Time the moon, or moon it - chase those illusions away!

At the June meeting of the Prime Time program Michael Twomey gave a report on the complex and puzzling phenomena of the moon illusion where the moon appears so large in the sky near the horizon, yet we know it is really much smaller. It's nt as simple an illusion as you may think and it got me thinking about a fun project. Why not measure the full moon by timing its rise? I’ve never tried this, but my first question to anyone wanting to give it a whirl is:

How long should it take the full moon to rise?

Hint: This is a new idea to me, but it seems to me that we know the size of the moon to be roughly 30 minutes, or half a degree. Knowing that, and knowing the Earth spins on its axis 360 degrees in 24 hours, it should be easy to calculate roughly how long it would take from the point the moon first breaks the horizon to the point it actually is fully visible.

This will vary somewhat with how far the moon is from us and what portion of the horizon it is on, but if we put this to the test and the answer is near our calculation, then it seems to me this is one more way to show that the apparent “huge” size of the moon when near the horizon is an illusion. Or perhaps it will confirm the illusion as unavoidable? I don’t know. But if you try it – or have some thoughts on it – please send me an email at gstone@umnassd.edu.

This month the full moon will rise about as far to the south as it gets. The full moon always appears directly opposite the sun in our sky. The Sun on June 30, the date of full moon, will set in the northwest very close to as far north as it gets, so it follows that the full moon will rise in the southeast and be close to as far south as it gets. Now comes the trick – how do you find it? That is, if you are going to time its rising, then you want to have a stop watch handy and you want to catch its first appearance on the horizon, so you need to be looking right where it is expected to rise. In fact, I plan to try this and I’ll scan the expected area with binoculars.

What is that area? Very close to azimuth 127 degrees, at least for Westport, MA. I plan to use a compass and wide field binoculars to scan the horizon in that vicinity on June 30, 2007 between 8:50 and 9 pm EDT. (My software predicts moon rise for 8:58, but I’m never entirely confident that will be right on the dot for the location I choose. I want a location with a clear eastern horizon, of course. )

As to the moon illusion phenomena, Michael suggested a couple neat tricks. The first – simply look at the moon through an open cardboard tube, or for that matter, make such a tube with your hand. See if that reduces its size. Or if you’re young and flexible, moon the moon – turn your back on it, bend down, and look at it through your legs!

For more on this, Michael suggests this Web site – note it is for the year 2005 but, of course, the information on the moon illusion remains timely.