“Wow! Look at that moon!” people say, when they see the full moon rising over the horizon, especially on warm summer and cool fall evenings. To our eyes, the moon seem gigantic, like somehow it had gotten closer to the earth recently. It’s a wonderful sight to behold, full of awe and wonder. And it’s another illusion. The moon isn’t bigger or closer, it’s just our eyes playing tricks on us again.
As I wrote yesterday, our eyes trick our brain into seeing what’s not really truly there. Yes, explanations sort of ruin the fun, but sometimes it’s nice to know. And the effect is the Moon Illusion.
There is an excellent article by Dr. Tony Phillips over on Science @ NASA that talks about the moon illusion, and how you might get fooled by it this weekend, when the full moon rises in the southeastern sky in the evening.
The full moon for June 30th rises at it’s southernmost point of the year. With the sun at it’s highest point in the sky for us in the northern hemisphere (the Summer Solstice was just over a week ago) the moon, which when it is in the “full” phase is 180° from the sun, is at it’s lowest point. Hence, the moon will rise in the southeast sky, and not get very high above the southern horizon all night.
Sunset on Saturday evening (for those in the Grand Rapids area) is at 9.25pm. The moon will rise at 10.07pm.
“Now wait” you say. “If the moon is opposite the sky from the sun, why doesn’t it rise at the same time the sun sets?” Good question. And for Saturday night, it is an easy answer. Even though the moon looks full, it really isn’t. The “point” where the moon is directly opposite the sun from earth takes place Saturday morning at 9.48am, over twelve hours before the moon rises. In that time, the moon moves just enough in its orbit to cause it to rise later than if it were exactly 180° from the sun.
But no matter the scientific and technical explanations, going out and seeing the huge, full moon rising in the evening is a sight to behold. And something to share with that special someone.
(Update: Astronomer Phil Plait, who owns the Bad Astronomy site, has written about the June 30th “big moon.” Jump on over and take a look at what he has to say about it.)