Jellyfish enemies, Balanced gravity fields, Glow-in-the-dark
Q: I've read the natural enemy of the jellyfish
is a centephus. What is this? (Linda, Bourbon, Missouri)
Jellyfish. Photo courtesy of Rich Harbison,
Woods Hole Oceanographic Institution
A: I've consulted reference
books, surfed the Internet and was able to find only one reference to 'centephus'
ó ThinkQuest, which said, "the jellyfish's natural enemy
is the centephus", without describing
So I asked
P. Madin, world renowned jellyfish authority
of Woods Hole Oceanographic Institution. "I've never heard of 'centephus'
he emailed. "There is a nudibranch called Cephalopyge trematoides that is a
predator on siphonophores [transparent floating medusa-like animals], and possibly some jellyfishes."
If Madin has never heard of the centephus, I
doubt it exists.
The enemies of most jellyfish are
Q: At what point
between the Earth and the Moon will their gravitational fields cancel each other (i.e., at what
point will a small mass experience zero net force)? (Nabeel, Montreal,
A real image of the Earth and the Moon (a barely visible dot at the far
right) showing the true-scale distance
between them. The white arrow shows the balance point of their two gravitational
Image courtesy of NASA/Jet Propulsion Laboratory/Arizona State University.
A: We can sneak up on the answer by
considering aspects of gravity. The greater the mass is, the greater
the force due to gravity. Thus, where Earth's and the Moon's gravitational
fields balance depends on their relative
masses. Earth is 81 times more massive than the Moon.
the balance point is far from Earth. How far? That depends also on the
distance squared, since both gravity fields decrease with the square of the
distance. Doubling the distance reduces the gravity force by one fourth.
Combining both effects (mass and distance
squared), we find the distance squared from Earth to the balance point is 81 times
the distance squared from there to the Moon. Thus the balance point is
about nine tenths of the way from Earth's center to the Moon's center
ó or, about 345,600 kilometers
Click here to see the
This 'balance point' is
one of the
'Lagrangian points' ó several positions where a small mass experiences no net
gravitational force from two massive bodies, in this case, Earth and the Moon.
The mass, however, will experience a force.
"It will be moved around as the Moon revolves around the Earth," emails
astronomer Robert Massey
of the Royal Observatory Greenwich in London. A
similar set of Lagrangian points exists between the Earth and the Sun; the
solar observatory SOHO is at one of them.
As a result, SOHO maintains an orbit locked to the
By the way, in his 1865 book, From Earth to
the Moon, Jules Verne predicted weightlessness at the gravitational balance
point. His concept, though, was slightly flawed because he overlooked
freefall effects. "Jules Verne
believed this would be the only point astronauts
on a journey from the Earth to the Moon would experience weightlessness," Massey
Gravitational Force, University of Pennsylvania
Moon, Royal Observatory Greenwich
solar and heliospheric observatory (SOHO), Royal Observatory Greenwich
Gravity, Astronomy Answer Book, University Utgrecht
Q: Is there something safe to add to food to make it glow? (Blane, Edmond, Oklahoma)
of luminous bacteria, photographed
by their own light. Photo
courtesy of J. Woodland Hastings, copyright, used with permission.
A: Toy makers add phosphor to toys to make
them glow. Zinc sulfate, a common toy additive, is not safe to eat.
On the other hand, people have been eating
glow-in-the-dark seafood for some time, by and large safely. The uncooked
food produces a bright, blue-green light in the dark, due to bacteria that emit
light (a phenomenon called bioluminescence).
In 1991, for example, a woman in Santa Barbara,
California bought raw, red snapper fillets for dinner. "She cut ends off
the fish and put them into her cat's dish and cooked the rest. In the
middle of the night she got out of bed to put her cat outside and noticed the
fish in her cat's dish emitted a green light," reports microbiologist
Patricia Sado of the US Food and Drug Administration at Bothell, Washington.
Sado analyzed the sample and found two strains of bioluminescent bacteria: P.
phosphoreum and V. logei. Both types live in deep sea fish.
Cooking kills the bacteria, so cooked seafood doesn't glow unless it's been
contaminated with raw bioluminescent seafood.
"So far as is known, ingestion of luminous
bacteria of any species is not harmful to humans (or cats)," emails biologist
Woodland Hastings, a Paul C. Mangelsdorf Professor of Natural Sciences at
Harvard University. "However, an infection can be harmful (or even fatal)
in humans, as is true for many bacteria, which might otherwise be harmless."
One unfortunate 72-year old man died of an
infection thirteen days after receiving a puncture wound in his leg while
cleaning fish on the Gulf of Mexico. The responsible bacteria were V. vulnificus, a bioluminescent.
Our future may hold safe bioluminescent beer and
champagne. A biotechnology company, Prolume, based in Pinetop, Arizona,
plans to use jellyfish luciferase, the chemical
that produces a
jellyfish's glow. Calcium in the beer
or champagne will trigger light emission.
Seafood by Patricia N. Sado, FDA
Growth and luminescence of the bacterium Xenorhabdus luminescens from a
human wound by Pio Colepicolo, Ki-woong Cho, George O. Poinar and J.
Woodland Hastings, Harvard University
Research by J. Woodland Hastings
sea by April Holladay, WonderQuest
glows: Why, What, How long by April Holladay, WonderQuest
(Answered Nov. 13, 2006)