Fiery Venus is a wonderful planet to look at, but you wouldn’t want to live
there! This is a good time to keep an eye on the second planet from the Sun
as it approaches Earth and delivers a dazzling sky show.
February 20, 2001 — During the “Golden Age of Science Fiction” in the
1930’s and 40’s, Venus was a frequent setting for space adventure stories.
After all, cloud-covered Venus is nearly the same size as Earth and it’s
only a little closer to the Sun than our planet is. Readers and writers
alike fancied Venus as an enticing safari planet — a steaming world-wide
jungle filled with unknown and exotic creatures.
Nowadays we know better, thanks to a parade of U.S. and Soviet spacecraft
that visited Venus dozens of times between 1961 and 1997.
Venus is indeed warm, but more so than early sci-fi authors suspected. The
surface temperature is ~860 F (460 C) — hot enough to melt lead! The air is
thick and steamy, too. The atmospheric pressure is about 90 times that of
Earth. And the steam …. it’s sulfuric acid, a corrosive mist that floats
cloud-like through Venus’s 96% carbon dioxide atmosphere.
Carbon dioxide is a greenhouse gas that traps infrared radiation beneath
Venus’s thick cloud cover. A runaway greenhouse effect is what makes Venus
even hotter than Mercury! The clouds also hide a forbidding terrain, strewn
with craters and volcanic calderas. There are no rivers, lakes, or oceans on
Venus — like Mars, Venus is bone dry.
If you were deposited on Venus by some unscrupulous space-tour company, you
would immediately suffocate, melt and be crushed. Which might happen first
is debatable, but it hardly matters. Venus is an awful vacation spot no
matter how you arrange the itinerary.
The best way to see Venus — surely one of the most hellish worlds in the
solar system — is from afar. And now is a great time to do just that.
On Thursday, February 22nd, Venus reaches its maximum brightness (visual
magnitude -4.6) this year. Hovering about 30 degrees above the western
horizon after sunset, Venus will be at least 7 times brighter than any other
star or planet in the night sky. It can even cast weak shadows! You can see
them before the Moon rises if you happen to live in a very dark area.
Venus is frequently mistaken for a bright star, an airplane, or even a UFO.
Indeed the planet is probably the most-often reported Unidentified Flying
Object! But if you peer at Venus for more than a few seconds it’s easy to
see that it must be a planet. Venus doesn’t twinkle like a star, nor does it
move rapidly across the sky as an airplane or a flying saucer might.
Why is Venus so bright? Although it’s not the largest planet in the solar
system, Venus is usually the one nearest to Earth. Furthermore, Venusian
clouds — the same ones that hide Venus’s fiery surface from inquisitive
astronomers — are excellent reflectors of sunlight. As much as 72% of the
light that shines down on Venus is bounced back into space.
When the Sun goes down on February 25th, sky watchers can spot Venus and the
slender crescent Moon pleasingly close together in the western sky. Be sure
to look before 8 p.m. local time, because that is roughly when the Moon will
set at mid-northern latitudes. If city lights are a problem in your area,
don’t worry. The pair are so bright that even city-dwellers can enjoy the
show.
Using a telescope you can see that there are actually two crescents in the
sky that night — the Moon and Venus. Even small telescopes are adequate to
reveal what Galileo first saw through a spyglass 400 years ago: Venus looks
like a croissant!
Like the Moon, Venus has a full range of phases. It can be “Full” when Venus
is on the far side of the Sun, “New” when Venus is between the Sun and
Earth, and a crescent at points in between.
Venus will become New on March 30th. Its darkened night side will face Earth
as it moves almost directly between our planet and the Sun — a situation
astronomers call inferior conjunction. During the days around inferior
conjunction, when Venus is as close as it can be to us, its whisker-thin
crescent will appear through the lens of a 34-power telescope as large as
the Moon does to the unaided eye!
But be careful, because Venus in late March will be less than 10 degrees
from the Sun. It is possible to see Venus during the day through a
telescope, and the temptation to look can be irresistible to amateur
astronomers. Observers should take precautions not to accidentally sweep
their telescopes across our star — focused sunlight can cause severe eye
damage.
If you do take care to watch Venus safely on March 30th — twilight is the
best time — you could be rewarded by something fantastic. At inferior
conjunction, the horns of Venus’s fading crescent can join tip-to-tip to
form a complete circle — the result of sunlight trickling through the upper
layers of Venus’s cloud cover. The short-lived phenomenon depends critically
on the Sun-Earth-Venus geometry and how light is bent through Venus’s
dynamic atmosphere — there’s no guarantee it will happen at all. But with a
little luck you can spot the rare and marvelous illusion of a ring-shaped
planet.
It seems those early sci-fi writers were right: Venus truly is an exotic
world, even if we prefer to appreciate it from a safe distance!
To learn more about Venus’s evolving phases and how to view them, see
“Venus’s Evening Sky Show” by Adrian Ashford in the March 2001 issue of Sky
& Telescope.
Editor’s Note: The author mentions that Venus is Full when it is on the
opposite side of the Sun. Can we ever see such a thing? Wouldn’t the Sun’s
disk hide that phase of the second planet? In fact, we can see a Full Venus
through a telescope, although it can be difficult with the bright Sun so
nearby. The Sun’s disk rarely blocks Venus, though, because that planet’s
orbit is tilted 3.3 degrees with respect to our own.
