Sun & Moon - Eclipse Glossary
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Solar Eclipse Glossary Terms

Would you like to learn some fundamental terms and facts so you can impress your friends and family on eclipse day? We've gathered everything you need to know about this year's eclipse, including relative eclipse keywords and how you can catch this monumental event.

Altitude (of a celestial object above horizon)

Altitude is simply how high an object is above the horizon. So, when you're looking up at a star, or any other celestial object, the angle measured upwards from the horizon is its altitude. For example, on object that appears to be right on the horizon would have an altitude of 0° and one that appears directly above your head, also known as the zenith, has an altitude of 90°.

Azimuth

Azimuth of a celestial object is the angle, left or right, measured along the North or South point of the horizon that the object appears. For example, if you are facing directly North, 90° is due East, 180° is due South, and 270° due West.

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Angular Diameter

The angular diameter of an object can also be described as its apparent size. From your point of view, it describes how large an object appears, given in an angular measurement. So instead of describing the Sun using its actual diameter, you could describe its angular diameter as 0.5° or 30 arcmin, for example.

Angular Distance

The angular distance is the separation angle between two points. For example, the angular distance of the Sun from the horizon would just be the angular measurement, in degrees or radians, from the horizon to the Sun with the vertex at the center of the sphere.

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Magnitude

In astronomy, the magnitude describes the brightness of an object. It may seem odd, but the brighter the object, the lower the magnitude. For example, any stellar object with a negative magnitude is going to be brighter in the night sky than a positive magnitude object would be.

Eclipse magnitude is different. The magnitude of a solar eclipse is the fraction of the Sun's diameter that is covered by the moon when the eclipse is at its maximum coverage. Keep in mind, this describes the ratio of diameters of the two objects, not a measure of the Sun's surface area covered by the moon.

Obscuration

Obscuration is the fraction of the Sun's surface area that is covered, or occulted, by the Moon. Keep in mind that the previously defined eclipse magnitude describes the amount of the Sun's diameter that is occulted by the Moon, while the obscuration is the surface area that is covered when the eclipse is at its maximum. Eclipse obscuration may be expressed as either a percentage or a decimal fraction.

Aphelion & Perihelion

Since orbits are elliptical and not just a circle, objects in orbit around the Sun will, at some point, be momentarily furthest away, and at another location, be at its closest point. The aphelion describes the location of an object in orbit around the Sun when it is at its furthest point. The perihelion is the point in the object's orbit when it is nearest to the Sun.

Apogee & Perigee

The apogee and Perigee are similar to the descriptions of the aphelion and perihelion, except they describe the orbit of the Moon around Earth. When the Moon is at the point of orbit closest to Earth, the point is called perigee. When it is at its furthest, it known as apogee.

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Syzygy

A syzygy is when three celestial objects are oriented into a straight line. For example, during a solar eclipse, the Sun, Moon, and Earth will all be in syzygy. This term also applies to both conjunctions and oppositions when the three objects are in relative positions from each other. These terms are defined as:

Conjunction

A conjunction is when two or more celestial objects meet or pass each other. For example, during a New Moon, the Moon passes between the Earth and the Sun. From Earth, the Moon is in conjunction with the Sun as it slowly passes in front of it. During the solar eclipse, the Sun and Moon will be aligned on the same side as Earth.

Opposition

Two celestial objects are in opposition when they are on opposite sides of a focal point. For example, during a Full Moon, the Sun and Moon are located on opposite sides of the Earth, meaning they are in opposition from the perspective of Earth.

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Photosphere

The photosphere of a layer of the Sun that describes its visible “surface”. Keep in mind, since the Sun is actually a ball of plasma (excited gas), there is not actually a physical surface. Instead, this thin layer is approximately 100 km thick and has a temperature between 4,500 and 6,000 K.

Chromosphere

The chromosphere is the second of three main layers that make up the Sun's atmosphere. Located just above the photosphere, the temperature wIthin this layer rises from 6000°C to about 20,000°C. Being primarily composed of hydrogen and helium, the temperature within this layer causes the hydrogen to excite, emitting light in red wavelength of the visible spectrum. This is known as H-alpha emission. Although this is always true, during a solar eclipse, observers from Earth can visibly see the reddish hue around the edges of the Sun.

Corona

In addition to the Sun's photosphere and chromosphere is the corona. This outermost layer of the atmosphere is typically masked by the light emitted by the Sun's surface, but during a solar eclipse, the majority of the light is blocked out. Because of this, the gasses contained within the corona become visible around the edges of the Sun.

Sun Spot

Sunspots are visible dark spots on the photosphere of the Sun. Compared to the rest of the surface, these spots appear to be darker. This is because these regions are actually cooler spots than the surrounding areas. The Sun is a massive, magnetic ball with strong, unstable magnetic fields. These areas of reduced temperature are caused by magnetic field lines which drive convection from the core of the Sun, resulting in correlated hot and cool spots on various areas of the surface.

Filament & Prominence

The magnetic fields spontaneously looping around the surface of the Sun trap plasma within them. These looping structures are referred to as two separate names, depending on their location. When looking at the Sun, if the material trapped within the magnetic field is facing towards you, it appears to be a line on the surface. This is called a filament. When the material is suspended on the edge of the Sun, the material is seen as looping off of the “edge” of the Sun, called a prominence. Essentially, they are the same exact thing, just depending on your point of view.

Solar Flare

A solar flare occurs when the magnetic field containing solar material becomes unstable and suddenly collapses. The amount of energy released during a solar flare is the equivalent to millions of 100-megaton hydrogen bombs exploding at the same time, according to NASA.

Baily's Beads

During the event of an eclipse, as the Moon slowly blocks out the light from the Sun the small slice of visible Sun appears to be smaller blobs of light. These blobs are called “Baily's beads” named after the British astronomer Francis Baily (1774-1844).

Diamond Ring

As baily's beads subside and the beads slowly disappear, another event can be seen. A diamond ring refers to when just one bead is left visible. At this stage, the eclipse looks like a diamond ring.

Center Line

The center line if the eclipse is the track traced on the Earth caused by the Moon's shadow. The longest duration of the eclipse will happen when viewing the event on the center line since you will experience the Moon's full shadow width.

First Contact, Second Contact, Third Contact, Fourth Contact

The transit phases of the solar eclipse can be described using four terms:

First contact: The Moon begins to “touch” the edge of the Sun.

Second contact: The Moon transits to the inside of the Sun

Third contact: The Moon moves to the opposite interior edge of the Sun.

Fourth contact: The Moon moves out of the edge of the Sun.

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Shadow Bands

Shadow bands are often seen during the conclusion of a total eclipse. As the Sun's light slowly becomes visible again, thin lines of alternating light and dark can be seen wiggling on a plain surface. Shadow bands are caused by refraction of the Moon's shadow passing through the atmosphere.

Penumbra

The penumbra is the region where some or all of the Sun's light is covered, or obscured, by the Moon. An observer within the penumbra, a lighter edge of the Moon's shadow, would experience only a partial eclipse.

Umbra

Alternatively, the umbra describes the event in which the Sun is completely blocked by the Moon. An observer within the umbra, the darkest part of the shadow, would witness a total eclipse.

Total Eclipse

A total eclipse happens when the Moon passes directly between the Sun and the Earth, completely covering the disk of the Sun, blocking out its light. If you are located directly in the Moon's shadow, or umbra, you will experience a total eclipse.

Partial Eclipse

During a partial eclipse, the Moon only blocks out part of the Sun. Therefore, only the Moon's penumbral shadow is casted on the Earth. In this perspective, an individual would see only part of the Sun's light being blocked.

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Annular Eclipse

An annular eclipse happens when the Moon is too far from the Earth to completely cover the Sun. Because of this, the Moon only covers the Sun's center, making the Sun appear to be a bright ring, or annulus, around the Moon.

Hybrid Eclipse

A hybrid eclipse, also known as an annular-total eclipse, is a rare solar eclipse that appears to transition from an annular solar eclipse, to a total solar eclipse. As the Moon's shadow, umbral and antumbral, traverse across the surface of the Earth, observers on Earth, depending on your location relative to the shadow's path, experience either an annular or total eclipse.

Central Eclipse

A central eclipse can be either total, annular or hybrid. This type of eclipse occurs when the central axis of the Moon's shadow, either the umbra or antumbra, traverses the Earth. Additionally, part of the shadow misses Earth during the eclipse, resulting in a Northern or Southern limit.

Non-Central Eclipse

A non-central eclipse has just one limit as the central axis of the Moon's shadow misses the Earth and only a single edge of either the umbra or antumbra passes over the surface. Non-central solar eclipses can also be either total, annular or hybrid.

Greatest Eclipse

The greatest eclipse describes the instant when the axis of the Moon's shadow is closest to the Earth's center. It is a calculated point that is determined using the path of the eclipse. This moment of greatest eclipse is often a comparison value of the duration of totality between different eclipses.

Totality

Totality of an eclipse is the phase which occurs between the second and third point of contact when the Moon completely covers the disk of the Sun. At this point, only the solar corona is visible. Currently, totality can never last more than 7 min 32s.

Annularity

In contrast to totality which refers to a total eclipse, annularity is the maximum phase of an annular eclipse. Since the Moon is too far from the Earth to completely cover the the Sun during an annular eclipse, the Sun is instead seen as a bright ring. Annularity cannot lost more than 12 minutes 29s.

Solar Eclipse

A solar eclipse is an astronomical event in which the Moon passes directly between the Sun and the Earth. When this happens, the Moon effectively blocks out the light from the Sun. If you are in the right location on Earth so that you are located directly in the shadow, you will momentarily experience complete darkness, known as a total solar eclipse. If you are in a location where the light is not completely blocked by the moon, then you will experience a partial eclipse.

Lunar Eclipse

A lunar eclipse occurs when the Moon is directly behind the Earth, creating a straight line with the Earth, Moon and Sun. During this phase, the Earth blocks the light from the Sun, creating a red hue on the Moon caused from the Sun's light passing through the Earth's atmosphere.

Lunar eclipses occur when Earth's shadow blocks the sun's light, which otherwise reflects off the moon. There are three types — total, partial and penumbral — with the most dramatic being a total lunar eclipse, in which Earth's shadow completely covers the moon.

Transit (such as Venus transit)

A transit occurs when one celestial object passes in front of another. For example, when Venus transited the Sun, you could see the small planet appearing the move across the surface obscuring just a small part of the celestial body.

Universal Time

Many significant events, or calculations which are dependent on equivalent times, are determined using “Universal Time”, or UT. It is sometimes referred to as "Greenwich Mean Time", or GMT. This precise measure of time is needed for all civil timekeeping and is particularly important when determining eclipse predictions.