64 ASTRONOMY • JUNE 2020SECRET SKY
Rainbows, those bridges of prismatic wonder,
have enchanted observers since biblical times
— although the poet John Keats apparently
blamed Sir Isaac Newton for abolishing the rainbow’s
poetry by reducing the sight to a strict code of color.
While this is typical, rainbows can, under certain con-
ditions, be monochromatic. That’s not to say they’re
dull. On the contrary, a monochromatic rainbow is a
sight arguably more glorious than the more common
technicolor wonder itself, chief ly because it’s red.The cause
Red rainbows arise from the same atmo-
spheric conditions that cause the Sun to
change its color from white to red as it
sinks from a more lofty perch to within
about a degree of the horizon. As Dutch
astronomer Marcel Minnaert explains in
his classic The Nature of Light and Color in the Open
Air, “Nature is here showing us the spectrum of the
sunlight, and demonstrating how its composition
changes during sunset.” The cause is the scattering of
the shorter wavelengths (by air molecules, dust, and
aerosols) during light’s long trek into the lowest and
densest part of Earth’s atmosphere.
Because only a setting or rising red Sun will cause a
red rainbow in the opposite part of the sky, we see the
bow at or near its maximum height above the horizon,
42 °. The lower the Sun is in the sky, the higher the rain-
bow, and vice versa. To see the phenomenon then, anobserver needs to be between a low red Sun and a rain
shower looming in the opposite direction.A sighting
On occasion, a red rainbow can form with the Sun a few
degrees below the horizon — and that is what occurred
in Maun, Botswana, on January 9, 2020. My wife,
Deborah Carter, and I went outside to enjoy a post-
sunset sky infused with fiery hues. As the display faded,
we turned and saw a partial red rainbow in the east.
At first, a hint of yellow-green could be seen right
after sunset. But as the minutes ticked away, so too did
all colors except for red. We were equally amazed to see
a double rainbow companion with Alexander’s dark
band in between, as well as at least a first-order super-
numerary bow associated with the primary bow, all of
which glowed red. The bow gradually faded, disappear-
ing a full 19 minutes after sunset, with the Sun nearly
5 ° below the horizon.
Had we been in a more open environment without
trees, we would have likely seen the lower part of the
bow “screened off,” as Minnaert explains, by the shadow
of the horizon, “so that [the] bow appears to begin at
some height above the horizon.”Historical precedent
Our view was reminiscent of a
more spectacular display in
July 1877, which Silvanus P.
Thompson witnessed over
Lake Lucerne in Switzerland,
as described in the September
1881 issue of Nature. Thompson
says the rainbow “showed only
red and orange colours in place
of its usual array of hues. No
fewer than five supernumerary
arcs were visible at the inner
edge of the primary bow, and these showed red only.”
Thompson went on to surmise (curi-
ously) that the phenomenon could not be
very rare, “from the circumstance that in
pictures of the rainbow red and yellow are
frequently the only colours set down by
the artist.” As an example he notes a semi-
circular red and yellow rainbow that
appears in Raphael’s Madonna di Foligno,
painted from 1511 to 1512.
By the way, other natural phenomena can create a
red rainbow, particularly at night. For instance, I once
observed a red bow in an approaching rainstorm off the
southern coast on the Big Island of Hawaii — created
by the reddish-yellow glow of molten lava that had
erupted from Kīlauea.
As always, send your observations and thoughts to
[email protected].Spotting a single-colored arch is an unusual sight.
Red rainbows
ABOVE: The
photographer
snapped this image
of a red rainbow with
her iPhone five
minutes after sunset.
The image shows dim
supernumerary and
secondary bows.
DEBORAH CARTER
RIGHT: Raphael’s
Madonna di Foligno
shows a monochrome
rainbow. PINACOTECA OF THE
VATICAN MUSEUMS, ROME/
WIKIMEDIA COMMONS
Rainbows can,
under certain
conditions, be
monochromatic.BY STEPHEN
JAMES O’MEARA
Stephen is a globe-
trotting observer who
is always looking
for the next great
celestial event.BROWSE THE “SECRET SKY” ARCHIVE AT
http://www.Astronomy.com/OMeara