The blue supermoon

  • The last time moon conditions were similar was March 31st 1866

Many moons ago, before calendars were invented, people used to count the year by the full moon. And as there are usually 12 full moons each year then this was a convenient, if not very accurate, way of marking time.

Each of the full moons was given a name such as the Harvest Moon and Hunter's Moon for example. But as the period between full moons is about 29.5 days then it is possible for a month to have two full moons.

This extra full moon is called a Blue Moon. The last full moon occurred on January 2nd this year and so we are able to squeeze another full moon in to this month on January 31st, making it a Blue Moon.

But this one will be special as it is also a supermoon. The moon orbits around the Earth not in a true circle but in an ellipse and so at times it is closer to the Earth (perigee) and sometimes it is further away (apogee).

At perigee the moon is 225,623 miles and at apogee it is 252,088 miles, that's a difference of almost 26.5 thousand miles.

If perigee occurs within a day or so of when the moon is full it is classed as being a supermoon.

This Blue Moon will be at perigee just a few hours before it reaches full moon and so will appear much bigger and brighter than usual.

And if that wasn't enough to get excited about there will also be a total eclipse of the moon happening on the same day!

The Blue Supermoon will pass into the shadow cast by the Earth. And with Earth in between the Sun and the Moon, sunlight has to pass through the Earth's atmosphere, which scatters the green to violet light more than it scatters the red light.

So the Moon appears red - a Blood Moon.

The last time that all three events occurred together was over 150 years ago on March 31st 1866.

Unfortunately we won't get to see the eclipse here in Spain as it happens during our daytime and only people living on the other side of the world will get to see it.

But it will be worth trying to see the supermoon on both Monday 30th and Tuesday 31st as both nights are equally close to the exact point of full moon.

If you manage to see the moon just as it is rising then it will appear much larger than normal; this is an optical illusion and is thought to be caused because you have objects such as trees and houses in view to give a sense of scale to the moon.