Nightskies_09-26

Jeff Green | Jul 02, 2009

by Leo Enright

Veteran stargazers know that the pleasantly mild evenings and nights usually experienced locally in the month of July often provide the setting for enjoyable and relaxing nights under a great canopy of stars – something seldom available at other times of the year unless accompanied by much colder temperatures. Both permanent residents and vacationers in our area can wonder at the great display of starlight – usually without much interference from the annoying glare of city streetlights and other forms of urban light pollution. During July, though the day is still very long, both sunset and end of evening twilight arrive a bit earlier each evening – by a bit less than one minute per day – with sunset on July 1 being at 8:45PM EDT and on July 31 at 8:25PM. Similarly, in the mornings the beginning of twilight and sunrise arrive later each morning by a bit less than one minute per day.

Under our wonderfully dark rural skies, this month we can see, in the advancing evening twilight, that the great Milky Way is the dominant feature of the eastern half of the sky. It sweeps prominently from the northeast to the southern part of the sky. During twilight as the earth turns and brings the objects in the lower part of the eastern sky upward to where we have a better view of them, we can easily notice that the Milky Way is far from homogeneous; in some places, actually, it appears very dense and in other places very thin. In fact, to the southeast there is one streak within the Milky Way within which there appear to be almost no stars at all. Astronomers call this “the dark lane” or “the great rift”, and they tell us that it is a huge cloud of interstellar dust blocking the light from the very distant stars beyond it. Try to notice this prominent dark lane within the Milky Way during every clear night in July. Remember as you view the Milky Way that what you are seeing is one arm of the great Milky Way Galaxy and that our Sun is just one star within this incredible collection of over 200 billion stars. Beyond our own Milky Way Galaxy, the telescopes of both professional and amateur astronomers reveal countless millions of other galaxies in our truly mind-boggling universe.

As our gaze moves southward past the great dust lane and further along the enormous river of stars, we approach the southern horizon and should see a constellation or grouping of stars that resembles a teapot. This is the constellation known to our ancient ancestors as Sagittarius, but the modern stargazer, with a much less active imagination, sees in their arrangement, not a mighty archer, but only a simple teapot. Modern astrophysicists tell us that in this very dense region of the Milky Way is the core of our galaxy – packed with billions of stars and probably a black hole – the densest object in the universe and something that swallows up stars and prevents even their light from escaping its grasp. Truly a great deal to think about while our gaze wanders along the Summer Milky Way.

Among the five bright planets all of them may be seen this month, but, as is sometimes the case, Mercury is the most difficult, even though there are two times in the month when it may be seen and in two completely different situations. For those who look very low in the eastern sky about a half-hour before sunrise, Mercury may be spotted just above the horizon during the first four days of the month. After that it appears too close to the direction of the sun to be safely observed and at mid-month its orbit takes to the other side of the sun as viewed from Planet Earth, and it emerges into view of the other side of the sun readily visible low in the western evening sky for a short while each night in the last week of the month beginning a half-hour after sunset – IF you have a completely unobstructed westerly view right down to the horizon. Binoculars may well be needed to assist in these twilight observations.

Saturn continues its performance of last month being visible near the bright stars of the constellation Leo the Lion in the western evening twilight, but as the month of July advances, both the planet and the stars of the constellation are seen slightly lower each night. Those who have a small telescope will be happy to be able to see the rings of this planet, but they should enjoy the view as much as possible in the early part of the month, because, as mentioned in a previous column, the angle of Saturn’s rings changes very noticeably this year and when they appear “edge-on”, as viewed from Earth, they seem to disappear completely. That is what will happen next month; so, grab as many views of Saturn’s “disappearing rings” as you can THIS month.

Jupiter rises about 11PM in early July, and late in the month it rises considerably earlier. There is no mistaking Jupiter, since it is by far the brightest object in the sky until Venus rises much later. The proximity of Jupiter to the giant outer planet Neptune gives me a chance to mentions that planet which I normally never do since Neptune is much too faint to see with the naked eye. Those who have in the past used a small telescope to observe Jupiter and its equatorial bands and its four brightest moons have a very rare treat in store this month. Between July 7 and 12 Jupiter and Neptune appear close enough to each other to be seen in the same field of view in most telescopes. In the eyepiece, Neptune appears upward from Jupiter and a there is a distant star about halfway between them. (Of course these two planets are not really close together. They are merely in line as viewed from Earth, with Neptune being far more distant than Jupiter.) It is extremely interesting to know that Galileo who first used the telescope for astronomical purposes 400 years ago this year actually saw the planet Neptune very close to Jupiter just as it is this month. (Of course he did not know it was another planet. He thought it was a very distant star but he did record it in a drawing he made of the stars that appeared in his telescopic view of Jupiter and its four brightest moons). I would be interested in hearing from any readers who are able to observe Neptune also while observing Jupiter on the dates given above.

Dazzlingly bright Venus rises in the east a couple of hours before sunrise, and reddish Mars, though not nearly as bright as Venus, is easy to spot for it is up and to the right from Venus. Over the month, the movement of both these planets in their orbits carries them apart from each other as viewed from Planet Earth. Careful observers will notice that by month’s end they appear three times as far apart as they were on July 1. All observers should notice that both Venus and Mars appear to move about among the distant stars of the constellation Taurus the Bull which contains two large star clusters quite familiar to stargazers everywhere – the Pleiades (or Seven Sisters), and the Hyades (or the Head of the Bull). Venus is very near the Pleiades from July 3 to 8 and Mars is near that cluster from July 9 to 15.

For three mornings in mid-July the waning crescent moon joins this gathering of two planets and two famous clusters for a dazzling display in the morning sky about an hour before sunrise – that is, on the mornings of July 17 (when the moon is ABOVE the two planets and two clusters), on the 18th (when the moon appears to the LEFT of the Pleiades and Mars), and on the 19th (when the moon appears to the LEFT of the Hyades and Venus). There is another Moon-Planet conjunction that will draw the attention of many stargazers. On the evenings of July 24 and 25 the thin waxing Crescent Moon will be seen low in the western evening sky as it glides past the planet Saturn; observe them beginning a half-hour after sunset.

Meteor activity usually increases in July. Many people are familiar with the long duration of the famous Perseids, which reach their peak in August, though early members of this shower are routinely seen in July. There is another shower that peaks in late July – known as the Delta Aquarids. Though it is active for several nights, its absolute peak is on the night of July 27-28, with the best prospect for seeing some bright meteors before dawn on July 28. Look in the eastern sky, and record the number seen in an hour, if possible.