Go Stargazing! October Edition

Venus passes between the Earth and sun on October 29, an alignment known as inferior conjunction. ‘Superior’ conjunction occurs when Venus passes around the far side of the sun. As a result, in October 2010 we see Venus stop its apparent forward motion and shift back towards the sun—it will soon leave our evening skies.  For now, you can look for Venus low in the southwest at dusk. After next week, however, Venus sets during twilight.

Mars is above Venus (and much, much dimmer) as October opens; it remains low in the southwest at dusk after Venus is gone.

Saturn aligned with the sun on September 30 (i.e., it was at conjunction), so we haven’t gotten a good look at it in a while.  Near the end of this month, though, you can begin to look for the ringed planet low in the east at dawn.

Jupiter dominates this month’s evening skies.  Up literally all night long late last month, the king of planets is now well placed for observing in convenient evening hours.  It outshines all stars in the sky, so it’s easy to find.  Face southeast at dusk, and you can’t miss it.

The Big Dipper happens to be to the lower left of the North Star at dusk this month; you’ll need a clear northern horizon to get a good look at it.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is in the southwest.  Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, high in the west.   As familiar summer patterns shift to the west, the constellations of autumn take center stage.  The Great Square of Pegasus is high in the east at dusk.  The star in its upper left hand corner is also the head of Andromeda.  Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen.  Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from that Dipper.  In fall, while the Dipper is low, Cassiopeia rides high. The vast stretch of sky under Pegasus is largely devoid of bright stars—ancients called this the ‘Celestial Sea.”  The only first magnitude star in the entire region is Fomalhaut, in the Southern Fish.  Jupiter’s stark brilliance is even more remarkable against this dim backdrop.

Comet  Hyakutake

A comet may become visible to the naked eye later this month.  If you recall comets Hyakutake and Hale-Bopp from the ‘90s, this one won’t be quite that bright, but it should be visible from dark sites when no moon is out, and definitely visible in binoculars.  It is Comet Hartley 2, and it makes its closet approach to Earth, at just 0.12 AU, on October 20.  On that date it will appear near the star Capella in Auriga. Therefore, it will rise in the northeast at dusk on that evening and be visible all night long for us.

Moon Phases in October 2010:

New Moon                       October 7, 1:44 p.m.

1st Quarter                     October 14, 4:25 p.m.

Full Moon                        October 22, 8:37 p.m.

Last Quarter                  October 30, 7:46 a.m.

Saturday, October 16, is our annual Astronomy Day at the George Observatory. Come join us anytime from 3 to 10 p.m.  On Astronomy Day, it is free to look through even the main domes at George.  Before dusk, we will have solar observing, Challenger Center simulations, outdoor and indoor presentations (beginning at 4) and many other activities!  Surf to www.astronomyday.info for more information.

This month, see a ‘Hairy Star!’

An unexpected visitor graces our skies this month.  Comet Lulin is now visible through binoculars in late evening and morning skies.  It makes its closest approach to Earth on February 24, when it may even be dimly visible to the naked eye!

Comet Hale-Bopp
Creative Commons License photo credit: tlindenbaum

Comets are made of ice and dust and are often called ‘dirty snowballs.’ They are believed to be left over from the formation of the solar system.  As comets approach the sun, ice changes into gas and the dust embedded within the ice is released.  A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be a hundred thousand miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust off of the comet, forming tails pointing away from the Sun. The coma and tails of a comet reminded the ancient Greeks of hair; the Greek word ‘kometes’ means ‘hairy.’   

Astronomers traditionally name comets after their discoverers.  On July 11, 2007, Lin Chen-Sheng of Lulin Observatory in Nantou, Taiwan took some photographs of the sky.  The photos were part of the Lulin Sky Survey, in which astronomers search the sky for Near-Earth Objects which might pose a risk of colliding with Earth.  One of his students, Ye Quanzhi, spotted what he thought was an asteroid in three of the pictures.  Closer observation, however, revealed the coma of a comet.  Officially designated C/2007 N3, the comet was named Lulin after the observatory where it was discovered. 

Here are some interesting facts about Comet Lulin’s orbit:

The eccentricityof an orbit describes its shape.  Bound orbits are ellipses with eccentricities between 0 and 1; 0 is a perfect circle while 1 is a parabola.  Lulin has an eccentricity of 0.9999948, almost 1.  This indicates an orbit so oblong that Lulin won’t return to the inner solar system for about 50 million years.  Some sources indicate an eccentricity slightly greater than 1.  In that case, Lulin will never again approach the Sun.

Lulin was closest to the Sun (at perihelion) on January 10.  But it approached the Sun from the far side (from our perspective).  Thus, as Lulin recedes from the Sun, it approaches Earth, with closest approach on February 24.  Not to worry, though–even at its closest, Lulin will be about 150 times as far away as the Moon.

Many comets’ orbits are highly inclined to ours.  (An inclination of 0 degrees would describe an orbit in the same plane as Earth’s orbit.)  Comet Lulin has an inclination of 178.37 degrees.   This inclination of almost 180 degrees puts Lulin back in the plane of the solar system, orbiting backwards compared to the planets’ orbits. 

Since Lulin orbits almost in Earth’s orbital plane, we see not only a tail but an ‘anti-tail.’  This is dust and debris left behind as the comet moves on its path.  Lulin is now moving away from the Sun, so the dust it leaves behind seems to point towards the Sun. The true tail of a comet always points away from the Sun (and therefore, the tail leads the comet as it moves away from the Sun). 

The Hale-Bopp Comet
Creative Commons License photo credit: Wolfiewolf

Because Lulin is roughly in the plane of the solar system, traveling backwards, it appears against the same zodiac band where we find the Sun, Moon, and planets in our sky.  As I type this, Lulin is among the stars of Virgo, the Virgin, moving towards Leo, the Lion. 

As we pass more or less between the Sun and Lulin next week, we’ll see it in Leo, first near Saturn and then near the bright star Regulus.  Lulin will be rising in the east at about dusk, highest in the sky about midnight, and setting in the west just before dawn.  Since Lulin and Earth are going in opposite directions, we see Lulin move quite noticeably night to night. 

This page has some finder charts for Lulin.  Some observers have reported seeing Lulin naked-eye, at the threshold of visibility.  You must get far from city lights, therefore, to see it without binoculars or a telescope.  Remember to scan the sky for a diffuse object about half as big across as the full Moon (and much dimmer than that), not a point of light.  Those who saw the spectacular comets Hyakutake and Hale-Bopp in the ’90s should keep in mind that Lulin will be barely (if at all) be visible to the unaided eye and will not come close to their displays.  If you find Lulin, see if you can follow it as it gets dimmer but higher in the evening sky in March. 

Once it fades away, we’ll never see it again.