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From: Frank Reed <FrankReed@HistoricalAtlas.com>
To: luabel@ymail.com
Sent: Wednesday, June 19, 2013 9:58 AM
Subject: [NavList] Re: Perigee Full Moon 23 June

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Oh my god... it's ENORMOUS!! You can expect to hear a lot of that in the next few days. :)

Ever since the phrase "Supermoon" entered the popular lexicon in March 2011 (a mere 27 months ago, following the earthquake/tsunami/meltdown in Japan), the usual suspects for exaggeration and hype have been playing up Perigee Full Moons. Just out of curiosity, where did you hear about it, Brad? For some entertainment, read the blog entry on this topic on the Weather Channel's web site:
http://www.weather.com/news/science/space/largest-supermoon-year-peaks-weekend-20130618. It's funny!

And I should add, for casual observers this is visually satisfying regardless of reality because the Moon will most likely be seen by most people right after it rises. The Moon illusion kicks in and makes it appear huge, no matter what its objective, measurable size. In reality, the real, measurable size difference is well below the threshold of detection by casual observation. The Moon is the only celestial body that most people can identify (the Earth and Sun don't count!), so despite the scientific "silliness" of these stories about moon names, like "supermoon" and "blue moon", they have significant popular appeal.

From the perspective of celestial navigation data, the HP of the Moon peaks at 61.42 minutes of arc at 12:00 UT on June 23 (the geocentric SD is, as always, 27.24% of the HP). The maximum lunar distance, by the way, occurs less than an hour earlier at about 176° 18'. Compare with last month. On May 26 near 0:00 UT, the Moon's HP peaked at 61.18' about a day after Full Moon. The difference from last month to this month is 0.24' corresponding to a difference in SD of only 0.07 minutes of arc (4 seconds of arc). Note that this is considerably smaller then the augmentation of the SD with altitude which has a range of 0.3'. So a southern hemisphere observer, who would have an opportunity to see the Moon high above the horizon, could have seen the Moon bigger a month ago than a mid-latitude northern hemisphere observer this coming weekend.

SD May 26: 16.67'
SD June 23: 16.73'
SD July 21: 16.67'

You'll find some sources suggesting that this perigee Full Moon will enhance the tides. It's true to a limited extent that has been well-understood since the earliest scientific tidal theories and properly accounted for in tidal calculations since the last 19th century. But the idea the exact timing of perigee at the instant of Full Moon yields the largest enhancement is incorrect. The tides are driven oscillators. There is a phase lag between the the Spring tides and the date of syzygy. On average it's about a day. On the US East coast, Spring tides occur a day AFTER Full Moon or New Moon. Along much of the coast of the North Sea, it's two to three days. The amplification due to perigee matters when perigee matches the phase lag. In other words, perigee on the exact date of Full Moon produces slightly less tidal range than perigee a day (or more) later. And in general, this is all mixed up with latitude effects that make it almost impossible to see any small variations of the perigee effect independently. Indeed the fact that this Full Moon occurs at a low southern declination, but well above the ecliptic is more significant for the tides than the small variation in perigee distance.

For casual observers, the declination factor is also lost in the noise of these "Big Moon" stories. It's far more significant for the pleasing view of the June Full Moon (for mid-latitude northern hemisphere observers!) that it occurs at low southern declination. The Full Moon is nearly opposite the Sun. So in June when the Sun rides high at noon, the Moon hangs low in the South at midnight. Its light is more mellow (extincted a bit by altitude and the greater haze of summer), and its light is spread across the landscape, under trees and through windows, creating a very pleasant moonlit experience. By contrast, the December Full Moon (again for mid-lat N hemisphere observers), is high overhead at midnight, even reaching the zenith for points as far north as 28.6° N in some years. With all the focus on size, casual observers will miss out on the most important astronomical feature of the June Full Moon-- its low arc across the southern sky.

-FER

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