Eclipse Session 5 Vocabulary - PHYS 108

Angular Diameter

The angle subtended by an astronomical body against the sky.

Annular Eclipse

A central solar eclipse in which the angular diameter of the moon is too small to cover the sun. With proper filtration the moon appears to have a ring of sun surrounding it.

Annulus

It is Latin for ring.

Anomalistic Month

The amount of time it takes the moon to complete two perigee or apogee passages. Its period is 27.55455 days.

Aphelion

The farthest position to the sun of an astronomical body in an elliptical orbit around the sun.

Apogee

The farthest point to the Earth in the moon's elliptical orbit around the Earth.

Ascending Node

With respect to eclipses, it is the location where the moon's orbital plane passes from below the ecliptic plane to above the ecliptic plane.

Baily's Beads

The light phenomenon which occurs just moments before second contact or moments after third contact in a central solar eclipse. Sunlight shining through lunar valleys on the moon's limb gives the sun a beaded texture.

Broken Annular Eclipse

A central solar eclipse in which the angular diameter of the moon is just small enough in comparison to the sun to cause the entire limb of the sun to be broken into a Baily's Beads structure during the moment of annularity. It is the rarest of solar eclipses.

Centerline

The line across the surface of the Earth where the apex of the moon's umbra points.

Central Eclipse

A total solar or an annular eclipse.

Chromosphere

The thin layer of solar atmosphere that lies between the photosphere and the corona which is visible as a pinkish hue just before totality. Its color is due to hydrogen emission.

Corona

The tenuous outermost atmosphere of the sun visible to the unaided eye during a total solar eclipse.

Descending Node

With respect to eclipses, it is the location where the moon's orbital plane passes from above the ecliptic plane to below the ecliptic plane.

Diamond Ring

The visual appearance of the sun about 10-15 seconds before or after a total solar eclipse. The diamond is created by the photosphere or chromosphere, and the ring by the corona.

Eclipse

It is from the Greek word, 'ekleipsis,' and it means to hide or cover.

Eclipse Chaser

A person who travels to see a total solar or annular eclipse.

Eclipse Season

The time when the moon is either new or full and also at a node. The time interval between two eclipse seasons is 173.3 days.

Eclipse Year

The time interval between two eclipse seasons occurring at the same node—346.6 days.

Ecliptic

The plane of the Earth's orbit projected into space. The motion of the sun against the starry background resulting from Earth's orbital motion around the sun.

First Contact

The first moment of tangency of the moon's limb with the sun's limb in a partial or central solar eclipse. The first moment of tangency of the moon with the Earth's umbral shadow in a partial or a total lunar eclipse.

Fourth Contact

The last moment of tangency of the moon's limb with the sun's limb in a central solar eclipse.

Last Moment of Tangency

The last moment of tangency of the moon with the Earth's umbral shadow in a total lunar eclipse.

Full Moon

The lunar phase which occurs when the moon is opposite to the sun and its entire nearside is completely illuminated.

Great Eclipse

A total solar eclipse in which the length of totality is equal to or greater than five minutes.

Greatest Eclipse

The moment when the moon's shadow passes closest to the center of the Earth.

Hybrid Eclipse

A solar eclipse which has a portion of its path seen as an annular eclipse and part of it path viewed as a total solar eclipse.

Limb

The apparent boundary of the sun, moon, or any astronomical object which has a true angular diameter in the sky.

Limb Darkening

The decrease in the amount of light intensity from the center to the limb of the sun, due to a decrease in the depth of the penetration into the photosphere towards the limb.

Line of Apsides

In the case of eclipses, the major axis of the moon's elliptical orbit.

Lunar Eclipse

An astronomical event in which the moon is totally or partially hidden by the Earth's umbral or penumbral shadows.

Major Axis

The longest axis of an ellipse. In an elliptical orbit, the major axis intersects the foci of the ellipse, the center of the ellipse, and the closest and farthest positions of the orbiting body.

Negative Shadow Zone

The area within the path of a central solar eclipse where the eclipse appears to be ringed.

New Moon

The phase of the moon in which the hemisphere of the moon facing Earth is in darkness. A condition where the elongation of the moon equals zero.

Node

The intersection point of two orbital planes.

Nodical/Draconic Period

With respect to eclipses, the period of time necessary for the moon to complete two successive crossings of the same node.

Northern Limit

The northernmost boundary along the path of a central solar eclipse where the eclipse appears total or annular.

Partial Eclipse

It is the portion of a solar eclipse where only a portion of the moon is covering the sun. It is the portion of a lunar eclipse where only part of the moon is immersed within Earth's umbra.

Partial Penumbral Eclipse

A lunar eclipse in which the moon only partly enters the penumbral shadow of the Earth.

Penumbra

The secondary shadow of a body created when it is only partially blocking a light source.

Perigee

The closest point from the Earth along the path of the moon's elliptical orbit.

Perihelion

The closest position of a body to the sun in its elliptical orbit around the sun.

Pinhole Projector

A brute force solar projection device in which a small hole, usually made by a pin, acts like a lens projecting an image of the sun to the opposite end of an enclosed tube or long box.

Prominence

A long lasting coronal event related to magnetic fields in which large amounts of plasma are made to condense and glow.

Regression of the Moon's Nodes

The westward motion of the moon's orbital intersection points with the ecliptic, completing a cycle in 18.61 years.

Revolution

The period of time it takes for an object of lesser mass to orbit around a body of greater mass.

Revolution of the Moon's Apsides

The period of time necessary for the major axis of the moon's elliptical orbit to complete one full cycle through the heavens, lasting 8.85 years.

Saros

The 18 year, 9, 10, 11, or 12 day period between the repetitions of similar solar or lunar eclipses, varying due to the number of leap years.

Second Contact

In a central solar/lunar eclipse, it is when totality or annularity begins; in a partial eclipse, it is when the moon is tangent to the sun's disk for the second time.

Shadow Bands

Faint ripples of low contrast light that can undulate across a landscape a minute or so before and after a total solar eclipse.

Shadow Cone

The primary or umbral shadow cone of the moon or the Earth.

Sidereal Period of the Moon

The amount of time it takes the moon to complete one revolution around the Earth, 27.321661 days.

Similar Eclipses

Eclipses of a particular saros cycle which have nearly the same characteristics.

Solar Eclipse

An astronomical event in which part of the moon or the entire moon hides the sun.

Solar Filter

An optical device for safely dimming the sun's brightness to a level where it can be viewed with the human eye.

Solar Projection

A safe solar observing technique where the sun's image is projected through the eyepiece of an unfiltered telescope onto a white screen.

Southern Limit

The southernmost position along the path of a central solar eclipse where the eclipse appears total or annular.

Sunspots

Darker, cooler regions in the sun's photosphere where an area of intense magnetic field is preventing the sun from cooling at its normal rate.

Synodic Period of Moon

The amount of time it takes the moon to complete one phase cycle, 29.5306 days.

Third Contact

The moment when totality or annularity is over.

Total Eclipse

An astronomical event where the moon completely covers the photosphere of the sun or is completely immersed in the primary shadow of the Earth.

Totality

The portion of a solar or lunar eclipse in which the moon completely hides the sun or is entirely immersed within the shadow of the Earth.

Umbra

The primary shadow cone of an astronomical body, where the eclipse appears to be total.