The Dog Days of Summer
What are the “dog days” (also called the canicular
days;’ which means the same thing)? Most people know that
they are supposed to be hot and humid days of summer. Some
folks would venture to guess that they occur in August, the
month with the greatest reputation for humidity. And many
people believe that these muggy, sultry days get their name
from the idea that dogs can go mad under such meteorological conditions.
The truth about the name and the exact timing of the dog
days is far more complex and interesting. The dog days are
actually named after the Dog Star, the brightest star in
the constellation Canis Major–the Big Dog–and
by far brightest star in all the heavens. The Dog Star is
more properly known as Sirius (pronounced like “serious”),
a name from ancient Greek which apparently means some thing
like “scorching.” But Sirius is visible in the night sky in
winter. What would it have to do with the dog days of
summer? A star that is in the sky on winter nights is also
in the sky on summer days. Of course, even Sirius is not
bright enough to be spotted readily with the naked eye in
the midst of broad daylight. But ancient skywatchers were
able to figure out that Sirius must be accompanying the sun
across the daytime sky in the summer. And apparently as far
back as hundreds of years before Christ, one explanation of
summer’s heat was that brilliant Sirius was adding extra
warmth to that of the sun at this time of year.
No one is quite sure which exact dates should mark the
start and end of the dog days. Should the central date of
the period be when the sun is passing due north of Sirius
or when they rise at the same time? The former occurs on
July 2, whatever the observer’s latitude–but only in
our part of history. The latter–when the sun and
Sirius rise together depends on your latitude. From 40
degrees north latitude (Philadelphia, Denver, San
Francisco) the sun and Sirius come up together on August 4
and by August 15 Sirius rises a full hour before the sun.
(From 30 degrees, it’s about a week earlier.)
In light of the complexity of the situation and the
vagueness of how close the sun and Sirius have to be for
them to supposedly combine heat, it’s not surprising to
hear that many dates have been suggested for the dog days.
One tradition has them occurring from July 3 to August 11.
But other lore authorities claim they begin later (even as
late as July 30) and the duration claimed for them ranges
from about four to six weeks.
At one time or another, most of us have seen a dragonfly
and marveled at it: hovering or darting near us, it looks
like a brightly colored needle with big whirring wings.
Perhaps we should not be surprised to learn that so
fanciful-looking a creature really does have a strange life
Two questions often asked about dragonflies are whether
they bite or sting people and whether they have any
practical benefits for humanity. The answers are No and
Yes. Dragonflies, and their generally smaller, more fragile
relatives the damselflies, can’t sting and are not known to
bite people. They do have jaws, however, and they are quite
voracious in their use of them to bite and eat a variety of
other insects–including mosquitoes and flies, which
human beings consider pests. Which of course answers the
second question: Yes, dragonflies are quite beneficial to
us by eliminating large numbers of insects that we find
undesirable. In flight, they actually grab bugs and collect
them in their curve–dunder legs, as if in a bucket.
Dragonflies lay eggs near or in the water and in their
early, nymph stages feed on aquatic creatures and breathe
with gills–until the time comes and the adult
dragonfly bursts forth from its shell. You may have noticed
large numbers of dragonflies in August, darting back and
forth over a field after flies and mosquitoes. Is this a
concentration of the dragonflies that live around the
neighborhood? No. These impressive displays of dozens or
hundreds of dragonflies are groups from far away pausing to
feed during migration. Some of the larger kinds of
dragonfly do migrate–not thousands of miles as is
common for birds, but at least hundreds of miles.
Dragonflies can be distinguished from damselflies because
they rest with wings outstretched while most damselflies
rest with wings folded. Both varieties have four thin,
transparent, many–veined wings.
By far the most common and conspicuous dragonfly is the
green darner. This, fast, green-bodied dragonfly is most
common in the East but can be found almost anywhere in
North America that there is sufficient water.
Green darners can have bodies over three inches long and a
wingspan up to about four inches across. But dragonflies
are ancient insects–fossil remains date back to
250–300 million years ago–and some of the early
varieties were enormous. In the fossil record are
dragonflies with two and a half-foot wingspans!
Saturn’s Razor-Thin Rings
In August, the rings of Saturn turn
sideways–“edgewise”–to Earth for a second
amazing time. But unlike in May, this time the sunlit face
of them bursts into view and the planet soon after makes
its closest approach of the year.
Getting a good view of Saturn’s rings these next few months
is tricky (much depends on having a good telescope and a
night of steady atmosphere up on high). But if you like
astronomy at all you should make the effort to see this.
Not for over 40 years will similar views be visible! If you
don’t have even a small telescope of your own, seek out the
public night of the nearest planetarium or amateur
astronomy club (a huge listing of planetarium and club
addresses and phone numbers can be found in the September
issue of Sky & Telescope magazine, which
should be available at newsstands and libraries by the time
you read this).
What will you see under excellent conditions through a
superb telescope? Until August 10, an oddly flattened
yellow globe with one or two thin dark lines across it (the
darkened, nearly edge–on rings and their shadow).
Then, in the weeks after that date, the rings slowly come
back into light and into enough tilt to become visible
first as a needle, then as a thicker line of light. You may
see moons of Saturn as little star-like beads lining up on
that needle or string of radiance.