Jeff Green | Jun 28, 2007
The Night Skies of July 2007:Venus and Saturn drift apart in July's night sky
by Leo Enright
The pleasantly mild evenings and nights usually associated with the month of July often provide a setting for the kind of relaxed stargazing that is seldom found at other times of the year. For summer vacationers as well as residents in our area, there are often July nights with beautiful star-filled skies, without the unwanted glare of city streetlights and other forms of urban light pollution. Locally, in July, sunset and the end of evening twilight arrive earlier each night by a bit less than one minute per day, with sunset on July 1 being at 8:45 p.m. EDT, and on July 31 at 8:25 p.m. Similarly in the mornings the beginnings of morning twilight times and sunrise times are arriving later each morning by about the same amount, that is, a bit less than one minute per day.
Under very clear, dark, rural skies of July, the great Milky Way is a dominant feature of late evening twilight as it sweeps prominently from the northeast to the southern part of the sky. Careful observation of it will show that there is a noticeable dark area within it – made up, we are told by astronomers, of interstellar dust which blocks the light from very distant stars behind it. (Try to notice this ‘dust lane’ as you sweep your gaze over the Milky Way high in the eastern sky in late evening.) Remember, as you look at this huge thing called ‘The Milky Way’, you are actually seeing one great arm of our Milky Way Galaxy, and our Sun is only one star in that vast river of stars that is collectively called The Milky Way. Astronomers tell us that there are over 200 billion stars in this galaxy. Beyond OUR galaxy, astronomical telescopes, even those of amateur astronomers, show that there are vast numbers of galaxies to be seen in almost every direction in the sky. Again, astronomers tell us something that is truly mind-boggling – that there are countless billions of galaxies in our truly incredible universe. As our glance sweeps southward over the Milky Way, we may easily notice that it thickens as we approach the southern horizon and see a constellation of stars that seem to form the shape of a simple teapot. This is the constellation that was known to ancient peoples as Sagittarius. To modern observers with less active imaginations, it looks not at all like an archer bearing a sheaf of arrows, but rather like a simple teapot with its handle to the left and spout to the right, and above the handle some people see also a group of four stars that appear in the shape of a small spoon, sometimes called “The Teaspoon”. On seeing these stars on the clear nights of July, we should remember that the glow of the Milky Way in, and around, the Teapot of Sagittarius is from many billions of stars at the core of the Milky Way Galaxy. Scientists tell us, too, that at the very centre of these stars there is probably a black hole, the densest object in the universe, and something that swallows up massive stars and prevents even their light from escaping its grasp. Try to recall what is really there the next time you see the Great Teapot in July’s southern Milky Way.
Among the five bright planets, Venus, the brightest, will probably be the first one seen in the western sky right after sunset. On Canada Day, July 1, we have the very unusual sight of the closest two-planet conjunction that is easily visible this year, since right beside Venus, on its right, we can see the planet Saturn, which many people have noticed drawing closer to Venus over the past month. On July 1, they appear less than one degree apart. (One degree is the width of a fingernail held up at arm’s length.) These two planets are not physically near each other; Saturn is actually far more distant than Venus, but their line of sight, as seen from Earth, is in the same direction. Watch both of them over the month of July as they do in reverse what was done during June. They will again appear to move apart from each other, night by night, and when they first come into view after sunset, they will seem to be closer to the western horizon and to take less time to set each evening as the month goes by. Careful observers may note that Venus reaches greatest brilliancy at mid-month, being bright enough to be seen during daylight, if one knows exactly where to look, and observers with a small telescope may notice that, as the month passes, Venus shows a larger disk, but its phase, like that of the moon, changes from a thin crescent to a very thin crescent. This means that the orbit of Venus is bringing it closer to Earth this month, and we can see less and less of the sunlit half of its disk. Jupiter is the extremely bright object in the southern sky, second in brightness only to Venus, and so in the evening twilight, it may be seen shortly after Venus is spotted and before Saturn is seen beside Venus. Jupiter is just above the distinctive star pattern of the southern constellation Scorpius, and not far above Antares, that constellation’s brightest star, which happens to be one of the reddest of the very bright stars in the sky. Again observers with small telescopes will be fascinated by the ‘night-to-night dance of the moons’ as the four brightest of Jupiter’s many moons are seen orbiting around this planet, a sight to fascinate the modern telescope owner, just as it amazed Galileo when he discovered those four moons 400 years ago. The fourth planet to become visible to the unaided eye each clear night will be reddish Mars which rises in the east at about 2 a.m. in early July, and a bit earlier each night until it may be seen above the eastern horizon by 1 a.m. in late July. The fifth bright planet, Mercury, is not seen in the first half of the month, but after July 15, it may be seen rising in the east about an hour before sunrise. Binoculars may be helpful in finding it since the glow of morning twilight will be evident in the eastern sky as Mercury rises.
The moon provides several interesting conjunctions this month. On the evening of July 16, beginning about 40 minutes after sunset, the western evening sky will feature a crescent moon appearing between brilliant Venus on its left and Saturn on its right. Regulus, the brightest star in the constellation Leo the Lion, will be just to the upper right of Venus. The following night, July 17, will have the crescent moon just above and to the left from the Venus-Regulus-Saturn grouping. Try not to miss either of those post-sunset displays. About a week later, there are two nights with interesting conjunctions in the southern sky. On July 24 from one hour to two hours after sunset, try to notice the gibbous moon below and to the right from the planet Jupiter and the star Antares and the other bright stars in the “Claw of Scorpius”. At the same time the following night, July 25, the moon will be below and to the left from Jupiter and those stars of Scorpius.
Much more information about observing and enjoying the summer sky is to be found in a popular and easy-to-understand book called The Beginner’s Observing Guide, which is now available at Sharbot Lake Pharmacy.