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EnglishWhat Is a Lunar Eclipse and Why Doesn't It Happen Every Full Moon?
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A lunar eclipse happens when the Sun, Earth, and Moon line up so that the Moon passes through Earth's shadow. This can only occur at full moon, but most full moons produce no eclipse because the Moon's orbit is tilted about 5° relative to Earth's orbit around the Sun. An eclipse requires the full moon to occur near one of the two points where the orbits cross (the nodes). When that condition is met, the Moon enters Earth's shadow, and the type of eclipse depends on how deep it goes.
The geometry: Earth's two shadows
Earth blocks sunlight and casts two nested shadow cones:
- Umbra — the inner cone where direct sunlight is fully blocked.
- Penumbra — the outer cone where sunlight is only partially blocked.
The Moon's appearance depends on which shadow it enters and how far into it the Moon travels.
Three types of lunar eclipse
| Type | What happens | What you see |
|---|---|---|
| Penumbral | Moon passes only through the penumbra | A subtle dimming, often hard to notice without comparing photos |
| Partial | Part of the Moon enters the umbra | A dark "bite" taken out of one edge of the Moon |
| Total | The entire Moon enters the umbra | The Moon turns a dim coppery red or orange (a "blood moon") |
During totality the Moon is not black. Sunlight refracted through Earth's atmosphere — mainly red wavelengths, because blue light is scattered out — still reaches the lunar surface. The exact color and brightness vary with atmospheric conditions such as dust and volcanic aerosols.
Why not every full moon?
If the Moon orbited in exactly the same plane as Earth's orbit around the Sun, every full moon would be a total lunar eclipse. In reality the lunar orbit is inclined by roughly 5°. At most full moons the Moon passes above or below Earth's shadow cone and is simply fully lit.
Eclipses cluster around eclipse seasons, the periods when the Sun is near one of the lunar nodes. Only then can the full moon fall close enough to the shadow to enter it. This is also why lunar and solar eclipses tend to come in pairs or close together in the calendar: both require the Sun to be near a node.
Who can see it, and do you need equipment?
A lunar eclipse is visible from the entire night side of Earth at the time of the event — a much wider area than for a solar eclipse. No filters or special equipment are needed; the Moon is safe to look at directly, and binoculars or a small telescope simply show surface detail and the shadow edge more clearly. The main practical requirement is that the Moon is above your horizon and the sky is reasonably clear.
Checking an eclipse for your location
MoonCalc (mooncalc.org) is an interactive tool for Moon data at a chosen location and date. Its page shows fields for latitude, longitude, height, and time zone, plus a time slider and a map view, and it reports Moon rise, culmination, set, distance, altitude, azimuth, and shadow length. The site also offers a lunar calendar and a date range extending into the distant past, and its help text explains that the yellow curve traces the Moon's path across the sky, with the Moon drawn higher toward the center when it is higher above the horizon.
To use it for eclipse planning:
- Enter your coordinates (or set them on the map) and time zone.
- Select the eclipse date.
- Move the time slider to the eclipse's peak and check the Moon's altitude and azimuth — a positive altitude means the Moon is above your horizon.
- Confirm rise and set times so you know the visible window.
The site does not state a price on the page; a voluntary donation option via PayPal is mentioned, but that should not be read as a statement that all features are free or require no login.