COTM: Capricornus

Darren McDowell

Well, this is my first go at doing a COTM so if it is less complete than other writers please excuse me. All estimates come from either the HB Astroatlas, Hartung, or the Collins Pocket Guide to Stars and Planets.

Capricornus is an ancient Greek constellation depicting a sea goat, and is quite prominent in a dark sky as a triangular constellation behind Sagittarius. It covers an area of 414 sq degrees and midnight culmination is on the 5th August. This constellation gave its name to the Tropic of Capricorn as the Summer solstice (for the Southern hemisphere) occurred in this constellation around 2,500 yrs ago. This is where the Sun reaches its most southerly point in the summer sky and is overhead at noon. While the solstice has now moved to Sagittarius we still keep the old name. So what does the constellation have to offer? Not much really for the deep sky observer, the only notable object being M30 which is quite easily seen in a small telescope. There are however a number of double stars which I will look at below. I used an 8-inch (20cm) f/6 Newtonian.

Alpha 1&2 (20h 18.1, -12 33') Mag. 4.2 & 3.6. These two make a nice visual double as well as being a double in their own right. Both are reddish in colour with dim companions. At 49x both the secondaries are just visible while at 203x they make a nice foursome in the eyepiece. The separation between the two primaries is 378" and about 35" for each of the secondaries.

Just to the west of Alpha is Nu (20h 20.6, -12 45') Mag. 4.8. Around 35" separation these two stars are easily visible at 203x, however not so at 49x. Sigma (20h 19.4, -19 07') Mag. 5.3. Easy split at both 49x and 203x with nice colour difference, the secondary being a reddish colour. Separation is again around 35".

The next three stars form a nice triple system within a 1 degree field, visible entirely in my 25mm eyepiece. Pi (20h 27.3, -18 13') Mag 5.2, 8.3 Sep. 3.4" PA 148. Unfortunately this star was unsplittable at 203x however I believe this was due to the merely fair seeing conditions. Rho (20h 28 9, -17 49') Mag. 5.0, 10.0 Sep 1.0" P.A. 199. As with the previous star I could not split them. Omicron (20h 29 9, -18 35') Mag. 5.9, 6.7 Sep 18.9", P.A. 239. Easily split in both the 25 and 6mm eyepiece (49x and 203x respectively).

Zeta (21h 26 7, -22 25') Mag. 3.7, 12.3, Sep. 21", P.A. 13. Very difficult for small apertures due to the brightness of the primary. I was unable to detect the companion in my 8" scope.

Now for the only notable deep sky object M30 (21h 40.4, -23 11) Mag 7.5. At 49x and 72x the cluster is compact with little resolution in the central region. With the 6mm, 203x, resolution is pronounced and the internal structure can be seen. It is probably best suited to a dark location being about midrange in size and brightness.