Wednesday, September 6, 2017 The Nugget Newspaper, Sisters, Oregon
17
Stars over Sisters
By Ron Thorkildson
Correspondent
The magnificence of last
month’s total solar eclipse
will likely stay with, perhaps
for a lifetime, the many thou-
sands of Central Oregon resi-
dents who witnessed it.
Though the weather was
mostly cloud-free, obscuring
smoke from wildfires hung
in the air in many areas. For
those in the path of totality,
the event delivered on all that
was promised, and right on
time. When the moon com-
pletely hid the face of the
sun, Sol’s corona popped
into view, along with a few
reddish-colored prominences
that thrilled onlookers. It is
hoped that those who live in
the Beaver state did all they
could to observe the total
solar eclipse, because the
next opportunity won’t come
again to Oregon until 2108.
We know there won’t be
a solar eclipse in September
because it’s not listed in any
eclipse tables. But let’s see if
we can figure out why such
an event can’t occur.
Solar eclipses always hap-
pen during a new moon, in
other words when both the
sun and moon have the same
celestial longitude. But they
don’t happen on every new
moon because the sun, moon
and the earth don’t always
lie along a straight line (a
condition called “syzygy”).
The reason for this is that the
moon’s orbit about the earth
is inclined about 5 degrees
to the plane of Earth’s orbit
round the sun, known as the
ecliptic. Such an alignment
can only occur when the
sun and moon are very close
to one of the two nodes, or
places where the ecliptic and
the moon’s orbit intersect.
Even when all the ingredi-
ents for an eclipse are there, a
total solar eclipse is still not
guaranteed. In its monthly
orbit around the earth, the
moon comes within about
225,622 miles (perigee) of
our home planet and moves
to as far as 252,087 miles
(apogee.) When it is near
apogee, the size of moon isn’t
quite big enough to com-
pletely cover the sun, result-
ing in a ring or halo around
the moon. This is called an
annular eclipse.
Why no eclipse in
September? New moon
occurs at 10:29 p.m. PDT on
PHOTO BY JIM HAMMOND
Diamond ring effect and outer corona captured by Jim Hammond from
Rimrock Ranch.
September 19. If an eclipse
were to occur, it would appear
on September 20 to inhabit-
ants somewhere on the other
side of the world from Central
Oregon. The moon’s distance
from the earth on September
19 will be approximately
237,266 miles, slightly less
than its average distance. The
size of the moon’s disk, there-
fore, would likely be large
enough to produce a total
eclipse somewhere.
The problem, then, must
be in the syzygy alignment.
And indeed it is. The celes-
tial latitude of the moon on
September 20 is 4 degrees,
39 minutes, while that of the
sun is 1 degree, 6 minutes.
This means that the moon’s
shadow overshoots the sun
(is above it) by more than 3
degrees. And since both the
sun and moon measure only
about half a degree of the
sky, this is not nearly close
enough to produce an eclipse.
The dwindling daylight
hours this month signal an
upcoming change of season.
At 1:02 p.m. PDT on Friday,
September 22, autumn
will arrive in the Northern
Hemisphere as the sun slips
across the celestial equator on
its journey south toward the
Tropic of Capricorn. When
this happens the duration of
day and nighttime hours will
be roughly equal everywhere
on the earth.
The celestial sights on tap
for September can’t begin
to compare to the August
eclipse; few, if any, can. Still,
there are a couple of very fine
early autumn objects worth
mentioning.
Globular clusters M2
in Aquarius and M15 in
Pegasus are both considered
by experienced observers to
rank in the top dozen bod-
ies of their type in the north-
ern sky. These spherically
shaped clusters are normally
ADAM BLOCK/MOUNT LEMMON SKY CENTER/UNIVERSITY OF ARIZONA
Globular star cluster M15, found in the constellation of Pegasus, lies at a
distance of about 33,600 light-years.
comprised of very old stars
that are thought to have
formed shortly after the uni-
verse came into being some
13.8 billion years ago.
At a diameter of 175
light-years, M2 is one of
the largest known globular
clusters. It is rich, compact,
somewhat elliptical and lies
about 37,500 light-years
away at the outer edge of
our Milky Way galaxy. M15
is some 33,600 light-years
distant and is one of the
most densely packed globu-
lars. Astronomers believe it
has undergone a contraction
known as “core collapse” and
contains an enormous num-
ber of stars surrounding what
may be a central black hole.
Both clusters are thought to
be approximately 13 billion
years old.
To learn more about the
night sky, or to share your
knowledge with others, con-
sider attending the next Stars
over Sisters Star Watch to be
held on Saturday, September
23. Proceedings begin at 7:30
p.m. with a presentation in
the Sisters Park & Recreation
District (SPRD) building.
After the talk, all are invited
outside to observe the night
sky through telescopes pro-
vided by local amateur astron-
omers. The event is free.
On Tuesday, September
19, the Sisters Astronomy
Club will hold its monthly
meeting at the SPRD build-
ing. It begins at 7 p.m. and all
are invited.
Fresh Take on the Old West
E x p e r i e n c e t h e h e a r t o f S i s t e r s
i n o u r 1 0 5 - y e a r - o l d , r e s t o r e d ,
h i s t o r i c b u i l d i n g .
Our food is fresh,
locally sourced
& chef-created.
Always,
family friendly.
Enjoy live music
Saturday nights
541-549-RIBS | SistersSaloon.net
190 E. Cascade Ave., Sisters
Hope for a child. Change for a nation.
There are a million perfectly understandable reasons not to help.
Thankfully, love trumps them all.
$37 a month. All the difference in the world. Sponsor a child with a local organization at
HopeAfricaKids.com
This ad sponsored by The Nugget Newspaper.