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75, 50 & 25 years ago.

March-April 1939

Invisible Glass "The amount of light reflected from a clean glass surface is small; if the reflected light could be entirely eliminated the glass would become invisible.... The trick is accomplished by coating the glass with a thin transparent film about 1/200,000 of an inch thick....

"The thickness of the film is critical. By doubling its depth the effect can be completely reversed. The reflections are intensified. Even this result may be of value, by making possible more perfect reflection from a silvered or aluminized mirror surface....

"Astronomers have been struggling for ages to get more of that precious commodity, starlight, into its most useful place, and every increase in efficiency is welcome.... How far-reaching the new discovery will be and to what new uses it may be put await for the future to decide. "

While still a graduate student, soon-to-be optical guru James G. Baker was reporting a startling discovery just announced by researchers at MIT and General Electric. It led in just a few years to the wide use of anti-reflection coatings on lenses and enhanced reflectivity coatings on mirrors.

March 1964

Comet's Demise?

"Dots [in the accompanying graph] represent absolute magnitudes of Encke's comet at each observed return since 1786.... Smaller dots are less accurate values. The curve fitted by F. L. Whipple predicts an increasingly rapid fading, leading to eventual disappearance about the year 1993. "

This bold prediction by a noted comet expert did not come to pass. Last November, with Comet ISON in its death dive near the Sun, Comet Encke was a nice binocular target at nearly 7th magnitude.

March 1989

Remote Remnant

"Robert A. Fesen (University of Colorado) and colleagues have recovered the visible remains of S Andromedae, the first supernova ever recorded in an external galaxy [in 1885]. The CCD image... was taken in the light of an iron line at 3860 angstroms with the 4-meter reflector atop Kitt Peak. The long-sought remnant appears as a dark spot... in absorption against M31's bright central region....

"The 1885 blast was a Type I supernova that left behind an expanding cloud made largely of iron. The debris is now about 0.3 arc second, or about 1 light-year, across, consistent with an expansion speed of 4,000 to 5,000 kilometers per second since 1885. "

On August 17, 1885, a bright star appeared in what was then called the Andromeda Nebula. It reached 6th magnitude a few days later and then faded slowly during the next six months. Astronomers would not figure out what a supernova was until the 20th century.

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Author:Sinnott, Roger W.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Mar 1, 2014
Words:431
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