- Angstrom: 10-8 cm, or 0.1 nm. It is
abbreviated as Å. The Angstrom is the natural unit for atomic
physics, as the ground-state diameter of the hydrogen atom is about 1 Å.
Visible light has wavelengths between about 4000 and 7000 Å.
Apastron: The point in a binary orbit when the
stars are furthest apart. This is not defined for a circular orbit
(eccentricity = 0). See periastron.
Aphelion: The point in an orbit where the body is at
the maximum distance from the Sun. See perihelion.
Arc minute: 1/60 of a degree. The symbol is
'. Also known as a minute of arc, or arcmin.
Arc second: 1/3600 of a degree. The symbol is
". Also known as a second of arc, or arcsec.
- Astronomical Unit: The mean distance
Earth and Sun, or 1.496x1013 cm.
Barycentric: Refers to the barycenter, or center of
mass, of a system. In the case of the solar system, this is usually, but not
always, inside the Sun.
The barycentric corrections are similar to the heliocentric
corrections, but are used when more accuracy is required.
Conjunction: An alignment of two objects. For binary
stars, conjunction occurs at orbital phases 0 or 0.5.
In an eclipsing binary, eclipses occur at conjunctions.
In the solar system, a conjunction is an alignment of two bodies as seen from
the Earth. Planetary conjunctions involve the Sun (the hour angle is that of
the Sun). In an inferior
conjunction the planet lies between the Sun and the Earth; in a
superior conjunction the planet lies on the far side of the Sun.
The new moon is an inferior conjunction.
Culmination: The highest and lowest elevations
observed in an astronomical object. The upper culmination, with the
the object crossing the merian, the transit, occurs at
hour angle=0. The lower culmination
occurs at hour angle=12h.
Declination: The equivalent of latitude
in the celestial coordinate system. DEC is
measured in units of degrees; the north celestial pole is at +90o and
the south celestial pole is at DEC=-90o.
- Eccentricity: (A-B)/A, where A and B are the
lengths of the semi-major and semi-minor axes,
respectively, of the orbit. The
eccentricity (e) of a circular orbit is 0, because A=B.
An ellipse has e<1. A Parabolic orbit has e=1.
A hyperbolic (unbound, or positive energy) orbit has negative e.
Echelle: An echelle is a coarse grating (few
grooves per mm) which is used in high order.
Because of the high order, the wavelengh dispersion is high, but order overlap
is severe. The order overlap can be removed by dispersing the light a second
time, using a standard low resolution grating dispersing light perpendicular
to the echelle dispersion, called a cross disperser. An
echelle illuminating a cross disperser forms an echelle spectrograph.
This results in a series of nearly parallel spectra, called orders.
Echelle spectrographs permit one to obtain high dispersion spectra over a
significant wavelength range using a small-format 2-dimensional detector.
Ecliptic: The path traced by the Sun around the sky
over the course of the year. The ecliptic plane is the plane of the
Earth's orbit around the sun. It is the natural plane of reference for the
ecliptic coordinate system.
Elevation: Height above the horizon (in degrees).
Elongation: The angular separation between two
components of a binary system. At maximum elongation the components
are most widely separated on the sky. Maximum elongation corresponds
Equinox: The times when the Sun crosses the
celestial equator. The vernal equinox, the first day of spring, occurs
when the Sun is at the
(oh RA, 0o
DEC), on about March 23. The autumnal equinox, the first day of fall,
occurs when the Sun is at 12h RA, 0o
DEC), on about September 23.
- First point of Aries:
oh RA, 0o DEC . The origin of the equatorial, or
celestial coordinate system.
- Flux: The energy received (or emitted), per
unit time per unit area. Typical units are
erg cm-2 s-1.
- Great Circle: A circle formed by
the intersection of the sphere with a plane containing the center of a sphere.
This is the largest circle that can be drawn on a sphere. The shortest distance
between two points on a sphere is always a part of a great circle.
- Heliocentric: Refers to the sun. A heliocentric
correction is applied to transform measurements to correspond to what an
observer at the center of the Sun would see.
Common heliocentric corrections are for the up-to-8 minute light travel time
across one astronomical unit, or for the 30 km/s orbital velocity of the Earth.
The barycentric correction to the center of mass of the
Solar System is more physically meaningful, but is harder to compute and is
unwarranted for many purposes.
Hour Angle: The difference between the
local sidereal time and the
Light Year: The distance light travels in a year, or
about 1018 cm.
Magnitudes: The negative of the logarithm
(base 2.5) of the flux. See here for more
details. The brightest star in the sky has magnitude about -1.4; the
faintest star visible to the naked eye at a dark site has magnitude about 6.
The Sun is magnitude -26.8; the faintest objects seen by the
Hubble Space Telescope are about
Meridian: The great circle
passing through the
celestial poles and the zenith.
Nadir: The point on the celestial sphere
180o from the zenith.
- Nodes: An orbit lies in a plane. The plane
of the sky is normal to the line of sight, and passes through the center of
mass of the system. The intersection
of the plane of the orbit with the plane of the sky is the line of nodes.
The nodes are the two intersections of the line opf nodes with the
orbit. The ascending node is the node at which the star is approaching
the observer; the descending node is the node at which the star is
receding from the observer.
Opposition: An object at opposition is at an hour
angle of 12 hours from the Sun. It is up all night. The full moon is at
Parallax: The apparent shift in the position of
a foreground object relative to background objects due to the motion of the
observer. The parallax is inversely related to the distance. Astronomers
use the 1 AU baseline of the Earth's orbit to measure the
parallax of nearby stars. The parallactic angle is half the angular
shift of the star on the sky over the course of a year. The inverse of
the parallax in arcsec is its distance in
Parsec: The distance at which an object has a
trigonometric parallax of one second of
arc, or 3 x 1018 cm. One parsec (pc) is 3.26
Periastron: The point in a binary orbit when the
stars are closest together. This is not defined for a circular orbit
(eccentricity = 0). See apastron.
Perihelion: The point in an orbit where the body is at
the minimum distance from the Sun. See aphelion.
Proper Motion: The motion of an object perpendicular
to the line of sight (i.e., in the plane of the sky). The units are usually
arcsec/year. This term is not used to refer to periodic
motions of binary stars, or to motions within the solar system.
Quadrature: An object is at quadrature when its
hour angle is 6h from the Sun.
To a good approximation, for an object
in the ecliptic plane, the angle
between the Earth, the Sun, and the object is 90o.
may also be used to refer to phases 0.25 and 0.75 of a binary star system.
Rayleigh: 106 photons/cm2/s/Sr,
or 1.58x10-11/lambda(nm) W cm2/Sr, where lambda is
the wavelength of the line in nanometers.
Right Ascension: The equivalent of longitude
in the celestial coordinate system. RA is
measured in units of time (1 hour = 15 degrees) from the first point of
Aries. RA increases towards the east.
Semi-major axis: Half the length of the long
asix of an ellipse (or of an elliptical orbit).
Similarly, the semi-minor axis is half the length of the short axis
of an ellipse.
Sidereal: Refers to the stars. A sidereal period
is one measured with respect to the stars. It differs from the
There is one more sidereal day than synodic days in a year.
Solstice: The times of the maximum and minimum
solar declination. Summer Solstice
occurs on about June
23, when the declination of the sun is +23.5 degrees, and marks the first
day of summer. Winter Solstice occurs on about December 23,
when the declination of the sun is -23.5 degrees, and marks the first
day of winter. These are, respectively, the longest and shortest days of the
Synodic: Refers to the local rest frame. The synodic
month is the length of time it takes the moon to circle the earth and return
to the same longitude, or about 29 days. The sideral lunar
month is 27.2 days. Similarly, the synodic period of a planet is the time for
a planet to return to the same place in the sky, as seen from Earth.
Note that the synodic period is constant only for circular orbits.
Transit: A crossing of the
objects transit when their
right ascension equals the
local sidereal time,
or their hour angle = 0.
Twilight: The time between sunset or sunrise and
the when the night sky is dark. Civil twilight is when the sun
in less than 12o below the horizon; Astronomical twilight is
when the sun is between 12o and 18o below the horizon.
Umbra: The darkest part of the shadow cast by an
astronomical object. The umbra of a solar eclipse is the region wherein the
eclipse is total. The penumbra surrounds the umbra, and is the region
which sees a partial eclipse.
Zenith: The point in the sky directly overhead.