How to measure 2014 biggest ‘supermoon’: diopter, theodolite, or just eye it?

Can casual astral observers really tell that the so-called ‘supermoon’ – the space rock’s closest perigee to earthlings – looks bigger than, say, a harvest moon? Verdict’s still out.

J. David Ake/AP/File
Traffic streaks by the supermoon as it rises behind the Memorial Bridge in Washington on July 12, 2014. The full moon may seem huge, but it's just an illusion caused by its position in the sky.

Everybody’s all excited about Sunday’s big supermoon 2014, but can a casual observer really tell if the moon seems larger – or if they’re just looking at regular old “moon illusion,” 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’s evolutionary offspring, the theodolite – or surveying scope – 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’s 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’s 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’t “look noticeably bigger or brighter than average, unless you measure it pretty carefully,” writes Maria Temming on “The Moon’s orbit is only a little bit elliptical.”

NASA astronomer Tony Phillips writes in a recent blog post that telling the difference is “tricky.” He adds, “There 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.”

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, “noted its unusual angular size, that is, its size as it appears on the dome of Earth’s sky.” After checking an ephemeris, or moon table, he writes: “[T]he reason was obvious: perigee was close and the moon’s distance only 223,000 miles! That’s compared to the moon’s average distance of 239,000 miles.

“And 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.”

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

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

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