Eclipse glossary of words

On Monday, August 21, 2017, all of North America will be treated to an eclipse of the sun. Anyone within the path of totality can see one of nature’s most awe inspiring sights - a total solar eclipse. This path, where the moon will completely cover the sun and the sun's tenuous atmosphere - the corona - can be seen, will stretch from Salem, Oregon to Charleston, South Carolina. Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun's disk. NASA created this website to provide a guide to this amazing event. Here you will find activities, events, broadcasts, and resources from NASA and our partners across the nation.

Eclipses only occur if the Moon is located within 0.5 degrees of the plane of the ecliptic, on a line that passes through the center of the Sun and the Earth. The Moon travels along an orbit that is inclined by 5 degrees to the ecliptic plane, so there are only two opportunities each month when it passes through the plane of the ecliptic. These points are called the ascending and descending nodes. Eclipses of the Sun only occur if new moon occurs when the Moon is near of one of these nodes. A similar argument explains why lunar eclipses do not occur every full moon at the node opposite the Sun from the Earth.

UT or "Universal Time" is a single time standard that applies to all locations on Earth as opposed to your local time which will vary depending on your location (longitude). For all practical purposes, UT is equivalent to GMT (Greenwich Mean Time) which establishes a line of longitude that passes through Greenwich England (called the Prime Meridian) as the standard from which all other times are measured and UTC (Coordinated Universal Time) which is based on International Atomic Time (time measured very precisely as vibrations of a cesium atom).

How Eclipses Work

Eclipses, whether solar or lunar, occur because of the periodic alignments of the sun, Earth, and moon. These three bodies, orbit in space in very predictable paths (yes, the sun orbits too. It orbits the galaxy once every 200 million years!). Ever since the days of Kepler and Newton, we have been able to predict the motion of planetary bodies with great precision. So, why do eclipses happen

Solar Eclipses Happen when the moon moves between Earth and the sun. You might think that this should happen every month since the moon’s orbit, depending on how it is defined is between about 27 and 29 days long. But our moon’s orbit is tilted with respect to Earth’s orbit around the sun by about five degrees. Not much, you say? Yes, but the moon, itself, is only about ½ degree in width in the sky, about ½ the width of your pinky finger held at arm’s length. So, sometimes the moon misses too high and sometimes too low to cause a solar eclipse. Only when the sun, moon, and Earth line up close to the “line of nodes”, the imaginary line that represents the intersection of the orbital planes of the moon and Earth, can you have an eclipse.

The Moon orbits the Earth in the months prior to the August 21, 2017 total solar eclipse. Viewed from above, the Moon's shadow appears to cross the Earth every month, but a side view reveals the five-degree tilt of the Moon's orbit. Its shadow only hits the Earth when the line of nodes, the fulcrum of its orbital tilt, is pointed toward the Sun.

This is true for both solar and lunar eclipses. This situation is somewhat unique as no other moon in the solar system orbits roughly in the plane of the “ecliptic”,  Earth’s orbital plane, that the planets more or less follow.  

When the moon does eclipse the sun, it produces two types of shadows on Earth. The umbral shadow is the relatively small in diameter point on Earth where an observer would see a total eclipse. The penumbral shadow is the much larger area on Earth where an observer will see a partial eclipse. Here, the sun is not completely covered by the moon.

There are Four Types of Eclipses

Depending on your location and on the specific geometry of the sun-Earth-moon system, you may experience one of four types of solar eclipses; total, partial, annular and hybrid.

A TOTAL ECLIPSE happens when the moon completely covers the sun. Here, the observer is standing under the umbral shadow of the moon. In a total solar eclipse, the sun’s outer atmosphere can be seen.

The brighter stars and the planets come out. Animals change their behavior. Birds and squirrels nest. Cows return to the barn. Crickets chirp. There is a noticeable drop in both light level and air temperature. It is an eerie feeling. Totality can last for no more than about seven and a half minutes but is usually less than three minutes long.

A PARTIAL ECLIPSE occurs when the moon passes in front of the sun, off center and only a portion of the sun’s disk is obscured. Here, the observer is standing in the penumbral shadow of the moon

AN ANNULAR ECLIPSE occurs when the moon passes dead center in front of the sun but, because the moon’s orbit is elliptical and so is sometimes closer and sometimes further from Earth, it appears too small to fully cover the disk of the sun.

Here, a bright ring called the “ring of fire” appears around the dark disk of the moon. In an annular eclipse, the moon’s umbral shadow comes to focus – to a point – above the Earth’s surface.

A HYBRID ECLIPSE is a combination of total and annular eclipses. The eclipse begins as one type and ends as another.

1)ANNULAR 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.                     

2              ANOMALISTIC MONTH

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

3              ANTUMBRA

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

4              APHELION

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

5              APOGEE

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

6              ASTRONOMICAL UNIT

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

7              BAILY’S BEADS

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

8              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.                                          

9              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.                                  

10           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.                                     

11           CORONA

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

12           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.                                              

13           ECLIPSE

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

14           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.                               

15           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).                                            

16           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.                                     

17           GIBBOUS

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

18           HYBRID ECLIPSE

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

19           LUNAR ECLIPSE

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

20           LUNAR MONTH

See synodic month.                                       

21           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 maximum phase.                                            

22           MARE

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

23           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.                                   

24           OBSCURATION (of a solar eclipse)

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

25           PARTIAL LUNAR ECLIPSE

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

26           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.                                   

27           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.                                               

28           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.                                   

29           PENUMBRAL ECLIPSE

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

30           PERIGEE

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

31           PERIHELION

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

32           PHOTOSPHERE

The bright, visible surface of the sun.                                   

33           PROMINENCE

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

34           RAYLEIGH SCATTERING

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

35           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.                                       

36           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.                                           

37           SHADOW BANDS

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

38           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.                                               

39           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.                                

40           SOLAR FLARE

An explosive eruption in the sun's atmosphere.                                              

41           SUNSPOT

A magnetic disturbance on the sun that appears as a dark blotch on its surface.                                               

42           SYNODIC MONTH

The time from one full moon to the next, which takes 29.53 days. Also called a lunar month.                                    

43           TERMINATOR

Theedge between night and day on the moon or a planet.                                         

44           TOTAL LUNAR ECLIPSE

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

45           TOTAL SOLAR ECLIPSE

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

46           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.                               

47           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.