Some terms relating to solar eclipses

Words in italics have their own entries in the glossary. Some definitions have extended explanations for those who want more details about the entry.

altitude Angular distance from horizon vertically up to celestial object. Examples: halfway up is 45°; overhead (the zenith) is 90°. An average hand width is about 7°.

annular (solar) eclipse An eclipse of the Sun where a ring ("annulus") of sunlight is seen around the Moon because the Moon is too far away for its disk to completely hide the Sun. Partial eclipses always precede and follow the annual phase. About one-third of all solar eclipses are annular.

azimuth Angular distance measured around the horizon usually starting at north as the zero point and measured toward east. Hence, the azimuth of the four cardinal points (north, east, south and west) would be 0°, 90°, 180° and 270° respectively. See bearing.

Baily's beads Observed just before (and after) totality. Final, thin solar crescent appears broken up into string of beads due to unevenness of lunar edge.

bearing Direction or heading. See azimuth.

chromosphere ("color sphere") Inner part of solar atmosphere. Appears as thin, pinkish glow around lunar edge during totality. Can be difficult to see. Glowing hydrogen gas produces reddish color.

corona Outer part of solar atmosphere. The "crowning glory" of a total solar eclipse. Appears as a white halo around lunar disk during totality. May extend out to many times solar radius.
    The corona's form varies from eclipse to eclipse (depends on solar activity). The corona is difficult to see when the Sun in not in total eclipse because its brightness is similar to the daytime sky. The temperature of the corona exceeds one million Kelvins (about two million Fahrenheit). For comparison, the temperature of the Sun's surface or photosphere ("light sphere") is about 6,000 K (or 11,000 F).

diamond ring A phenomenon of solar eclipses observed just before (and after) totality. Appears as brilliant sparkle of sunlight shining through a valley on the Moon's edge. See also Baily's beads.

elongation Angular distance on the sky between the Sun (usually) and another celestial object. Hence, elongation is used to give the apparent separation between the Sun and another celestial body.
    If the object is east of the Sun, the elongation is said to be east (e.g., 10°E); If the object is west of the Sun, the elongation is said to be west (e.g., 10°W).
    The elongations of Mercury and Venus (called inferior planets) never exceed 28° and 47° respectively. (The greatest elongation of Mercury actually varies from about 18° to 28°.) The elongation° of the Moon and remaining planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) can reach 180°.

first contact Instant when Moon's disk first touches the Sun's disk during a solar eclipse (partial eclipse begins). Appears as a "first bite" out of the Sun's disk. The partial phases of a total solar eclipse have a typical duration of about 1½ hours both before and after totality. See also last contact.

fourth contact See last contact.

kelvin (K) The SI (Système International or International System) unit of temperature. One kelvin (K) is the same temperature interval as one degree on the Celsius (Centigrade) scale. The Kelvin scale, however, begins at -273.16° Celsius (C). On the Kelvin scale, water freezes at 273.17 K (0° C) and boils at 373.13 K (100° C). Temperatures recorded on the Kelvin scale are not written degrees kelvin (°K) but simply as kelvin (K) without the degree symbol.
    The kelvin is formally defined as a unit of thermodynamic (i.e., absolute) temperature equal to 1/273.16 of the thermodynamic temperature of the triple point of water-the temperature at which steam, ice and water are at equilibrium. The zero point on the Kelvin scale is known as absolute zero (0 K) and so this scale was formerly known as the absolute temperature scale. On this scale temperatures are directly related to the energy possessed by matter. Thus, kelvins are regarded as a measure of thermodynamic temperatures. As such, the Kelvin scale is preferred when discussing relations dealing with the physical properties of materials.

last contact (also fourth contact) Instant when Moon's disk last touches the Sun's disk during a solar eclipse (partial eclipse ends). Appears as a "last bite" out of the Sun's disk. See also first contact.

limb The extreme edge of the visible disk of a celestial object (e.g., the Moon's limb).

magnitude (solar eclipse) Fraction of the Sun's diameter covered by the Moon at moment of greatest eclipse. For example, the 1998 February 26 eclipse magnitude was 1.04 in the Caribbean Sea where the eclipse was total but only 0.37 (37%) In Gainesville, Florida, where the eclipse was partial. See also obscuration.

magnitude (stellar) The brightness of a celestial object. On a magnitude scale, the lower numbers indicate brighter objects. The naked eye limit is roughly mag. +6½ (dark, clear sky) but in a small city is probably no fainter than about magnitude +3 to +4. There are about twenty stars brighter than about magnitude 1.5. They are usually called "first magnitude" stars.
    Modern magnitudes are standardized so a difference of five magnitudes is a brightness ratio of one hundred to one. Hence, a difference of one magnitude equals a brightness ratio corresponding to the fifth root of one hundred (about 2.512).
    Examples: Sun, -27; Venus, -4.5; Sirius, -1.5 (brightest nighttime star); Polaris +2.0 (North Star).

obscuration (solar eclipse) Fraction of the Sun's area covered by the Moon at moment of greatest eclipse. For example, the 1998 February 26 eclipse obscuration was 1.08 in the Caribbean Sea where the eclipse was total but only 0.26 (26%) in Gainesville, Florida, where the eclipse was partial. See also magnitude (solar eclipse).

occultation The passage of one celestial body in front of another so it is hidden from view. A total eclipse of the Sun is an occultation of the Sun by the Moon.

partial (solar) eclipse An eclipse of the Sun where the Sun does not appear completely hidden because the Moon is not centrally located over the Sun. Partial phases precede all total and annular solar eclipses. About one-third of all solar eclipses are partial. See also total (solar) eclipse and annular eclipse.

penumbra (of a shadow) A partial shadow—the space between regions of complete shadow and complete illumination. During a solar eclipse, observer's in the Earth's penumbra see the Sun partially covered by the Moon (see partial solar eclipse.)

penumbra (of a sunspot) The lighter, cooler outer portion of a sunspot surrounding the umbra. Small spots may only have an umbra but not a penumbra. However, mature sunspots usually have a well developed penumbra that may cover 70% of the sunspot's area.

photosphere ("light sphere") The visible surface of the Sun or star. It is the layer of the Sun, about 500 km (300 miles) thick, that emits the light we see.
    The photosphere's temperature is about 6,000 Kelvins (11,000 Fahrenheit). The color of the photosphere (i.e., the "color of the Sun") is very close to white. (The Sun's color, however, becomes reddened when it is near the horizon due to scattering of the Sun's bluer colors by the Earth's atmosphere.)
    In addition, careful observations of the Sun's disk show the Sun's edge (limb) appears slightly dimmer and redder than the center of the Sun's disk. Some eclipse photographers increase exposure times slightly when taking pictures of the slender solar crescent during the partial phases of a solar eclipse to compensate for this effect.

prominences Appear as small, bright, reddish, cloud and flame-like structures in the solar chromosphere and inner corona during totality. (Actual size can be many Earth diameters!) Prominences seem to peak out from edge of the Moon's edge as small, reddish flames or "tongues" after the Moon completely covers Sun.
    The position of prominences around Moon's disk, their number and structure vary from eclipse to eclipse. During totality, the visibility of prominences changes as the Moon's disk passes over the Sun's disk. Binoculars can help make them more evident and reveal their structures.

saros The cycle of about 18 years over which a sequence of similar solar (or lunar) eclipses repeats. Any two eclipses separated by one saros cycle have similar characteristics.
    This period of time (close to 6585.32 days or about 18 years, 11 days and 8 hours) has been known since ancient Babylonian times. After this period, the Earth, Sun and Moon return to nearly the same relative positions so that two successive eclipses in the same saros cycle occur at the same time of year, have similar durations, tracks and occur at similar latitudes on Earth. However, the second eclipse is shifted westward by about 120° due to the 1/3 day in the saros interval. A given saros cycle does not last indefinitely because lunar and solar cycles get out of step.
    The 2001 June 21 solar eclipse belongs to Saros 127 (member 57 of 82), which began in 0991 A.D. and ends in 2452 A.D. The last eclipse of Saros 127 occurred 1983 June 11, the next after the 2001 eclipse is 2019 July 2.

second contact Instant when Moon's disk first completely covers the Sun's disk during a total eclipse. (Thus, the total phase of the eclipse begins at second contact.) See also third contact.
The duration of totality can range from zero to about 7½ minutes but most total eclipses typically last only a few minutes. The length of totality also depends where the observer is along the Moon's shadow path and how far from the center line.
Maximum duration for the 2001 June 21 eclipse is 4 minutes and 6 seconds for a position west of Angola in the Atlantic Ocean. In Zimbabwe, where Continental Capers Travel Center plans to observe the eclipse, totality should last about 3 minutes and 24 seconds.

shadow bands A phenomenon sometimes seen briefly several minutes before and after totality as rapidly shimmering, irregular bands of shadow on the ground and walls. (A white surface helps make them more visible since shadow bands have low contrast.)
    Bands may be a few centimeters (1/2 to 2 inches) apart, up to a meter (a few feet) apart and travel a few meters per second (about 10 feet per second).     The cause of shadow bands is probably the refraction or distortion of light from the thin solar crescent by the Earth's atmosphere. Even veteran eclipse observers do not always see them. They are also difficult to photograph. (Use fast film and a fast lens with short exposure times—1/250 second or less.) Most intense bands seem to occur over dry, warm areas rather than from coastal and shipboard areas. (Water surfaces may help to thermally stabilize air layers that would otherwise cause the bands.)

sunspots Localized regions of the Sun's surface (photosphere) where the temperature is approximately 1,500 Kelvins (about 3,000 Fahrenheit degrees) cooler than adjacent areas of the Sun's surface.
    Sunspots appear as dark, irregular small spots on the solar disk, sometimes occurring in pairs. (Many are actually much larger than the Earth!) Sunspots contain a central umbra sometimes surrounded by a penumbra. The numbers of sunspots wax and wane with the approximate eleven year solar activity cycle. Magnetic regions within the Sun probably cause the "cooling" effect.

third contact Instant when Moon's disk last completely covers the Sun's disk during a total eclipse (i.e., the total phase of the eclipse ends). See also second contact.

total (solar) eclipse An eclipse of the Sun where the Moon's disk is large enough to completely hide the Sun. Partial eclipses always precede and follow the period of totality when the Sun is totally obscured by the Moon. Nearly one-third of all solar eclipses are total. See also partial eclipse and annular eclipse.

umbra (of a shadow) The completely dark portion of the shadow cast by an object such as the Earth or Moon. During a solar eclipse, observer's in the Earth's umbra see the Sun completely (totally) covered by the Moon (see total solar eclipse.)

umbra (of a sunspot) The darkest, coolest region of a sunspot. The umbra is often not uniformly dark but may contain lighter umbral dots—penumbral grains that have invaded the umbra.

zenith The overhead point (altitude 90°).