annular eclipse
A type of solar eclipse in which a ring (annulus) of solar photosphere remains visible.
apparent magnitude
The brightness of a star as seen by an observer, given in a specific system in which a difference of five magnitudes corresponds to a brightness ratio of one hundred times; the scale is fixed by correspondence with a historical background.
autumnal equinox
Of the two locations in the sky where the ecliptic crosses the celestial equator, the one that the Sun passes each year when moving from northern to southern declinations.
Baily's beads
Beads of light visible around the rim of the Moon at the beginning and end of a total solar eclipse. They result from the solar photosphere shining through valleys at the edge of the Moon.
celestial equator
The intersection of the celestial sphere with the plane that passes through the Earth's equator.
celestial poles
The intersection of the celestial sphere with the axis of rotation of the Earth.
celestial sphere
The hypothetical sphere centered at the center of the Earth to which it appears that the stars are affixed.
corona
The outermost region of the Sun (or of other stars), characterized by temperatures of millions of kelvins.
declination
Celestial latitude, measured in degrees north or south of the celestial equator.
diamond-ring effect
The last Baily's bead glowing brightly at the beginning of the total phase of a solar eclipse, or its counterpart at the end of totality.
ecliptic
The path followed by the Sun across the celestial sphere in the course of a year.
Gregorian calendar
The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that are divisible by 100 but not by 400.
international date line
A crooked imaginary line on the Earth's surface, roughly corresponding to 180� longitude, at which, when crossed from east to west, the date jumps forward by one day.
Julian calendar
The calendar with 365-day years and leap years every fourth year without exception; the predecessor to the Gregorian calendar.
lunar eclipse
The passage of the Moon into the Earth's shadow.
magnitude scale
The scale of apparent magnitudes and absolute magnitudes used by astronomers, in which each factor of 100 in brightness corresponds to a difference of 5 magnitudes.
midnight sun
The Sun seen around the clock from locations sufficiently far north or south during the suitable season.
penumbra
(a) For an eclipse, the part of the shadow from which the Sun is only partially occulted; (b) of a sunspot, the outer region, not as dark as the umbra.
phase
(a) Of a planet, the varying shape of the lighted part of a planet or moon as seen from some vantage point; (b) the relation of the variations of a set of waves.
pole star
A star approximately at a celestial pole; Polaris is now the pole star; there is no south pole star.
precession
The very slowly changing position of stars in the sky resulting from variations in the orientation of the Earth's axis; a complete cycle takes 26,000 years.
right ascension
Celestial longitude, measured eastward along the celestial equator in hours of time from the vernal equinox.
solar year (tropical year)
An object's complete circuit of the Sun; a tropical year in between vernal equinoxes.
solstice
The point on the celestial sphere of northernmost or southernmost declination of the Sun in the course of a year; colloquially, the time when the Sun reaches that point.
twinkle
A scintillation - rapid changing in brightness - and slight changing in position of stars as their light passes through the Earth's atmosphere.
umbra (umbrae)
(a) Of a sunspot, the dark central region; (b) of an eclipse shadow, the part from which the Sun cannot be seen at all.
vernal equinox
The equinox crossed by the Sun as it moves to northern declinations.
zenith
The point in the sky directly overhead an observer.