Watch for the famous ‘moon illusion’ with tonight’s Harvest Moon

With these being the very last few days of summer, the full moon that graces our skies tonight becomes the Harvest Moon. So named because it helped farmers to bring in their crops on time, it also marks a special time for sky watchers and gives a great opportunity to see an amazing optical illusion that makes the moon look exceptionally big.

The Harvest Moon has a long history, and it has special significance not only for different cultures, but also for astronomers and other sky watchers. The good people of Science@NASA put together this great video to explain all about this particular full moon:

The official time of the full moon will actually be at 7:13 a.m. ET tomorrow morning, so technically after sunrise for anyone in the eastern time zone. However, it will be close enough to full all night that most people won't even notice, so the 'late arrival' of the phase shouldn't diminish anyone's view.

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As the video mentions, the curious optical illusion known as the 'Moon Illusion' will likely be especially noticeable to anyone watching the moon as it rises tonight or sets in the morning, making the moon look huge when it's near the horizon.

There's been plenty of explanations for this effect, the most common one being that the atmosphere acts like a lens, magnifying the moon so that it looks bigger. However, that's not actually what's happening.

The effect is deeply rooted in how our brain interprets what our eyes see, and it's a great example of the Ponzo illusion. Check out the little image I put together in Photoshop to experience this illusion for yourself. At first glance, the upper red line probably looks longer than the lower red line, doesn't it?

It all comes down to how our brain perceives what we see, since it always interprets objects on the horizon to be farther away than objects overhead, even if those objects are exactly the same distance away from us. So, when we see the moon on the horizon, our brain thinks that it's far, far away and the same effect that creates the Ponzo illusion makes our brain pump up the moon to make it look huge.

If the weather is nice and you have a clear sky, try to get out to watch the moon rise tonight or the moon set tomorrow morning, to check it out for yourself. Even knowing what causes the effect, it doesn't diminish the sight of that huge swollen moon.

(Photo courtesy: Wikimedia Commons)

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