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- Aberration of
starlight
The apparent displacement
of a star from its true position in the sky, due
to the fact that light has a definite velocity
(299,792-5 km per second). The Earth is moving
round the Sun, and thus the starlight seems to
reach it "at an angle" The apparent
positions of stars may be affected by up to 20.5
second of arc.
- Absolute magnitude
The apparent magnitude that
a star would have if it were observed from a
standard distance of 10 parsecs, or 32.6 light-years.The absolute magnitude of the
sun is 4.8.
- Air glow
The faint natural
luminosity of the night sky, due to reactions going on it the Earth's upper
atmosphere.
- Airy disk
The apparent size of a star's disk produced even by a
perfect optical system. Since the star can never
be focused perfectly, 84 per cent of the light
will concentrate into a single disk, and 16 per
cent into a system of surrounding rings.
- Albedo
The reflecting power of a planet or other non-luminous body. A
perfect reflector would have an albedo of 100 per
cent.-
- Altitude
The angular distance from
the observer's horizon, usually taken to be that
horizon that is unobstructed by natural or
artificial features (such as mountains or
buildings), measured directly up from the horizon
toward the zenith; positive numbers indicate
values of altitude above the horizon, and
negative numbers indicate below the horizon ---
with negative numbers usually being used in terms
of how far below the horizon the sun is situated at a given time [for
example, the boundary between civil twilight and
nautical twilight is when the sun is at altitude
-6 degrees]. -
- Aperture
The size of the primary optical surface of an
astronomical instrument (telescope),
usually given in inches, centimeters, or meters.
In the case of a reflecting
telescope, the aperture usually refers to the
size of the main mirror; in the case of a refracting telescope (of
which binoculars are one example), the aperture
refers to the size of the primary lens (which in
binoculars is usually given in millimeters). -
- Aphelion
For an object orbiting the sun, the point
(distance and time) where/when the object is
furthest from the sun in its elliptical orbit.
- Arc,
degree of
One three hundred and
sixtieth part of a full circle (360°)
- Arc,
minute of
One sixtieth part of a
degree of arc. One minute of arc (1') is in turn
divided into 60 seconds of arc(60")-
- Arc,
second of
One sixtieth part of a
minute of arc. One second of arc: 1".
- Ashen light
The faint luminosity of the
night side of the planet venus, seen
when venus is in the crescent stage. It is
probably a genuine phenomenon rather than a mere
contrast effect, but its cause is not certainly
known.-
- Astronometry
The careful, precise
measurement of astronomical objects, usually made
with respect to standard catalogues of star positions. For comet orbit
computations, astrometry good to 1" or
2" (1 or 2 arc seconds), or better, is the standard
nowadays. -
- Astronomical
unit
Approximately equal to the
mean earth-sun distance, which is about
150,000,000 km or 93,000,000 miles. Formally, the
AU is actually slightly less than the earth's
mean distance from the sun (semi-major axis)
because it is the radius of a circular orbit of
negligible mass (and unperturbed by other planets) that revolves about the sun in
a specific period of time. -
- Atmosphere
The gaseous mantle
surrounding a planet or other body.
It can have no definite boundary, but merely
thins out until the density is no greater than
that of surrounding space.-
- Azimuth
Angular distance measured
clockwise around the observer's horizon in units
of degrees; astronomers usually take north
to be 0 degrees, east to be 90 degrees, south to
be 180 degrees, and west to be 270 degrees.
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- Baily's Beard
Brilliant points seen along
the edge of the moon's disk at a total solar eclipse, just before totality and again
just after totality has ended.-
- Barycentre
The center of mass of a
system of bodies, such as the solar
system. When a comet, for example, is well outside
the orbit of neptune (the farthest major planet),
it sees the sun and major planets essentially as a single object
of summed mass, and the center of this mass
(called the barycenter of the solar system) is
offset somewhat from the sun;
"original" and "future"
orbits of long-period comets are computed for
this barycenter, while perturbed, osculating orbits of
currently-observed objects in the inner solar
system are computed for heliocentric orbits.
The center of gravity of the earth-moon system.
Because the earth is 81 times more massive than
the moon, the barycentre lies within the
terrestrial globe.-
- Barycentric Dynamical
Time (TDB)
Differing from TDT only via
periodic variations, TDB is used in ephemerides
and equations of motion that refer to the barycenter of the solar
system.-
- Besselian
year
A quantity introduced by F.
W. Bessel in the nineteenth century that has been
used into the twentieth century. Bessel
introduced a system whereby it would be
convenient to identify any instant of time by
giving the year and the decimal fraction of the
year to a few places, but the starting times of
the year was not convenient for dynamical studies
that utilize Julian dates, differing by 0.5 day, and the
Besselian year varies slowly. The recent change
to Julian year usage in dynamical astronomy (and
the J2000.0 equinox) took effect in solar-system ephemerides of the Minor Planet
Center and Central Bureau for Astronomical
Telegrams on Jan. 1, 1992. (See Julian year.) -
- Black hole
A region of space
surrounding a very massive collapsed star, or
"collapsar" from which not even light
can escape.-
- Bode's Law
An empirical relationship
between the distances of the planets from the
Sun, discovered by J.D.Titius in 1972 and made
famous by J.E. Bode. The law seems to be
fortuitous, and without any real scientific
basis.
- Bolide
A brilliant meteor, which may explode during its
descent through the Earth's atmosphere.
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- Cassegrain
reflector
A type of reflecting
telescope (Reflector) in which the
light from the object under study is reflected
from the main mirror to a convex secondary, and
thence back to the eyepiece through a hole in the
main mirror.-
- CCD
Charge-coupled device, a
very sensitive electronic device that is
revolutionizing astronomy in the 1990s. CCD
cameras are composed of silicon chips that are
sensitive to light, changing detected photons of
light into electronic signals that can then be
used to make images of astronomical objects or to
analyze how much light is being received from
such objects. CCDs require computers for
reduction of data, so the expense can be much
greater than for, say, photography --- but CCDs
can detect much fainter objects than can
photographs. Unfiltered CCDs tend to be more
red-sensitive than the human eye. .-
- Celestial
Sphere
An imaginary sphere
surrounding the Earth, concentric with the
Earth's center.-
- Circular velocity
The velocity with which an
object must move, in the absence of air
resistance, in order to describe a circular orbit around its primary.-
- Colures
Great circles on the
celestial sphere. The equinoctial colure, for
instance, is the great circle which passes
through both celestial poles.-
- Coma
A comet's atmosphere surrounding its
nucleus. The coma is rather tenuous (except very
close to the nucleus), and stars can be occasionally easily seen
through it, shining from behind.-
- Comet
A celestial body orbiting the sun
(though some may be ejected from the solar system by
planetary perturbations) that displays (at least
during a portion of its orbit)
some diffuseness and/or a "tail" of
debris that points generally in the anti-solar
direction. A more detailed explanation is
available in the Press Information Sheet on comet
C/1995 O1 (Hale-Bopp).
- Cybernetics
The study of methods of
communications and controls which are common to
machines and to living organisms.
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- Declination
One element of the astronomical coordinate system
on the sky that is used by astronomers.
Declination, which can be thought of as latitude
on the earth projected onto the sky, is usually
denoted by the lower-case Greek letter delta and
is measured north (+) and south (-) of the
celestial equator in degrees, minutes, and
seconds of arc. The celestial equator is defined
as being at declination zero (0) degrees; the
north and south celestial poles are defined as
being at +90 and -90 degrees, respectively. When
specifying a comet's location on the sky, one must
state the right ascension and declination (with
equinox), along with date and time (since a comet
moves with respect to the background stars).
The angular distance of a celestial body north or
south of the celestial equator. It may be said to correspond to
latitude on the surface of the Earth.
- Degree
A unit used in the
measurement of angles, heavily used particularly
in astronomy. Due to ancient Babylonian
mathematics, we still divide a circle into 360
even units of arc and call each of these units one
degree. The entire sky, therefore, spans 360
degrees. Up to about 180 degrees of sky is
visible from any given point on earth with an
unobstructed horizon (as measured from, say, east
to west, or north to south). The degree is used
to make measurements of distance, or position (as
with declination) in astronomy. In turn, a degree
is composed of 60 minutes of arc, and also of 360
seconds of arc.
- Dichotomy
The exact half-phase of Mercury, Venus or the Moon.
- Diffraction rings
Concentric rings
surrounding the image of a star as seen in a
telescope.
- Diurnal motion
The apparent daily rotation
of the sky from east to west. It is due to the
real rotation of the Earth from west to east.
- Doppler effect
The apparent change in the
wavelength of light caused by the motion of the
observer.
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These are of two kinds:
solar and lunar. (sun or moon)
- Ecliptic
The projection of the Earth's orbit on to the celestial
sphere.
It may also be defined as the apparent yearly
path of the Sun against the stars".-
- Ecosphere
The region around the Sun in which the
temperatures are neither too hot nor too cold for
life to exist under suitable conditions. Venus
lies near the inner edge of the Ecosphere; Mars
is near the outer edge.-
- Electron
A fundamental particle
carrying a unit negative charge of electricity.-
- Elongation
Angular distance of a
celestial object from the sun in the sky. In standard ephemerides, this is usually denoted by the
Greek letter epsilon (or by the abbreviation
"Elong."). A celestial (usually solar-system) object's "phase
angle" is the elongation of the earth from
the sun, as would be seen by an observer on that
third celestial object.-
- Ephemeris
Ephemeris (plural:
ephemerides). Pronounced ee-FEM-er-is. A table
listing specific data of a moving object, as a
function of time. Ephemerides usually contain
right ascension and declination, apparent angle
of elongation from the sun (in degrees), and magnitude
(brightness) of the object; other quantities
frequently included in ephemerides include the
objects distances from the sun and earth (in AU), phase angle, and moon phase.-
- Ephemeris Time (ET)
Determined in principle
from the sun's apparent annual motion, ET is the
numerical measure of uniform time, which is the
independent variable in the gravitational theory
of the earth's orbital motion, coming from Simon
Newcomb's Tables of the Sun. In practice, ET was obtained by
comparing observing positions of the Moon with
gravitational ephemerides calculated from
theories. In 1992, standard (apparent geocentric)
ephemerides of comets and minor planets changed from
using Ephemeris Time to Terrestrial Dynamical
Time (TDT, or TT).-
- Equator, celestial
The projection of the
Earth's equator on to the celestial
sphere. It
divides the sky into two equal hemispheres.-
- Equatorial mount
A Telescope mounting in which the instrument
is set upon an axis which is parallel to the axis
of the Earth; the angle of the axis must be equal
to the observer's latitude.-
- Equinox
Either of the two points
(vernal, autumnal) on the celestial sphere where
the ecliptic (which is the apparent path of the sun on the sky) intersects the
celestial equator. Due to precession, this point
moves over time, so positions of stars in catalogues and on atlases are
usually referred to a "mean equator and
equinox" of a specified standard epoch. For
the purposes of the positions of objects dealt
with in these ICQ/CBAT/MPC Web pages, the
positions are almost always given for
"equinox J2000.0", meaning that the
reference system is that at the beginning of the
year 2000; prior to 1992, most astronomers were
using "equinox B1950.0". Many older
star atlases and catalogues still in use refer to
equinox 1950.0, so observers must be careful when
plotting positions (and when reporting positions)
to note the proper equinox. (The "B"
and "J" preceding the equinox years
indicate "Besselian" and
"Julian", respectively. See separate
definitions for Besselian year and Julian
year.) The
differences in an object's position when given in
equinoxes 1950.0 and 2000.0 amounts to several
arc minutes. -
- Extinction,
atmospheric
The diminishing of light
from astronomical objects due to the earth's
atmosphere, in which molecules (air, dust, etc.)
of the atmosphere absorb, reflect, and refract
light before it reaches the ground. Extinction
becomes a severe problem for astronomers when
objects are viewed close to (especially within 20
degrees of) the local horizon. There are various
methods that have been developed for astronomers
to try and compensate for this extinction, but it
is always best to make measurements of
astronomical objects when they are as high in the
sky as possible (to minimize errors).-
- Exosphere
The outermost part of the
Earths atmosphere. It is very rarefied, and has no
definite upper boundary, since it simply
"thins out" into space.
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- Faculae
Bright , temporary patches
on the surface of the sun, usually (though not
always) associated with sunspots.-
- Flare stars
Faint red dwarf stars which may brighten up by several
magnitudes over a period of a few minutes, fading
back to their usual brightness within an hour or
so. Typical flare stars are UV Ceti and AD
Leonis.-
- Flocculi
Patches on the Sun,s
surface, observed by instruments based on the
principle of the spectroscope. Bright flocculi are composed of
calcium; dark flocculi are made up of hydrogen.-
- Fraunhofer lines
The dark absorption lines
in the Sun's spectrum, named in honor of the
German optician J. von Fraunhofer, who first
studied and mapped them in 1814.-
- Focal
length
The distance between a lens
(or mirror) and the point at which the image of
an object at infinity is brought to focus. The
focal length divided by the aperture of the mirror or lens is termed
the focal ratio.-
- Fringe region
The upper part of the exosphere
. Atomic
particles in the fringe region have little chance
of collision with each other, and to all intents
and purposes they travel in free orbits, subject to the Earth's gravitation.
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- Gravitation
The force of attraction
which exists between all particles of matter in
the universe. Particles attract each other
with a force which is directly proportional to
the product of their masses and inversely
proportional to the square of the distance
between them.
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Heliocentric
Referring to the sun. A heliocentric orbit is one
based on the sun as one of the two foci of the
(elliptical) orbit (or as the center of a
circular orbit); a heliocentric magnitude is the
brightness of an object as would be seen from a
heliocentric distance of 1 AU (which means a distance of 1 AU
from the sun).
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Julian date (JD)
The interval of time in
days (and fraction of a day) since Greenwich noon
on Jan. 1, 4713 BC. The JD is always half a day
off from Universal Time, because the current definition
of JD was introduced when the astronomical day
was defined to start at noon (prior to 1925)
instead of midnight. Thus, 1995 Oct. 10.0 UT = JD
2450000.5.-
- Julian
year
- Exactly 365.25 days, in
which a century (100 years) is exactly 36525 days
and in which 1900.0 corresponds exactly to 1900
January 0.5 (from the Julian-date system, which is half a day
different from civil time or UT). The standard
epoch J2000.0, now used for new star-position
catalogues and in solar-system-orbital calculations, means 2000 Jan.
1.5 Barycentric Dynamical Time (TDB) = Julian
Date 2451545.0
TDB. When this dynamical, artificial "Julian
year" is employed, a letter "J"
prefixes the year.
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km
- kilometer = 0.6 miles.
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- Light Pollution
- The emission of stray light
or glare from lighting fixtures in manners that
counter the purpose of the light (which is to
light what is below); also known as the waste of
money and energy in the form of electric light,
usually meant in the form of outdoor night
lighting. Such light trespass causes severe
safety problems for motorists, pedestrians, and
cyclists at night from lighting that shines onto
streets and highways and sidewalks from
poorly-designed or poorly-mounted lighting. Such
glare also imposes on privacy, by shining
brightly into bedroom windows at night and into
backyards where adults and children are trying to
observe the night sky. While most people have
accepted such bad, glare lighting without
question and assumed that nothing could be done
about it, dedicated groups of volunteers around
the world are now showing that effective laws and
guidelines can be instated at the local and
regional levels of government (and in planning
and engineering offices), which mean that proper
outdoor night lighting can be a norm so that
everybody benefits --- auto drivers, sleeping
residents, government budgets, and skygazers
alike. Laws mandating full-cutoff light fixtures
are already in place in states such as Maine and
Connecticut and are pending elsewhere. For more
information on the Web, see http://cfa-www.harvard.edu/cfa/ps/nelpag.html.
-
- Light
Year
- The distance traveled by
light in one year. It is equal to
9,460,000,000,000 km.
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- m(sub)1
- Total, integrated magnitude
of a comet's head (meaning coma + nuclear
condensation). This can be estimated visually, as
the comet's "total visual magnitude".
The variable m(sub)1 is usually found in
ephemerides predicting a comet's future motion,
position on the sky, and brightness. See also
definition for "Magnitude", below.
[Note that m(sub)1 is also used by stellar
spectrophotometrists to define a "metal
index" on the Stroemgren ubvy photometric
system.]
-
- m(sub)2
- The magnitude value
measured (or predicted) for a comet's nuclear
condensation. Note that the true comet nucleus is
rarely, if ever, directly observed from the earth
because of the large amount of gas and dust that
is ever-present in the inner coma close to the
nucleus, serving to hide the true nucleus'
surface. So-called "nuclear magnitudes"
are therefore fraught with problems as to true
meaning, especially because such nuclear
magnitudes are extremely dependent upon
instrumentation (aperture, focal-ratio,
magnification) and wavelength. Nuclear magnitudes
are chiefly used for astrometric purposes, in
which predictions are made for the brightness of
the comet's nuclear condensation so that
astrometrists can gauge how faint the
condensation is likely to be and thus how long an
exposure is needed to get a good, measurable
image. (Astrometrists are only concerned about
measuring the nuclear condensation, which is
considered to be the site of the main mass of any
comet.) See also definition for
"Magnitude", below.
-
- Magnitude
- The units used to describe
brightness of astronomical objects. The smaller
the numerical value, the brighter the object. The
human eye can detect stars to 6th or 7th magnitude on a
dark, clear night far from city lights; in
suburbs or cities, stars may only be visible to
mag 2 or 3 or 4, due to light pollution. The
brightest star, Sirius, shines at visual
magnitude -1.5. Jupiter can get about as bright
as visual magnitude -3 and Venus as bright as -4.
The full moon is near magnitude -13, and the sun
near mag -26. Comet C/1996 B2 (Hyakutake) reached
magnitude about 0 in late March 1996. The
magnitude scale is logarithmic, with a difference
of one magnitude corresponding to a change of
about 2.5 times in brightness; a change of 5
magnitudes is defined as a change of exactly 100
times in brightness. In the case of comets, we
speak of a magnitude that is
"integrated" over an observed coma
diameter of several arc minutes; this is called
the comet's "total (visual) magnitude",
and is usually denoted by the variable m(sub)1.
Thus, a 7th-magnitude comet is much harder to see
than a 7th-magnitude star -- the latter having
all its light in a pinpoint, and the former
having the same amount of light spread out over a
large area (imagine defocussing a 7th-magnitude
star to the size of a diffuse comet). Typically,
however, when comets become very bright, their
apparent coma sizes shrink so that the majority
of visible light is in a small, intense core of
the comet's head (and the comet may appear
starlike with a tail emanating from the
comet's head).
In ICQ/CBAT/MPC publications, ephemerides for
solar-system objects usually give
predicted/projected magnitudes of comets and
minor planets in the last column, denoted m(sub)1 and m(sub)2 for cometary "total"
and "nuclear" magnitudes, or V for
minor-planet V-band ("visual")
magnitudes.
-
- Meteor
- Small rocky and/or icy
particles that are swept up by the earth in its orbit about the sun. Also called "shooting
stars", they travel across the sky in a very
short time, from less than a second to several
seconds, and they do so because they are only a
matter of tens of miles above the surface of the
earth. Meteor showers are generally thought to be
produced by the debris left by comets as the latter orbit the sun.
(Comets, on the other hand, are not in our
atmosphere but are much further away than is our
own Moon; therefore, comets do not
"streak" across the sky as do meteors
-- a common misconception among the general
public.)
A small particle which enters the earth's atmosphere (at a velocity of anything up to
72km, per second) and becomes heated by friction,
so that it destroys itself in the streak of
luminosity known as a shooting-star.
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Nuclear
magnitude
See definition for m(sub)2.
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- Orbit
The path of an artificial
or natural celestial body.-
- Orbiral elements
Parameters (numbers) that
determine an object's location and motion in its orbit about another object. In the
case of solar-system objects such as comets and planets, one must ultimately account for
perturbing gravitational effects of numerous
other planets in the solar system (not merely the
sun), and when such account is made, one has what
are called "osculating elements" (which
are always changing with time and which therefore
must have a stated epoch of validity). Six
elements are usually used to determine uniquely
the orbit of an object in orbit about the sun,
with a seventh element (the epoch, or time, for
which the elements are valid) added when
planetary perturbations are allowed for; initial
("preliminary") orbit determinations
shortly after the discovery of a new comet or
minor planet (when very few observations are
available) are usually "two-body
determinations", meaning that only the
object and the sun are taken into account ---
with, of course, the earth in terms of observing
perspective) work with only the following six
orbital elements: time of perihelion passage (T)
[sometimes taken instead as an angular measure
called "mean anomaly", M]; perihelion
distance (q), usually given in AU; eccentricity
(e) of the orbit; and three angles (for which the
mean equinox must be specified) --- the argument
of perihelion (lower-case Greek letter omega),
the longitude of the ascending node (upper-case
Greek letter Omega), and the inclination (i) of
the orbit with respect to the ecliptic.
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- Parallax
The apparent displacement
or the difference in apparent direction of an
object as seen from two different points not on a
straight line with the object (as from two
different observing sites on earth).-
- Parsec
The distance at which a
star would show parallax of one second of arc. It is equal to 3.26 light-years, 206,265 astronomical
units, or
30,8000,000,000,000 km. (Apart from the Sun, no
star lies within one parsec of us.)-
- Perigee
The point where (and when)
an object's orbit about the earth in which it is
closest to the earth; only applicable to objects
orbiting the earth (not to objects orbiting the sun --- a common error).-
- Perihelion
The point where (and when)
an object orbiting the sun is closest to the sun.-
- Perturbations
Gravitational influences
("tugging" and "pulling") of
one astronomical body on another. Comets are strongly perturbed by the
gravitational forces of the major planets,
particularly by the largest planet, Jupiter.
These perturbations must be allowed for in orbit
computations, and they lead to what are known as
"osculating elements" (which means that
the orbital element numbers change from day to
day and month to month due to continued
perturbations by the major planets, so that an
epoch is necessarily stated to denote the
particular date that the elements are valid.-
- Phase
angle
For a solar system object
besides the earth and sun, the angle between the
earth and the sun (or the earth's elongation from
the sun) as seen from that third object. The
phase angle is given in ephemerides on IAU
Circulars and Minor Planet Circulars is denoted
by either of the lower-case Greek letters beta or
phi.-
- Phases
The apparent changes in
shape of the moon and some planets depending upon
the amount of the sunlit hemisphere turned toward
us.-
- Photometry
In astronomy, the
measurement of the light emitting from
astronomical objects, generally in the visible or
infrared bands, in which a specific or general
wavelength band is normally specified. An
excellent reference on this topic is Astronomical
Photometry: A Guide, by C. Sterken and J.
Manfroid (1992, Dordrecht: Kluwer Academic
Publishers). -
- Photosphere
The bright surface of the
Sun.-
- Planet
A non-luminous body moving
round a star. There are nine known planets in
our Solar
System, some of
which are attended by satellites. Experience the planets of the
solar system.-
- Precession
A slow but relatively
uniform motion of the earth's rotational axis
that causes changes in the coordinate systems
used for mapping the sky. The earth's axis of
rotation does not always point in the same
direction, due to gravitational tugs by the sun
and moon (known as lunisolar precession) and by
the major planets (known as planetary precession).
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- r.
The alphabetic letter
("variable") used to denote the
distance between the sun and the object being
discussed, also called the object's heliocentric distance; in most ephemerides of objects such as comets and minor planets, r is given in
AU. Similarly, the upper-case Greek
letter Delta gives the distance between the
object and the earth (its geocentric distance). -
- Reflector
A telescope that uses as its primary optical
element a mirror. Nearly all large telescopes in
use today by amateur and professional astronomers
are reflecting telescopes.-
- Refractor
A telescope that uses as
its primary optical element a lens. Binoculars
are a type of refractor. In general, refractors
are much more expensive to build and buy than are
reflectors.-
- Right ascension
One element of the
astronomical coordinate system on the sky, which
can be though of as longitude on the earth
projected onto the sky. Right ascension is
usually denoted by the lower-case Greek letter
alpha and is measured eastward in hours, minutes,
and seconds of time from the vernal
equinox. There
are 24 hours of right ascension, though the
24-hour line is always taken as 0 hours. More
rarely, one sometimes sees right ascension in
degrees, in which case there are 360 degrees of
right ascension to make a complete circuit of the
sky. When specifying a comet's location on the sky, one must
state the right ascension and declination (with
equinox), along with date and time (since a comet
moves with respect to the background stars).
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- Satellite
A secondary body moving
round a planet.-
- Solar
System
The system made up of the sun, the planets, satellites, comets, asteroids,
meteoroids, and interplanetary dust and gas. Experience
the solar system.-
- Spectroscope
An instrument used to
analyze the light from a star or other luminous
object.-
- Star
A self-luminous gaseous
body. The sun is a typical star.-
- Sublimation
The change of a solid (such
as ice) directly into a gaseous state (bypassing
the liquid state). This happens in the vacuum of
space with comets, as the heating effects of solar
radiation cause ices in comets to "steam
off" as gasses into space. The ice molecules
present in the nucleus actually break up (or
dissociate) into atoms or smaller molecules after
leaving the nucleus in gas form.-
- Sunspots
Darker patches on the solar
photosphere; their temperature is around
4000șC(as against about 6000șC for the general
photosphere), so that they are dark only by
contrast; if they could be seen shining on their
own, their surface brilliance would be greater
than that of an arc-light.
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- Telescope
The main instrument used to
collect the light from celestial bodies, thereby
producing an image which can be magnified. (see reflector telescope)-
- Terrestrial Dynamical
Time (TDT or TT)
Time scale used in orbital
computations; TDT is tied to atomic clocks
(International Atomic Time, TAI), whereas
Universal Time is tied to observations. Prior to
1992, Ephemeris Time (ET) was used in publications of the
ICQ/CBAT/MPC; since then, TT has been used. The
difference between TDT and UTC in 1994 was 60
seconds (i.e., UT + 60 seconds = TDT).-
- Total (visual) magnitude
Total, integrated magnitude
of a comet's head (meaning coma + nuclear condensation). This
can be estimated visually, as the comet's
"total visual magnitude". The variable m(sub)1, usually found in comet ephemerides, is used to denote the total
(often predicted) magnitude.
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- Universal Time (UT, or
UTC)
A measure
of time used by astronomers; UT conforms (within
a close approximation) to the mean daily
(apparent) motion of the sun. UT is determined from
observations of the diurnal (daily) motions of
the stars for an observer on the earth. UT
is usually used for astronomical observations,
while Terrestrial Dynamical Time (TDT, or simply
TT) is used in orbital and ephemeris computations that
involve geocentric computations. Coordinated
Universal Time (UTC) is that used for broadcast
time signals (available via shortwave radio, for
example), and it is within a second of UT.
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- Universe
It is best to admit that we
have no knowledge whatsoever about the origin of
the universe. We do not know how matter came into
existence; neither does it help to claim that the
universe has always existed, so that there was no
actual moment of creation.
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Vernal equinox
The point on the celestial sphere
where the sun crosses the
celestial equator moving northward, which
corresponds to the beginning of spring in the
northern hemisphere and the beginning of autumn
in the southern hemisphere (in the third week of
March). This point corresponds to zero (0) hours
of right ascension.
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Zenith
The point directly overhead in
the sky.
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