The Big Full Moon

When I was a kid, I had the pleasure of seeing a huge, bright orange moon low on the horizon one autumn evening.  We were heading back after a day spent out on the boat and right in front of us was this huge gorgeous moon.  Only time will tell if I see another like it.

I would imagine that most people have seen similar large moons at some point in their lives.  But why does the moon sometimes look so much larger?

One's first thought might be that the moon is closer when it appears larger.  While this is true, it does not have much of an effect on the above phenomenon.  The moon's orbit, like those of the planets, is slightly elliptical rather than perfectly circular.  At the moon's minimum distance to Earth on its orbit (perigee), it is 26,465 miles (10.5%) closer than at its maximum distance (apogee).  When photos are taken comparing the size of the moon at both perigee and apogee side-by-side, the difference in size is noticeable.  However, it is too small of an effect for you to notice it by just looking up at the moon on any given night.

So if the moon's size isn't changing much, why can it appear so much larger at times?

Think about where the moon was in the sky the last time it noticed it being larger than usual.  Was it close to the horizon?  Were there any objects (buildings, trees, etc) nearby?  You most likely will have answered yes to one, if not both, of those questions.  This is the key to an explanation.

The large moon effect is simply an optical illusion.  When the moon is high above the horizon, nothing appears to be close to it.  The area surrounding it is vast and open making it appear smaller.  In contrast, the moon looks much bigger when it is seen next to a building or tree.  You know how large those objects are, even if they are distant.  Your brain can compare the moon's size with that of the objects near it, making it seem much larger than normal.  In the picture above, I managed to catch a plane and its contrails near a gibbous moon.  This gives you something with which to compare its size.

You can test this out quite easily.  On a night when the moon is full (or close to it), it will be low on the horizon just after sunset.  Go outside and compare it to the size of your thumb held up at arm's length.  A few hours later on the same night, go outside again and repeat the test.  You should find that the moon is still the same size compared to your thumb.

The other effect described at the beginning is the moon being bright orange in color rather than white.  This will be discussed in the next blog.

Reference
Earth's Moon: Facts & Figures by NASA