On the night of September 27 at 10:47 p.m. EDT (September 28, 3:47 a.m. BST), we will be treated to a beautiful "Blood Moon" eclipse. This is more accurately known as a total lunar eclipse and occurs when Earth’s shadow completely covers the Moon, with the Sun behind us in respect to the Moon.
During the event, the particular refraction of sunlight through Earth’s atmosphere can cause the Moon to appear red. This is because the more of our atmosphere sunlight passes through, the more it shifts towards the red end of the spectrum, just like how a sunrise or sunset appears red. If Earth had no atmosphere, the Moon would appear dark during the eclipse.
With all three bodies in a line, the sunlight refracts twice and hits the Moon, casting red light upon it. How red it will appear varies from eclipse to eclipse, although it can look a quite brilliant color in the night sky.
The term Blood Moon seems to have come out of nowhere, though, and doesn’t have any scientific significance. This particular event is the last in a row of four total lunar eclipses, known as a lunar tetrad. The first was April 14 2014, the second October 7 2014 and the third April 4 2015.
Some have claimed this event heralds the end of the world, or other nonsense. Lunar tetrads are not that uncommon: there will be a total of eight this century, with one already having occurred in 2003-4. There have been more than 50 since the first century AD and, as you can probably guess, none of them caused Armageddon (as far as we can tell). This isn't the first "end of the world" scare this year: last month, NASA issued a statement that an asteroid would also not be destroying Earth any time soon.
That doesn’t take anything away from this event, though. It’s an amazing scientific phenomenon, showing the majesty of how Earth, the Sun and the Moon interact.
In the words of the late Leonard Nimoy in The Simpsons, the cosmic ballet goes on.