2015 full blue moon

The “Full Moon” is never full

Every time I take pictures of the full moon up close I notice it’s never quite full.  It always looks full with the naked eye but you can never be sure, it looks like it’s slightly off.  Turns out it is.  You’re not imagining things, a full moon is almost never full.

I took some astrophotography pictures through my telescope this weekend hoping to get a nice picture of this year’s “Blue Moon”.  Which isn’t blue BTW, it just means the second full moon of the calendar month; in other words, just another full moon.  But it is a cool event and a reason to take a picture.

Working the picture in Post Production I noticed the same slight edge of the moon not being full I always seem to get.  I took it on the day of the “full moon” but because of my helpful SunSurveyor app I knew ahead of time it wasn’t at 100%.  According to the app it was 99.8% full when I shot it, and would be 99.86% at moonset; the next morning when it rose it would be 99.26% and waning.

2015 full blue moon
The July 31st 2015 Blue Moon

I noticed this happens all the time so I started to wonder if it ever was 100% at some place on the earth after it set for me.  Turns out it’s not, due to orbital mechanics the moon is rarely at 100% full from the earth.  The orbit of the moon is tilted 5 degrees from the earth’s orbit to the sun, so the times that the sun, earth, and moon form a perfect line are kind of rare… And causes lunar eclipses.

So a true 100% full moon is a lunar eclipse (hey, I did get a picture of a few of those!), if you want a nice bright perfect circle full moon take your shot right before or after a lunar eclipse.

Lunar Eclipse
The cloudy night create bookends for the eclipse event.

Or just accept that you’ll always have a rough edge on your full moon shots.


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