Rating a Lunar Eclipse - The Danjon Scale

Lunar eclipses can vary a good bit in brightness and appearance and so it became necessary to create a way of rating the Moon when it was in totality. Early in the twentieth century, the French astronomer André Louis Danjon devised a clever five-point scale for rating the darkness of a total lunar eclipse. Danjon's scale worked so well that it has since gone on to become the standard by which all total lunar eclipses are judged.

Why is there such variation in the appearance of the Moon during an eclipse? It has mostly to do with the Earth's atmosphere. Earth itself blocks all direct light from the Sun but our atmosphere refracts some of the sunlight into the shadow.

The contents of Earth's atmosphere vary too and this affects refracted sunlight also. There are varying amounts of water from clouds and precipitation in the atmosphere and also solid particles such as dust, organic particles and most importantly volcanic ash. This material filters the sunlight before it gets refracted into the Earth's shadow.

As an example, if there has been a recent volcanic eruption on Earth, spewing lots of volcanic ash into the atmosphere, lunar eclipses would likely be very dark for as long as several years. Lunar eclipses can also be darkened by substantial cloud coverage along Earth's limb.

Below is the Danjon scale for rating lunar eclipses.

You can rate the lunar eclipse on the Danjon scale by using binoculars, a small telescope or just your eyes. The time to make your observations is during mid-totality. You may also want to observe the Moon just after it has passed into the penumbra and just before the end of totality. This allows you to assign an 'L: value to the outer umbra.

When you make your observations be sure to record how you made it, such as with a telescope or naked eye. You also need to record the time of your observation. It is also useful to make notes about variation of colour as the Moon passes through different parts of the umbra, and how sharp the edge of the shadow is. Details can also be noted about the visibility of lunar features.

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