Sky and Telescope - August 2018 - 48

AUGUST 2018 OBSERVING
Celestial Calendar by S. N. Johnson-Roehr

Peak Perseids
Plan for optimal conditions for the
year's most popular meteor shower.

tars fell like weaving in the south, unceasingly
through the night." So a city gazetteer printed in
Shanxi, China, described the sky above Fenyang on August
10, 1862. Calculating backward, scholars have determined
that the "weaving stars" witnessed by the townspeople
were in fact Perseid meteors, falling at a time when the
shower's radiant, the point from which the meteors
appear to emanate, lay low in the sky.
The Perseids are associated with the short-period
Halley-type comet 109P/Swift-Tuttle, which was independently discovered by American astronomers Lewis Swift
and Horace Tuttle as it approached perihelion in July 1862.
When Earth crosses Swift-Tuttle's orbit, bits of dust and
rocks left behind by the comet hit the planet's atmosphere, creating the light show we know as the Perseid
meteor shower. In years when Swift-Tuttle reaches perihelion, the number of visible meteors significantly increases,
which explains the dramatic display of August 1862. Since
the comet has a 133-year orbital period and last visited the
inner solar system in 1992, we've got another 107 years to
go before that possibility arises again. Occasional bumpups in the number of detected meteors do occur when

48

AUGUST 2 018 * SK Y & TELESCOPE

This composite image is made of several exposures shot
at Sunset Crater, Arizona, over nearly 2 hours on the
morning of August 12, 2016. The image shows 48 Perseids - including two spectacular fireballs - and 5 sporadics (meteors not associated with the Perseids, identified by the trails not in alignment with the majority).

Earth passes close to dust trails left behind during even earlier
visits. However, the next such major outburst isn't predicted
until 2028. Even so, the "ordinary" Perseids make for a strong
shower and an impressive spectacle. Under ideal, dark skies,
viewers may see more than 50 meteors in an hour.
The extant Chinese records for the 1862 event all come from
northern China, suggesting unfavorable weather in the south
that year. Here and now in 2018, we hope for good weather
for the summer's greatest shower, because viewing conditions
should otherwise be perfect. The shower's predicted peak falls
on the evening of August 12th, soon after new Moon (9:58 UT
August 11th). Weather and light pollution (and possibly mosqui-

J ER EM Y PER E Z

"S



Sky and Telescope - August 2018

Table of Contents for the Digital Edition of Sky and Telescope - August 2018

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Sky and Telescope - August 2018 - Cover1
Sky and Telescope - August 2018 - Cover2
Sky and Telescope - August 2018 - 1
Sky and Telescope - August 2018 - Contents
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