How solar and lunar eclipses happen: the simple science

Darkness falling in the middle of the day, or the full moon turning red — in the old days such sights frightened people. Today we know that solar and lunar eclipses are a play of shadows between the Sun, the Earth and the Moon, and scientists can give the exact dates of eclipses hundreds of years ahead. Here is the simple science behind them.

How does a solar eclipse happen?

When the Moon comes between the Sun and the Earth, the Moon's shadow falls on part of the Earth, and from there the Sun looks fully or partly covered. This can only happen at new moon (amavasya), because only then is the Moon between the Sun and the Earth. A remarkable coincidence makes it possible — the Sun is about 400 times bigger than the Moon but also about 400 times farther away, so in the sky the two look almost the same size.

How does a lunar eclipse happen?

When the Earth comes between the Sun and the Moon, the Earth's shadow falls on the Moon. This happens only at full moon (purnima). In a total lunar eclipse the Moon does not vanish but turns a coppery red, called a "blood moon". The reason is that as sunlight passes through the Earth's atmosphere, its blue part is scattered and the red light bends round to reach the Moon — the same science that makes sunsets red. Read Why is the sky blue and the sunset red.

Did you know? About 1,500 years ago the Indian mathematician and astronomer Aryabhata explained in his "Aryabhatiya" that eclipses are caused not by Rahu and Ketu swallowing the Sun or Moon but by the shadows of the Earth and the Moon. Read the Aryabhata satellite and the story of its name.

Why isn't there an eclipse every month?

There is a new moon and a full moon every month, yet not every one brings an eclipse. The reason is that the Moon's orbit is tilted by about 5 degrees to the Earth's orbit. In most months the Moon passes slightly above or below the Sun–Earth line. An eclipse happens only when a new or full moon falls just as the Moon is near one of the two points where the orbits cross (called Rahu and Ketu in Indian tradition). That is why there are usually only 4 to 7 eclipses a year.

How to watch an eclipse safely

• Never look at a solar eclipse with the naked eye — even in a partial eclipse, the Sun's glare can permanently damage the retina.
• Sunglasses, X-ray film and smoked glass are not safe. Use only glasses with certified solar filters.
• The easiest way: make a small hole in a piece of card and look at the Sun's image on the ground or on paper (a pinhole projector).
• A lunar eclipse is completely safe — it can be watched with the naked eye, binoculars or a telescope.

Memorable eclipses in India

The total solar eclipse of 22 July 2009 was the longest total solar eclipse of this century; in many parts of India — such as Varanasi, Patna and Guwahati — day turned to darkness for a few minutes. Observations made in India during the eclipses of 1868 and 1898 also led to big discoveries about the Sun — during the eclipse seen at Guntur in 1868, signs of a new element were found in the Sun, later named "helium".

Test yourself: Class 5 GK quiz: the solar system · Read more: Science.