Feature Article September 9, 2004
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Three Bright Planets in the Early Morning SkyDuring September, the change in the length of day and night is quite noticeable. Locally sunset on the first day of the month was at about 7:45 p.m. Eastern Daylight Time, and astronomical twilight ended at about 9:30 p.m. During September, the sunset time moves about two whole minutes earlier each day, until by the end of the month, it is at about 6:45 p.m., with twilight lasting until about 8:30 p.m. There is also a change of over a half-hour in the mornings for the beginning of dawn and of sunrise. On the first day of the month, dawn or morning twilight, began at 4:45 a.m. with sunrise at 6:30, but by months end these events will occur at 5:25 a.m. and 7:05 a.m. Hence, over the whole month, there is over an hour and a half daily decrease in daylight, and of course, over an hour and a half increase in the time of darkness.
In the late evening hours throughout September, the beautiful Milky Way still sweeps overhead from northeast to southwest. We should continue to look for the constellation called Cygnus, the Swan or the Northern Cross, flying almost directly overhead as twilight ends. This huge starry bird flying southward along the Milky Way may remind us of our Canada Geese as they prepare for their annual migration, but the great Milky Way through which it appears to be flying should remind us of our place in the universe. The Milky Way that we see stretching from horizon to horizon is one great arm of the Milky Way Galaxy, and our own sun is just one star among this galaxys 200,000,000,000 stars. On these September evenings from a rural setting we have a chance for a great view of the central portion of the galaxy itself, if we look low to the southwestern sky to where the Milky Way appears thickest. This core of the galaxy is seen just to the right of the distinctive teapot-shaped group of stars that are commonly called the constellation Sagittarius. The ancient peoples may have seen a creature called a centaur or someone carrying a sheaf of arrows, but the modern viewer with her/his less active imagination usually sees only a teapot in the arrangement of the stars. As we look at this huge cloud of stars, let us remember that they form the core of the galaxy and are about 40,000 light-years away. (By comparison, our sun is just over 8 light-minutes away from Earth.) Above our heads the Summer Triangle is still seen in the evening sky. If we face southeastward and then look almost straight upward, we will see these three bright stars forming a large triangle. The brightest of them all is Vega, brightest star in the constellation Lyra, the Lyre, and at the upper right of the triangle. Second brightest is Altair, brightest star in the constellation Aquila, the Eagle, at the bottom of the triangle. At the upper left of the triangle is the third brightest, Deneb, brightest star of the constellation Cygnus, the Swan. Stellar astronomers are able to tell us some amazing facts regarding this trio. Vega, a blue-white star over 25 light-years away, is about 58 times as luminous as our Sun. Imagine the incredible glare if our Earth had its orbit around that star! Now consider Deneb, which is said to be 60,000 times as luminous as our Sun!! It actually appears fainter than Vega, simply because of its great distance, being about 1600 light-years away, that is, over 63 times as far away as Vega is. Of the trio, Altair is the nearest to us, being about 16.8 light-years away. Just a few amazing facts to keep in mind as we admire Summers Triangle! As the night advances and the earth rotates, this triangle moves westward and is about to set as morning arrives.
Among the five bright planets, only three of them are easily viewed this month. Mars and Jupiter are now on the other side of the sun, as viewed from our position on the planet Earth, and so may not be seen at all for most of the month. The other three bright planets, Venus, Mercury, and Saturn are all to be seen in the morning sky, or at least by those who observe well after midnight. Throughout the month, Venus is stunningly bright and may be seen rising in the east between 2:00 a.m. and 3:00 a.m. It dominates the sky almost until sunrise. Note its apparent downward motion among the stars as the month advances. Saturn is easily found above Venus and below the twin stars of Gemini, that is, Castor and Pollux. In fact, it forms almost a straight line with those two stars. Mercury, which is found well below Saturn and Venus, may be seen very low in the east beginning about an hour or so before sunrise. Near mid-month it is very near the bright star Regulus in the constellation Leo, the Lion. During the latter part of the month, Mercury becomes slightly brighter each morning, but lower and lower in the sky in the hour before sunrise, until in the last week of the month it is almost impossible to see in the morning twilight.
Early morning skywatchers should make a point of catching a view of the waning crescent moon passing Saturn and Venus on the mornings of September 9, 10, and 11, and then passing Mercury and Regulus on the 12th and 13th, though catching the thin crescent on the 13th will be a real challenge. Set the alarm for about 5:00 a.m.
This months Full Moon on September 28 is known as the Harvest Moon, the one nearest the Autumnal Equinox, since its earlier than usual rising times over several nights in a row have traditionally provided an additional light, almost an extended daylight, for farmers to complete their harvesting chores. In fact, for centuries the farmers of Europe counted on its rising during evening twilight, knowing it would rise only about 20 minutes later from night to night during the week of the Harvest Moon, whereas the average over the entire year is about 50 minutes or more. Try to witness for yourself this age-old phenomenon beginning on the evening of September 27 or 28 and over the next 5 or 6 evenings. Record for yourself the moons rising times each evening.
More information about the Harvest Moon and about observing stars, constellations, and planets may be found in a book called The Beginners Observing Guide, available at Sharbot Lake Pharmacy.
