Century's longest lunar eclipse on 27 July: How is redness of the Moon measured

The appearance of the Moon during a total lunar eclipse can vary from one eclipse to the next.

With the longest total lunar eclipse of the 21st century scheduled for 27 July – and other celestial events also lined up this month – stargazers have a busy calendar in the coming weeks. And if the skies remain clear, this can be a spectacle of a lifetime.

The eclipse will be visible in parts of South America, most of Africa, and West and Central Asia. For viewers in India, the eclipse, both partial and the total, will be visible in their entirety from all parts of the country. The partial eclipse of the Moon will start around 11.52 pm IST, with the total eclipse beginning at 1 am on 28 July.

The lunar eclipse, which will last one hour and 43 minutes, will also feature a "Blood Moon" — which is a non-scientific term used to refer to the red tinge on a fully eclipsed Moon. It will be preceded and followed by partial eclipses lasting over an hour.

Why is it called a 'Blood Moon'

The appearance of the Moon during a total lunar eclipse can vary enormously from one eclipse to the next.

Atmospheric conditions during an eclipse (including dust, organic debris and temperature) decide what shade of colour, in the spectrum of copper brown to deep red, the Moon dons. This is because the shorter-wavelengths in sunlight (like violets, blues and greens) get scattered while passing through the edges of Earth's atmosphere.

The 'Super Blue Blood Moon' on 31 January. Reuters

The 'Super Blue Blood Moon' on 31 January. Reuters

However, longer wavelengths (like yellows, oranges and reds), are refracted, or bent around the circumference of our planet and reach the Moon's surface while under Earth's shadow. This effect, which is known as Rayleigh scattering, gives the Moon its characteristic reddish-orange tinge during a lunar eclipse. Rayleigh scattering is the same phenomenon that explains the blue colour of the sky, and the red sunsets.

How is redness measured?

French astronomer A Danjon proposed a five-point scale to evaluate the visual appearance and brightness of the Moon during total lunar eclipses. Values for the letter 'L', which stands for various luminosities, are defined as follows:

  • L = 0: Very dark eclipse
    Moon almost invisible, especially at mid-totality.
  • L = 1: Dark eclipse, gray or brownish in colouration
    Details distinguishable only with difficulty.
  • L = 2: Deep red or rust-colored eclipse
    Very dark central shadow, while outer edge of umbra is relatively bright.
  • L = 3: Brick-red eclipse
    Umbral shadow usually has a bright or yellow rim.
  • L = 4: Very bright copper-red or orange eclipse
    Umbral shadow has a bluish, very bright rim.

The assignment of an 'L' value to lunar eclipses can be done using the naked eye, binoculars or a small telescope near the time of mid-totality.

Lunar eclipses are completely safe to view with the naked eye.

An eclipse of the moon takes place only at full moon. Whenever the sun, Earth and Moon come in a perfect straight line, as the sun's rays fall on the Earth and its shadow falls onto a patch of space, and only when moon enters that patch of shadow can we see a lunar eclipse.

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