Nightskies_09-30

Jeff Green | Jul 30, 2009

by Leo Enright

With August annually bringing days that are noticeably shorter and nights that are longer than July, stargazers are able to enjoy late evening views of the sweeping panorama of the Great Milky Way as well as the year’s most famous meteor shower.

Locally at the beginning of August sunset is at about 8:30 p.m. EDT, and evening astronomical twilight comes to an end at about 10:30 p.m., almost a whole hour earlier than it did a month ago. By the end of August our sunsets will be at about 7:45 p.m., and twilight will end at about 9:30 p.m. As can be seen from these numbers, sunset is averaging about a minute and a half earlier per evening in August and ‘end of twilight’ is averaging about two minutes earlier per evening.

August stargazers, both those who are permanent residents in this area and those who are annually drawn to the region because of many attractions – that may well have included dark skies unpolluted by the light pollution of our major cities – should take advantage of every possible opportunity to explore and learn more about the Summer Milky Way and the riches within its numerous constellations.

In the advancing darkness, as the time approaches that of the end of twilight, as was explained in last month’s column, we can still see, in the northeast, the distinctive “W” outline of Cassiopeia, marking the beginning of the trail of the “Summer Milky Way”, but the next very large constellation, Cygnus The Swan, or the Northern Cross, is markedly higher than is was in early July, being in fact, almost directly overhead. Directing our attention thence to the south-southwest, we should be able by now easily to recognize the “Dark Lane” or “the Great Rift”, to which readers were introduced last month, as the location of a huge cloud of dust in our galaxy, obscuring our view of millions of stars beyond it. Moving our gaze right down to the far southwestern sky, we should recognize the “teapot-shape” outline of the constellation Sagittarius – the very special constellation that marks the centre and core of our galaxy and the place where the stream of the visible Milky Way appears at its widest. While tracing out its broad expanse in the August evenings, try to keep in mind that our solar system with its planets and their many moons form just one ‘star system’ within this Milky Way which has about 200 BILLION other stars – some of them definitely with planetary systems, and some of them perhaps with planetary systems similar to our own.

Among the five bright planets, all may be seen this month, but some will be viewed more easily than others, and one will certainly dominate for several reasons. Firstly, Saturn and Mercury may be seen low in the western evening sky beginning about a half-hour to an hour after sunset, if one has an unobstructed view in that direction. Saturn appears above and Mercury below and for the first three weeks of the month, they appear to approach each other. On August 22nd, they are side by side, with the slender crescent moon appearing to their left – a very memorable lineup, low in the evening sky.

Within an hour and a half after sunset, Saturn and Mercury will have set and the month’s main planetary attraction, Jupiter, will have appeared in the east. No mistaking Jupiter. It is now brighter than it has been in a long time and completely dominates the night-time sky – reaching what astronomers call ‘opposition’ on the 14th, meaning that it is exactly opposite the sun, as viewed from Earth, and it is at its closest point to the Earth. Readers who own a small telescope may view its cloud-strewn surface and see the ever-changing positions of its four brightest moons, just as Galileo did 400 years ago. It is also just possible that such telescope owners may be lucky enough to observe something that is truly a rare event. About one month ago a comet impacted Jupiter, in an event that was similar to the impact on Jupiter of Comet Shoemaker-Levy 9 in 1994, and this impact has created a black scar in the southern hemisphere – that has been seen by many amateur astronomers. It is an exciting time for “Jupiter specialists”, just as it was 15 years ago, when it was especially so for my friend, David Levy, who had co-discovered that famous comet. If trying to see “the black scar” on Jupiter, remember that it is not always visible, since Jupiter rotates about once every 9 hours and 55 minutes.

Those who tried to “star-hop” from Jupiter (following instructions in last month’s column) to the planet Neptune (a planet which I have seen hundreds of times, but normally do not mention in this column because it is not a naked-eye planet) may continue this month to find it in their binoculars or telescopes. The reference star, Mu Capricorni, mentioned last month as being just above Jupiter in the telescope or binocular field-of-view now appears twice as far away, and TO THE LEFT, from Jupiter. Neptune is still above and to the right from that star, but a bit more to the right than it was last month.

Venus and Mars are the “morning planets” this month – seen in the eastern sky in the hour or so before sunrise. Mars spends most of the month among the distant stars of the constellation Taurus the Bull, and brilliant Venus spends the month among the stars of the constellation Gemini the Twins. Both are seen by those who look directly east about an hour or so before sunrise. Perhaps the best Moon-Planet conjunctions are those of the mornings of August 15th to 18th. On the 15th the waning crescent moon appears above Mars, and on the 16th the moon is slightly below and to the left of Mars. On the 17th, the slender crescent is above Venus and on the 18th the very thin crescent is well below that dazzling planet. Well worth getting up very early on all four mornings!

The year’s most famous meteor shower, the Perseids, reach their peak on August 12th, but this shower is quite active for many days before, and a few days after, the actual peak of activity. Even though there will be some moonlight interfering this year, I would still recommend observing this shower in the hours between 1:00 a.m. and morning twilight on the mornings of August 10th, 11th, and 12th. Avoid looking in the direction of the moon, and record the number of “shooting stars” seen per hour.