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Transcript
II
June 2007
Highways in the Sky (Asterisms)
The most frequently asked question I’m hearing at stargazing
meetings is: “Where is it?” The stars are up there and for all
practical purposes they stay put. Our Earth, however, is rotating
around its axis.
Look at a star and take note of its location. One hour later the
star has moved 15 degrees, going around the polar star (Polaris),
just like the Sun. Relative to other stars the star's position has not
changed at all. All stars circle around Polaris at 15 degrees/hr. And
that is because the Earth rotates that fast and its axis lines up with
Polaris!
To make things interesting, our earth goes also around the Sun
in one year. That's 30 degrees per month (roughly 1 degree per
day). If I look up (to the south) at a star at 9:00 PM, then at 11:00
PM it has turned westward by 30 degrees. But if I look a month
from now at the same star at 9:00 PM again, it has done the same!
Think about it and you might get the idea that star charts can be
drawn with time and date markings on it; you'll find some details
further explained on our web-site (http://www.wyalusing.org);
under <star gazing>, click <Lessons>, and select "Highways and
Coordinates".
There are two problems with the star charts. The constellations
are so nicely drawn on the maps, but no such lines or pictures in
the sky to help you out. The second problem lies in the fact that we
see the stars as projected on a sphere, the celestial sphere or simply
‘The Sky’. Maps are drawn on flat pieces of paper. Pounding a
sphere into flat pieces causes distortions, which make things hard
for beginners. Laser pointers are a great help for me in pointing
objects out to you. Good sky charts are even better, but you have to
practice to use them. Let me show you:
Earth’s axis points right to the pole star… well almost. It misses
it by ‘that much’, about ¾ degree. Where is Polaris? Do you
remember the trick? Find the Big Dipper, that pan with a handle;
extend the line going from the bottom end star of the pan to the top
star of the pan, away from the handle. At about 6-7 times that
distance along the line is Polaris. The end of the handle is about
straight overhead in early June around 10 PM and the pan is to the
West.
Circumpolar drwg
Can we use the Big Bear for other directions in the sky? Yes,
we can! You do not think it looks much like a bear? Neither do I,
so let’s call the constellation just UMA (remember Ursa Major?),
pronounce it as ‘you Ma’. UMA is the constellation and Big
Dipper (that ‘pan’ of 7 stars) is the “asterism”. The brightest one,
alpha, at the far end and top of the dipper (also called Dubhe, like
DO-be) and ‘eta’ (or Alkaid, like all-KAID) at the end of the
handle, mark 25 degrees of arc in the sky. By the way, the sun and
the moon are ½ degree-of-arc wide in the sky. Your pinky’s nail,
held at arm’s length, covers 1 degree of the sky. The rest of your
outstretched hand can be used too, three fingers closed together is
5 degrees; ring and index finger spread out is 10; pinky and index
finger, spread as far as you can, cover 15 degrees; and finally
check that pinky-to-thumb matches the 25 degrees from Dubhe to
Alkaid . Checked it?
Now we can go to the sketch and you should be able to find the
stars in the clear night sky, using your calibrated hand. Is the
sketch a bit too small? For me it is; send me an e-mail and I’ll send
you the JPEG file so that you can print it out B I G.
Big Sketch
Since it is summer we go for the ‘summer triangle’. This is
another great road sign or asterism in the sky.
Let us find Deneb, an easy-to-find and bright star. We go from
beta to alpha UMA (that is the side of the dipper-pan away from
the handle), continue 15x this distance (through Polaris again) and
then curving through the 2 stars forming the SE side of the
‘house’-like asterism (in Cepheus), that is 60 degrees of arc or two
pink-to-thumb distances plus a little. There is Deneb or alpha
CYG, the tail of the Swan (or the top of the Northern Cross). As a
check, you can imagine a circle around Polaris from Alkaid (end of
Big Dipper’s handle) going 50 degrees to Vega (will be at your
Zenith on Aug 1 at 10 PM). Along that circle continue 20 degrees
clockwise and you should be at Deneb again. Early September at
10 PM Deneb will be in Zenith, straight above, and Vega will have
moved on to the West.
Deneb and Vega (alpha LYR) are 20 degrees of arc apart. Now
make a triangle, point down and you come to Altair (all-TER) or
alpha-AQUILA (ACK-will-uh), in the constellation The Eagle.
Don’t think it looks like an eagle? Then likely you did not see a
swan in CYGNUS either. You have to use a lot of imagination.
Otherwise just see the triangle with its 20 degrees base and 30 arcdegrees sides; that’s our road sign anyway.
More “highways” around the Big Dipper are in the drawing,
with distances expressed in degrees. Use your stretched arm and
calibrated hand and be on your way.
Now go way down along the Deneb-Altair line, twice that
distance, and you might see the Teapot (in Sagittarius, like sai-jeTER-ee-us, the Bowhunter) just above the horizon. It has only
second and third magnitude stars; if it is not yet above the horizon
wait a while; it is a neat constellation. You can find the Keystone-4
of Hercules between Vega and Arcturus. These four are only third
magnitude stars, but the Keystone quadrangle is still easily
recognizable. Going from Vega through Deneb, twice that distance
and you’ll notice the fuzzy M31 or large Andromeda galaxy (looks
interesting through binoculars or telescopes).
On the line from Vega to Altair you’ll find a little parallelogram
of third and fourth magnitude stars. With the bright Vega so
nearby, you can easily miss the one in the corner near Vega. Try
hard or use binoculars. We will come back here later!
At our latitude of almost 43 degrees, Polaris is way down (47
degrees) and North of Zenith. Begin July Vega is right above you
at midnight. If you draw a large circle in the sketch with Vega as
center, you have the picture of what you see in the sky. To the
South you’ll be able to see a little (20 degrees if clear near the
horizon) farther. Two hours later and also begin August at
midnight; you should draw the circle around Deneb.
Likely you remember: ‘follow the arch of the Big Dipper’s
handle to Arcturus (in Bootes or ice-cream cone), continue and spy
Spica in Virgo’. Practice a bit more and you’ll never be lost again
in the night sky. You know your way around!
From November to early April you have also Orion to guide
you. Try it using this drawing.
Highways around Orion (winter)
You might consider buying the “Seasonal Star Chart”, published
by Discovery Channel, $ 20. The 18” by 12” map has the horizoncircle as a cutout and you can rotate the star chart behind it. Timeof-night and day-of-month markings help to reveal the right
“view”. Rand McNeally publishes a smaller one for $7, also called
a ‘plani-sphere’. However, you might be better off spending $20
on SKY&Telescope’s Pocket Sky Atlas by the well-known
cartographer Roger Sinnott. With your highways-in-the-sky skills
you just mastered, you’re ready for it! That little atlas has 8 groups
of 10 maps, very logically and clearly organized, showing 30,000
stars and 1500 interesting deepsky objects. It is 7” by 9”, requiring
thus a rather big pocket.
With the binocs (binoculars) you saw a lot more stars and much
easier, but the stars were still just little point sources of light. How
come? Our eyes have lenses with 3/8 to ½ inch diameter (pupils).
The binocs have 1.5 to 2 inch lenses and can bring the light so
caught into your eyes, gathering thus 15 times as much light as
compared to looking without the binocs. Remember the 2.5 rule of
a mag 1 increase? 15 x corresponds with 2.5 x 2.5 x 2.5, this means
we can now see to mag 8 instead of 5! The stars are so far away,
many light-years (1 l-y = 6 trillion miles!), that you still see them
as points.
A useful trick for orientation on star-charts is via the RA of your
meridian (line due South from your zenith). You’ve noticed that
the latitude on Earth relates to declination in the sky and longitude
is akin to “RA”, Right Ascension (and no, I don’t know where Left
Ascension is). RA is measured in hours and minutes. Since 24
hours comes to 360 degrees (one full rotation of Earth), 1 hr RA
equals 15 degrees; and 1 RA-minute is 15 arc-minutes. There is a
small correction since Earth revolves around the Sun in one year;
you get in effect one “extra” rotation in one year. You can figure
out how small the correction is: 360 degrees in 365 days, 1 year,
amounts to almost 1 degree per day. That is 1/15-th of 1 hr RA or 4
RA-minutes. The sidereal day, star-day, is only 4 minutes shorter
than our Earth-day. But keep in mind that in the course of a whole
year the star-time is falling 24 hrs short of our regular clock.
On January 1 at 5:21 PM your “South” is at 0 h RA at WSP
(Wyalusing State Park at 91 deg. longitude); believe me. For every
month later you add 2 h (and add for days 4 min/day) to it and for
every hour past 5:21 PM, you add 1 (you figured that out already,
I’m sure; and 1 min/min of course)). When there is daylight
savings time you have to subtract 1 from your answer (don’t blame
me, I hate DST).
You want an example? OK: July 7th here at the Park at 10:00
PM (note that daylight savings time is in effect). From January 1 to
July 7th is 6 months and 6 days; add 12 h-24 min. From 5:21 PM to
10:00 PM is 4 h-39 min; DST makes you subtract 1. My due south
point is thus at RA = 0 + 12:24 + 4:39 - 1 = 15:63 = 16:03. If this
RA-result is larger than 24, subtract 24, of course.
You’re almost pros now! You’ve just calculated SIDEREAL
TIME! You can do it almost by heart. The meridian on your star
chart at that time and place is thus the N-S line having that RA!
You should be able now to locate, on the charts and in the sky, the
objects that are being mentioned in future columns here or
elsewhere. On our big sketch, as an example, draw your meridian
through Polaris and the RA-value listed on the outer circle; the RA
that you just calculated for Wyalusing and the time and date of
your observation. Line up that South with the actual S, likely you
know where that is or use a compass. Have fun crisscrossing the
Sky with you Highways sketches.
Clear Skies
GW
1 page from Sinnott’s Pocket Sky Atlas