Danjon scale
Encyclopedia
The Danjon Scale of lunar eclipse
brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon
during a lunar eclipse. It was proposed by André-Louis Danjon when he was measuring the Earthshine on the Moon
. An eclipse's rating on the Danjon Scale is denoted by the letter L.
.
is important, but so too are the current conditions of the Earth's atmosphere
. While the Earth's shadow blocks any direct light from striking the Moon during a lunar eclipse, some light is refracted through the Earth's atmosphere giving a Moon a red hue.
The amount of light refracted affects the brightness of the moon at mid-eclipse, and this depends on several factors. Volcanic eruptions are one of the most significant - eruptions which spew significant amounts of volcanic ash
into the air are generally followed by several years of dark, deep red eclipses. The effect of the eruption of Mount Pinatubo
on subsequent lunar eclipses was very noticeable, with the eclipse of 9 December 1992 rated 0 on the Danjon Scale by many observers.
It is also thought that the solar cycle
has some effect on the darkness of lunar eclipses.
Lunar eclipse
A lunar eclipse occurs when the Moon passes behind the Earth so that the Earth blocks the Sun's rays from striking the Moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can only occur the night of a...
brightness is a five-point scale useful for measuring the appearance and luminosity of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
during a lunar eclipse. It was proposed by André-Louis Danjon when he was measuring the Earthshine on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. An eclipse's rating on the Danjon Scale is denoted by the letter L.
The scale
The scale is defined as follows:| L value | Description |
|---|---|
| 0 | Very dark eclipse. Moon almost invisible, especially at greatest eclipse. |
| 1 | Dark Eclipse, gray or brownish in coloration. Details distinguishable only with difficulty. |
| 2 | Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright. |
| 3 | Brick-red eclipse. Umbral shadow usually has a bright or yellow rim. |
| 4 | Very bright copper-red or orange eclipse. Umbral shadow has a bluish, very bright rim. |
Determining the value of L
Determination of the value of L for an eclipse is best done near mid-totality with the naked eye. The scale is subjective, and different observers may determine different values. In addition, different parts of the Moon may have different L values, depending on their distance from the center of the Earth's umbraUmbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
.
Factors affecting the value of L
Many factors can affect the appearance of the Moon during a lunar eclipse. The Moon's path through the Earth's umbraUmbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
is important, but so too are the current conditions of the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. While the Earth's shadow blocks any direct light from striking the Moon during a lunar eclipse, some light is refracted through the Earth's atmosphere giving a Moon a red hue.
The amount of light refracted affects the brightness of the moon at mid-eclipse, and this depends on several factors. Volcanic eruptions are one of the most significant - eruptions which spew significant amounts of volcanic ash
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
into the air are generally followed by several years of dark, deep red eclipses. The effect of the eruption of Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...
on subsequent lunar eclipses was very noticeable, with the eclipse of 9 December 1992 rated 0 on the Danjon Scale by many observers.
It is also thought that the solar cycle
Solar cycle
The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...
has some effect on the darkness of lunar eclipses.