EclipseWise - Glossary of Solar Eclipse Terms

by Fred Espenak

annular eclipse - A solar eclipse in which the Moon's antumbral shadow traverses Earth (the Moon is too far from Earth to completely cover the Sun). During the maximum phase of an annular eclipse, the Sun appears as a blindingly bright ring surrounding the Moon.

annularity - The maximum phase of an annular eclipse during which the Moon's entire disk is seen silhouetted against the Sun. Annularity is the period between second and third contact during an annular eclipse. It can last from a fraction of a second to a maximum of 12 minutes 29 seconds.

anomalistic month - The time required for the Moon to make one revolution around its orbit with respect to the perigee; the length of the mean anomalistic month as calculated for the year 2000 is 27.55455 days (27d 13h 18m 33s); the actual duration can vary by several days due to the gravitational perturbations of the Sun on the Moon's eccentric orbit.

antumbra - The antumbra is that part of the Moon's shadow that extends beyond the umbra. It is similar to the penumbra in that the Sun is only partially blocked by the Moon. From within the antumbra, the Sun appears larger than the Moon which is seen in complete silhouette. An annular eclipse is seen when an observer passes through the antumbra.

Besselian elements - The Besselian elements are a series of time dependent variables used to calculate various aspects of a solar eclipse. They describe the movement of the Moon's shadow with respect to the fundamental plane. This plane passes through the center of Earth and is oriented perpendicular to the Moon's shadow axis. Next, the shadow cone is projected onto Earth's surface including the effects of Earth's rotation, the flattening of Earth and the latitude, longitude and elevation of the observer. The local circumstances at the observer's position can then be calculated including the eclipse contact times, eclipse magnitude and the duration of totality (or annularity).

center of figure - The center of figure of a celestial body (e.g., planet, moon) is the apparent center of the object with respect to its surface and takes into account irregularities in its shape. If the distribution of mass is not uniform, then the center of mass does not coincide with the center of figure. In the case of the Moon, the offset between the center of mass and center of figure is ~0.5 kilometers.

center of mass - In orbital mechanics, the equations of motion of celestial bodies (stars, planets, moons, etc.) are formulated as point masses located at the centers of mass. In other words, the motion of a celestial body can be predicted assuming the object's entire mass is concentrated into one single point called the center of mass.

central eclipse - A solar eclipse in which the central axis of the Moon’s shadow cone traverses Earth thereby producing a central line in the eclipse track. The umbra or antumbra falls entirely upon Earth so the ground track has both a northern and southern limit. Central solar eclipses can be either total, annular or hybrid.

central eclipse (one limit) - A solar eclipse in which the central axis of the Moon’s shadow cone traverses Earth. However, a portion of the umbra or antumbra misses Earth throughout the eclipse and the resulting ground track has just one limit. Central solar eclipses with one limit can be either total, annular or hybrid.

central line - The central line is the locus of points of intersection of the axis of the Moon's shadow with the surface of Earth. It is sometimes called the "center line" or "centerline".

eclipse magnitude - Eclipse magnitude is the fraction of the Sun’s diameter occulted by the Moon. It is strictly a ratio of diameters and should not be confused with eclipse obscuration, which is a measure of the Sun’s surface area occulted by the Moon. Eclipse magnitude may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50). By convention, its value is given at the instant of greatest eclipse.

eclipse obscuration - Eclipse obscuration is the fraction of the Sun’s area occulted by the Moon. It should not be confused with eclipse magnitude, which is the fraction of the Sun’s diameter occulted by the Moon. Eclipse obscuration may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50).

eclipse season - An eclipse season is a period during which the Sun appears close enough to one of the Moon’s nodes to permit an eclipse to occur. Each season lasts approximately 34 days and repeats at about 173-day intervals. Thus there are always two full eclipse seasons each year. The 173-day interval is the time it takes the Sun to travel from one lunar node to the next. Because the Moon’s synodic period is shorter than an eclipse season, there is a least one solar and one lunar eclipse during every eclipse season. (see Wikipedia)

eye safety - The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Moon completely covers the disk of the Sun. It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques. Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause permanent retinal damage, especially when viewed through binoculars or other optical aids.

first contact - The instant when the partial phase of an eclipse begins.

fourth contact - The instant when the partial phase of an eclipse ends.

gamma - Gamma is the distance of the Moon’s shadow axis from Earth’s center in units of equatorial Earth radii. It is defined at the instant of greatest eclipse when its absolute value is at a minimum.

geocentric - As seen from the center of Earth; the geocentric coordinates of a planet is the position of the planet as seen from Earth's center.

greatest duration - Greatest duration (for total eclipses) is defined as the instant when the duration of totality reaches a maximum along the path of a total eclipse. The calculation of greatest duration requires an accurate lunar limb profile to account for the effects of mountains and valleys around the circumference of the Moon on the duration of totality. The length of totality calculated at greatest duration may differ by 1-2 seconds compared with greatest eclipse, and the geographic location may differ by a hundred kilometers or more.

greatest eclipse - Greatest eclipse is defined as the instant when the axis of the Moon's shadow cone passes closest to Earth's center. The computation of the duration of totality at this point is typically done using a smooth edge for the Moon that ignores the effects of mountains and valleys along the lunar limb. For total eclipses, the instant of greatest eclipse offers a good approximation (1-2 seconds) to the maximum duration of totality along the entire eclipse path. Far more rigorous calculations using the Moon's limb profile are required to predict the instant of greatest duration (of totality) and a more accurate value for the maximum length of totality. For annular eclipses, the instant of greatest duration may occur either near the time of greatest eclipse or near the sunrise and sunset points of the eclipse path.

hybrid eclipse - A solar eclipse in which the Moon's umbral and antumbral shadows traverse Earth (the eclipse appears annular and total along different sections of its path). Hybrid eclipses are also known as annular-total eclipses. In most cases, hybrid eclipses begin as annular, transform into total, and then revert back to annular before the end of their track. In rare instances, a hybrid eclipse may begin annular and end total, or vice versa.

node - The Moon's orbit is tipped about 5.1° to Earth's orbit around the Sun. The two points where the Moon's orbit crosses Earth's orbit are called the nodes. The ascending node is where the Moon crosses to the north of Earth's orbit. The descending node is where it crosses to the south.

non-central eclipse (one limit) - A solar eclipse in which the central axis of the Moon’s shadow cone misses Earth. However, one edge of the umbra or antumbra grazes Earth thereby producing a ground track with one limit and no central line. Non-central solar eclipses can be either total, annular or hybrid. (Partial eclipses can also be considered non-central eclipses in which only the penumbral shadow traverses Earth's surface)

partial eclipse - A solar eclipse in which the Moon's penumbral shadow traverses Earth (umbral and antumbral shadows completely miss Earth). During a partial eclipse, the Moon appears to block part (but not all) of the Sun's disk.
From the prospective of an individual observer, a partial eclipse is one in which the observer is within the penumbral shadow but outside the path of the umbral or antumbral shadows.

penumbra - The penumbra is the weak or pale part of the Moon's shadow. From within the penumbra, the Sun is only partially blocked by the Moon as in the case of a partial eclipse. This contrasts with the umbra, where the Sun is completely blocked resulting in a total eclipse.

saros - The periodicity and recurrence of solar (and lunar) eclipses is governed by the Saros cycle, a period of approximately 6,585.3d (18yr 11d 8h). When two eclipses are separated by a period of one Saros, they share a very similar geometry. The eclipses occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, the Saros is a useful tool for organizing eclipses into families or series. Each series typically lasts 12 or 13 centuries and contains 70 or more eclipses.
For more information, see Eclipses and the Saros. The Saros Catalog of Solar Eclipses provides complete details for all current Saros cycles.

second contact - The instant when the total or annular phase of an eclipse begins.

shadow bands - Shadow bands are caused by the thin slit-like solar crescent illuminating Earth's atmosphere moments before and after a total solar eclipse. They appear as thin parallel undulating lines of alternating light and dark that race across the ground. Their movement is caused by atmospheric winds.

third contact - The instant when the total or annular phase of an eclipse ends.

total solar eclipse - A solar eclipse in which the Moon's umbral shadow traverses Earth (Moon is close enough to Earth to completely cover the Sun). During the maximum phase of a total eclipse, the Sun's disk is completely blocked Moon. The Sun's faint corona is then safely revealed to the naked eye.

totality - The maximum phase of a total eclipse during which the Moon's disk completely covers the Sun. Totality is the period between second and third contact during a total eclipse. It can last from a fraction of a second to a maximum of 7 minutes 32 seconds.

umbra - The umbra is the darkest part of the Moon's shadow. From within the umbra, the Sun is completely blocked by the Moon as in the case of a total eclipse. This contrasts with the penumbra, where the Sun is only partially blocked resulting in a partial eclipse.