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Total lunar eclipses explained

The Moon does not have any light of its own, but its surface reflects the Sun's rays. During a total lunar eclipse, the Earth moves between the Sun and the Moon.

Illustration image
Total lunar eclipses occur on full Moon nights.
©iStockphoto.com/wrangel

Because of this, 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.

Visualizing a total lunar eclipse

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.

Upcoming Total Lunar Eclipses

DatesVisibility Map/Path of the eclipse
Oct 8, 2014
Sep 28, 2015
Jan 31, 2018
Jul 27, 2018
Jan 21, 2019
May 16, 2022
Nov 8, 2022

More details about upcoming Eclipses

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.

What happens if the Earth is not precisely between Sun and Moon?

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.

What happens if the Moon passes through the Earth's penumbra?

A tetrad of lunar eclipses

The April 15, 2014 total lunar eclipse is the first in a series of four consecutive total lunar eclipses, also known as a tetrad. The next three eclipses will occur on October 8, 2014, April 4, 2015 and September 28, 2015.

Italian Astronomer Giovanni Schiaparelli calculated that the occurrence of such tetrads is variable over centuries – some 300 year intervals have several total lunar eclipses in a series, while other such intervals do not have any. For instance, the years between 1852 and 1908 did not have any tetrads, whereas the next 3 centuries will have 17 tetrads.

According to NASA the current century – 2001 to 2100 will have 8 tetrads. The first eclipses in each of these tetrads will occur between the months of March and May.

What is a blood Moon?

The term, Blood Moon, has recently become popular while referring to the total lunar eclipses of the tetrad that will occur in 2014 and 2015. This term has no technical or astronomical basis and it is unclear where such a description comes from.

One theory about why these eclipses are being called Blood Moon eclipses comes from the fact that the full Moon can sometimes appear bright coppery red during a total lunar eclipse.

Many religious people believe that the upcoming tetrad has special significance because the eclipses coincide with important Jewish festivals. The two April lunar eclipses in 2014 and 2015 occur at the same time as Passover, while the October and September eclipses occur during the Feast of Tabernacle. Astronomers and scientists however do not believe that this is of any significance.

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.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

In this Article

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All about lunar eclipses

  1. Types of solar and lunar eclipses
  2. Total lunar eclipses
  3. Partial lunar eclipses
  4. Penumbral lunar eclipses
  5. How to view a lunar eclipse

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