[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[at-l] Nekked Eye Astronomy


Starry Night's Fall Sky Tour: Easy Targets in the Northern Sky
Tue Oct 1, 9:29 AM ET
By Robert Roy Britt
Senior Science Writer, SPACE.com

While much of the sky offers daunting challenges to backyard astronomers, t=
he northern fall sky
includes familiar stars and star patterns that will make even the most caus=
al stargazer feel

We ended yesterday=92s tour of the western sky at the handle of the Big Dip=
per, so let=92s begin
there. Three bright stars -- Alkaid, Mizar and Alioth -- form the handle, o=
ff toward the northwest
and somewhat low on the horizon in late evening.

The Dipper itself is not a constellation, but it resides in one called Ursa=
 Major (the Great
Bear). Ancient cultures saw the Big Dipper=92s star pattern as a bear with =
a long tail. The idea of
a large ladle in the sky appears to be a modern American invention. [ More =
about Big Dipper lore]

In the Dipper, Alioth is the brightest star, shining at magnitude 1.75 on a=
 scale that measures
the apparent brightness of objects. The lower the number, the brighter the =
object. The brightest
stars are zero or first magnitude. Negative numbers are reserved for the mo=
st brilliant objects:
the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26=
.7. The faintest stars
visible under dark skies are around +6.

Even under bright city lights you should be able to spot Alioth and its fel=
low Big Dipper stars.
The next brightest is Dubhe, at magnitude 1.78.

North Star & Little Dipper

Another familiar name in the northern sky is Polaris, the North Star. To fi=
nd it, locate the Big
Dipper star called Merak, at the bottom right of the bowl. Draw a line from=
 Merak up through Dubhe
and continue it up into the sky. Stretch your arm out full length and measu=
re about three
fist-widths. There, just to the right of your line, is Polaris.

Polaris is a moderately bright star. It is called the North Star because th=
e North Pole of Earth=92s
axis of rotation points up toward it. For this reason, Polaris remains rela=
tively fixed in the sky
during the night, and Earth=92s spin causes the other stars to rotate aroun=
d Polaris.

Polaris is also the outermost star on the handle of the Little Dipper, whic=
h seems to pour into
the Big Dipper. The stars of the Little Dipper are dimmer, but under reason=
ably dark skies you=92ll
see them stretched out to the left of Polaris. Another moderately bright st=
ar, Kochab, serves as
the outer lip of the Little Dipper=92s bowl. [ More about the Little Dipper=

There=92s another easy-to-spot star pattern in the northern sky. Off to the=
 right of Polaris and
slightly higher are five stars that form a W, called Cassiopeia. This time =
of year, the W is on
its side in the evening. [The Fall Sky Tour will focus on the region around=
 Cassiopeia Tuesday,
Oct. 8]

Finally, off to the northeast and very low to the horizon at dusk is the br=
ightest star in the
northern sky, Capella, which shines at magnitude 0.06. Capella is the sixth=
 brightest star in the
entire sky. It resides in the constellation Auriga, the Charioteer.

Watch the sky move

If you have the opportunity to go outside twice during the night, you can u=
se some of the stars
discussed here to observe the sky=92s apparent counterclockwise motion arou=
nd the North Star.

Use the map on this page around 8 p.m. to locate Polaris. Find the Big Dipp=
er, which before 9 p.m.
will be slightly higher than shown on the map. Capella will be skimming the=
 horizon (in fact
you=92ll need a clear view of the horizon to find it before 9 p.m.).

Go out a couple hours later, say at 10 or 11 p.m., and you=92ll find Polari=
s in the same spot. The
Big Dipper will have dipped lower toward the horizon and scooted to the rig=
ht. Capella will have
drifted higher and to the right.

By midnight, the Big Dipper is directly underneath Polaris and hard to find=
 along the horizon.
Capella is high in the sky, at roughly the same elevation as Polaris and of=
f to the right. By
dawn, the Big Dipper has moved up and to the right, into the position Capel=
la held during the
evening hours.

Capella is now above Polaris.

In fact, Capella is almost directly overhead just before dawn, near the ima=
ginary point in the sky
called the zenith. In that region of the sky, Capella finds a rival for the=
 brightest "star." This
bright planet Saturn (magnitude zero right now) is not visible in the early=
 evening sky. It rises
in the east well after 10 p.m. and is best viewed during morning hours. But=
 we=92ll talk about that
tomorrow, when our Fall Sky Tour continues with a look at Easy Targets in t=
he Eastern Sky.

David Addleton
vocate atque non vocate deus aderit

Do you Yahoo!?
New DSL Internet Access from SBC & Yahoo!