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Eclipses Have Advanced Our Knowledge of the Sciences in More Ways Than One

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FEATURE ARTICLE Masquerade of the Spheres Aditi Ghose CLIPSES are treasure troves of scientific explorations. They have almost always led to new insights into the Ephysics, chemistry, biology and mathematics of the world around us. But how do meticulous calculations work so that scientists and their equipment wind up in the right place at the right time? What happens if they don't? Predicting whether our place will experience this year’s last solar eclipse in December is akin to a challenge – and like all challenges, the rewards of getting it right are huge too! Advice to an Umbraphile "Before you die, you owe it to yourself to experience a total solar eclipse." That was an advice from an astronomer to David Baron, a science writer and much later, self-confessed eclipse evangelist. Baron was skeptical of such a profound and intimate counsel. Eclipses have advanced our knowledge of the sciences in more ways than one. Because a total solar eclipse, like the one next due in India on 20 March 2034, is the only time a masked Sun allows us a glimpse into its outer atmosphere. Moon's shadow seen falling on Cyprus and coastal Eclipse experiments have included, but never been Turkey, as seen on 29 March 2006, 230 miles above limited to tests of general relativity, studies in coronal physics the planet from the International Space Station. and chemistry, investigations of solar prominences, diameter (NASA) 26 | Science Reporter | July 2021 What does Totality Entail? (Edwin L. Turner, A Partial Eclipse is Interesting, 2017) Geometry of the Moon’s shadow during solar eclipse. With the moon at M1, an observer between A’B’ sees an annular eclipse. With the moon at M2, an observer between AB sees a total eclipse; at C, a partial eclipse is seen (J.B. Zirker, Total Eclipses of the Sun, 1980). Far from seeing a hole in the sky, a total solar eclipse is a multi-sensory experience. • The solar corona ̶ the halo of the gas visible around the Sun during totality is at a temperature of a few million degrees, far hotter than the solar surface. It glows with a light unlike seen with white sunlight and has a completely different hue. Appearing slight bluish white to the eye, it makes our everyday objects look weird and vaguely unreal. • Much blacker than the normal night sky, the black of the Moon's night side (the black circle inside the halo of white light) is a profoundly dark black. Little or no artificial lighting and clear atmospheric conditions help to appreciate it to the fullest. • The eclipse is accompanied by a gusty winds and a 360° twilight 'frame' all about the horizon. • Animals and birds behave weird, day-timers prepare to return while nocturnal come out. • Temperatures drop suddenly ̶ around 10 to 20° in a few minutes. • Optical effects like 'Diamond Ring', 'Bailey's Beads' and 'Shadow Bands' come into play. If observed from an opportune location like a high terrain, the Moon's shadow can be seen to race past across the lower valley at 1,000 miles per hour ̶ close to the speed of a jet plane. People who've experienced parallel it to being in some sort of other-worldly CGI and even a drug- induced hallucination that feels, and in this case is, totally real. Artist’s sketch of the scene at Bekal Fort, on the southwest Indian coast, on 12 December 1871 (The Illustrated London News 1872). Local inhabitants were seen preparing a bonfire to encourage the darkening Sun to become bright again. Much to the relief of observers from the British Association for the Advancement of Science, gathered on the watch tower specifically to observe the Sun’s corona during the eclipse, it was put out by a squadron of police (Barbara Ryden, Total eclipse, partial failure: Scientific expeditions don’t always go as planned, 2017). July 2021 | Science Reporter | 27 measurements, search for interplanetary dust, study of gravity waves in the Earth's atmosphere, effects on aerosols in the earth's atmosphere, measurement of water-quality in seas and experiments on the biological effects of animals and humans (J.B. Zirker, Total Eclipses of the Sun, 1980). Physicist Baron knew any given place on Earth experiences a total solar eclipse once in 400 years. This totality is visible on a narrow path, merely 100 miles wide. Then again, if the Moon is further away, a solar eclipse, like that in December 2067, will feature a very small umbra and only for 8 seconds of totality (Caleb Scharf, Eclipse: It’s all about the Umbra, 2017). But the moon definitely casts this shadow on some region of Earth every six months. So, to improve his odds of heeding the astronomer's counsel, Baron decided to travel to a place due to experience the next total eclipse — the Caribbean in 1998. What he experienced there changed his life forever. "... pictures just don't do it justice. It's not a ring or halo around Prime Minister Narendra Modi watching Kozhikode's live feed the sun; it's finely textured like it's made out of strands of of the 26 December 2019 annular solar eclipse. silk." Of course, he was only talking about the solar corona there ̶ the muse of scientists and artists alike. But Baron It was noticed that after roughly half a year, the Moon had claims that it was this moment that transformed him into a an opportunity to cross the Earth-Sun plane within the fabled life-long Umbraphile ̶ the eclipse chaser. So what does it take 17° alignment. This second eclipse season could be expected to predict these awe-inspiring eclipses accurately, and how at about 173.3 days after the first. It did not exactly amount deep does it hurt to mess up? to half a year as the location of lunar crossing nodes shifts 19° in longitude per year. This led to around 11% successive Getting the Date & Venue Right solar eclipses occurring roughly a synodic month apart, 23% There have been 11,898 solar eclipses in the past 5 millennia occurring 5 synodic months apart and the remaining 66% (Caleb A. Scharf, 11,898 Solar Eclipses in 5,000 Years) being 6 synodic months apart. But the celestial masquerade and even before the mastery of orbital natures and three- was only just beginning. dimensional state of our Universe, we have been doing a pretty good job at pinpointing them ̶ almost. The mechanics are The Symphony of Saros subtle. The Moon crosses the Earth-Sun plane, at the 'node' On a longer timescale, the 'Saros Cycle' comes into play twice in each lunar orbit ̶ because the lunar orbit is inclined (Fred Espenak, Eclipses and the Saros). Derived from the by 5.1° to the Earth's orbit around the Sun. With the classic Babylonian term 'sar', an interval of 3600 years, the Saros New Moon orientation of the Sun and Earth encompassing cycle as an eclipse period is much shorter. Chaldeans the Moon, there will be a visible solar eclipse, partial or observed that just like the harmonies wafting across a good total, within around 17° (actually 15.4° to 18.6° owing to masquerade, lunar eclipses, much alike solar eclipses, seemed the elliptical orbits of the Earth and Moon) on the sky of this to repeat themselves periodically. crossing. The Sun takes 34.5 days to appear to cross this 17° The beat frequency of this Saros Cycle recurs periodically radius eclipse zone. The Moon's 'synodic period' of 29.53 over 6585.3 days (18 years, 11 days and 8 hours). The days with respect to this alignment, slightly longer than its harmonies owe their periodicity to the fact that the Moon's actual orbital period of 27.3 days, allows at most 2 solar orbital periods share a natural harmony. The Synodic Month eclipses during this 'season'. The challenge is then to figure (average period of the Moon's orbit with respect to the line out the next ones. joining Sun and Earth) of 29.53 days is quite similar to the Geometry of the solar and lunar apparent disks near a node in the lunar orbit. If the moon in its eastward motion overtakes the Sun at S (the eclipse limit), it eclipses only the tangent point. If the moon overtakes the Sun closer to the node (for instance, at S1), a total annular eclipse can occur. 28 | Science Reporter | July 2021 Anomalistic Month (time the Moon takes to go between its cycle. For solar eclipses, this entails the westward shift of closest approach to Earth on its slightly elliptical orbit) of each successive eclipse path by 120°. The Saros series returns 27.55 days and the Draconian Month (time between passages to the same geographic region every 3 saroses (54 years and through the same orbital node) of 27.21 days. One Saros, as a 34 days). result, is equal to 223 Synodic months. So are 239 Anomalistic No Saros series lasts indefinitely. With the Moon's Months and 242 Draconian Months (to within a few hours). node shifting eastward by ~5° with each cycle, the typical 223 Synodic Months (New Moon to New Moon) Saros series for a solar eclipse begins when the new Moon = 223 x 29.530589 days = 6585 days 07 hours 43 occurs ~18° east of a node. Suppose the first eclipse occurs minutes at the Moon's descending node, the Moon's umbral shadow 239 Anomalistic Months (Perigee to Perigee) will pass ~3500 km below the Earth, with a partial eclipse = 239 x 27.554550 days = 6585 days 12 hours 54 visible from the south polar region.
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