SkyandTelescope.com August 2014 67Tuesday the 12th for the North American time zones. (For
more on this year’s bright-Moon Perseids, see page 50).
For viewers near 40°° north latitude, the radiant will be 15°
high in the north-northeast at the end of twilight.
The main reason why you see fewer meteors from a
low radiant is the same reason the ground is less bright
under slanting sunlight than when the Sun is overhead.
Particles coming in at a low angle get spread over a larger
area than they would if they were coming straight down.
That’s true whether we’re talking about photons hitting
the ground or meteoroids hitting the upper atmosphere.
The same eff ect is what causes the low Sun of winter to
heat the land less than the high Sun of summer.
Although you may see only one or two earthgrazers
in an evening, if you’re lucky, each is a memorable event.
For a chance at catching a true horizontal skimmer — a
rare but truly remarkable sight — you can begin watching
even a little before a shower’s radiant rises. Earth’s grav-
ity bends the trajectory of incoming meteors enough to
elevate a radiant that’s near the horizon by a few degrees
compared to its nominal position among the stars. (The
exact amount depends on the meteoroid stream’s veloc-
ity.) You’ll be able to follow such a long fl yer for perhaps
15 seconds or longer, a seeming eternity compared to
typical meteors. In theory, a large skimmer passing from
horizon to horizon, and arriving at only a little more than
Earth’s escape velocity, can shine in an observer’s view for
about a minute.
Watching for earthgrazers widens your scope to
include showers that we normally think of as Southern
Hemisphere specialties. So, northerners might add
the rich Eta Aquariids around May 6th to their meteor
calendars. On the northern side of the sky, the January
Quadrantids, with their north-circumpolar radiant that
lingers along the horizon all evening, off er excellent pros-
pects for evening earthgrazers in years when the timing
is right (the Quadrantid peak lasts just a few hours).
Similarly, skywatchers as far south as Australia can
keep an eye out for stray northern specialties such as the
Perseids, Geminids, and Quadrantids.
And you gain wider perspectives to the east and west
too. One of the year’s stronger showers is the Daytime
Arietids, which lasts for nearly the fi rst half of June.
During that time, it’s supplemented by the weaker but
similarly protracted Daytime Zeta Perseids from a radi-
ant nearby. The “daytime” label is not quite true. Both
radiants rise above the east-northeast horizon just before
dawn for the mid-northern latitudes. When the radiants
are high in the middle of the day, they might produce 40
to 80 meteors visible per hour if daylight weren’t pres-
ent — which suggests that they off er fair prospects for an
earthgrazer or two at the beginning of dawn. Have you
ever seen a meteor from a “daytime” shower? ✦Alan MacRobert had no trouble counting Perseids as a child
in the light-polluted suburbs of Boston.Whatever its brightness, any meteor that lasts long and travels
far is likely to be noticed by more people than a short, quick one.
Artist Frederic Church saw, and later painted, the great horizon-
tal skimmer that made news across the northeastern United
States on the evening of July 20, 1860 (S&T: July 2010, p. 28).