July 21, 2017
We want to prepare you for the total solar eclipse that will sweep across the United States on August 21, 2017. So that we’re all speaking the same language, here's a brief glossary, which includes a few illustrations.
altitude — the angular height of a point or celestial object above the horizon measured from 0° (on the horizon) to 90° (at the zenith).
angular diameter — the apparent size of a celestial object, measured in degrees, minutes, and/or seconds, as seen from Earth; for example, the average angular size of the Sun and the Moon, as seen from Earth, is 31 arcminutes, or 0.52°.
angular distance — the distance between two celestial bodies expressed in degrees, minutes, and/or seconds of arc.
aphelion — the position of an object in solar orbit when it lies farthest from the Sun.
(Aphelion, Earth’s greatest distance from the Sun, happens near the Fourth of July. Perihelion, our planet’s closest approach to the daytime star, takes place around January 4. // Holley Y. Bakich)
apogee — the position of the Moon or other object in Earth orbit when it lies farthest from our planet.
azimuth — the angular distance (from 0° to 360°) to an object measured eastward along the horizon from north to a line passing through the object and at a right angle to the horizon.
(This graphic illustrates azimuth. North is 0°, east is 90°, and so on. // Holley Y. Bakich)
Baily’s beads — during a total solar eclipse, the effect seen just before and just after totality when only a few points of sunlight are visible at the edge of the Moon, caused by the irregularity of the lunar surface.
(Baily’s Beads form just before and just after totality when gaps between lunar mountains allow tiny areas of sunlight to pass through. // Mike Reynolds)
chromosphere — the region of the Sun’s atmosphere between its photosphere and its corona; sometimes briefly visible just before or after totality as an intense red glow at the Moon’s edge.
centerline — the midpoint of the width of the Moon’s shadow on Earth; the centerline is the location for the maximum duration of totality.
corona — the shell of thin gas that extends out some distance from the Sun’s surface normally visible only during totality.
diamond ring — the effect just prior to second contact or just after third contact of a total solar eclipse when a small portion of the Sun’s disk plus its corona produce an effect similar to a ring with a brilliant diamond.
disk — the visible surface of any heavenly body.
ecliptic — the circle described by the Sun’s apparent annual path through the stars; the plane of Earth’s orbit around the Sun.
first contact — during a solar eclipse, the moment that the Moon makes contact with the Sun; the beginning of the eclipse.
fourth contact — during a solar eclipse, the moment that the disk of the Moon breaks contact with the Sun; the end of the eclipse.
magnitude — the amount of the Sun's diameter the Moon covers during an eclipse; see obscuration.
New Moon — the phase where the Moon seems completely unlit; solar eclipses can occur only at New Moon.
(The phases of the Moon. // Holley Y. Bakich)
nodes — regarding solar eclipses, the two points at which the Moon’s orbital plane intersects the ecliptic; eclipses can occur only near nodes.
obscuration — the amount of the Sun's area the Moon covers during an eclipse; see magnitude.
penumbra — the less dark outer region of the Moon’s shadow; an observer in the penumbra sees a partial solar eclipse.
perigee — the position of the Moon or other object in Earth orbit when it lies closest to our planet.
perihelion — the position of an object in solar orbit when it lies closest to the Sun.
photosphere — the visible surface of the Sun where it emits light.
prominence — a large-scale, gaseous formation above the surface of the Sun usually occurring over regions of solar activity such as sunspot groups.
(The reddish blobs of light scattered around the Moon’s edge are solar prominences. // Mike Reynolds)
Saros cycle — a time period equal to 6,585.3 days between which similar eclipses occur.
second contact — during a total solar eclipse, the moment the Moon covers 100 percent of the Sun’s disk; the instant totality begins.
shadow bands — faint ripples of light sometimes seen on flat, light–colored surfaces just before and just after totality.
solar flare — a sudden burst of particles (protons, electrons, etc.) and electromagnetic energy from the solar photosphere most often seen silhouetted against space at the edge of the Sun, although they also can be seen against the Sun's disk.
solar telescope — a telescope whose design is dedicated to observing the Sun.
sunspot — a temporarily cooler (and therefore darker) region on the Sun’s photosphere caused by magnetic field variations.
(A sunspot group. // NASA)
syzygy — the lineup of three celestial bodies; for a solar eclipse, the lineup is the Sun, the Moon, and Earth.
third contact — during a total solar eclipse, the instant totality ends.
umbra — the dark inner region of a shadow cast by a solar system object illuminated by the Sun.
(The light outer part of the Moon’s shadow is the penumbra. The dark inner part (shown here as a yellow circle to make it visible) is the umbra. // Holley Y. Bakich)
Universal Time (UT) — also known as Greenwich Mean Time (GMT); standard time kept on the Greenwich meridian (longitude = 0°); astronomers use UT to coordinate observations of celestial events.