Is it not actually true that the moon should measure largest when it is directly overhead? - we’re standing on a (near)sphere, so when the moon is on the horizon, it is further away by about the distance of the radius of the Earth (probably this doesn’t have a significant effect), assuming a circular orbit.
Yes, by 0.8%, less at high latitudes.
Any refraction due to thickening of the atmosphere should make the moon appear smaller on the horizon.
If you use callipers like QED suggests, the moon will appear smaller directly above compared to the horizon. That is because the callipers are closer to the eye when held above due to body mechanics (try it with a tape measure) and gravity compressing the joints (a bit).
That’s what I love about this site, although I must say I’m a little disappointed that nobody has explored the effects of relativity on this experiment (when you’re looking at the moon on the horizon, your arm is tangential to the rotation of the Earth, when you’re looking at the moon overhead, it is perpendicular.
The Moon has often been in zodiacal constellations near the horizon recently when full; this often happens in summer, but the Moon has been slightly south of the ecliptic at times, as well.
Many people have noted that it has looked red recently; this is nothing to do with the planet Mars, Planet X or chemtrails, but is just a beautiful astronomical phenomenon.
SF worldbuilding at
http://www.orionsarm.com/main.html
It did cross my mind that it might have been a partial lunar eclipse (where the moon passes through the shadow of the Earth, rather than the other way around, which is a solar eclipse).
During a lunar eclipse, there is a moment when the moon is illuminated by light that is grazing the Earth and being filtered by our atmosphere; this apparently makes it look red or orange (I’ve never witnessed it personally as the UK can be relied upon to completely cloud over when any spectacular astronomical event is taking place).
But there was no lunar eclipse on the date in the OP, as far as I can tell.
In summary of the cites provided by commasense and Q.E.D., here is the simplified mathematical explanation for the moon illusion.
A few moments thought will confirm that Retinal Image Size = Actual Size divided by Actual Distance. (I=S/D) Manipulating the formula, S=I*D.
Optical illusions of size rely on distance cues “fooling” us (like the Ponzo effect). So, if the Perceived Distance (D’) is different than the Actual Distance, then the Perceived Size (S’) is different than the Actual Size. S’=I*D’.
The “sky dome” effect (which has been experimentally verified) that people perceive the sky straight up to be closer than the sky at the horizon, explains the moon illusion. At the horizon, the moon is perceived to be further away (D’) than high above, therefore the Perceived Size (S’) is larger, due to S’=I*D’.
The retinal image (I) does not change. Or more correctly, does not change enough to matter.
This photograph captures the appearance of the Moon and Mars this month as they rise over a blacked out New York City.
Thanks for the pic, Squink! There is definitely a difference in the appearance of moon size vs. Mars size. Color me red, as in embarrassed!!
That bright orange low-hanging moon scares the crap put of me. Once when I was 10 years old I was riding my bike to a friend’s house about two blocks away to borrow an encyclopedia or something and it was just hanging there, staring at me. Then, I had to ride home knowing it was behind me. According to my mom, when I got home I was white as a sheet. Even today, 20 years later, I still gibber whenever I see a low orange moon.
–Cliffy
Here’s another size comparison 
Maybe Mars will have a near collision with earth like Venus did several thousand years ago,
i.e. if you believe Velikovsky
te he
More neat stuff…
Moon’s appearance over the course of a lunar month…with noticeable libration…
I’m looking for a cite with a good explanation of the ecliptic and why the full moon is lower in the summer and higher in the winter (which is opposite of the Sun’s respective positions)…
I disagree completely. Earth is the only planet whose moon appears roughly the same size as its primary. Thus, we’re unique in having solar eclipses in which the Sun, and only the Sun, are blocked out. Everywhere else, the satellite is either too large or too small.
OK. First, I wrote a debunk of the “Mars as big as the Moon” thing.
Second, the Moon (and Sun) sometimes look reddish when rising or setting due to several effects. The big one is scattering; blue light is scattered by nitrogen molecules in the atmosphere, spreading it out all over the sky, which is why the sky looks blue. Blue light scatters more then red. However, when you look toward the horizon, you are looking through more air, which can also scatter the green and yellow light from the Moon, leaving it looking red. Also, dust tends to absorb blue light and let red through, and toward the horizon you are looking through more dust.
Third, the Moon is not bigger on the horizon. In fact, it’s smaller: it’s farther away by about the radius of the Earth than when it’s overhead (draw a picture). Also, the Earth’s air tends to distort objects near the horizon, squashing them. This is easy to see with the Sun.
The Moon is not bigger on the horizon; it’s an illusion. Donal Simanek has the best explanation I have seen, barring the chapter in my book about this.
Thanks The Bad Astronomer.
I will forward your site to people who send me that misleading email.
Sorry to resurrect this thread from the dead, but I had to share this picture, taken at high magnification, showing Mars “rising” behind the Moon. Cool or what.
Thanks for sharing, r_k! That’s an incredible shot.
A link on the page r_k cites above has this striking graphic “proof” that the moon is not larger when close to the horizon.
If you understand why the Sun is higher up in the day in the summer, then just turn it around and realize that the full Moon is opposite the Sun.