June 17, 2019

Astro-Event: The Smallest Full Moon of the Year!!!

Can you see the difference? (Photo by Author).

The Full Hunter’s Moon this Tuesday night ushers in the ultimate non-event (well, next to the 2012 “galactic-axis alignment doomsday scenario!”) but unlike like that whole debacle, this one is interesting and real. Sure, everyone commemorates the largest apparent Full Moon of the year, but few mark this month’s, which is its antithesis as the smallest.

Not that there’s much variation in the apparent size of the Moon; the smallest the Moon can appear to us is 29.3’ in size, while the largest is about 34.1’, about a 14.1% reduction in size. We neared this earlier this past spring during the much touted “super-Moon.” A good rule of thumb is that the Moon is about 30’ or half a degree of arc in apparent size; this means you could line up 720 (=360 x 2) Moons around your local horizon. And yes, the Sun has roughly the same apparent size as the Moon when viewed from Earth, although its much farther away. This is a happy coincidence of our current epoch as the Moon recedes from us; uber-cool events such as total solar eclipses will cease to occur millions of years from now!

Said Full Moon occurs at 2:07UT on Oct 12th, with an apogee of 406,434km occurring just 9 hours and 37 minutes later at 11:44UT. This is the smallest since September 22nd, 2006, and won’t be topped again until November 28th of 2012 (which is also the date of a shallow penumbral eclipse). As a consequence, the New Moon on October 26th will also be exceptionally large as it occurs within 7 hours of perigee… Too bad there are no solar/lunar eclipses this month, as a near-perigee New Moon would make for an extra-long solar eclipse, and a slower moving distant apogee Full Moon would also lengthen a total lunar eclipse! Of course, this all occurs because the Moon’s orbit isn’t precisely elliptical, but instead has an eccentricity of about 5.5%.

The October Moon is known as the Hunter’s Moon, allowing a few extra hours of illumination for tracking prey into the autumn night. Perhaps it’s for this reason the names Blood or Sanguine Moon are also applied; other alternate names include the Travelers or Dying Grass Moon. Note that as the Moon passes the equinoctial point leading up to Full this week, its angle to the horizon for mid-latitude observers becomes very oblique, causing it to rise as little as 20 minutes apart on successive evenings. Late Fall and Wintertime Moons tend to ride high in the sky as a consequence of its high northerly declination, as the Sun recedes to the South. Also, expect high tides to be more lackluster than usual around the Full Moon this month, and the reverse to be true around the New Moon… but the burning question is, what should we call the smallest Full Moon of the year? The largest is always well-heralded as the “Super” or Proxigean Full Moon, but perhaps the smallest merits a name as well… how ‘bout the Mini-Moon? Micro-Moon? Tiny-Weenie Moon? Bizzaro Moon? Just think of the legions of future astronomy students that will love us for yet one more obscure astronomical term to endure!

Lunar Halo pic taken from Astroguyz HQ.

The Astro-Word for this week is Lunar Halo. This is an interesting atmospheric feature that often accompanies a bright Moon shining through a high thin cirrus cloud. Of course, these cloud fronts are usually harbingers of inbound storm systems in a few days, and it’s no coincidence that olden-time farmers saw them as signs of rain. Lunar halos usually substend from about 22 degrees of arc from the main body of the Moon, and are caused by moonlight shining through ice crystals embedded in the clouds. The Moon is usually bright enough to cause a visible lunar halo from about 1st Quarter phase to Last Quarter waning, and they can occur any time of year but always seem to be more prevalent during the long winter nights simply because we get to see more of our nearest celestial neighbor. Rarer but related phenomena include Moon-dogs (similar to sundogs) and lunar coronas extending around the limb of the Moon, which may even be multi-colored. Keep an eye out this winter during weeks on either side of Full phase and the odds are that you’ll soon see a Lunar Halo around the Moon. Photography of such phenomena is a pretty straight forward affair, but you’ll have to over-expose or block out the Moon using a nearby structure for best effect. Shutter speeds of about 3-5” seconds work best for a Van Gough-like ethereal effect… good luck with your lunar quest!

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  1. [...] fans and followers of this space know, last Tuesday’s Hunter’s Moon also marked the visually smallest of the year, as Full phase was reached only hours before apogee. While that particular Moon [...]

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