why do we have eclipses on earth

AN graphic by Greg Smye-Rumsby. Solar eclipses are relatively rare; they happen when the Moon moves directly and precisely in front of the Sun. Why are they so infrequent, and why do solar eclipses always come in pairs with lunar eclipses? It is all down to a mix of orbital alignments and cosmic coincidences. The Moon is able to cover the Sunвs disc precisely because the Sun is about 400 times bigger than the Moon, and also about 400 times further away, meaning they have the same size in our sky. Thatвs a huge coincidence! With all that sky above our heads though, what causes the Sun and the Moon to be in the same position in the sky at the same time? Itвs all a question of orbits. Earth orbits the Sun along a plane that we call the ecliptic в all of the planets orbit along this plane, more or less. The Moon, however, orbits Earth at an angle that is tilted to the ecliptic, by about five degrees.


This means that on each orbit around Earth (a lunar orbit lasts 27 days), the Moon only crosses the ecliptic at two locations and these are the only two opportunities for an eclipse, be it a solar or a lunar eclipse. A picture of the partial eclipse of 23 October 2014 taken using an HTC mobile phone at the eyepiece of a filtered 125mm refractor. Image: Nick James. Now, an eclipse of either variety can only happen when the Sun, Moon and Earth are in a line. Usually, the Moon crosses the ecliptic when they are not in a line, so we do not see an eclipse. However, on odd occasions в roughly once every 18 months on average в the Moon crosses the ecliptic at a time when it is aligned with Earth and the Sun. If this crossing takes place when the Moon is between Earth and the Sun, we see a solar eclipse. Two weeks later the Moon moves to the other side of Earth, with the alignment still the same, causing the Moon to slip into Earthвs shadow.


Here on Earth, we see a lunar eclipse. So solar and lunar eclipses always come in pairs. This year, following the dramatic total solar eclipse on 20 March, a total lunar eclipse will occur on the night of 4/5 April, which will be visible from the Southern Hemisphere, in particular Australia and New Zealand. In the, find out everything you need to know about eclipses. Also this month, we go behind the scenes at Stargazing Live and celebrate 25 years of the Hubble Space Telescope. Get your copy in the shops or. Never miss an issue by. Also available for
and. A new NASA video explains what determines when Earth experiences solar eclipses and why they don t happen every month, as the moon crosses between the sun and the Earth.


To, which will cross the continental United States on Aug. 21, NASA illustrated how the moon orbits the Earth and why the natural satellite s shadow sometimes lines up just right to move across Earth s surface, effectively blotting out the sun for a short period of time. Because the moon s orbit wobbles up and down with respect to the Earth, the satellite s shadow can be too high or too low to cause a solar eclipse. If the moon partially covers the sun for a given location, that area experiences a partial solar eclipse. But if the moon fully covers the sun, it will cause either a total solar eclipse or ; that s when a bright ring of light is still visible in the sky around the dark silhouette of the moon. Email Sarah Lewin at slewin@space. com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article on Space. com.

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