Glossary of Eclipse Terminology

ECLIPSE GLOSSARY

altitude -- the angle (in degrees) above the level horizon where an object in the sky appears. (The object's azimuth is also needed to pinpoint its position.)

annular eclipse -- a solar eclipse that occurs when the apparent size of the Moon is not great enough to completely cover the Sun. A thin ring of sunlight can be seen around the black disk of the Moon.

annular-total eclipse -- a solar eclipse that has both annular and total phases. (Also called a central eclipse.)

anomalistic month -- the time it takes for the Moon to travel from apogee to perigee and back again (about 27.6 days).

aphelion -- the point in the Earth's orbit that is farthest from the Sun. Currently the Earth reaches aphelion in early July.

apogee -- the point in the Moon's orbit that is farthest from the Earth.

ascending node -- the point in the orbit of the Moon where it passes from below the ecliptic plane to above (see node).

Aubrey holes -- the 56 chalk-filled holes (named for John Aubrey) that mark the outer ring of Stonehenge. These holes may have served as "counters" to help in marking the cycles needed to predict eclipses.

azimuth -- the compass direction (in degrees) where an object in the sky appears. (The object's altitude is also needed to pinpoint its position.)

Baily's beads -- the effect seen just before and just after totality when only a few points of sunlight are visible at the edge of the lunar disk.

canon -- in ancient times, an historical record of events. In modern astronomy, a canon is a listing of celestial events, such as eclipses, over a period of time.

central eclipse -- in some references, a central eclipse refers to an eclipse that has both annular and total phases. (See annular-total eclipse.)

chromosphere -- the lower atmosphere of the Sun that appears as a thin rosy ring around the edge of the solar disk during a total eclipse.

corona -- the upper atmosphere of the Sun that appears as a halo around the Sun during a total eclipse.

contact -- one of the instances when the apparent position of the edges of the Sun and the Moon cross one another during an eclipse. They are designated as first contact, second contact, third contact, and fourth contact.

descending node -- the point in the orbit of the Moon where it passes from above the ecliptic plane to below (see node).

draconic month -- the time it takes for the Moon to return to a node (about 27.2 days).

eclipse -- the alignment of celestial bodies so that one is obscured, either partially or totally, by the other.

eclipse season -- the period of time when the Sun is near alignment with a lunar node, during which eclipses may take place. For solar eclipses, this time window of 37-1/2 days occurs every 173 days.

eclipse year -- the length of time it takes for a lunar node to return to its original alignment with respect to the Sun (about 346.6 days).

ecliptic -- the plane of the Earth's orbit around the Sun. As seen from the Earth, the Sun appears to move across the ecliptic during one year.

equinox -- either of the two days when the periods of daylight and darkness are of equal length. The vernal equinox is usually March 21; the autumnal equinox is usually September 23.

first contact -- the beginning of a solar eclipse marked by the edge of the Moon first passing across the disk of the Sun.

fourth contact -- the end of a solar eclipse marked by the disk of the Moon completely passing away from the disk of the Sun.

G.M.T. -- Greenwich Mean Time. The time at Greenwich, England, which is used as the basis for standard time throughout the world.

heel stone -- the large upright boulder (or menhir) at Stonehenge that is aligned with the summer solstice sunrise.

latitude -- distance on the Earth (measured in degrees) north or south of the equator.

longitude -- distance on the Earth (measured in degrees) east or west from a reference line, usually the line running between the poles passing through Greenwich, England.

lunar eclipse -- the passage of the Moon into the shadow of the Earth, always occurring at a full Moon.

negative shadow -- the extension of the umbra of an annular eclipse that delineates the path from which observers may see the ring of Sun of the annular eclipse.

node -- the two points where a tilted orbit intersects a geometrical plane. The Moon's orbit intersects the ecliptic plane at the ascending node and the descending node.

partial eclipse -- an eclipse during which only the partial shadow touches the Earth (for a solar eclipse) or the Moon (for a lunar eclipse).

path of totality -- the path (up to 200 miles wide) that the Moon's shadow traces on the Earth during a total solar eclipse.

penumbra -- the part of a shadow (as of the Moon) within which the source of light (the Sun) is only partially blocked out.

perigee -- the point in the orbit of the Moon that is closest to the Earth.

perihelion -- the point in the orbit of the Earth that is closest to the Sun. Currently the Earth reaches perihelion in early January.

prominence -- a large-scale gaseous formation above the surface of the Sun.

regression -- the movement of points in an orbit in the direction opposite from the motion of the orbiting body. For example, the Moon travels from west to east, but its nodes are regressing from east to west.

saros -- the eclipse cycle with a period of 223 synodic months, or 6,585.32 days (18 years and about 11 days).

second contact -- the beginning of the total phase of a solar eclipse marked by the leading edge of the Moon first completely obscuring the Sun.

shadow bands -- faint ripples of light sometimes seen on flat, light-colored surfaces just before and just after totality.

solar eclipse -- the passage of the new Moon directly between the Sun and the Earth when the Moon's shadow is cast upon the Earth. The Sun appears in the sky either partially or totally covered by the Moon.

solstice -- the day when the noontime Sun is either highest in the sky (summer solstice is June 22) or lowest in the sky (winter solstice on December 22).

spectroscope -- a scientific instrument that breaks light into its component wavelengths for measurement.

sunspot -- a magnetic disturbance on the Sun that appears as a dark blotch on its surface.

synodic month -- the time from one full Moon to the next (about 29.5 days).

third contact -- the end of the total phase of a solar eclipse marked by the trailing edge of the Moon first revealing the Sun.

total eclipse -- an eclipse during which the umbra touches the Earth (for a solar eclipse) or completely engulfs the Moon (for a lunar eclipse).

totality -- the period during a solar eclipse when the Sun is completely blocked by the Moon. (Totality for a lunar eclipse is the period when the Moon is in the complete shadow of the Earth.)

umbra -- a complete shadow (as of the Moon) within which the source of light (the Sun) is totally hidden from view.

zodiac -- the division of the ecliptic into twelve equal parts; each of these parts or "signs" is identified by a name and symbol.

** Material adapted from ECLIPSE by Bryan Brewer ($14.95 plus S&H). ECLIPSE Book Order Form. **