ANNULAR ECLIPSE (Annular Solar Eclipse)

A solar eclipse that occurs when the apparent size of the moon is not large enough to completely cover the sun. A thin ring of very bright sunlight remains around the black disk of the moon. MORE »

ANOMALISTIC MONTH

The time it takes for the moon to orbit Earth from apogee to perigee and back to apogee.

ANTUMBRA

That part of the moon’s shadow that extends beyond the umbra. An annular eclipse is seen by an observer in the antumbra.

APHELION

The point in an object’s orbit when it is farthest from the sun. Currently, Earth reaches aphelion in July.

APOGEE

The point in an object’s orbit when it is farthest from Earth.

ASTRONOMICAL UNIT

The average distance between Earth and the sun (149,597,870 km or 92,955,807 miles)

BAILY’S BEADS

The effect seen just before and just after totality during a total solar eclipse when only a few points of sunlight are visible through valleys around the edge of the moon.

The moon’s limb is not perfectly smooth because of the mountain ranges and canyons that pepper the moon’s circumference as viewed from Earth. Shortly before the moon fully blocks the disk of the sun during a total solar eclipse, flashes of light can often be seen around the circumference of the moon’s blackened disk. These are caused by sunlight passing through the canyons around the limb of the moon.

The namesake for these ‘diamond ring’ flashes is Francis Baily; a prominent English astronomer and four-time president of the Royal Astronomical Society. His vivid description of the phenomenon (following an eclipse on May 15, 1836) caused it to be associated with his name in 1836, but he was not the first historically-named person to discover this phenomenon. More than a century earlier, the famous English astronomer Sir Edmond Halley (discoverer of Halley’s Comet) described this spectacular phenomenon and also gave a correct explanation for it during an eclipse in 1715: “About two minutes before the Total Immersion, the remaining part of the Sun was reduced to a very fine Horn, whose Extremities seemed to lose their Acuteness, and to become round like Stars … which Appearance could proceed from no other Cause but the Inequalities of the Moon’s Surface, there being some elevated parts thereof near the Moon’s Southern Pole, by whose Interposition part of that exceedingly fine Filament of Light was intercepted.”

Thanks to the results from the NASA Lunar Reconnaissance Orbiter, which measured details across the entire moon to 2-meter accuracy, we can now predict exactly when and where these brilliant flashes of light will appear as a total solar eclipse takes place because now we know where and how deep the lunar limb canyons will be. Still, despite our abilities to predict it, this lovely effect and its diamond ring-like character will continue to mesmerize observers for all times to come!

CENTRAL ECLIPSE

A solar eclipse in which the central axis of the moon’s shadow traverses Earth. Central solar eclipses can be total, annular, or hybrid.

This is also a total lunar eclipse during which the Moon passes through the center of Earth’s shadow, contacting the antisolar point. This type of lunar eclipse is relatively rare. MORE »

CHROMOSPHERE

The lower atmosphere of the sun just above the photosphere that appears as a thin crimson ring around the edge of the sun during a total solar eclipse.

CONTACT

One of the instances when the apparent position of the edges of the sun and the moon (for eclipses) and the sun and a planet (for transits) cross one another. They are designated as first, second, third, and fourth contact.

CORONA

The upper atmosphere of the sun. It appears as a halo around the sun during a total solar eclipse. MORE »

DIAMOND RING

The effect seen in the few seconds just before and after totality of a total solar eclipse when there is a single point of sunlight brilliantly shining through a valley on the limb of the moon.

ECLIPSE

The alignment of celestial bodies so that one is obscured, either partially or totally, by the other. MORE »

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 31-37 days occurs every 173.3 days.

ECLIPSE YEAR

The length of time it takes for the apparent motion of the sun to take it from one node of the moon to the other and back to the original node (about 346.6 days).

ECLIPTIC

The plane of Earth’s orbit around the sun. As seen from Earth, the sun appears to move along the ecliptic during the course of a year.

GIBBOUS

Phase of the moon when it appears more than half illuminated.

HYBRID ECLIPSE (Hybrid Solar Eclipse)

A solar eclipse which appears annular or total along different sections of its path. MORE »

LUNAR ECLIPSE

The passage of the moon into the shadow of Earth, which can only occur at a full moon. MORE »

LUNAR MONTH

See synodic month.

MAGNITUDE (of a solar eclipse)

The fraction of the apparent diameter of the sun covered by the moon. By convention, it is usually quoted at the maximum phase.

MARE

A large flat area on the moon formed by volcanic material.

NODE

The two points where a tilted orbit intersects a geometrical plane. For example, where the moon’s orbit intersects the ecliptic, that is the plane that contains Earth and the sun.

OBSCURATION (of a solar eclipse)

The fraction of the sun’s area covered by the moon.

PARTIAL LUNAR ECLIPSE

A lunar eclipse where a portion—but not all—of the moon enters Earth’s umbra. MORE »

PARTIAL SOLAR ECLIPSE

A solar eclipse seen from within the moon’s penumbra. The moon appears to block part—but not all —of the sun’s photosphere. MORE »

PATH OF TOTALITY

The path (up to about 270 km or 168 miles wide) that the moon’s shadow traces on Earth during a total solar eclipse.

PENUMBRA

The part of a shadow—as of the moon or Earth—within which the source of light, such as the sun, is only partially blocked. Also, it refers to the lighter outer area of a sunspot.

PENUMBRAL ECLIPSE (Penumbral Lunar Eclipse)

An eclipse of the moon when the moon enters the penumbra of Earth’s shadow. MORE »

PERIGEE

The point in an object’s orbit when it is closest to Earth.

PERIHELION

The point in an object’s orbit when it is closest to the sun. Currently, Earth reaches perihelion in early January.

PHOTOSPHERE

The bright, visible surface of the sun. MORE »

PROMINENCE

A large-scale gaseous formation above the surface of the sun shaped by the sun’s magnetic field. MORE »

RAYLEIGH SCATTERING

The scattering of light by particles smaller than the wavelength of the light, resulting in the separation of colors. This causes the sky to be blue and sunsets to be reddish.

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, which has a period of 223 synodic months or 6,585.32 days—the equivalent of 18 years and about 11.3 days.

Because 223 synodic months is not identical to 239 anomalistic months or 242 draconic months, The 18-year saros periods do not endlessly repeat. Each series begins with the Moon’s shadow crossing Earth near the north or south pole, and subsequent events progress toward the other pole until the Moon’s shadow misses Earth and the series ends. A full series from start to finish lasts about 1,300 years. The August 21, 2017 eclipse is part of Saros Series 145 that includes 77 eclipses of which the August eclipse is number 22. All eclipses in this series occur at the ascending node of the lunar orbit. The series began with the partial solar eclipse of January 4, 1639 visible at the North Pole. The series will end with the partial solar eclipse of April 17, 3009 visible from the South Pole. The length of this series is 1,370 years. By the way, the total solar eclipse of August 21 is preceded two weeks earlier by a partial lunar eclipse on August 7, 2017, which occurs during the same eclipse season when the sun is nearest this node.

SHADOW BANDS

Faint ripples of light sometimes seen on flat, light-colored surfaces just before and just after totality of a total solar eclipse.

SIDEREAL MONTH

The time it takes for the moon to make one orbit of Earth with reference to the fixed stars—a total of 27.32 days.

SOLAR ECLIPSE

The passage of the new moon directly between the sun and Earth when the moon’s shadow is cast upon Earth. The sun appears in the sky either partially or totally covered by the moon. MORE »

SOLAR FLARE

An explosive eruption in the sun’s atmosphere.

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, which takes 29.53 days. Also called a lunar month.

SYZYGY

Apart from being a wonderful word to use in the game of Scrabble, this astronomical term is an event in which one astronomical object is lined-up with another. This leads to the pithy aphorism: all eclipses are syzygies but not all syzygies are eclipses. For example, the Full moon and New Moon are syzygies involving the lining up of the Sun, Earth, and Moon, therefore, lunar and solar eclipses are syzygies. When a planetary moon passes across the face of another body but does not eclipse it, this is called a transit. From Earth, the small disks of Venus and Mercury can be seen passing across the face of the sun during transits of Venus and Mercury. These also involve the straight-line alignment of the Sun, Earth, and each planet. On June 3, 2014, the Curiosity rover on Mars observed the planet Mercury transiting the sun, marking the first time a planetary transit has been observed from a celestial body besides Earth. Previously, the Curiosity rover has captured images of the Martian moons Phobos and Deimos transiting the sun.

TERMINATOR

The edge between night and day on the moon or a planet.

TOTAL LUNAR ECLIPSE

A lunar eclipse where the moon completely enters Earth’s umbra. MORE »

TOTAL SOLAR ECLIPSE

A solar eclipse seen from within the moon’s umbra. The moon appears to completely block the sun’s photosphere. MORE »

TOTALITY

The period during a solar eclipse when the sun’s photosphere is completely covered by the moon and the period for a lunar eclipse when the moon is in the complete shadow of Earth.

UMBRA

A complete shadow—such as that of the moon or Earth—within which the source of light, such as the sun, is totally hidden from view. Also, it refers to the dark inner area of a sunspot.