Sky & Telescope - June 2021 - 4

SPECTRUM

by Peter Tyson

An Exquisite Precision

The Essential Guide to Astronomy
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EDITORIAL
Editor in Chief Peter Tyson
Senior Editors J. Kelly Beatty, Alan M. MacRobert
Science Editor Camille M. Carlisle
News Editor Monica Young
Associate Editor Sean Walker
Observing Editor Diana Hannikainen
Consulting Editor Gary Seronik
Editorial Assistant Sabrina Garvin
Senior Contributing Editors
Dennis di Cicco, Richard Tresch Fienberg,
Robert Naeye, Roger W. Sinnott
Contributing Editors
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Forte, Steve Gottlieb, David Grinspoon, Shannon Hall,
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Lakdawalla, Rod Mollise, James Mullaney, Donald W.
Olson, Jerry Oltion, Joe Rao, Dean Regas, Fred Schaaf,
Govert Schilling, William Sheehan, Mathew Wedel, Alan
Whitman, Charles A. Wood, Richard S. Wright, Jr.
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DENNIS DI CICCO

one of astronomy's signature traits: the ability of its practitioners to
predict celestial events with extraordinary exactitude. Throughout
every issue of Sky & Telescope - most noticeably in the observing
section starting on page 41 - you'll encounter precise times, positions, brightnesses, and other measurements that attest to this characteristic.
But to grasp just how painstaking such calculations can be, one need look no
further than at eclipse forecasting.
A quick glance at the tables found in Joe Rao's cover story (page 34), for
instance, will show how meticulous the timings are for this month's annular
eclipse. But get a ﬁ x on this: Such to-the-instant timings already exist for every
eclipse that has occurred or will occur between 2000 BC and AD 3000. Created by Fred Espenak and Jean Meeus, the Five Millennium Canon of Solar Eclipses (https://is.gd/5mcanon)
tells us that, during that 5,000-year stretch, our planet
will experience exactly 11,898 eclipses of the Sun. That
includes 3,173 total, 3,956 annular, 569 hybrid, and
4,200 partial eclipses.
Consider annulars alone. Want to know the longestlasting annular eclipse of this century? That happened
on January 15, 2010, and it lasted 11 minutes, 8 secS The ring (Latin: annulus) that defines an
onds. (That's the duration of the " ring " effect caused by
annular eclipse, this one
the Moon mostly but not completely covering the Sun.)
from 1994
Don't worry if you missed it: This century's second-longest annular will take place on January 26, 2028 (10 minutes, 27 seconds). Such
precision holds even further out. The lengthiest annular of the ﬁfth millennium
AD? That will transpire on January 20, 4885 (11 minutes, 8 seconds).
Espenak and Meeus are careful to note that these extremely rigorous predictions aren't perfectly precise. The largest uncertainty in their calculations, they
say, comes from lunar tidal friction causing ﬂuctuations in Earth's rotation,
which results in a drift in apparent clock time. But for your and my purposes,
they might as well be perfect.
How can astronomers be so deﬁnite? In broad strokes, the process they use to
determine exactly when an eclipse will begin and end for any spot on the planet
essentially boils down to this: Take the sizes of Earth, Moon, and Sun, factor in
how the three bodies move in relation to one another, and run the numbers.
Of course, there's much more to it than that, and eclipse aﬁcionados like
Joe Rao love to parse the particulars. But you don't need to
be so fastidious to appreciate an eclipse. When the Moon
comes between you and the Sun, just grab your eclipse
glasses and enjoy the spectacle.

AMERICAN ASTRONOMICAL
SOCIETY
Executive Ofﬁcer / CEO, AAS Sky Publishing, LLC
Kevin B. Marvel
President Paula Szkody, University of Washington
Past President Megan Donahue, Michigan State
University
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Space Research Association
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State University
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Institute of Technology
Treasurer Doris Daou, NASA Planetary Science Division
Secretary Alice K. B. Monet, U.S. Naval Observatory (ret.)
At-Large Trustees Marcel Agüeros, Columbia University;
Kelsey Johnson, University of Virginia; Tereasa Brainerd,
Boston University; Hannah Jang-Condell, University of
Wyoming; Edmund Bertschinger, MIT

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Sky & Telescope - June 2021

Table of Contents for the Digital Edition of Sky & Telescope - June 2021