Total Lunar Eclipse
During a total lunar eclipse the Moon passes through the Earth's shadow. It is then often visible as a copper-red orb in the night sky.
Total lunar eclipses explained
Although the Moon is a dark object, it can be seen in the sky most of the time because its surface reflects the Sun's rays back to Earth. During a total lunar eclipse, the Earth moves between the Sun and the Moon, so no direct sunlight gets through to the Moon and its entire visible surface is enveloped in the darkest, central part of the Earth's shadow - the umbra.
Unlike solar eclipses, which can only be seen along a narrow path on Earth, total eclipses of the Moon can be observed all across the night side of Earth because observers are situated on the same celestial body that casts the shadow. For this reason, the probability to witness a lunar eclipse from any one point on Earth is much higher compared to solar eclipses, even though both occur in similar intervals.
What does it look like?
The Moon is still visible in the night sky during totality. Although the Earth blocks all direct sunlight from the Moon's surface, some rays find their way onto the Moon via the Earth's atmosphere. Parts of the sunlight's spectrum are blocked or filtered out during this process, red being the remaining frequency under normal circumstances. This accounts for the usual appearance of a totally eclipsed Moon as a copper-red orb. However, depending on the amount of dust and clouds in the Earth's atmosphere, the color can change to yellow, orange, or brown.
Before totality, the Earth's shadow slowly grows across the Moon, like during a partial lunar eclipse. After totality, this process is reversed.
When does a total lunar eclipse occur?
A total lunar eclipse can be observed at night and during Full Moon when
- the Moon is near one of its orbital nodes so Sun, Earth and Moon form a straight line,
- and the observer is located on the night side of Earth.
The Moon's orbit and lunar nodes
The Earth revolves around the Sun and the Moon circles the Earth. During Full Moon, the Earth passes roughly between Moon and Sun. However, in most cases the three celestial bodies do not form a completely straight line, so the Moon is not eclipsed.
The reason why total lunar eclipses do not happen every Full Moon is that the lunar orbital plane - the imaginary flat surface whose outer rim is formed by the Moon's path around Earth - runs at an angle of approximately 5 degrees to the Earth's orbital plane around the Sun (ecliptic). The points where the two orbital planes meet are called lunar nodes. Only if the Moon appears near one of the two lunar nodes during Full Moon can a total lunar eclipse be observed from the Earth's night side.
The Earth's shadow
Like any other object's shadow, the Earth's shadow consists of three different areas: the innermost and darkest part (umbra), the lighter, outer part (penumbra), and a partly shaded area beyond the umbra (antumbra). During a total lunar eclipse, the Moon passes through the Earth's umbra.
Did you know...?
The size of the eclipsed portion of the Moon's surface (magnitude) is the same irrespective of the observer's location on the night side of Earth. However, because observers on the southern hemisphere stand “upside-down” compared to observers on the northern hemisphere, they also see the Moon “upside-down”. The orientation of a lunar eclipse and the direction in which the shadow appears to move across the Moon's surface can therefore vary according to latitude.