Eclipse 2028 practical guide

Eclipse Glossary: Terms Explained in Plain Language

The guides on this site use precise astronomical terms deliberately, because loose language is exactly where eclipse misinformation starts. This page defines each one in plain words.

By Mark Mayo, who has stood in the Moon's shadow at three total solar eclipses. Published 27 September 2026. Updated 27 September 2026.

Quick answer

Umbra and penumbra describe the two parts of the Moon's shadow; totality, magnitude and obscuration describe how completely the Sun is covered; contacts, centre line and Saros cycle describe timing and recurrence; the rest cover safety and calculation terms used throughout the site.

Every term on this page ultimately traces back to these two shadow regions.

Shadow and coverage

Umbra: The Moon's full, dark shadow. An observer standing inside the umbra sees a total solar eclipse. It is a narrow region, typically only tens to a few hundred kilometres wide, that sweeps across Earth's surface as the Moon moves.
Penumbra: The Moon's partial shadow, surrounding the umbra and much larger. An observer inside the penumbra but outside the umbra sees a partial eclipse: some fraction of the Sun remains visible throughout.
Totality: The period during a total eclipse when the Moon's disc completely covers the Sun's photosphere. Only during totality is it safe to remove solar viewers and only during totality is the corona visible.
Magnitude: The fraction of the Sun's diameter covered by the Moon at maximum eclipse, expressed as a decimal. A magnitude of 1.0 or greater at a given coordinate means that point lies inside the path of totality.
Obscuration: The fraction of the Sun's visible area covered at maximum. Unlike magnitude, which is a one-dimensional measurement across the disc, obscuration accounts for the full two-dimensional overlap between the Sun and Moon's discs.

Timing and geometry

First contact: The moment the Moon's disc first touches the Sun's disc, marking the start of the partial phase.
Second contact: Inside the path of totality only: the moment the Moon's disc fully covers the Sun, marking the start of totality.
Third contact: Inside the path of totality only: the moment the trailing edge of the Sun reappears, marking the end of totality.
Fourth contact: The moment the Moon's disc fully clears the Sun's disc, marking the end of the eclipse.
Centre line: The line running through the middle of the path of totality, where totality lasts longest for that eclipse. Duration falls the further a point sits from the centre line, reaching zero at the path's edge.
Path of totality: The narrow band on Earth's surface that the umbra crosses during a given eclipse. Use the interactive map to check whether a specific coordinate falls inside it for 22 July 2028.
Saros cycle: A period of about 18 years and 11 days after which the Sun, Moon and Earth return to approximately the same relative geometry, producing a similar eclipse shifted roughly 120 degrees in longitude. Eclipses in the same Saros series are numbered sequentially; see the Dunedin 1163 eclipse page for how this explains long gaps between eclipses at one location.

What you see

Corona: The Sun's faint outer atmosphere, normally invisible against the photosphere's brightness. It becomes visible only during totality, when the photosphere is fully covered.
Diamond ring effect: A brief, bright point of remaining sunlight visible just before second contact or just after third contact, resembling a diamond on a ring of the thin solar corona.
Baily's beads: Beads of sunlight visible for a few seconds around second and third contact, caused by sunlight passing through valleys on the Moon's uneven limb just before or after totality.
Annular eclipse: A different type of eclipse, not occurring in 2028, where the Moon is far enough from Earth in its orbit that its disc is too small to fully cover the Sun, leaving a visible ring even at maximum.

A schematic diamond-ring flare at second contact; the animation pauses automatically if your system prefers reduced motion.

Safety and calculation

ISO 12312-2: The international safety standard for solar viewers and eclipse glasses. See the solar viewer guide for how to verify a pair meets it.
Besselian elements: A set of mathematical coefficients that describe an eclipse's geometry over time, used to calculate contact times, magnitude and duration for any coordinate. This site's calculation method explains how they're applied.
Delta T (ΔT): The small, slowly-changing difference between Terrestrial Dynamical Time (used in astronomical calculations) and Universal Time (used on clocks), arising from the gradual, slightly irregular slowing of Earth's rotation. Eclipse predictions must estimate its future value.
Ephemeris: A table of calculated positions for an astronomical object at successive times, the underlying data type behind any eclipse-timing calculation.

Frequently asked questions

Do I need to know these terms to use the site?

No. The city guides and interactive map are written to be usable without any background. This page is here for readers who want to understand exactly what a term like 'magnitude' or 'centre line' means once they encounter it.

Why does obscuration differ from magnitude?

Magnitude is a one-dimensional measure, the fraction of the Sun's diameter covered. Obscuration is a two-dimensional measure, the fraction of the Sun's area covered. The two numbers are close but not identical because the Sun and Moon rarely have exactly equal apparent size.

This page follows the site's correction policy: factual errors are fixed and the update date above changes so the fix is visible, rather than silently edited.

Put these terms to use

Check a specific coordinate against the path of totality on the interactive map, read the full calculation method behind this site's numbers, or start with how eclipses work for the fuller explanation behind these terms.