WWW.ASTRONOMY.COM 47
showers, of which 112 have
been established by repeated
observations. With only 88
constellations in the sky, many
of these showers come from
the same constellations.
Rather than designating
them “Perseids,” “Perseids 2,”
and so on, astronomers name
the shower after the visible
star nearest the radiant —
for example, the Alpha
Capricornids. They also
use other modifiers, so you
also may encounter the
Northern Daytime Omega
Cetids and the Southern
Daytime Omega Cetids.
2020 forecast
Although this year’s Perseids
peak August 11/12, the shower
actually begins when Earth
reaches the edge of the mete-
oroid stream July 17 and con-
tinues until our planet exits it
August 26. Of course, the fur-
ther you observe from the peak,
the fewer Perseids you’ll see.
The main question each
year for any meteor shower
is: “How many will we see?”
Meteor observers start with
a number called the zenithal
hourly rate (ZHR). The ZHR
for any shower is the number
of meteors per hour that an
observer could see under a
dark sky (no scattered light),
assuming the radiant is at the
zenith (the overhead point).
The Perseids’ ZHR is 110.
As the saying goes, though,
your mileage will vary. If you
start observing on the 11th
at the end of astronomical
twilight (the moment no
sunlight remains in the sky),
you can expect to see perhaps
20 Perseids per hour. Thatnumber will increase as the
radiant climbs higher. When
the Moon rises, its light will
mask some of the fainter
meteors, although the bright
ones will still be easy to see.
Observers generally record
higher hourly rates after local
midnight because Earth has
rotated so its night side is
heading into the stream.
Before midnight, meteors
must catch up with Earth. So,
after balancing the increased
meteor rate after midnight
against the extra light from
the Moon rising higher, a goodguess would place the maxi-
mum hourly rate at between
50 and 75. Not too shabby!The plan
As much as or more than any
other astronomical observ-
ing, meteor watching is a
social event. You can watch for
10 minutes or 10 hours. The
atmosphere is casual, so you
can carry on conversations,
although your personal taste
in music may be distracting for
most observers, so please don’t
crank up your stereo. (I once
almost got into a fight withThis bright Perseid cast a colorful reflection on a lake at Cherry Springs State Park in Coudersport, Pennsylvania,
August 6, 2016. LARRY KUHN