelion:
the point in an
orbit
that is nearest to the Sun.
period:
the interval of time required to complete one revolution in an
orbit
or one cycle of a periodic phenomenon, such as a cycle of
phases
. (See
phase
.)
perturbations:
1.
Deviations between the actual
orbit
of a celestial body and an assumed reference orbit.
2.
The forces that cause deviations between the actual and reference orbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the reference orbit to obtain the precise coordinates.
phase:
1.
The name applied to the apparent degree of illumination of the disk of the Moon or a
planet
as seen from Earth (crescent, gibbous, full, etc.).
2.
The ratio of the illuminated area of the apparent disk of a celestial body to the entire area of the apparent disk; i.e., the fraction illuminated.
3.
Used loosely to refer to one
aspect
of an
eclipse
(partial phase, annular phase, etc.). (See
lunar phases
.)
phase angle:
the angle measured at the center of an illuminated body between the light source and the observer.
photometry:
a measurement of the intensity of light, usually specified for a specific wavelength range.
planet:
a celestial body that is in
orbit
around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit. (See
dwarf planet
.)
planetocentric coordinates:
coordinates for general use, where the $z$-axis is the mean axis of rotation, the $x$-axis is the intersection of the planetary
equator
(normal to the $z$-axis through the center of mass) and an arbitrary
prime meridian
, and the $y$-axis completes a right-hand coordinate system. Longitude of a point is measured positive to the prime
meridian
as defined by
rotational elements
. Latitude of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point.
planetographic coordinates:
coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the
prime meridian
defined by a clearly observable surface feature. Latitude of a point is the angle between the planetary
equator
(normal to the $z$-axis and through the center of mass) and normal to the reference surface at the point. The
height
of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.
polar motion:
polar wobble:
pole, celestial:
either of the two points projected onto the
celestial sphere
by the Earth's axis. Usually, this is the axis of the
Celestial Intermediate Pole (CIP)
, but it may also refer to the instantaneous axis of rotation, or the angular momentum vector. All of these axes are within $0.\!\!^{\prime\prime}1$ of each other. If greater accuracy is desired, the specific axis should be designated.
pole, Tisserand mean:
precession:
the smoothly changing orientation (secular motion) of an orbital plane or the
equator
of a rotating body. Applied to rotational dynamics, precession may be excited by a singular event, such as a collision, a progenitor's disruption, or a tidal interaction at a close approach (free precession); or caused by continuous torques from other solar system bodies, or jetting, in the case of comets (forced precession). For the Earth's rotation, the main sources of forced precession are the torques caused by the attraction of the Sun and Moon on the Earth's equatorial bulge, called precession of the equator (formerly known as lunisolar precession). The slow change in the orientation of the Earth's orbital plane is called precession of the
ecliptic
(formerly known as planetary precession). The combination of both motions — that is, the motion of the equator with respect to the ecliptic — is called general precession.
prime meridian:
proleptic calendar:
proper motion:
the projection onto the
celestial sphere
of the space motion of a star relative to the solar system; thus the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in
right ascension
and
declination
per
year
or century.
proper place:
quadrature:
a configuration in which two celestial bodies have apparent longitudes that differ by 90° as viewed from a third body. Quadratures are usually tabulated with respect to the Sun as viewed from the center of the Earth. (See
longitude, ecliptic
.)
radial velocity:
the rate of change of the distance to an object, usually corrected for the Earth's motion with respect to the solar system
barycenter
.
radius vector:
an imaginary line from the center of one body to another, often from the heliocenter. Sometimes only the length of the vector is given.
refraction:
the change in direction of travel (bending) of a light ray as it passes obliquely from a medium of lesser/greater density to a medium of greater/lesser density.
refraction, astronomical:
the change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of
refraction
the observed
altitude
of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.
refraction, horizontal:
retrograde motion:
for orbital motion in the solar system, motion that is clockwise in the
orbit
as seen from the north pole of the
ecliptic
; for an object observed on the
celestial sphere
, motion that is from east to west, resulting from the relative motion of the object and the Earth. (See
direct motion
.)
right ascension:
rotational elements:
typically, a set of six time-dependent parameters used to describe the instantaneous orientation (attitude) and the instantaneous spin (angular velocity) of a celestial body. When the orientation and spin are described in inertial space, the set of rotational
elements
is often chosen to comprise the two angular coordinates of the direction of the north (or positive) pole and the location of the
prime meridian
at a
standard epoch
, and the time derivatives of each of those three angles. Additional parameters may be required when the object is a non-rigid body.
second, Systeme International (SI):
the duration of 9$\ $192$\ $631$\ $770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.
selenocentric:
with reference to, or pertaining to, the center of the Moon.
semidiameter:
the angle at the observer subtended by the equatorial radius of the Sun, Moon or a
planet
.
semimajor axis:
SI second:
sidereal day:
sidereal hour angle:
sidereal month:
sidereal time:
solar parallax:
solar time:
solar time, apparent:
the measure of time based on the
diurnal motion
of the true Sun. The rate of diurnal motion undergoes seasonal variation caused by the
obliquity
of the
ecliptic
and by the
eccentricity
of the Earth's
orbit
. Additional small variations result from irregularities in the rotation of the Earth on its axis.
solar time, mean:
a measure of time based conceptually on the
diurnal motion
of a fiducial point, called the fictitious mean Sun, with uniform motion along the
celestial equator
.
solstice:
either of the two points on the
ecliptic
at which the apparent longitude of the Sun is 90° or 270°; also the time at which the Sun is at either point. (See
longitude, ecliptic
.)
spectral types or classes:
categorization of stars according to their spectra, primarily due to differing temperatures of the stellar atmosphere. From hottest to coolest, the commonly used Morgan-Keenan spectral types are O, B, A, F, G, K and M. Some other extended spectral types include W, L, T, S, D and C.
standard epoch:
a date and time that specifies the reference system to which celestial coordinates are referred. (See
mean equator and equinox
.)
stationary point:
sunrise, sunset:
the times at which the apparent upper
limb
of the Sun is on the
astronomical horizon
. In
The Astronomical Almanac
they are computed as the times when the true
zenith distance
, referred to the center of the Earth, of the central point of the disk is 90°50$^{\prime}$, based on adopted values of 34$^{\prime}$ for
horizontal refraction
and 16$^{\prime}$ for the Sun's
semidiameter
.
surface brightness:
the visual
magnitude
of an average square arcsecond area of the illuminated portion of the apparent disk of the Moon or a
planet
.
synodic month:
synodic period:
the mean interval of time between successive
conjunctions
of a pair of
planets
, as observed from the Sun; or the mean interval between successive conjunctions of a satellite with the Sun, as observed from the satellite's primary.
synodic time:
syzygy:
1.
A configuration where three or more celestial bodies are positioned approximately in a straight line in space. Often the bodies involved are the Earth, Sun and either the Moon or a
planet
.
2.
The times of the new moon and full moon.
T$_{\bf eph}$:
TAI:
TCB:
TCG:
TDB:
TDT:
terminator:
the boundary between the illuminated and dark areas of a celestial body.
Terrestrial Dynamical Time (TDT):
Terrestrial Ephemeris Origin (TEO):
Terrestrial Intermediate Origin (TIO):
Terrestrial Intermediate Reference System (TIRS):
Terrestrial Time (TT):
topocentric:
with reference to, or pertaining to, a point on the surface of the Earth.
topocentric place (or position):
transit:
1.
The passage of the apparent center of the disk of a celestial object across a
meridian
.
2.
The passage of one celestial body in front of another of greater apparent diameter (e.g., the passage of Mercury or Venus across the Sun or Jupiter's satellites across its disk); however, the passage of the Moon in front of the larger apparent Sun is called an
annular eclipse
. (See
eclipse, annular
;
eclipse, solar
.)
transit, shadow:
The passage of a body's shadow across another body; however, the passage of the Moon's shadow across the Earth is called a
solar eclipse
.
true equator and equinox:
TT:
twilight:
the interval before
sunrise
and after sunset during which the scattering of sunlight by the Earth's atmosphere provides significant illumination. The qualitative descriptions of astronomical, civil and
nautical twilight
will match the computed beginning and ending times for an observer near sea level, with good weather conditions, and a level
horizon
. (See
sunrise, sunset
.)
twilight, astronomical:
the illumination level at which scattered light from the Sun exceeds that from starlight and other natural sources before
sunrise
and after sunset. Astronomical
twilight
is defined to begin or end when the geometric
zenith distance
of the central point of the Sun, referred to the center of the Earth, is 108°.
twilight, civil:
the illumination level sufficient that most ordinary outdoor activities can be done without artificial lighting before
sunrise
or after sunset. Civil
twilight
is defined to begin or end when the geometric
zenith distance
of the central point of the Sun, referred to the center of the Earth, is 96°.
twilight, nautical:
the illumination level at which the
horizon
is still visible even on a moonless night allowing mariners to take reliable star sights for navigational purposes before
sunrise
or after sunset. Nautical
twilight
is defined to begin or end when the geometric
zenith distance
of the central point of the Sun, referred to the center of the Earth, is 102°.
umbra:
the portion of a shadow cone in which none of the light from an extended light source (ignoring
refraction
) can be observed.
Universal Time (UT):
a generic reference to one of several time scales that approximate the mean
diurnal motion
of the Sun; loosely,
mean solar time
on the
Greenwich meridian
(previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or to
Coordinated Universal Time (UTC)
; in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it to
sidereal time
. Thus, UT1 is observationally determined by the apparent diurnal motions of celestial bodies, and is affected by irregularities in the Earth's rate of rotation. UTC is an atomic time scale but is maintained within 0$^{\rm s}\!\!.$9 of UT1 by the introduction of 1-
second
steps when necessary. (See
leap second
.)
UT1:
UTC:
vernal equinox:
vertical:
the apparent direction of gravity at the point of observation (normal to the plane of a free level surface).
week:
wobble, polar:
1.
In current practice including the phraseology used in
The Astronomical Almanac
, it is identical to
polar motion
.
2.
In certain contexts it can refer to specific components of polar motion,
e.g.
Chandler wobble or annual wobble. (See
polar motion
.)
year:
a
period
of time based on the revolution of the Earth around the Sun, or the period of the Sun's apparent motion around the
celestial sphere
. The length of a given year depends on the choice of the reference point used to measure this motion.
year, anomalistic:
year, Besselian:
the
period
of one complete revolution in
right ascension
of the fictitious mean Sun, as defined by Newcomb. Its length is shorter than a
tropical year
by 0.148×T
seconds
, where T is centuries since 1900.0. The beginning of the Besselian
year
occurs when the fictitious mean Sun is at
ecliptic longitude
280°. Now obsolete.
year, calendar:
year, eclipse:
the
period
between successive passages of the Sun (as seen from the geocenter) through the same lunar
node
(one of two points where the Moon's
orbit
intersects the
ecliptic
). It is approximately 346.62
days
.
year, Julian:
year, sidereal:
the
period
of revolution of the Earth around the Sun in a fixed reference frame. It is the mean period of the Earth's revolution with respect to the background stars. The sidereal
year
is approximately 20 minutes longer than the
tropical year
.
year, tropical:
the
period
of time for the
ecliptic longitude
of the Sun to increase 360 degrees. Since the Sun's
ecliptic
longitude is measured with respect to the
equinox
, the tropical
year
comprises a complete cycle of seasons, and its length is approximated in the long term by the civil
(Gregorian) calendar
. The mean tropical year is approximately 365
days
, 5 hours, 48 minutes, 45
seconds
.
zenith:
zenith, astronomical:
the extension to infinity of a plumb line from an observer's location.
zenith, geocentric:
The point projected onto the
celestial sphere
by a line that passes through the geocenter and an observer.
zenith, geodetic:
the point projected onto the
celestial sphere
by the line normal to the Earth’s geodetic ellipsoid at an observer's location.
zenith distance:
angular distance on the
celestial sphere
measured along the great circle from the
zenith
to the celestial object. Zenith distance is 90° minus
altitude
.