Asterisms
An asterism is a pattern
created by reasonably bright stars and is often times even a
subset of a constellation. See below for an example of
the asterism the Big Dipper found in the constellation Ursa
Major.
Note:
As is the case with the
constellation line drawings the asterism lines are very subjective and there really isn't any one "standard"
set to choose
from, so far as I know. They change with culture and many
people simply do not see
them the same as others.
As a source
for many of my asterisms I used the book
"The Cambridge Guide to
the Constellations" written by Michael
E. Bakich and published by
Cambride University
Press as a reference. Because I could not find
any one source showing what they "should" look like there may be
error in my ways.
Although the
book above used TheSky astronomy software for creating the
finder charts for the constellations there wasn't a
graphical source for what the asterisms look like. A
list of the stars found in the asterism were included for
most and I took it from there. Artistic license was
held to a minimum I assure you and if something doesn't
appear "right" let me know!
Example. The Big Dipper Asterism in Ursa Major. Here is a
screen shot of TheSky display using the characture drawing
of the Great Bear Ursa Major. And again, there are
several representations even
for
just the Big Bear. <--- click here for details and
other line drawings.
Big Bear
Ursa Major/The Big
Dipper
The Big
Dipper Asterism within the constellation Ursa Major is shown
below. And the list of stars making up the asterism
from the Cambridge Guide are as follows,
50-Alpha Ursae
Majoris (Dubhe), 48-Beta Ursae Majoris (Merak), 64-Gamma
Ursae Majoris (Phecda), 69-Delta Ursae Majoris (Megrez),
77-Epsilon Ursae Majoris (Alioth), 80 Ursae Majoris (Alcor),
and 80 Ursae Majoris (Alkaid).
This is then
translated into the following "asterism" when these stars
are "connected".
Asterism "The Big Dipper"
(or Plough)
Subset "asterism" in Ursa Major
"The Big Dipper"
Close up
showing stars use in the "Big Dipper"
Star names and Greek
designations shown
To further
complicate this example the Big Dipper is also seen as a
"plough" in the UK. I have chosen to supply the
Big Dipper but the "plough" could be yet another asterism. However, when plotting more than one line
drawing depiction things get a bit confusing to say the least.
Plough
example
Available in the Asterism
file
(commented out by default!)
The asterisms
are included with the
latest update to TheSky.
WARNING! If you
have made your own changes, corrections or additions to the
Asterisms & Other text file/SDB make sure you backup
those changes or they will be lost.
The newer SDB files names Large Asterism Drawings and Large
Asterism Labels must be "added" to your existing active list
of SDB's (sky databases) using TheSky's
Data| Sky Database
Manager.
To the best of
my knowledge there are no rules or regulations here. And drawing your
own asterisms is quite easy to do, so have fun. The section
on drawing your own constellation lines applies directly to
asterisms.
Click
here for details on exactly how this is done.
NOTE:
Because of a recent email from someone expressing concern
along these lines I am currently researching this further. If anyone can supply me details on rules/regulations or
potential issues regarding the creation of and/or the naming of
asterisms please email me
so this page can reflect any pertinent changes. Thanks, I certainly don't want to break any rules or step on
any toes here.
Plotting the
asterisms is zoom level dependent. The asterism
drawings SDB files
work as follows. First I chose to break out the larger
asterisms like the Summer Triangle, The Winter Triangle, The
Diamond of Virgo, The Keystone, The "W" in Cassiopeia and other larger asterisms into their own
separate SDB file named Large asterisms. The smaller
asterisms are found in the default "Asterisms & Other"
SDB.
This way the larger (many degrees
in size) asterisms only plot when the zoom level is further
out. When the display is narrowed the smaller
asterisms will "kick" in (field of view smaller than
40
degrees). This is also true for their respective labels. See example below demonstrating exactly this.
Because The
Kite and The Trapezoid are both sharing stars in
Bootes the smaller
Trapezoid is not plotted until the field of view is smaller
than 40 degrees. Outside of this the larger Kite is
plotted. This is how the larger asterisms are
currently being handled.
Outside
40 degrees
and Inside of 40 degrees
The wider field asterisms are
there own separate SDB file. To search for these use
Edit | Find and then expand the Sky Database (SDBs) "Large
Asterism labels" as shown below.
Search
for a Wide field asterism
Large Asterism labels
The object type for the
Asterism drawings is "Reference Lines". This allows you to
plot them independent of the Constellation Lines. You
can also make a new Object Type called Asterisms if you
like. Then re-compile the data as object type "Asterisms"
and set the Asterisms attributes accordingly.
Click here for exactly
how this is done!
Make sure that object type
"Reference Lines" (or your own type) is set to Visible. Use View | Display Explorer and type in Reference Line (or
enter your own type by name) and
you will be taken directly there.
View | Display Explorer
Enable Reference Line(s)
plotting here
Now to set the attributes for
Reference line (color of the line, size of the line, type of
line, and font for the labels) use the Display properties
for Object Type Reference Lines as shown.
Set line
attributes here
Pick line color and Font
type and color for labels
Using
reference lines instead of Constellation lines allows you to
plot them both at the same time or one or the other. You can
also make a new Object Type called Asterisms if you like.
Then re-compile the data as type Asterisms and set the
attributes accordingly.
Click here for exactly
how this is done!
Use Edit |
Find and the entry for the Asterisms is located under the
Sky Databases (SDB'S) as shown below. These are the
smaller asterisms. The larger wide field asterisms have
their own SDB file named Large Asterism Labels.
See below for
details. If you are using
the Frame Object option the default frame for objects
without a size is 5 arcminutes!
Asterisms listed here
Find Dialog
NOTE: If you
are using the option "Frame Object" on the Find dialog
as shown above you
need to be aware of the following. The asterism labels
do not currently have a size associated with them.
Therefore "Frame Object" will not comfortably size TheSky
display accordingly as it does with other objects like
non-stellar data (i.e. NGC1976, NGC6611, etc.) because the
data has that data has the size of the object specified. However, if you choose
to over-ride the default zoom level from 5 arcminutes to
something larger like several degrees that can be done. See below.
When finding
(Edit | Find) asterisms in the list if you choose Find rather than center
and frame the field of view will not change from the
current. If you center and frame the objects you will
be placed zoomed in to the default field of view 5
arcminutes unless changed. Meaning you will be well inside
the asterisms for the most part and you will have to zoom
out.
The default
frame size for objects that do not have a size associated
with them is 5 arcminutes. This can be changed to a
much larger field to accommodate the Center and Frame
feature for asterisms by changing the default size using the
View | Non Stellar Options as shown here to accommodate
larger objects like the asterisms.
Unknown
object size framing
Choose something larger
than default
3600 arcminutes is 60
degrees
Make sure the Common
Non-Stellar labels are enabled using View | Display
Explorer. The asterisms fall into the category of
Common Name under the heading Non-Stellar labels.
View | Display Explorer
Enable Common Name Labels
Now the labels will appear. Remember that for clarity/clutter the smaller asterism
labels DO NOT show until you are in a field of view smaller than 40
degrees! However you can still search for them using
Edit | Find.
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