The definition
For a solar eclipse, magnitude measures how far the Moon’s disk reaches across the Sun’s disk, as a fraction of the Sun’s diameter. It is measured along the line joining the centers of the two disks.
- 0.0: the Moon just touches the edge of the Sun.
- 0.5: the Moon covers half of the Sun’s diameter.
- Close to 1.0: only a thin sliver of Sun remains.
- 1.0 or more: the Sun is completely covered, which is totality.
Eclipse predictions usually quote the magnitude at maximum eclipse for a location, the moment when coverage is greatest. The value changes from place to place, so a single eclipse might have a magnitude of 0.3 in one city and 0.9 in another.
Magnitude vs obscuration
Many eclipse tables also give obscuration, the fraction of the Sun’s area that is covered. It is the better measure of how much sunlight is blocked. Because the Sun is round, obscuration is always lower than magnitude for a partial eclipse.
| Magnitude | Approximate obscuration |
|---|---|
| 0.10 | About 4% |
| 0.25 | About 14% |
| 0.50 | About 39% |
| 0.75 | About 69% |
| 0.90 | About 87% |
| 0.95 | About 94% |
These figures assume the Moon and Sun look about the same size, which is close to true for most eclipses.
So “half covered” in the sense of magnitude still leaves about 60 percent of the Sun’s surface shining. That is one reason people are often surprised at how bright the day stays during a partial eclipse. The light only drops noticeably once most of the Sun’s area is hidden.
Magnitude in total and annular eclipses
For central eclipses, magnitude takes on a slightly different meaning: it becomes the ratio of the apparent diameters of the Moon and the Sun.
- In a total solar eclipse, the Moon looks larger than the Sun, so the magnitude is above 1, for example 1.03 or 1.07. Larger values mean the Moon’s shadow is wider and totality tends to last longer.
- In an annular solar eclipse, the Moon looks smaller, so even at the center of the path the magnitude stays below 1, for example 0.94. The obscuration at that moment is about the square of the magnitude, so a 0.94 annular eclipse blocks about 88 percent of the Sun’s area and leaves a bright ring.
The Moon’s apparent size changes because its distance from Earth varies by more than 10 percent around its orbit. The Sun’s apparent size also changes slightly, as Earth is closer to it in January than in July.
Magnitude for lunar eclipses
Lunar eclipses use the same idea, but measured against Earth’s shadow:
- Umbral magnitude is the fraction of the Moon’s diameter inside Earth’s dark umbra. Above 1, the eclipse is total; between 0 and 1, it is partial.
- Penumbral magnitude is the fraction of the Moon’s diameter inside the fainter penumbra. A penumbral eclipse has an umbral magnitude below 0.
A total lunar eclipse with a high umbral magnitude places the Moon deep in the shadow, which usually makes it darker and the totality longer.
Reading an eclipse listing
When you see a line such as “partial eclipse, magnitude 0.62” for your city, it means that at maximum, the Moon will cover 62 percent of the Sun’s width. Check the eclipses page for upcoming events, and remember that any partial phase requires eye protection, as explained in solar eclipse eye safety.