Glossary of Solar Eclipse Terms

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.

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 - During a central solar eclipse, the central axis of the Moon’s shadow cone traverses Earth's surface. The track produced by the shadow axis is called the central line of the eclipse. The duration of a total or annular eclipse is longest on the central line (neglecting Earth's curvature and effects introduced by the direction of the shadow with respect to the Equator) and drops to 0 as the observer moves to the path limits.

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).

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,

greatest duration - Greatest Duration (GD) is defined as the instant when the length of the total (or annular) phase reaches a maximum along the central path of a solar eclipse. The computation of the eclipse duration is typically done using a smooth edge for the Moon that ignores the effects of mountains and valleys along the lunar limb. Although the location of Greatest Duration may be relatively close to the point of Greatest Eclipse, it differs because of a combination of factors including the relative motion of the Moon's shadow with respect to the curvature of Earth's surface and to Earth's Equator. The length of the total (or annular) eclipse calculated at Greatest Duration typically differs by ~1-2 seconds compared with Greatest Eclipse, and the geographic location may differ by ~10-100 kilometers or more. Under certain conditions, the instant of Greatest Duration for some annular eclipses can occur at either end of the eclipse path at sunrise or sunset. There is no explicit or analytical solution for the determination of Greatest Duration. It may be calculated though an iterative series of approximations. When the highest accuracy is needed, a Corrected Greatest Duration (GDC) must be calculated that includes the effects of the Moon's limb profile, which may differ by a couple seconds from the uncorrected value.

greatest eclipse - For solar eclipses, Greatest Eclipse (GE) 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 the total (or annular) phase 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 (typically ~1-2 seconds) to the Greatest Duration of totality along the entire eclipse path. The instant of Greatest Eclipse is easily calculated for total, annular and partial eclipses, and is the standard time used for comparing different eclipses with each other.
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.
For lunar eclipses, Greatest Eclipse is defined as the instant when the Moon passes closest to the axis of Earth's shadow.

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.

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: Saros 0 - 180 provides complete details for all current Saros cycles.

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

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

total 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.

Compiled by Fred Espenak

Originally published by NASA https://eclipse.gsfc.nasa.gov/SEhelp/SEglossary.html