Sky Watching — September 2011

The bright stars of the Summer Triangle shine from high in the southern skies during the evenings in September.  Three bright stars — Vega, Deneb, and Altair —  that are actually part of the their own constellations, mark the “corners” of the triangle.  Deneb is one of the brightest stars in the sky.  Shining with the brightness of 170,000 suns, this star is 2,500 light years away.  The combination of actual brightness and distance makes it the 19th brightest star seen in the sky at night.  Compared to its neighbor, Vega appears as the 5th brightest star, although it is only 60 times brighter than our sun and 25 light years distant.  The third star, Altair, ranks as the 12 brightest star with an actual brightness of about 11 suns with a distance of 17 light years.  The brightness that a star appears in our sky is related to the star’s actual brightness and distance.

Deneb is part of the constellation Cygnus, the swan.  It represents its tail.  The head of the swan is marked by Albireo.  Look at this star through a small telescope.  It will reveal a wonderful double star, one golden, one blue.

On September 3, the moon appears near the bright star Delta Scorpii as seen from mid-latitudes.  The moon can been seen covering the star from the southeastern US.  This link provides more details.  For others, the moon appears very close to the star with the bright star Antares nearby.

The Autumnal Equinox occurs at 4:05 a.m. CDT on September 23.  At this time, the sun is directly above the earth’s equator.  On this date at noon the sun will go directly overhead for people living at the equator.  For residents of mid-latitudes, the sun will about halfway up in the south at noon, rising at the east direction point and setting at the west cardinal direction.  The equinox also brings equal daylight and darkness at 12 hours each.  From this date until the Vernal Equinox in March, the length of nighttime is longer than daylight hours.

Mercury is usually difficult to see as it rapidly shuttles from morning sky to evening sky.  During early September, Mercury makes a brief appearance in the morning sky.  The chart above shows Mercury and Regulus, the bright star in Leo, at 6 a.m. on September 9.  Locate an observing spot with a clear horizon.  Looking with binoculars locate Mercury and Regulus in a close pairing.

Venus begins its evening appearance late in the month.  Venus passed behind the sun in mid-August.  Venus and Earth are like two cars on a race track; Earth is in an outside lane and Venus an inside lane.  Along with a shorter course, Venus moves faster than Earth.  In September, Venus is nearly on the other side of the track and the infield spectators (the sun) are blocking our view of the planet.  It will move faster and catch up to our planet and move between us in the sun (inferior conjunction) in June 2012.

As the chart above indicates, the sunset time between Venus and sun will be between about 30 minutes and 45 minutes throughout September.  Sharp observers may note it very close to the horizon in the west after sunset.  Binoculars will be helpful.  As Venus begins to catch up to Earth, it will appear longer and longer  in the western evening sky, outshining all other objects in the night sky besides the moon.  It can be easily mistaken for lights on an airplane.

As Venus closes in on Earth, it will grow brighter until April 30.  The greatest separation (marked greatest elongation on the diagram) between Venus and the sun is March 27, 2012, with Venus setting nearly 7 hours after the sun.  It’ll be a spectacular sight in the spring night sky, when the tilt of the solar system provides marvelous views of the inner solar system.  Venus will make interesting viewing as it passes bright stars and other planets during its evening appearance.  We will note them here in future postings.

Mars is visible in the eastern, predawn sky.  Late in the month, the moon appears near Mars.  The chart above shows Mars with the moon and bright distant stars (Pollux, Castor, and Procyon) on September 22 and 23.  During September, Mars shines nearly equal to the bright stars in its background.  While distinctly reddish-orange, the moon helps identify it late in the month.

Jupiter shines brightly from the eastern sky during late evenings as it rises  in the east around 10 p.m. early in the month.  It is in the sky until sunrise and it dominates the southern sky just before sunrise.  Late in the month, it rises in the east around 9 p.m.  The chart above shows the moon and Jupiter on September 15 and September 16.  The bright star Hamal is nearby.

Saturn disappears into the sun’s bright sunlight as it moves behind the sun in October.

We’d appreciate reading what you are observing.  Please post any interesting observations in the comments section.