A lunar eclipse occurs when the Earth, moon and sun line up with the Earth in the middle. Since the moon's light is a reflection of the sun, during an eclipse, the Earth blocks this light and the moon is seen in a different colour. There are phases in a lunar eclipse, depending on how far the moon is from the Earth's shadow region. When the moon comes fully inside the shadow, it is called the full moon or blood moon eclipse.
Given the nature of the Earth and moon's orbits, eclipses are bound to happen every year, at least a minimum of four times. Lunar eclipses are more commonly visible than solar eclipses, because they last longer -- the moon's shadow on the Earth is much smaller than the Earth's shadow on the Moon.
The moon's orbit around the Earth is kind of similar to the Earth's orbit around the sun, in that it is elliptical in shape and there are times when the moon is closest and times when it is farthest. These times are known as the perigee -- closest to Earth -- and apogee -- farthest from Earth, and currently the Moon is two days away from its perigee position.
On Sunday night around 8.58 pm, the moon will slowly start inching towards the Earth's shadow, beginning the eclipse process. First, as a section of the moon enters the shadow, it will appear like a faint darkening of the moon's outer layer. Then, at about 10 pm, the partial lunar eclipse will occur, where only half of the moon will be visible. The exact time of the full lunar eclipse is 11 pm, when the moon will start appearing red in colour.
The reason for this redness is the scattering of white light, which is a phenomenon responsible for everything from pretty sunset colours to rainbows to more complex astronomical tasks like identifying celestial objects through space telescopes.
The atmosphere scatters white light in different wavelengths to reach our eyes; the sun itself emits white light; however, we see it as orange, red or yellow depending on how long it takes for the light to reach us. Colours like blue scatter more easily, which is the reason for blue skies during the day, but red light has a long wavelength, so we see it during sunset when the sun is farther from the Earth.
In similar fashion, the Moon usually appears grey because the sun's light reaches it directly with minimal scattering. During an eclipse, the sun's light has to go through the Earth to reach the moon. With this much atmospheric scattering, the only light that has a wavelength long enough to reach is red, and so we see the blood-red moon.
"It's as if all the world's sunrises and sunsets are projected onto the moon (during an eclipse)," explains the NASA website.
While a lunar eclipse is entirely safe to view from the naked eye and can be seen by anyone from their house, the Astronomical Society of India has released a list of public viewings of the eclipse across the country.
In Delhi, a group named Night Sky Hunters is organising a viewing in Pushp Vihar community park. Other astronomy enthusiasts and sky watch groups in Bengaluru, Mumbai, Mysuru and Chennai are also organising viewings as well as live streaming the event.
A red moon can often be seen without an eclipse, which happens when there is pollution, haze or dust particles in the atmosphere, which don't let white light through.
An interesting fact about eclipses is that they never come alone -- after the lunar eclipse on 7-8 September, a partial solar eclipse is expected to follow on 21 September. However, it won't be visible in India, and will be relegated mainly to Australia, New Zealand and Antarctica.