Eclipse pages often include numbers that sound similar but measure different things. Magnitude, obscuration, and gamma are three of the most useful. Together, they explain how deep an eclipse is, how much sunlight is covered, and how centrally the Moon's shadow passes Earth.
They are easy to mix up because all three describe eclipse geometry. The simplest way to remember the difference is this: magnitude is about a line, obscuration is about an area, and gamma is about the shadow's path relative to Earth.
Eclipse magnitude
Solar eclipse magnitude describes the fraction of the Sun's diameter covered by the Moon at maximum eclipse.
If the Moon covers half the Sun's diameter, the magnitude is about 0.5. If it covers the full diameter, the magnitude is about 1.0. In a total eclipse, the Moon may appear slightly larger than the Sun, so the event's maximum magnitude can be greater than 1.
Magnitude is not the same as brightness drop. Covering half the Sun's diameter does not cover half its area. That is why obscuration exists as a separate number.
Eclipse obscuration
Solar eclipse obscuration is the fraction of the Sun's visible disk area covered by the Moon.
This is often closer to what people expect when they ask "How much of the Sun is blocked?" A partial eclipse with a magnitude of 0.5 covers a line across half the Sun's diameter, but the covered area can be much less than half depending on the overlap.
During totality, obscuration reaches 100% because the Sun's bright disk is fully covered. During an annular eclipse, obscuration can be very high, but it is still less than 100% because a ring of photosphere remains visible.
That is also why an annular eclipse is never safe to view without certified solar filters. A small uncovered ring still emits intense sunlight.
Gamma value
Gamma describes how far the Moon's shadow axis passes from Earth's center at greatest eclipse. It is measured in Earth radii.
A gamma near 0 means the shadow axis passes close to Earth's center, so the eclipse is very central. A larger positive or negative gamma means the axis passes farther north or south of Earth's center. If the shadow misses Earth entirely, there is no central total or annular path.
Gamma is an event-level geometry number, not a local viewing statistic for your town. It helps explain why some eclipses have long central paths while others skim Earth near the poles or remain partial.
Why the numbers disagree
Magnitude and obscuration can feel like they should move together, and often they do, but they are not interchangeable.
Imagine sliding one circle across another. The line overlap grows differently from the area overlap. Near the beginning of a partial eclipse, a small bite may have a noticeable diameter but not much area. Near maximum, a small change in alignment can affect the remaining bright area strongly.
Gamma tells a different story again. It describes where the shadow axis passes relative to Earth as a whole. A low gamma can produce a central eclipse, but your local magnitude and obscuration still depend on where you stand inside or outside the path.
Local numbers matter most for planning
For eclipse planning, the local values are what you use on the ground:
- Contact times tell you when the eclipse begins, reaches maximum, and ends.
- Magnitude tells you how deep the disk overlap becomes.
- Obscuration tells you how much of the Sun's area is covered.
- Sun altitude tells you whether terrain or buildings may block the view.
- Duration tells you how long totality or annularity lasts, if your location is inside the central path.
Those values change from one place to another. A city near the centerline can have a much longer totality duration than one near the edge. A city outside the path may still have high obscuration but no totality at all.
How to read an eclipse map
Start with the path. If you want totality or annularity, you need to be inside the central path. Then check local contact times and Sun altitude. Finally, use magnitude and obscuration to compare partial-eclipse locations or to understand how deep the eclipse will be where you are.
Gamma is most useful when comparing eclipses in a catalog. It helps explain whether the shadow passes centrally across Earth or grazes it.
Sources and related guides
- NASA GSFC's eclipse publication notes define magnitude, obscuration, and gamma.
- NASA GSFC explains how eclipse predictions compute local circumstances from the underlying geometry.
- NASA's eclipse guide explains why totality occurs only inside a long, narrow path.
- Related SolarWatch guides: how eclipse predictions work, the Moon's elliptical orbit, eclipse contact times, and the path of totality.
See it in SolarWatch
SolarWatch shows local eclipse circumstances for the selected point on the map. Use magnitude, obscuration, contact times, Sun altitude, and duration together instead of relying on one number to decide whether a location is worth observing.