A Solar Eclipse Glossary of Terms

With April 8 fast approaching, levels of interest in the last total solar eclipse to cross North America in the next 20 years are skyrocketing.

There’s a lot of terminology that gets bandied around relating to eclipses, so here’s our guide to some of the main terms you’ll likely encounter.

Entries marked ‡ are phenomena or data that you will find supported in the Photo Ephemeris, either in our maps, the local circumstances display, or the eclipse simulator.

You can explore many of these solar eclipse phenomena in either Photo Ephemeris Web or Photo Ephemeris iOS.

Annular Eclipse ‡

During an annular eclipse, magnitude is less than 1.00 at all times, and obscuration never reaches 100%, but the Moon moves entirely within the limb of the Sun for observers on the central path, producing the so-called ‘ring of fire’ effect.

Baily’s Beads ‡

The moments around C2 and C3 where, in certain circumstances, multiple ‘beads’ or ‘pearls’ of photosphere remain or become visible. Named after Francis Baily, who described the phenomenon in Scotland in 1836. Baily’s beads typically last no more than a few seconds for observers located well within the central path. But for observers located near the path edges, or during eclipses where the maximum magnitude is close to 1.00, they may remain visible for the entirety of the time between C2 and C3.

Central Path ‡

The path across the Earth’s surface within which an annular or total eclipse can be observed. Path location, width, and length vary for each eclipse. In some circumstances where an eclipse occurs in polar regions, a northern (ɣ ~+1) or southern (ɣ ~-1) limit may not be defined as the Moon’s shadow cone ‘misses’ the Earth.

Central Line (or Centerline) ‡

The Central Line is the locus of points during an eclipse at which the Moon’s disk is observed passing directly over the center of the Sun’s disk. Magnitude is greatest for a given longitude when on the Central Line. Additionally, eclipse duration is longest and C2 and C3 are observed to occur at opposite points on the Moon’s disk.

Corona ‡

The outer atmosphere of the Sun, consisting of plasma that extends millions of miles above the Sun’s surface. This is one of the most visually striking elements of a total solar eclipse. Its appearance can be predicted in advance to a limited degree, but only by a few days. During solar minimums, when the Sun’s activity is near a low point during the 11-year cycle, the corona is typically seen to extend farthest from the Sun’s poles. Near the time of solar maximum, the corona’s extent around the solar limb is often more even.

Chromosphere ‡

The Sun’s chromosphere is visible as a very thin pale pink line. The chromosphere is another striking visual effect seen during totality and is easily captured by cameras (no filters required). Don’t mistake it for chromatic aberration in your lens - it’s the Sun’s inner atmosphere. The pink color arises from H-alpha emissions.

Chromospheric Beading ‡

Breaks in the chromosphere observed during totality, arising due to lunar mountains. They have the same underlying cause as Baily’s beads, but the irregular lunar limb affects the chromosphere rather than the photosphere.

Chromospheric Beading before C3, April 20 2023, Exmouth, Australia

Delta-T ‡

Measured in seconds, ΔT is the difference between ‘terrestrial time’ (TT) and universal time (UT). This difference is determined by variation in the speed of Earth’s rotation on its own axis. It cannot be determined analytically, but must be measured and predicted. It is of importance to eclipse chasers because a change in ΔT results in an eastward or westward shift in the path of the eclipse. Generally, ΔT predictions are reliable in the near term and the magnitude of any expected changes is of significance only to those planning to be located very close to the edge of the Central Path.

Diamond Ring ‡

The moments just before C2 and just after C3 when the last or first glimpse of the Sun’s photosphere is visible.

Double Diamond Ring ‡

The moments just before C2 and just after C3 when the photosphere converges or emerges into two prominent Baily’s Beads. Due to the geometry of eclipses, the two beads are necessarily located near to one another on the lunar limb, although the exact distance varies by circumstances. Double diamond rings (and even triples and more!) are visible in the Photo Ephemeris eclipse simulator.

First Contact, C1 ‡

The moment when the Moon starts to obscure the Sun, i.e. the start of the partial phase of the eclipse. Magnitude is 0.00 and obscuration is 0.00% at this moment.

Fourth Contact ‡

The end of the partial eclipse, when the Moon no longer covers any portion of the Sun’s disk. Magnitude is 0.00 and obscuration is 0.00% at this moment.

Gamma, ɣ

Formally, gamma is the “ minimum distance from the axis of the lunar shadow cone to the center of the Earth, in units of the equatorial radius of the Earth. ” (Meeus, Elements of Solar Eclipses, 1951-2200). If ɣ=0, the center of the Moon’s shadow is at the equator at greatest eclipse. If ɣ=+1, it is at the North Pole. If ɣ=-1, it lies at the South Pole. (In fact, the values are ±0.997 rather than ±1, due to the flattening of the Earth at the poles.)

Greatest Eclipse

The time and location where the greatest magnitude of an eclipse is observed. Local circumstances can be calculated for any location on Earth, including the point of greatest eclipse, but there is only one location where greatest eclipse occurs.

Hybrid Eclipse ‡

A hybrid eclipse is one which is annular on some points along the central path but total at others. Hybrid eclipses may be annular-total (starting annular, typically in the west, then becoming total), total-annular, or annular-total-annular. The next hybrid eclipse will occur on Nov 14, 2031 and is of type a-t-a, beginning annular in the northwest Pacific, becoming total in the mid-Pacific, then annular once again as it approaches Panama. For an observer at any point along the Central Path, a hybrid eclipse is either annular or total.

Lunar Limb ‡

The visible edge of the Moon as seen by an observer. The lunar limb appears uneven, due to prominent mountains and deep valleys on the Moon’s surface. This gives rise to Baily’s Beads and chromospheric beading. The exact profile of the lunar limb varies by time and observer location due to lunar libration. To predict Baily’s Beads, it is necessary to know the lunar libration for a given time and observer location and then the corresponding lunar limb profile.

Lunar Libration

Lunar libration refers to periodic variation of the Moon’s position as observed from Earth. It is made up of multiple components: libration in lunar longitude and latitude, up to 7.9° and 6.7° respectively (although libration in latitude experienced during an eclipse is more constrained due to geometry), and parallax libration arising from differences in the observer’s location on Earth.

Magnitude ‡

The fraction of the Sun’s angular diameter covered by the Moon. Values range from 0.00 to ~1.12. In general discussion of an eclipse, magnitude typically refers to the highest magnitude observed for the event at maximum eclipse (either globally or at a specific location), but it can be calculated for any instant of time during the eclipse. In The Photographer’s Ephemeris, we display the magnitude at the time of maximum eclipse in the timeline and the instantaneous magnitude in the eclipse simulator.

Maximum Eclipse ‡

The moment of greatest magnitude for the eclipse at the selected location. This typically applies to the local circumstances, and is distinct from Greatest Eclipse, which is the time and location where the largest magnitude of an eclipse is observed.

Obscuration ‡

The percentage of the Sun’s disk (photosphere) that is obscured by the Moon, between 0% and 100%. Like magnitude, the term may refer to obscuration at the time of maximum eclipse, or to instantaneous obscuration.

Partial Eclipse ‡

In a partial eclipse, magnitude is less than 1.00 at all times, and obscuration never reaches 100%. Unlike annular eclipses, in a partial eclipse the Moon never moves entirely within the limb of the Sun’s photosphere. During any Total, Annular or Hybrid eclipse, only a partial eclipse or no eclipse is visible to observers outside the central path.

Photosphere

The outer boundary of the Sun or a star, from which light is emitted. Do not look directly at the photosphere without certified eye protection. Use a solar filter to protect cameras and telescopes. It is safe to observe totality without eye protection because, by definition, the photosphere is fully obscured by the Moon.

Second Contact, C2 ‡

The moment in a total eclipse when the Moon completely obscures the Sun — this is the start of totality. In the case of an annular eclipse, this is when the Moon moves completely within the bounds of the Sun’s disc.

Third Contact ‡

The end of totality or annularity. The Moon no longer completely obscures the Sun (total) or its limb moves outside the bounds of the Sun’s disk (annular).

Total Eclipse ‡

A solar eclipse where the magnitude exceeds 1.00 and obscuration reaches 100% - i.e., the Sun’s photosphere is fully obscured by the Moon for some period of time for observers located along a contiguous path on the Earth’s surface, and where the eclipse is not annular at any point (see Hybrid eclipse). For other locations a partial or no eclipse will be visible.