This is Dudley Observatory’s Skywatch Line for Friday, July 5 through Sunday, July 7, written by Sam Salem.
On Friday, Sun rises at 5:20am and sets at 8:35pm; the waxing crescent Moon rises at 8:30am and sets at 11:07pm. Look for the crescent Moon on Friday at dusk, in the western sky, about two finger-widths from the first magnitude star Regulus in constellation Leo.
Jupiter and Saturn are the sole remaining evening planets as both Mars and Mercury now lost in twilight’s glare. Jupiter, at magnitude –2.6, can be spotted low in the southeast as soon as dusk begins to fade. Your best chance for steady, detailed views occurs when Jupiter culminates at around 11pm. Saturn, at magnitude 0.1, rises only a few minutes after sunset. Saturn reaches the meridian roughly at 1am. Venus, at magnitude –3.9, is very slowly sinking into morning twilight and now rises just 50 minutes before the Sun. However, Venus can be seen only a few minutes before sunrise, when it sits just a little more than 5 degrees above the east-northeast horizon.
Late evenings this weekend are fine time for viewing star clusters and nebulas. The Coathanger asterism lies not far from the distinctive little constellation, Sagitta, the arrow. The asterism is made up of 10 stars ranging from 5th to 7th magnitude forming a straight line of 6 stars with a “hook” of 4 stars on the south side. An additional 30 or so fainter stars are sometimes considered to be associated as well. It does look like a coat hanger hanging upside down. The grouping is also known as Brocchi’s Cluster. The cluster was first described by astronomer Al Sufi in his Book of Fixed Stars in 964. The status of this group as a star cluster has changed in recent years. Several independent studies since 1998 have now determined that this object is not a true cluster at all, but rather a line-of-sight effect.
Look for the constellation Corona Borealis, also known as the Northern Crown. You’ll need a pretty dark sky to see it. The constellation makes the shape of the letter C. During the evening hours in July, you’ll be looking high overhead. Corona Borealis constellation looks like a half-circle, in the middle of which is a white jewel of a star called Alphecca or Gemma. Alphecca is Arabic, short for “the bright (star) of the broken (ring of stars)”. It is also called Gemma, which is Latin for “jewel”. The Crown is located more or less along a line between two bright stars, Arcturus in the constellation Bootes the Herdsman, and Vega in the constellation Lyra the Harp. Arcturus has already passed its highest point in the evening at this time of year and is slowly descending to the west. Vega is still high in the east on July evenings. Corona Borealis can be found between these two stars, closer to Vega. Alpha Coronae Borealis, Alphecca, is an eclipsing binary star, similar to Algol (Beta Persei). The periodic eclipses result in a magnitude variation of +2.21 to +2.32, which is hardly noticeable to the unaided eye.