**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 between the Earth and Sun, or 1.496x10^{13}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=12^{h}. -
**Declination**: The equivalent of latitude in the celestial coordinate system. DEC is measured in units of degrees; the north celestial pole is at +90^{o}and the south celestial pole is at DEC=-90^{o}. **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 to quadrature. -
**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 (o^{h}RA, 0^{o}DEC), on about March 23. The**autumnal equinox**, the first day of fall, occurs when the Sun is at 12^{h}RA, 0^{o}DEC), on about September 23. **First point of Aries**: o^{h}RA, 0^{o}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 right ascension. -
**Jansky**: 10^{-26}W/m^{2}/Hz. -
**Light Year**: The distance light travels in a year, or about 10^{18}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 30^{th}magnitude. -
**Meridian**: The great circle passing through the celestial poles and the zenith. -
**Nadir**: The point on the celestial sphere 180^{o}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 opposition. -
**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 parsecs. -
**Parsec**: The distance at which an object has a trigonometric parallax of one second of arc, or 3 x 10^{18}cm. One parsec (**pc**) is 3.26 light years. -
**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 6^{h}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 90^{o}. Quadrature may also be used to refer to phases 0.25 and 0.75 of a binary star system. -
**Rayleigh**: 10^{6}photons/cm^{2}/s/Sr, or 1.58x10^{-11}/lambda(nm) W cm^{2}/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 synodic period. 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 year. -
**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 meridian. Astronomical 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 12^{o}below the horizon;**Astronomical twilight**is when the sun is between 12^{o}and 18^{o}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.