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You are hereTHIS MONTH'S SKY - September 2010

THIS MONTH'S SKY - September 2010

THIS MONTH'S SKY
Celestial Events -- September 2010

 

JUPITER AND URANUS AT OPPOSITION

On September 21, both Jupiter and Uranus are at opposition to the Sun.  That is, they are both opposite in the sky from the Sun -- Jupiter and Uranus rise at sunset, and set at sunrise.  As a result they are up all night and quite near each other.  Jupiter is the brightest object in the night sky after the Moon and Venus, so it needs no help being seen, but Uranus is difficult to find in a light polluted sky, so this provides an opportunity to easily find it.

Geometrically, Sun, Earth, Jupiter and Uranus lie in that order along a nearly straight line. (Jupiter's opposition precedes that of Uranus by five hours).  Earth, moving faster on a smaller orbit, is passing Jupiter and Saturn. As seen from Earth planets being passed seem to move the "wrong way" (Figure 1) in the sky -- from east to west with respect to background stars.

Jupiter is the largest planet in our solar system.  Through a telescope two horizontal "belts" can usually be seen, separated by lighter "zones".  Recently however the southern belt faded from view!  This has happened before and skywatchers are checking for its eventual return. Ironically, the belt's disappearance may make it easier to spot the Great Red Spot, which normally appears in the same region as the southern belt.  To see when the Great Red Spot is facing Earth, check out http://www.skyandtelescope.com/skytel/beyondthepage/91731334.html.

Jupiter's four bright moons are always a hit.  Easy to spot from any location they rapidly change their positions in their speedy orbits around.  To see what moons are visible at a given time, go to the website http://www.skyandtelescope.com/observing/objects/javascript/jupiter
Figure 1
Retrograde Motion of Jupiter and Uranus
 


Figure 2
Close-up of Jupiter and Uranus
in September

Uranus is much fainter.  At magnitude 5.7 it may be seen with the naked eye from a dark location.  From the city binoculars or a telescope are required.  Fortunately, Jupiter's proximity this month makes Uranus easy to find.  Use Figure 2.  Uranus can be spotted by its blue-green color.  A telescope with magnification 100x or more should show Uranus as a little ball in space rather than a star.

EVENING PLANET GATHERING IN WEST NEARS AN END

For several months we have chronicled the movements of Venus, Mars and Saturn as they approached each other.  The planets were closest in August but now become more difficult to spot as they set earlier after sunset.

Saturn is the first of the three to be snared by the Sun.  As September begins it is a whopping 20° west of Mars and Venus.  Within a week it is lost in the Sun's glare.  It will return to view in late September's morning sky.

Venus and Mars have a little dance to perform before they leave the scene. Joining them is the bright star Spica.  On the first day of the month Spica lies between the two planets, 1¼° above right of Venus and 3½° below left of Mars.  It is closest to Mars (2°) on the 5th

The Moon comes by on the 10th and 11th.  On the 10th it forms a diamond shaped grouping with Mars highest, Venus to the left, Spica to the right and the Moon lowest.

For awhile Venus moves farther to the left of Mars but by the 20th this changes, as Mars moves to Venus' upper right.  This is due to Venus "rounding the bend" and approaching the Sun more quickly.  This leads to a conjunction of Mars and Venus on the 30th.

Although Mars is higher than Venus, the latter is still much easier to spot because of its brightness.  For the first few days of October it will still be possible to spot Venus (and Mars, with more difficulty).  By mid-October both planets will be swallowed up by the Sun's light.


Figure 3
Planets in the West
8 PM    September 1
 


Figure 4
Planets in the West
8 PM    September 10

 

 


SEPTEMBER'S EVENING SKY

Click to get sky charts and a description of the sky for this month.
 

PLANETS IN SEPTEMBER

 

MERCURY is at inferior conjunction with the Sun on the 3rd.  It rapidly moves into the morning sky and can be seen with binoculars by the 10th.  It is 5½° below Regulus on the 13th, and should be visible to the naked eye, having brightened to magnitude 1.1.  Because the slope of the ecliptic is favorable, Mercury rises a full hour and a half before the Sun at its greatest elongation from the Sun on the 19th.  On that date its magnitude is -0.4 and its disk, though waxing,  is still under 50% lit.  Mercury is still visible the rest of the month, into the first week of October.

VENUS (magnitude -4.6 to -4.8, diameter 28.3" to 43.8", phase 42% to 20% lit) is still visible in the western sky at dusk. See above for more info.

MARS (magnitude +1.5, diameter 4.4" to 4.2") like Venus is holding out in the early evening sky.  See above.

JUPITER (magnitude -2.9, diameter 49.1" to 49.7"), is at opposition on the 21st.  It appears as big as it will be for the next 12 years.  Nearby Uranus keeps its company.  See above.

SATURN (magnitude +1.0 to +0.9, diameter 15.9" to 15.7") is getting swallowed up by the Sun.  Early in the month you might see it in binoculars.  On September 1st it is 17°  to the right of Mars.  Soon afterwards is it lost in the Sun's glare.  On the 30th it is conjunction with the Sun and enters the morning sky.

URANUS (magnitude +5.7, diameter 3.7") is at opposition on the 21st, the same day as Jupiter.. So the two are close together this month.  Use Jupiter to find Uranus, as described above.

NEPTUNE (magnitude +7.8, diameter 2.3") is in Capricornus.  It is currently retrograding.

PLUTO (magnitude +14.0, diameter 0.1") is still in western Sagittarius.

The MOON is at Last Quarter on September 1st and New on the 8th. Two days later it is near Spica, forming a diamond with Venus and Mars.  The Moon passes Antares on the 13th.  First Quarter is two days later. The Moon passes above Jupiter and Uranus on the 22nd.  The following evening is Full Moon, and Last Quarter is on the 30th.


THIS MONTH'S EVENTS
(Times in EDT)

September 1 Last Quarter Moon at 1:22 PM.
  Venus-Spica only 1¼° apart this evening.  Spica is only 3¼° from Mars.
September 3 Mercury at inferior conjunction, entering the morning sky.
SEPTEMBER 5 MARS AND SPICA ARE AT THEIR CLOSEST TONIGHT, JUST OVER 2° APART.
September 8 New Moon at 6:30 AM.
September 10 The crescent Moon is 3¾° below left of Spica this evening.
September 11 The Moon is 6° left and slightly higher than Venus.
September 13 Antares is 4° left of the Moon tonight.
September 14 Pluto is stationary, resuming direct motion.
September 15 First Quarter Moon at 1:50 AM.
September 19 Mercury at greatest elongation west (18°) in the morning sky.
SEPTEMBER 21 JUPITER AND URANUS ARE BOTH AT OPPOSITION TONIGHT.
September 22 Uranus is 0.9° above and left of Jupiter, which is 6° below the Moon.
  Autumnal equinox at 11:09 PM.
September 23 Full Moon at 5:17 AM.
  Venus is at greatest brilliancy (magnitude -4.8).
September 27 The Pleiades are 2° left of the Moon.
September 29 Venus is 6° below left of Mars.  They are getting very low now.
September 30 Saturn is in conjunction with the Sun, entering the morning sky.
  Last Quarter Moon at 11:52 PM.