Sky and Telescope - November 2015 - 8

Letters
Earth's limb on the way to the Moon. Our
database includes, for each timing, the corresponding position angle around the umbra,
so in principle one could look for a topographic eﬀect. But cloud formations around
Earth's limb also block sunlight below 10
km, and obtaining the weather data for a
proper study would be challenging.

Moon. I'd start by asking my students if
they knew the distance around Earth's
equator. Although I considered my
students to be some of the best on the
planet, very few could answer quickly. We
did some math, spent some time "estimating" (guessing), and ﬁnally arrived at
the idea that the Moon's distance could
be expressed in terms of Earth circumferences (it's roughly 10).
Then we tackled the speed of light.
Together we concluded that in one second, with "space mirrors" strategically
placed, light would travel around Earth
roughly seven times.
I particularly enjoyed discussing the
Earth-Sun distance with them. We agreed
that 93 million of anything is too much to
grasp. Then I'd ask, "What is the easiest number to think about?" Eventually
they'd settle on "1," so we identiﬁed the

Astronomical Numbers
I smiled while reading Peter Tyson's
"Take a Number" (S&T: June 2015, p. 6).
During 35 years of teaching secondary chemistry, physics, and astronomy,
I spent a great deal of time helping
students understand the meaning of
large and small values. The numbers
and measurements mentioned by Tyson
include some that I stressed because they
are particularly signiﬁcant.
Take, for example, the distance to the

75, 50 & 25 Years Ago

A billion seconds is about 31.7 years.
Assuming that a galaxy has 200 billion
stars, and you counted them at a rate of
200 per second starting in 1983, you'd be
ﬁnishing right about now. To me, that's
more understandable and even more
overwhelming. As for Abell 2065, well, if
one could cover 1 light-year in 1 second,
then 35 years later . . .
Reinhard Oberberger
Calgary, Alberta

For the Record
✹ June 2015, p. 55: The angular diameters
of the Moon at apogee and perigee were
inadvertently interchanged.

Roger W. Sinnott

November-December 1940
Kindred Orbits "Most astronomers . . . are as
much annoyed at the continued existence of
Encke's Comet as at its peculiar behavior. Moving in such a small orbit, almost like that of an
asteroid, the comet is activated by fairly intense
sunlight at all times. . . . How it can continue to
show indeﬁnitely as a hazy diﬀuse object and
not be completely dissipated is truly a mystery.
We now add to its list of peculiarities an association with the extensive Taurid stream of meteors
[even though] the planes of the orbits are tipped
about 12°° with respect to each other. . . .
A new mathematical theory for the perturbing eﬀect of Jupiter's attraction showed that
Encke's Comet does not keep the same plane for
long periods of time. In less than six thousand
years the orbit plane tips around like the rim of a
wobbling top. . . . The most reasonable conclusion . . . is not that the Taurid meteors arise
from Encke's Comet but rather that they both
have a common ancestor, some large comet
that broke up . . . some
ﬁve thousand to ﬁfteen
thousand years ago."
Harvard astronomer Fred Whipple
was famous for such
linkages. Some 43
years after writing

Earth-Sun distance as one "something"
- 1 astronomical unit.
Frank Lock
Gainesville, Georgia

November 2015 sky & telescope

these words, he startled other astronomers when
he tied the Sun-approaching asteroid 3200
Phaethon to the annual Geminid meteor shower.

relation still holds true, though energetic particles
from solar "superﬂares" can trigger carbon-14
creation; see page 22.

November 1965
Tree Rings "It has long been known that there
is a correlation between the sunspot cycle and
the growth of trees, as measured by the thickness of their annual rings. Now another association between solar activity and tree rings is
suggested in Science by Minze Stuiver of Yale
University's Radiocarbon Laboratory. . . .
"Dr. Stuiver has analyzed various wood
specimens, but especially a section from a
Douglas ﬁr which grew between 1687 and 1951
at an altitude of 6,000 feet. . . . His measurements show a clearcut negative correlation: the
radiocarbon content of tree rings tends to be
high in years when sunspots are few.
"There is an obvious explanation of this.
When solar activity is low, more cosmic rays
from outside the solar
system can reach
the earth, increasing the production
of neutrons in the
upper atmosphere.
The neutrons combine
with nitrogen nuclei to
make more C14."
The negative cor-

November 1990
Cosmic Steppingstone "The Virgo cluster is
the nearest rich collection of galaxies. [Hence]
its precise distance is a critical link in determining the distance scale of the universe. . . .
Recent estimates, using a wide variety of 'standard candles,' range from just under 40 million
light-years to somewhat over 70 million. (The
ﬁrst estimate, that made by Edwin P. Hubble
and Milton L. Humason in 1931, was a mere 6
million light-years!) . . .
"Now George H. Jacoby and Robin
Ciardullo (Kitt Peak National Observatory) and
Holland C. Ford (Johns Hopkins University)
have derived a new distance of 48 ± 3 million
light-years, using planetary nebulae as their
distance indicators. . . . "
The uncertainty
in the Virgo Cluster's
distance seems to be
settling down. In 2007
a clever statistical technique, based on surfacebrightness ﬂuctuations
in the cluster's elliptical
galaxies, yielded 54 million light-years.

Sky and Telescope - November 2015

Table of Contents for the Digital Edition of Sky and Telescope - November 2015