| Moondark for September: Shadows of the Earth and Moon | |
| Eclipses are the marquee astronomical event. The total solar and partial lunar eclipses of this past August are just the latest examples, garnering considerable media and internet attention, even live webcasts. Just as well since neither event was visible from Delmarva nor even from most of North America. A rather exacting alignment of three celestial bodies is a necessary condition for an eclipse. When the Sun, Moon and the Earth align, in that order and precisely enough that the shadow of the Moon falls on the Earth, a solar eclipse occurs. It is a partial eclipse wherever the Moon partly obscures the Sun. When the alignment is especially accurate, the Moon can completely obscure the Sun along a narrow track, but totality lasts only minutes anywhere along that line. It is surely spectacular and exhilarating: what other astronomical event drives such a huge astro tourism market? Thousands of astro-travelers fly, sail and trek to far corners of the globe for a few minutes in the shadow of the Moon. The rarity of the total eclipse experience results not only from an exactness of alignment but also from the remarkable coincidence of the angular size of the Moon and Sun, both about one-half degree. If the Moon is relatively far from Earth in it’s somewhat egg-shaped orbit, it appears slightly smaller than the Sun, and an annular or ring of fire eclipse will be seen by observers directly in line with the Sun and Moon. And because the Moon’s inclined orbit wobbles like a hula hoop, eclipses don’t occur twice a month, but only a handful of times in any given year. When the Sun, Earth then Moon align at Full Moon, a lunar eclipse may occur, and it may be total or partial depending on whether the Moon passes directly through the Earth’s shadow or not. Lunar eclipses are lower profile events, more gradual and visible from anywhere it’s nighttime. No eye protection, nor any astronomical equipment, are needed. So it may be the Staples of astronomical events (“That was easy”). Such eclipse configurations are straightforward to tabulate nowadays, but throughout history they have always been notable events. Megalithic observatories may have functioned as eclipse predicting machines. The ancient Chinese recorded a solar eclipse over four thousand years ago, and others are recorded in the Bible. The curved shadow of the Earth on the face of the Full Moon clearly told the Greeks the Earth was round, and they invented mechanisms of unprecedented sophistication to predict eclipses as well. Christopher Columbus impressed the natives in Jamaica, correctly foretelling a lunar eclipse in 1504. In 1919, Sir Arthur Eddington measured the minute shift of stellar positions caused by the gravitational deflection of star light by the Sun during a total solar eclipse, confirming Einstein’s predictions from general relativity and catapulting him to scientific celebrity. But for the last century or so, eclipses have been more for observing and less for marking of geopolitical transitions or scientific study. Eclipses are predictable but not too common, and there is inevitably some element of chance. Will the skies be clear? What will the Sun’s corona look like, will there be beads and prominences? How red will the Moon become, how dark will the sky appear? They are times to stop fiddling with the telescope and start enjoying something happening in the heavens. Given the fond memories of eclipses I’ve enjoyed, I’m not convinced of the appeal of webcasts. Tightly framed on the Sun and Moon, the change of light on the ground, the darkening of the sky and appearance of bright stars and planets are all missing. Most of us will have to suffice with remote eclipse watching for some time. Although up to seven eclipses can occur in any given year, location is everything, and we miss out on all six eclipses coming up next year. In fact, Delmarva’s string of misses continues through late 2010 with a total lunar eclipse on 21 December. The US solar eclipse dry-spell will end in nine years on 21 August 2017 when the Moon’s shadow sweeps from Oregon to South Carolina. Then during the total eclipse of 8 April 2024, another band of totality will sweep across the country from the southwest. Folks near Carbondale, Illinois are lucky enough to experience two total solar eclipses in just seven years! Mark your calendars: the paths of totality will be within a day’s drive of Delmarva for both events. While the web can't replace the real eclipse experience, it is where to find out about them: Mr. Eclipse and NASA sites are great resources for eclipse particulars, charts and even interactive maps. Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 24 August 2008. Text on this web page is free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses. |
In every astronomy text, you'll find an eclipse diagram like this one from Moulton's (1920) Introduction to Astronomy.  While the geometry is accurate, the scale is wildly distorted. Here, the Earth and Moon are in the proper relative size but should be separated by about 30 Earth diameters or about one foot. The Sun should be110 Earth diameters in size, say a yard across on this scale, and located more than a football field away. Little wonder then that eclipses are a rare and remarkable experience for Earth's inhabitants. |
