Baily's beads

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August 21 2017 solar eclipse baily beads TLR2.jpg
Baily's beads photographed 4 seconds before totality of the solar eclipse of August 21, 2017

The Baily's beads effect, or diamond ring effect, is a feature of total and annular solar eclipses. As the Moon covers the Sun during a solar eclipse, the rugged topography of the lunar limb allows beads of sunlight to shine through in some places while not in others. The effect is named after Francis Baily, who explained the phenomenon in 1836. [1] [2] The diamond ring effect is seen when only one bead is left, appearing as a shining "diamond" set in a bright ring around the lunar silhouette. [3]

Solar eclipse Natural phenomenon wherein the Sun is obscured by the Moon

A solar eclipse occurs when an observer passes through the shadow cast by the Moon which fully or partially blocks ("occults") the Sun. This can only happen when the Sun, Moon and Earth are nearly aligned on a straight line in three dimensions (syzygy) during a new moon when the Moon is close to the ecliptic plane. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.

Moon Earths natural satellite

The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits. The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.

Sun Star at the centre of the Solar System

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

Contents

Lunar topography has considerable relief because of the presence of mountains, craters, valleys, and other topographical features. The irregularities of the lunar limb profile (the "edge" of the Moon, as seen from a distance) are known accurately from observations of grazing occultations of stars. Astronomers thus have a fairly good idea which mountains and valleys will cause the beads to appear in advance of the eclipse. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, lasting 1–2 minutes.

Topography of the Moon

The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, including data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as the maria Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.

Terrain Vertical and horizontal dimension and shape of land surface

Terrain or relief involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word terra means "earth."

Impact crater Circular depression on a solid astronomical body formed by a hypervelocity impact of a smaller object

An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

After the diamond ring effect has diminished, the subsequent Baily's beads effect and totality phase are safe to view without the solar filters used during the partial phases. By then, less than 0.001% of the Sun's photosphere is visible.

Photosphere A stars outer shell from which light is radiated

The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3, or equivalently, a depth from which 50% of light will escape without being scattered. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Observers in the path of totality of a solar eclipse see first a gradual covering of the Sun by the lunar silhouette for over an hour, followed by the diamond ring effect (visible without filters) as the last bit of photosphere disappears. As the burst of light from the ring fades, Bailey's beads appear as the last bits of the bright photosphere shine through valleys aligned at the edge of the Moon. [4] As the Baily's beads disappear behind the advancing lunar edge (the beads also reappear at the end of totality), a thin reddish edge called the chromosphere (the Greek chromos meaning "color") appears. Though the reddish hydrogen radiation is most visible to the unaided eye, the chromosphere also emits thousands of additional spectral lines. [5]

Chromosphere 2nd of the 3 main layers in the Suns atmosphere (above the photosphere, below the solar transition region); 3000–5000 km deep, with a rosy red color only apparent during eclipses

The chromosphere is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses. The chromosphere sits just above the photosphere and below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy-appearing spicules rise from the homogeneous layer, some of which extend 10,000 km into the corona above.

Naked eye practice of engaging in visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"

Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical instrument, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is still considered "naked".

Spectral line optical phenomenon

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules. These "fingerprints" can be compared to the previously collected "fingerprints" of atoms and molecules, and are thus used to identify the atomic and molecular components of stars and planets, which would otherwise be impossible.

Observational history

Although Baily is often said to have discovered the cause of the feature which bears his name, Sir Edmond Halley made the first recorded observations of Baily's beads during the solar eclipse of 3 May 1715. [6] [Note 1] Halley described and correctly ascertained the cause of the effect [6] in his "Observations of the late Total Eclipse of the Sun[...]" in the Philosophical Transactions of the Royal Society :

Edmond Halley English astronomer, geophysicist, mathematician, meteorologist, and physicist

EdmondHalley, FRS was an English astronomer, geophysicist, mathematician, meteorologist, and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.

Solar eclipse of May 3, 1715

A total solar eclipse occurred on 3 May 1715. It was known as Halley's Eclipse, after Edmond Halley (1656–1742) who predicted this eclipse to within 4 minutes accuracy. Halley observed the eclipse from London where the city of London enjoyed 3 minutes 33 seconds of totality. He also drew a predictive map showing the path of totality across England. The original map was about 20 miles off the observed eclipse path, mainly due to his use of inaccurate lunar ephemeris. After the eclipse, he corrected the eclipse path, and added the path and description of the 1724 total solar eclipse.

<i>Philosophical Transactions of the Royal Society</i> Scientific journal published by the Royal Society

Philosophical Transactions, titled Philosophical Transactions of the Royal Society from 1776, is a scientific journal published by the Royal Society. In its earliest days, it was a private venture of the Royal Society's secretary. It became an official society publication in 1752. It was established in 1665, making it the first journal in the world exclusively devoted to science, and therefore also the world's longest-running scientific journal. The use of the word philosophical in the title refers to natural philosophy, which was the equivalent of what would now be generally called science.

About two Minutes before the Total Immersion, the remaining part of the Sun was reduced to a very fine Horn, whose Extremeties seemed to lose their Acuteness, and to become round like Stars ... which Appearance could proceed from no other Cause but the Inequalities of the Moon's Surface, there being some elevated parts thereof near the Moon's Southern Pole, by whose Interposition part of that exceedingly fine Filament of Light was intercepted. [6]

In media

Cosmas Damian Asam was probably the earliest realistic painter to depict a total solar eclipse and diamond ring. [7] His painting was finished in 1735.

The Baily's beads phenomenon is seen during the credit opening sequence of the NBC TV show Heroes , while the Diamond Ring effect is seen during the credit opening sequence of Star Trek: Voyager , albeit from a fictitious extrasolar body, seen from space.

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References

Notes

  1. Great Britain did not adopt the Gregorian calendar until 1752, so at the time of the eclipse, the date was recorded as 22 April 1715.

Citations

  1. Baily (1836). "I. On a remarkable phenomenon that occurs in total and annular eclipses of the sun". Monthly Notices of the Royal Astronomical Society. 4 (2): 15–19. doi:10.1093/mnras/4.2.15.
  2. Littmann, Mark; Willcox, Ken; Espenak, Fred (1999). Totality – Eclipses of the Sun. Oxford University Press. pp. 65–66. ISBN   978-0-19-513179-6.
  3. O. Staiger. "The Experience of Totality".
  4. Pasachoff, J. M. & Covington, M. The Cambridge Eclipse Photography Guide (Cambridge Univ. Press, 1993)[ page needed ]
  5. Pasachoff, Jay M. (2009). "Solar eclipses as an astrophysical laboratory". Nature. 459 (7248): 789–795. Bibcode:2009Natur.459..789P. doi:10.1038/nature07987. PMID   19516332.
  6. 1 2 3 Pasachoff, Jay M. (1999). "Halley and his maps of the Total Eclipses of 1715 and 1724". Journal of Astronomical History and Heritage. 2 (1): 39. Bibcode:1999JAHH....2...39P.
  7. Nemiroff, R.; Bonnell, J., eds. (28 January 2008). "A Solar Eclipse Painting from the 1700s". Astronomy Picture of the Day . NASA.