海角大神

Everybody鈥檚 all excited about Sunday鈥檚 big supermoon 2014, but can a casual observer really tell if the moon seems larger 鈥� or if they鈥檙e just looking at regular old 鈥渕oon illusion,鈥� or the magnifying impact of atmosphere and fixed objects when the moon first peeks above the horizon?

Greek astronomers used angle measuring tools like the diopter, which Euclid among others used to good effect, while subsequent researchers migrated to the diopter鈥檚 evolutionary offspring, the theodolite 鈥� or surveying scope 鈥� to inspect the moon and stars.

Both instruments work well for measuring angles against fixed object to ascertain distance, which is how astronomers first understood that the moon鈥檚 orbit is elliptical, and that it passes closer to the earth on some days than others. When that perigee occurs when no earth shadow falls on the moon, voila, the supermoon, also less often called the perigean moon.

But without a diopter, can the human eye actually tell whether Sunday鈥檚 moon over, say, Miami Beach is larger than an average one? Perhaps, perhaps not.

Sure, on paper, the supermoon is about 8 percent bigger and up to 30 percent brighter than a normal full moon.

Still, a supermoon doesn鈥檛 鈥渓ook noticeably bigger or brighter than average, unless you measure it pretty carefully,鈥� writes Maria Temming on SkyAndTelescope.com. 鈥淭he Moon鈥檚 orbit is only a little bit elliptical.鈥�

NASA astronomer Tony Phillips writes in a recent blog post that telling the difference is 鈥渢ricky.鈥� He adds, 鈥淭here are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full moon can seem much like any other.鈥�

But one amateur astronomer-blogger has claimed for several years that the perigee can actually be observed with the naked eye.

German space blogger Daniel Fischer writes that, in February 2011, he had no knowledge of a special full moon when he looked up in the sky and, as he writes, 鈥渘oted its unusual angular size, that is, its size as it appears on the dome of Earth鈥檚 sky.鈥� After checking an ephemeris, or moon table, he writes: 鈥淸T]he reason was obvious: perigee was close and the moon鈥檚 distance only 223,000 miles! That鈥檚 compared to the moon鈥檚 average distance of 239,000 miles.

鈥淎nd so I had found out by chance that the ellipticity, or oblong-ness, of the lunar orbit is actually pretty obvious to the unaided eye, with perigee full moons easily recognized as such without being told about their closeness beforehand.鈥�

Okay, so there鈥檚 the rub: The supermoon is most obvious when a viewer inadvertently stumbles on one with no foreknowledge. Unfortunately, there鈥檚 no chance of that with this Sunday鈥檚 supermoon, at least for anyone reading this.

Either way, 2014 has provided a bounty of opportunities to personally assess the moon鈥檚 stellar girth. The year has five supermoons. The most recent one was in July, the next one will come in September. But Sunday鈥檚 moon, a diopter would indicate, will skim the closest to Earth.