Area fraction and diameter fraction diverge
During a partial eclipse, the umbral boundary cuts a curved segment from the lunar disc. The covered area depends nonlinearly on the boundary position, so a diameter-based immersion value and an area-based coverage percentage generally differ. Fifty percent obscuration does not imply an umbral magnitude of 0.5, and the reverse conversion needs more geometry than one rounded display number provides.
For a total eclipse, the entire apparent lunar disc is inside the umbra and obscuration reaches 100 percent. Umbral magnitude can exceed 1 because it can express how far beyond full immersion the shadow extends along the reference diameter. Obscuration remains capped at the complete disc area and therefore cannot show that additional depth.
What the finder exposes
Each umbral eclipse has an obscuration fraction, shown as a rounded percentage. Penumbral eclipses do not have an umbral-covered area in the same sense.
Eclipse type, contact intervals, and obscuration are separate fields. Type states whether immersion reaches penumbral, partial, or total geometry; contacts state timing; obscuration states peak covered area. Keeping them separate prevents a high percentage from being mistaken for longer duration or better visibility at an observer's location.
Worked example
Worked obscuration comparison example
The calculator's neighboring 2026 results illustrate how the bounded obscuration field behaves:
- 3 March 2026 event
- total eclipse; 100% umbral obscuration
- 28 August 2026 event
- partial eclipse; 96.6% umbral obscuration
- Reported coverage unit
- fraction of apparent lunar-disc area
- Umbral magnitude
- not calculated or displayed by this tool
The total event saturates the area measure at 100 percent. A catalogue could still give it a magnitude above 1, but substituting 1.00 from obscuration would erase that diameter-depth meaning. The partial event's 96.6 percent likewise must not be read as magnitude 0.966.
Read before comparing
Reference conditions
- Obscuration describes global shadow geometry; local cloud, horizon, and daylight conditions are separate observer questions.
- Umbral-magnitude comparisons require that separate field from an identified external catalogue.
- Rounded percentages cannot be reversed into exact shadow-boundary positions or contact times.
Next step
Use the working calculator
Calculate another exact instant with the same reference frame and conventions described on this working sheet.