A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Nov 13, 11:13 -0800
Ed Popko, you wrote:
"I have never taken a night time lunar."
Well, that's a shame! Next time the Moon and Jupiter are conveniently arranged, you should certainly try. You can get excellent results with Jupiter. Just remember to "split" the small disk of Jupiter on the Moon's limb. Most clearing methodologies assume this. Many people also report excellent results with Venus. My results with Venus have not been as good as with Jupiter, but that may just be luck of the draw. Lunars with true stars are a little tougher. There are contrast problems between the bright moon and most of the available lunars stars. This can be offset by using shades on the Moon, but then you have to worry about shade error, resulting from any slight prismatic effect in the glass of the shade. In addition, true stars often look "blurry" or "spikey" to many observers since they tend to highlight the imperfections of vision. One solution here is to shoot lunars with true stars before your eyes dark-adapt (Bill B. has seen and commented on this effect a number of times).
"Perhaps November's super Beaver moon is the time to start."
Tonight's perigee full moon is somewhat better for lunars because the Moon is moving a bit more rapidly across the sky than its usual pace. It's moving faster due to Kepler's Second Law (or in modern terms, conservation of orbital angular momentum), which guarantees that its actual speed is higher when it's closer to the Earth, and it's moving faster across the sky because it is physically closer. By dawn tomorrow, it's moving about 38 minutes of arc per hour which is about 25% faster than normal. So if you can determine your GMT to within 30 seconds normally with lunars, then tonight/tomorrow you can expect about 24 seconds accuracy. This is good, but it's within the noise for most lunar experiments.
"Doing some quick references for reasonable near-ecliptic stars, Pollux and Hamal seem possible. Aldebaran is too close and Betelgeuse is perhaps too far off the ecliptic track."
Well, it depends on when you do your sights. If you get up just before dawn tomorrow (specific to the eastern USA) when the Moon is marginally closest to perigee, Hamal and Aldebaran are both about 19° from the Moon; earlier Hamal is too close, later Aldebaran is too close. You can decide which bodies are in range and visible using my web app here. See some results for an observer in southern New England here. If you decide to try your lunars tonight instead of waiting for dawn on Monday, then Altair and Fomalhaut are also good options.
For most navigators, the easiest way to understand the meaning of this perigee effect is to look at the Moon's HP. Tomorrow, around noon GMT, the Moon's HP peaks at 61.50 minutes of arc. That's quite high. For comparison, there was a full moon perigee last September (sep 28, 2015) which also occurred with a lunar eclipse. Then the HP was 61.43. Back on March 19, 2011, there was a full moon perigee that launched the earliest "supermoon" mania on the internet. On that date the Moon's HP peaked at 61.48. For upcoming dates, on January 1, 2018, the HP will again be 61.50. Finally, if we dig back to January 4, 1912, the HP of the Moon was 61.53. Clearly all of these dates are within a tenth of a minute of arc of the same maximum value: 61.5'. There's no significance for celestial navigation. The semi-diameter of the Moon may be calculated as 27.24% of the Moon's HP. Since that's a bit more than a quarter of the HP, a difference of a tenth of a minute of arc in HP is four times smaller in the range of SD values --effectively nil. On top of this, don't forget the augmentation which increases the SD by 0.3 minutes of arc when the Moon is at high altitude compared to low, near the horizon. That will yield a bigger change in the Moon's true size than these small differences in perigee distance.
To sum up, there's nothing genuinely extraordinary about tonight as far as lunars are concerned, but perigee does increase the accuracy of lunars slightly for a few days every month. And full moon perigees are always a little better still, since the Sun's tidal field "stretches" the Moon's orbit more than usual. These full moon perigees have come to be know as "supermoons" thanks to Internet mania in the past five years. Before 2011, the term was used only by a tiny handful of astrologers. You can see its sudden rise to prominence in the attached graphic from Google Trends. This sudden appearance was driven by a coincidental article on space.com predicting earthquakes "caused by lunar perigee" which appeared just before the tragic earthquake/tsunami/nuclear accident in Japan in March of 2011. Now it's an internet mania recurring every 14 months or so.