Page 61 - BBC Sky at Night - September 2017 UK

THE SKY GUIDE SEPTEMBER 61

THE SKY GUIDE CHALLENGE

The most southerly star you can detect from your regular observing site
-25° -26°
¡
-26° -27°
-27° -28°
-28° PISCIS AUSTRINUS -29°
-29° e -30°
_ Fomalhaut
-30° ¡ -31°
o
-31° a -32°
` + f
-32° b -33°
a p -34°
-33° _
-34° -35°
/
-35° -36°
a
-36° MICROSCOPIUM -37°
-37°° GRUS -38°
h -39°
-38°
e 1 -40°
-39° + 1 2
-40° l + 2 e -41°
-42°
-41° b 1
-42° b 2 -43°
Þ Fomalhaut is south at 01:20 BST (00:20 UT) on the 1st and four minutes earlier each following day. Gamma Gruis is south an hour before it

Our minds tend to should only appear for an relatively straightforward spot. Identify a number of stars
compartmentalise the sky instant when due south. Once you’ve found it, its host around your theoretical
into regions that are easy However, this threshold is constellation of Pisces limit throughout the year
to view and those which somewhat relaxed by Austrinus, the Southern Fish, and keep a log of your
are harder to examine. atmospheric refraction. can be used to push your attempts. Different results
One common example of The atmosphere refracts southern limits. Mag. +4.2 can be achieved using
the latter is the murky part (bends) light. The closer to Delta (b) Piscis Austrinii is binoculars and telescopes.
of the sky very close to the the horizon you’re trying 3º south-southwest of A camera on a fixed tripod
horizon, where the stars to see, the more refraction Fomalhaut at a declination pointing south can also be
appear dimmer and less occurs, allowing us to see of –32.5º and is dimmer, so it a great help. Capturing an
easy to identify. a little farther south than will be harder to find. South exposure between one and
This month, our challenge maths predicts. Refraction of the Fish’s tail is mag. +3.0 five minutes long should
asks how far south you can also varies according to Gamma (a) Gruis, in the produce a star-trailed image,
actually see. In theory this atmospheric conditions, so constellation of Grus, which which may make finding your
should be easy to answer the limit is not a constant. has a declination of –37º 17’, elusive southern declination
so long as you know your If your view to the south a much trickier prospect still. limit that much easier.
latitude. In the northern is built-up and obscured,
hemisphere, subtract 90 you’ll have a false elevated Gamma Gruis, just visible
from your latitude to give horizon and your limit will above a low cloud bank
the lowest theoretical be further north. A flat from 50.75ºN
declination visible due south. seaward southern horizon
For example, from 53ºN , the is about as close to perfect
lowest theoretical declination as you can get, but if you live
should be 37ºS (53 – 90 = –37). at altitude, an elevated view
Of course, it’s never that to the south should give you
straightforward. an additional advantage.
The first issue is to identify A good starting point is
a bright star exactly on your Fomalhaut (Alpha (_) Piscis
declination limit. You also Austrinii), the most southerly
need to consider how close to first-magnitude star. Shining
your limit you can go. For at mag. +1.2 and located at a PETE LAWRENCE X 3
example, given a perfect declination of –29.5º,
horizon a star on your limit Fomalhaut should be a

skyatnightmagazine.com 2017