Shadows: umbra and penumbra
Every eclipse involves a shadow. Because the Sun is a broad disk rather than a single point, the shadow of Earth or the Moon has two parts:
- The umbra, the dark central cone where the Sun is completely blocked.
- The penumbra, the lighter outer region where only part of the Sun is blocked.
Which part of the shadow falls on the Moon or on Earth decides the type of eclipse.
Solar eclipses
A solar eclipse happens at new Moon, when the Moon passes between the Sun and Earth and its shadow falls on Earth. It can only be seen from where the shadow lands, which is a limited part of the planet. See what is a solar eclipse for the basics.
| Type | What happens | What you see |
|---|---|---|
| Total | The Moon’s umbra reaches Earth | The Sun is fully covered; the corona appears |
| Annular | The Moon is too far to cover the Sun | A bright ring of sunlight around the Moon |
| Hybrid | Total in some places, annular in others | Depends on where you are along the path |
| Partial | Only the penumbra touches Earth, or you are outside the central path | A bite taken out of the Sun |
The difference between total and annular comes from distance. The Moon’s orbit is an ellipse, so its apparent size changes. Near its closest point it looks slightly larger than the Sun and can cover it completely. Near its farthest point it looks slightly smaller and leaves a ring.
Every solar eclipse, of any type, is partial for most observers, since the central path is narrow and the partial zone is wide. Looking at the Sun without proper protection is never safe except during the brief total phase of a total eclipse; read solar eclipse eye safety before any solar eclipse.
Lunar eclipses
A lunar eclipse happens at full Moon, when Earth sits between the Sun and the Moon and Earth’s shadow falls on the Moon. Anyone on the night side of Earth can see it, and it is safe to watch with the naked eye.
| Type | What happens | What you see |
|---|---|---|
| Total | The whole Moon enters Earth’s umbra | The Moon turns red or copper |
| Partial | Only part of the Moon enters the umbra | A dark, curved bite on the Moon |
| Penumbral | The Moon passes only through the penumbra | A subtle gray shading, often hard to notice |
Why eclipses do not happen every month
There is a new Moon and a full Moon roughly every 29.5 days, yet eclipses are much rarer. The Moon’s orbit is tilted about 5 degrees relative to the plane of Earth’s orbit, so most months the Moon passes above or below the Sun or Earth’s shadow.
Eclipses can only happen when a new or full Moon falls near one of the two points where the Moon’s orbit crosses Earth’s orbital plane. These windows, called eclipse seasons, come around roughly every six months. Each season usually brings one solar and one lunar eclipse about two weeks apart, and sometimes three eclipses.
Relative frequency of solar eclipse types
Over long periods, solar eclipses divide up roughly like this:
- Partial: about 35 percent
- Annular: about 33 percent
- Total: about 27 percent
- Hybrid: about 5 percent
For lunar eclipses, penumbral, partial and total eclipses each make up roughly a third of the total, with total eclipses slightly the least common, although the exact split varies from century to century.
The saros cycle
Eclipses repeat in a pattern called the saros, which lasts about 18 years, 11 days and 8 hours. After one saros, the Sun, Earth and Moon return to almost the same arrangement, so a similar eclipse occurs. The extra eight hours mean Earth has turned an extra third of a rotation, so the repeat is visible from a region about 120 degrees of longitude to the west.
Related events
A few events resemble eclipses but are classed separately. In a transit, a planet crosses the Sun’s face as a small black dot, as with the transit of Mercury and the transit of Venus. In an occultation, the Moon or a planet passes in front of a star or another planet and hides it completely. For dates of every upcoming solar and lunar eclipse, see the eclipse calendar.