Eclipse Terms and Definitions
Programmer: Bill Kramer
Last update: April 17 2015
Eclipse Chaser Terminology
-- A --
Alt-Azimuth - Altitude and Azimuth - a way to express spherical coordinates. Normally in degrees.
Altitude is almost always from zero to ninety with zero at the horizon and ninety directly overhead.
Azimuth can be expressed in numerous ways. A common format is to use the compass points and degrees such as N30E for 30 degrees from the North towards the East.
Allais Effect - Allais Effect - an anomalous change sometimes observed in a pendulum during a solar eclipse. First reported by Maurice Allais in 1954 and observed again during the 1959 eclipse. Repeating the observations has not met with consistent results thus the anomaly remains in question.
Angular size - Angular size - the size of something in sky is typically expressed in terms of degrees. You can think of the visible sky as 180 degrees. From the horizon to over head is ninety degrees.
For perspective, the angular sizes of the moon and sun are about one half of a degree.
Annular Eclipse - Annular Eclipse - A solar eclipse where the angular size of the moon is smaller than the angular size of the sun's photosphere. The umbra shadow of the moon does not touch the surface of the Earth.
Aperture - Aperture - The size of the light gathering lens or mirror. Typically expressed in millimeters or inches.
Aphelion - Aphelion - the furtherest possible point in the Earth's orbit about the Sun. Eclipses that occur near aphelion (early July) are always total because the apparent size of the Sun is smaller.
Apogee - Apogee - the further possible point in an orbit about the Earth for a body, such as the Moon. The closer to apogee an eclipse takes place, the shorter the duration.
ASE - Annular Solar Eclipse
-- B --
Baader film filter - Baader film filter - a commercial foil that can be used as a solar filter. Often used to create a filter for telescopes or binoculars.
Baily's Beads - Baily's Beads - The name given to the broken series of bright points at the very end or start of totality.
The photosphere shining through the valleys along the edge of the moon was described as "a row of lucid points, a string of beads" by Francis Baily, an English astronomer during the early 1800s.
Barlow Lens - Barlow Lens - a concave lens that elongates the focal length of an optical system.
Can be used in astrophotography as part of a 'negative projection' system.
Besselian elements - Besselian elements provide the position of the umbral axis with respect to the fundamental plane and the size of the umbra and penumbra at a particular instance in time. The fundamental plane is a plane cutting through the center of the Earth normal to the Earth-Sun line. Derived from lunar and solar ephemerides, the Besselian elements can be represented by 3rd and 4th order polynomials during the relatively short couple of hours of a solar eclipse.
-- C --
C1-C4 - C1 - First Contact, moon nibbles edge of sun.
C2 - Second Contact, moon covers sun.
C3 - Third Contact, moon done covering sun.
C4 - Fourth Contact, moon nibbling ends.
Cable release - Cable release - a photographer's tool for taking pictures without vibrating the camera on a tripod or rigid surface.
Calcium line filter - Calcium line filter - is a solar filter for viewing light centered around the 395 nanometer bandwidth, a deep violet color. As calcium looses energy near the surface of the photosphere it emits light at 393.3 nanometers and at 396.9 nanometers (the K and H lines). This light is at the near edge of the visual spectrum meaning that the views can be rather dark in the eyepiece.
Calcium line filters reveal details such as granulation and flares during the partial phases of an eclipse (and during normal clear days). They should not be used during totality.
Chromosphere - Chromosphere - a shell of the solar disk located just above the photosphere or brightest part of the sun. The chromosphere is the transition from the photosphere to the corona.
Clear Sky Clock - Clear Sky Clock - s a web site suppling astronomical tuned weather forecasts. They also produce local charts for many areas.
They are the best tool that amateur astronomer use for determining the viewing conditions.
Produced by Attila Denko and Allan Rahill
CME - CME - Coronal Mass Ejection. A form of solar eruption that creates a severely distorted corona.
Conical shadow - Conical shadow - A shadow in the shape of a cone. The sun being bigger than the moon creates a conical shadow projected away from the sun. The cone reduces in size and by the type it reaches Earth it is much smaller than the moon.
Contact - Contact - when the moon appears to touch an edge of the sun. 1st contact is the initial nibble of the sun by the moon. 2nd contact is the first diamond ring and the beginning of totality. 3rd contact is the second diamond ring and the end of totality. 4th contact is when the last bit of the moon is no longer seen against the sun's bright surface.
Cookie bite - Cookie bite - a description of the initial appearance of the partial eclipse. It really does look like something took a bite out of it.
Corona - Corona - a shell of the solar disk that extends deep in to the solar system. The corona (or crown) is the white glowing material seen around the eclipsed sun. The corona is only directly visible during totality.
Coronal Mass Ejection - Coronal Mass Ejection - a form of solar eruption that creates a severely distorted corona.
-- D --
Delta T - Delta T is the difference between Universal Time (used to compute the local time and based on solar transit timing) and the Terrestrial Dynamical Time (TDT) which is an atomic time clock, not subject to the rotational fluctuations of the Earth.
Diamond Ring - Diamond Ring - at 2nd and 3rd contacts when just a point or small sliver of the photosphere is visible it appears as a bright gem with the corona as the ring around the dark moon. Hence the diamond ring effect.
Duration - Duration - in eclipse chaser talk, duration almost always means the time between 2nd contact and 3rd contact. This is the totality time.
-- E --
Eclipse chaser - Eclipse chaser - one who makes specific travel plans to be in the path of a total solar eclipse. A niche market for the travel industry often serviced by those who want to share the event with others as well as professional travel agencies.
Eclipse fever - Eclipse fever - a strong desire to see another total solar eclipse. Also known as eclipse addiction, eclipse fever symptoms are pretty easy to spot. The most obvious symptom is incessant talking about the next eclipse and how to get to see it.
Virtually all eclipse chasers have eclipse fever to some degree.
Eclipse glasses - Eclipse glasses - solar filters fashioned into sometimes colorful eye glasses. Intended to be worn when looking at the partial phases of the eclipse.
Ecliptic - Ecliptic - the apparent annual path of the Sun on the celestial sphere.
Edge effects - Edge effects - the shadows on the ground just before and after totality, when the sun is over 50% obscured. When a tall shadow is cast on the ground, one side will appear sharp and distinct while the other side fuzzy and blurred.
Edge effects can also mean going to the edge of the path of totality in order to prolong the diamond ring at the expense of less totality time.
Equatorial - Equatorial - a type of telescope mount that tracks the movement the stars as the Earth rotates.
Exeligmos cycle - Exeligmos cycle is a cycle of three Saros cycles (four eclipses from the same Saros). At the completion of an exeligmos cycle an eclipse occurs at roughly the same position. An exeligmos cycle is about 56 years.
-- F --
Facula - Facula (plural: Faculae) - bright spots that form between solar granules as a result of magnetic field lines.
Field of View - Field of View or Angle of View - a value expressed in degrees that describes a relative size for items viewed through telescopes, cameras, and binoculars.
First contact - First Contact (1st contact) - the very start of the eclipse. When the moon first appears to touch the surface of the sun.
Flash spectrum - Flash spectrum - a valuable tool used to identify the elements that comprise the chromosphere and photosphere. Around the time of the diamond ring a flash spectrum is produced using a spectroscope or grated filter and a camera.
Fourth Contact - Fourth Contact (4th contact) - the very end of the eclipse. When the moon is no longer visible against the surface of the sun.
Fraunhofer - Fraunhofer - a Bavarian credited with the invention of the spectroscope and diffraction grating. Subsequently went on to discover absorption lines in the solar spectrum.
Fraunhofer lines are the absorption lines caused by various atoms in the sun. A later discovery by Kirchoff and Bunsen about the relationship between various absorption lines and atomic elements led to some of the greatest discoveries in astronomy.
-- G --
Gamma - Gamma - when analyzing a sequence of eclipses, as in a Saros series, Gamma is the smallest distance of the shadow axis relative to the Earth's center. Gamma is greater than zero when the axis is north of the center, negative when south.
Greatest Eclipse - Greatest Eclipse - Greatest eclipse is defined as the instant when the axis of the Moon's shadow cone passes closest to Earth's center. For total eclipses, the instant of greatest eclipse is virtually identical to the instants of greatest magnitude and greatest duration. However, for annular eclipses, the instant of greatest duration may occur at either the time of greatest eclipse or near the sunrise and sunset points of the eclipse path.
-- H --
Herschel Wedge - Herschel wedge - a prism that can be used in the construction of a white light telescope. Typically used as a diagonal replacement for refracting telescopes or in pairs. White light telescopes reduce the intensity of the light from the sun by reflecting just a small amount of the light to the eyepiece. Solar details are much more distinct as a result.
Hershel wedges are often used as laser beam splitters.
Horizon effects - Horizon effects - as the moon's shadow moves across the Earth it hits clouds and the atmosphere. Near the edge of the shadow the light from the sun hits the clouds often before it hits the ground near you. This creates a form of sunset or sunrise look to the horizon. As the shadow is approaching the horizon grows dark and as it moves away the horizon lightens.
Hybrid eclipse - Hybrid eclipse - a term often used to describe an annular/total solar eclipse.
For a total eclipse to exist across the entire path, the point of the umbra shadow must be past the center of the Earth. When the point is between the center of the Earth and the surface of the Earth it is possible to have a transitional eclipse where it starts as an annular then becomes a total and reverting back to an annular by the end of the path.
The term "hybrid" was first used to describe annular-total eclipses in van den Bergh's seminal work on the periodicity and frequency of eclipses in 1954.
Hydrogen alpha filter - Hydrogen alpha filter - a solar filter that only allows the light of the color at 656 nanometers through. That color is the red seen in the chromosphere. Hydrogen alpha filters provide an enjoyable view of the sun during the partial phases and under normal viewing conditions. They are not to be used during totality, a regular telescope or binoculars with out a filter work best during totality.
-- I --
Intervalometer - Intervalometer - photography tool for triggering exposures at predetermined times automatically.
-- L --
Limb corrections - Limb corrections - eclipse circumstance calculations are based on simplified geometry which produces results that are fairly accurate. However, the lunar limb profile is not a simple geometry. It is comprised of valleys and peaks. Valleys will shorten the time of totality while peaks will increase it. If you want to get the most accurate possible timing for the contacts, then the lunar limb corrections need to be applied. They are typically expressed as seconds to add or subtract from the calculated time.
Limb profile - Limb profile - the variable valleys and peaks along the edge of the moon from the observers location.
Lunar diameters - Lunar diameter - a convenient, but not highly accurate, measuring method normally used to express the size of the corona in any given direction. A lunar diameter is roughly one half of a degree in angular size.
-- M --
Magnitude - Magnitude of a solar eclipse - a definition of the depth of the eclipse. Magnitude is described as the fraction of the solar diameter that is covered by the moon. In eclipse calculations, magnitude is the ratio of the solar diameter over the distance of a line that goes through the center of both the sun and moon ending at the limbs of the sun and moon nearest the opposite center point (units are solar diameters). A magnitude of one or greater signifies a total solar eclipse. A magnitude less than one but greater than zero is a partial solar eclipse.
Mr Eclipse - Mr Eclipse is Fred Espenak, author of the NASA Canons of Eclipses. His web site, MrEclipse.Com, offers wonderful pictures of eclipses and lots of useful information for those wanting to know more about eclipses. Cool T-shirt designs too.
Mylar filter - Mylar filter - a solar filter made of the Mylar material.
-- N --
Node - Node - an intersection of the ecliptic and the path of the moon. The path of the moon is inclined by five degrees relative to the ecliptic.
-- O --
Objective lens - Objective lens - the light gathering lens of an optical system.
Obscuration - Obscuration of a solar eclipse - the fraction of the area of the solar disk that is covered or obscured by the moon. Obscuration can be computed from the eclipse magnitude.
Occultation - Occultation - when one object passes in front of another.
Orange peel - Orange peel - a colorful description of a partial eclipse close to totality as seen through an orange filter.
-- P --
Partial Eclipse - Partial Eclipse - every solar eclipse contains a partial eclipse. That is when the sun is partly obscured by the moon. Partial eclipses are expressed in percentages of coverage. Anything less than 100% is a partial eclipse.
Path - Path - in an eclipse chaser conversation, path means the track that the lunar shadow will make across the Earth. The path varies in width with the largest width being only about 100 miles. Being inside the path means that you will see a total solar eclipse.
Penumbra - Penumbra - the outer part of a shadow when the light source is only partially obscured.
Perigee - Perigee - the closest possible distance from the Earth to an object in orbit about the Earth, such as the Moon. When the Moon is closer to perigee it appears larger in the sky by a small amount resulting in longer totality durations.
Perihelion - Perihelion - the closest possible distance from the Sun to the Earth where the Sun's angular size is slightly larger. This results in shorter eclipses. Perihelion takes place in early January.
Photosphere - Photosphere - the bright surface of the sun.
Pin hole - Pin hole - a simple way to project the eclipse image. Use a piece of paper, put a small hole in it, and hold it so that it castes a shadow on the ground. Where the hole is located you should see an image of the partial eclipse.
Polar brush - Polar brush - a series of coronal spikes along the northern and southern poles of the sun.
Polar mount - Polar mount - a type of telescope (or camera mount) that tracks stars during the course of the night.
Projection - Projection - in eclipse chaser conversations a projection normally refers to a way of observing the sun safely during the partial eclipse phases. The image of the sun is projected onto a surface using some sort of optical device.
Prominence - Prominence - a eruption from the photosphere into the corona along the edge of the sun. Appears the same color as the chromosphere, bright red.
Prominence - loop - Prominence loop - prominences that follow magnetic lines forming a loop as viewed from the Earth. The material is ejecting from one side and falling back on the other side of the loop.
Prominence - spike - Prominence spike - a prominence that appears like a pole or spike.
PSE - PSE - Partial Solar Eclipse
-- R --
RA-DEC - RA-DEC - Right Ascension and Declination, polar coordinates in the sky.
-- S --
Saros Cycle - Saros Cycle - Approximately every 18 years, 11 days, and 8 hours circumstances for an eclipse repeat. This cycle was used in ancient times to predict lunar and solar eclipses. Given an observed eclipse one can calculate into the future when others will occur.
Second contact - Second Contact (2nd contact) - the very start of totality. When the moon covers the surface of the sun.
Shadow bands - Shadow bands - fast moving crescents caused by atmospheric scintillation that appear in the moments before and after totality.
Shadows - Shadows - During the time of the partial eclipse nearer to totality shadows will take on a strange appearance. Pinholes will look like little images of the eclipsed sun and the edges of long shadows will appear uneven.
Solar screen - Solar Screen - where the image of the sun is projected, normally through a telescope.
SOHO - SOHO - Solar and Heliospheric Observatory observatory in space.
Sun screen - Sun screen - a projection screen for viewing the sun.
Sun screen can also mean the lotion applied to protect the skin from the sun. A total solar eclipse only blocks out the sun for a few minutes - the partial eclipse can still result in a sun burn if you don't take precautions.
Spicules - Spicules - the smaller spikes of the chromosphere.
Solar filter - Solar filter - a filter for your eyes, camera, or a telescope that will block out the harmful radiation of the sun and only allow a very small amount of light through.
Sun spot - Sun spot - a form of solar event that appears darker against the photosphere. Sun spots are commonly associated with prominences when near the edge of the sun.
Streamers - Streamers - in eclipse chaser descriptions of the corona the term streamers refers to the elongated sections typically seen along the equatorial regions.
Syzygy - Syzygy - an alignment of three or more celestial bodies.
-- T --
Third contact - Third Contact (3rd contact) - the very end of totality. When the surface of the sun first appears from behind the moon.
Time line - Time line - normally refers to the eclipse times.
Time zone - Time zone - eclipse calculations are done using UT (universal time). The time zone is a numeric offset in hours for the local time.
Total Eclipse - Total Eclipse - when the sun is completely covered by the moon.
Totality - Totality - the brief period in time when the sun is completely eclipsed.
Tracking mount - Tracking mount - a telescope or camera mount that tracks the sky during the course of a night.
Transit - Transit - when an object passes in front of the Sun as seen from Earth, that object is considered to be in transit. Transits of Mercury and Venus are often interesting to eclipse chasers.
Tripod - Tripod - any device that will hold a camera steady.
TSA - TSA - Transportation Security Administration - due to the travel involved in eclipse chasing, most must use airlines and other public transportation that falls under the guide lines of the TSA. It is wise to learn in advance what things you can and cannot take with you plus how the security procedures in place may effect delicate hardware such as telescopes, film, and cameras.
TSE - TSE - Total Solar Eclipse
Two line elements - Two Line Elements - the basis for orbital prediction programs.
Eclipse chasers also like to capture other transitory astronomical events and usually together.
T.S. Kelso is the best source for current data
-- U --
U1-U4 - U1-U4 - Umbral contact times. Same as C1-C4 however often expressed in UT (universal time).
Umbra - Umbra - the darker part of a shadow where the light source is completely obscured.
Umbraphile - Umbraphile - a shadow lover. In terms of eclipse chasing, it means one who is addicted to the glory and majesty of total solar eclipses. This term is synonymous with eclipse chaser but implies a greater zeal.
Also the name of a software program created by Glenn Schneider for automated control of cameras allowing the operator the opportunity to observe and enjoy the glory and majesty of the total solar eclipse.
Universal time (UT) - Universal time (UT) - a time scale based on the rotation of the Earth. Originally it was based on the mean solar time at the meridian of Greenwich. Because of slight variations in the rotation of the Earth UT is now based on the atomic clock time known at UTC.
UTC - UTC - Coordinated Universal Time - time standard based on an International Atomic Clock. UTC always stays within 0.9 seconds of UT1, the official solar observed day to day time at the Greenwich Observatory.
UTM - UTM - Universal Transverse Mercator, A type of map projection. The grid lines you see on a USGS Quads represent a map projection known as Universal Transverse Mercator (UTM), and are 1000 meters apart.
Quad maps are used my many eclipse chases to determine viewing locations and roads to get there.
-- # --
#14 Welding filter - Welding filter - a #14 Welder's filter can be used for viewing the partial phases of the eclipse.
Eclipse chaser talk, select a term to learn more.
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Small Dictionary of Eclipse Terms - from IAU (International Astronomical Union)
PHP programming by Bill Kramer. Content Contributor(s): Alan Rifkin, Glenn Schneider, Michael Gill, Nico Hogeboom