In The Sky This Month – September 2010

This feature highlights a number of meteor showers, comets and asteroids which are visible during the month of September 2010.

September 2010 Highlights

* Venus, Mars and Spica together in evening sky at start of month
* The above 3 joined by the Moon on the 11th
* Jupiter at opposition (21st) with Full Moon (23rd) and Uranus nearby
* Mercury has a good morning apparition
* Comet 103P/Hartley 2 is bright enough for small telescopes and binoculars

Note: If anyone has pictures or observations of these objects/events and want to share them with my readers, send them to the Transient Sky at <transientsky1@yahoo.com>.

Planets

Moon – This month brings a new addition to the “In The Sky This Month” lineup. The table below lists important lunar dates for the month, including the phases of the Moon and nights of lunar-planetary and lunar-stellar conjunctions.

September 1 - Last Quarter
September 1 - Moon 1° South of Pleiades
September 1 - Moon 8° North of Aldebaran
September 4 - Moon 8° SSW of Pollux
September 5 - Moon 4° SSW of Beehive Cluster
September 8 - New Moon
September 9 - Moon 7° SSW of Saturn
September 10 - Moon 3° SSW of Spica
September 11 - Moon 5° SSW of Mars
September 12 - Moon 0.6° from Venus
September 14 - Moon 2° NNW of Antares
September 15 - First Quarter
September 23 - Full Moon
September 23 - Moon 7° from Jupiter
September 28 - Moon 1.5° from Pleiades
September 29 - Moon 8° North of Aldebaran

Saturn – Saturn is located low in the west during evening twilight. By month’s end the +1.0 magnitude planet will be too close to the Sun to be seen easily by most observers.

September 9 - Moon within 7° of Saturn

Mars – Mars is the second of three planets visible in the west after sundown. This month +1.5 magnitude Mars starts a few degrees to the upper right of brilliant Venus. Though the planets will slowly move apart they will stay within 7° of each other all month long.

September 11 - Moon passes within 5° of Mars

Venus – Venus is the third and by far the brightest planet visible in the early evening (at magnitude -4.6 to -4.8). For northern observers this is a poor apparition. Though the planet is at its brightest and located almost as far from the Sun as it gets, the angle between the ecliptic and the horizon results in the planet staying close to the horizon. A clear view of the WSW horizon is needed in order to see Venus (and Mars and Saturn, as well). The planet sets 1.5 hours after sunset at the start of September and less than an hour by the end of the month. See the notes in the Mars section above about seeing Mars in the vicinity of Venus.

September 1 - Venus within 1° of Spica
September 11 - Moon passes within 0.6° of Venus
September 27 - Venus at its brightest (magnitude -4.6)

Jupiter (and Uranus, too…) – This month Jupiter rules the heavens. On the 21st, the ‘King of the Planets’ will be at opposition (meaning it is located directly opposite the Sun on the sky). All month Jupiter will shine at its brightest (near magnitude -2.9). Opposition also means it will be visible all night long, rising in the evening, reaching its highest elevation around midnight and setting during dawn.

If you have a pair of binoculars or small telescope take a look at Jupiter. See if you can see any of its 4 bright Galilean moons or 2 large atmospheric belts. In addition, Jupiter is within a degree of +5.7 magnitude Uranus all month long.

September 18 - Jupiter and Uranus within 0.8° of each other
September 21 - Jupiter and Uranus at opposition
September 23 - Moon within 7 of Jupiter and 6 of Uranus

Mercury – Mercury starts the month too close to the Sun to be seen. By mid-month, it is in the midst of its best morning apparition (for northern observers).

September 19 - Mercury at Greatest Elongation West (18 from Sun)

Meteors

Meteor activity is near a yearly maximum in September. The year is usually split in 2 with January through June having low rates with few major showers while July through December (really through the 1st week of January) have high rates with many major showers.

Sporadic Meteors

Sporadic meteors are not part of any known meteor shower. They represent the background flux of meteors. Except for the few days per year when a major shower is active, most meteors that are observed are Sporadics. This is especially true for meteors observed during the evening. During September, 10-16 or so Sporadic meteors can be observed per hour from a dark moonless sky.

Major Meteor Showers

No major showers this month.

Minor Meteor Showers

Minor showers produce so few meteors that they are hard to notice above the background of regular meteors. Starting this month, info on most of the minor showers will be provided on a weekly basis by Robert Lunsford’s Meteor Activity Outlook.

Additional information on these showers and other minor showers not included here can be found at the following sites: Wayne Hally’s and Mark Davis’s NAMN Notes, and the International Meteor Organization’s 2010 Meteor Shower Calendar.

Comets

Naked Eye Comets (V < 6.0)

Binocular Comets (V = 6.0 – 8.0)

Comet 103P/Hartley 2

If it behaves as expected, Comet 103P/Hartley 2 should be the comet of the year. Currently magnitude +11.5, the comet is predicted to reach naked eye visibility (for dark sky observers) in October/November.

103P was discovered at Siding Spring Observatory (Australia) on March 15, 1986 by  Malcolm Hartley. With an orbital period of 6.47 years, the comet’s orbit currently stretches from 1.06 AU to 5.89 AU from the Sun. Though not an especially big or active comet, this year it passes 0.12 AU from Earth on October 21 allowing the comet to get much brighter than usual.

The comet starts the month in the faint constellation of Lacerta at a distance of 1.31 AU from the Sun and 0.39 AU from Earth. By mid-month it has moved into Andromeda and will be 1.21 AU from the Sun and 0.29 AU from Earth. By month’s end, the comet will just south of the brightest star in Cassiopeia (Alpha Cas) and 1.13 AU from the Sun and 0.19 AU from Earth.

[Note: The following paragraph has been updated to reflect recent observations.]

Now that the Moon is out of the morning sky, visual observers have been able to estimate the brightness of the comet. Observations over the past few days show the comet to be around magnitude 8.5. This is very close to its predicted brightness and suggests the comet may live up to its expected maximum brightness of 5th or 6th magnitude this Oct/Nov. The comet is large and diffuse with much of its brightness contained in a large, faint outer coma. As a result, the comet may appear much fainter to observers using large telescopes or observing from bright sites. By the end of the month the comet should be around magnitude 6.5 making it an easy binocular object.

In early November the NASA EPOXI (ex-Deep Impact) spacecraft will encounter the comet giving us close-up views of the comet’s nucleus.

A finder chart for Comet Hartley 2 can be found at Comet Chasing.

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Small Telescope Comets (V = 8.0 – 10.0)

Comet 10P/Tempel 2

’10P’ says it all. This was only the 10th comet to be observed at a 2nd apparition meaning we’ve been following this comet for a long time. Discovered by prolific German comet discoverer Ernst Wilhelm Leberecht Tempel in Marseille, France on July 4, 1873, Tempel 2 has been observed at nearly every return since then. The comet’s current orbit brings it to within 1.42 AU of the Sun on July 4 and to within 0.65 AU of Earth in late August.

The comet is currently at a brightness of 9.0 to 9.5 magnitude. This is a large diffuse object so it will be more difficult to see than your average 9th magnitude comet or deep sky object. From my moderately light polluted backyard and 12″ telescope, the comet was a difficult object and was estimated to be magnitude 10.0. From a dark site and 30×125 binoculars, the comet was much brighter (magnitude 9.5), larger and easier to see. The added brightness was probably due to the dark site allowing me to see much more of the comet’s coma.

At mid-month the comet will be 1.61 AU from the Sun and 0.67 AU from Earth. Tempel 2 is a morning object moving through the constellation of Cetus.

A finder chart for Comet Tempel 2 can be found at Comet Chasing.

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Asteroids

Binocular and Small Telescope Asteroids (V < 9.0)

(1) Ceres

Ceres is the biggest asteroid in the Main Belt with a diameter of 585 miles or 975 km. It is so big that it is now considered a Dwarf Planet. Classified as a carbonaceous (carbon-rich) Cg-type asteroid, there are suggestions that it may be rich in volatile material such as water. Some even propose that an ocean exists below its surface. Ceres is the other target of NASA’s Dawn spacecraft which is scheduled to visit it in 2015.

This month Ceres is an evening object moving from Ophiuchus into Saggitarius. The asteroid will start the month at magnitude 8.6 and fade to magnitude 9.0 by the end of the month.

A finder chart (needs to be flipped upside down for Northern Hemisphere observers) can be found at the Royal Astronomical Society of New Zealand. Finder chart for Ceres from Heavens Above.

(6) Hebe and (8) Flora

These 2 large inner Main Belt asteroids will move in tandem this fall. Both are S-type asteroids with similar compositions and albedos. (6) Hebe is the larger of the pair (205 x 185 x 170 km). Recent research suggests that it is the source of many ordinary chondrite meteorites and near-Earth asteroids. (8) Flora is a little smaller (136 x 136 x 113 km) and is the largest surviving member of a numerous asteroid family created by a long ago impact.

This month Hebe is retrograding in Cetus and will brighten from magnitude 8.0 to 7.7. Just across the border in Aquarius Flora will brighten from magnitude 8.4 and peak at magnitude 8.2 in early September before fading back to 8.4 at the end of the month.

A finder chart (needs to be flipped upside down for Northern Hemisphere observers) can be found at the Royal Astronomical Society of New Zealand. Finder chart for Hebe from Heavens Above.