Why in the news?

  • People of Asia and Europe witnessed the Blood Moon incident on September 7th, 2025.

Blood Moon

  • What is it?
    • A Blood Moon occurs during a total lunar eclipse, when the Earth comes directly between the Sun and the Moon.
    • Instead of turning completely dark, the Moon appears reddish in color.
  • Scientific Reason: The red tint is due to Rayleigh Scattering- Earth’s atmosphere filters shorter blue wavelengths and bends the longer red wavelengths towards the Moon.
  • How it happens?:
    • A Blood Moon takes place when the Earth positions itself directly between the Sun and the Moon.
    • Due to a phenomenon known as Rayleigh scattering, the Moon appears red rather than turning completely dark.
    • This effect occurs because Earth’s atmosphere scatters shorter blue wavelengths while allowing longer red wavelengths to pass through, casting a reddish glow on the Moon.
    • During the total phase of the eclipse, the Moon is fully engulfed in the Earth’s darkest shadow, called the umbra.
    • When inside the umbra, the Moon glows red-orange, giving rise to the term “Blood Moon.”
    • The exact hue- ranging from red to orange or copper- varies depending on atmospheric conditions and the level of light pollution.

Rayleigh Scattering

  • What is it?
    • Rayleigh Scattering is the scattering of light (or other electromagnetic radiation) by particles much smaller than the wavelength of light.
  • Scientific Basis
    • Proposed by Lord Rayleigh (1871).
    • Intensity of scattering is inversely proportional to the fourth power of wavelength
    • Shorter wavelengths (blue, violet) scatter much more strongly than longer wavelengths (red, orange).
  • Examples Other than Blood Moon
    • Blue Sky:
      • Sunlight interacts with nitrogen and oxygen molecules in the atmosphere.
      • Blue light scatters more strongly than red light, making the sky appear blue.
    • Red Sunsets/Sunrises:
      • When the Sun is low on the horizon, light travels through a greater thickness of the atmosphere.
      • Most shorter wavelengths are scattered away, leaving red/orange hues.
    • Ocean Colour
      • Partly due to Rayleigh scattering of light by water molecules (though absorption also contributes).