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Wednesday, February 06, 2013


Viewing Comet C/2011 PanSTARRS in the Southern Hemisphere

Comet C/2011 L4 PanSTARRS simulated in Stellarium. The view is on 15 February at 21:00 ACDST as seen from Adelaide looking out over the ocean. Similar views will be seen elsewhere in the southern hemisphere at the equivalent local time. Click to embiggen.This view is on 25 February at 21:00 ACDST as seen from Adelaide looking out over the ocean. Similar views will be seen elsewhere in the southern hemisphere at the equivalent local time. Click to embiggen.
This view is on 14 February at 21:00 ACDST as seen from Adelaide looking out over the ocean. It's at a smaller scale than the others to show the Moon. Similar views will be seen elsewhere in the southern hemisphere at the equivalent local time. Click to embiggen.Comet C/2011 L4 PanSTARRS simulated in Celestia. The comet (where the red arrows are), is below the plane of the ecliptic. Click to embiggen.

From around Thursday February 14 the comet C/2012 L4 PanSTARSS will be high enough above the western horizon to be potentially visible. Note the word "potentially". Recent observations have suggested it might be dimmer than predicted. This is Comet PanSTARRS first visit to the inner solar system, and the brightness of these "fresh" comets is highly unpredictable.

Now, at its brightest, the comet is predicted to be no brighter than delta Crucis in the Southern Cross, and may be as dim as epsilon Crucis (the fifth and dimmest star in the Southern Cross).

Or it might be brighter. In both cases it will be tricky to see in any case early on, as it is very close to the horizon and the horizon murk will make it harder to see. Also, there will be very few visible stars that you can use as a guide to its location.

On the 14th there is an opportunity to test your ability to see the comet. Start looking a little before an hour after Sunset (this is nautical twilight, when the brighter stars are visible, and some of the moderately bright stars are becoming visible).

You will need a really flat, level horizon, like the ocean or the dessert, to have a chance of seeing the comet. Off to the right of due west you will see the crescent Moon (see diagram above). If you turn to your left slowly, the first bright star you come to is Fomalhaut. Turning a little further left there is a pair of dimmer stars, the brightest stars in the constellation Grus. These stars are about as bright as comet PanSTARRS will be at its brightest.

If you can see these stars, then you will be in a good position to see the comet at its predicted brightest. If you follow the two stars down, then the comet should be below them (Alnair is indicated on the map from the 14th). It will be no more than 4 degrees above the horizon.  On the 14th, you may need binoculars to see it (or it may turn out to be too dim to see at all).

So, if you can't see the comet, what is the point of this exercise? Well, now you know where the guide stars are, and on subsequent nights you can find them without the help of the Moon. As the days go on, the guide stars sink lower, and the comet rises higher, so you can be prepared when the comet finally appears. By the 25th, the comet is now near the readily recognisable star Fomalhaut and the sky is darker. The comet should now be visible to the unaided eye. Whether it is just visible or easily visible is another thing entirely.

Note that the diagrams don't show a tail, this is because we have no idea what the tail would look like. It may be short and faint and lost in the twilight, or it may be long and sort of brightish. Unlike the spectacular tail of C/2006 P1 McNaught, or the fainter but still amazing C/2011 W3 Lovejoy, which streamed up into the sky and were visible even when the comet wasn't, any tail for C/2011 L4 PAnSTARRS will be almost parallel to the horizon, and will set not long after the comet itself.

So, why look for something you possibly won't see? Just in case the comet surprises us all and is brighter than predicted, or has an out burst, or it has a spectacular tail and then you can see something wonderful. And even if you see only a dim fuzzy dot, that dot has dropped in from the farthest reches of our solar system, and is whipping around closer to the Sun than Venus, so it's pretty amazing.

If you want to wait until March, the key dates are March 5, when the comet is closest to Earth, and March 10, when the comet is closest to the Sun.

I've also made a Celestia file for you. As usual, copy the code below and save as it as a text file 2011L4.ssc in the Celestia extras folder. For soem reason, the name and th orbit of the comet doesn't come up, even when other comets with very similar parameters, like Elenin, do. Still it will let you follow the comet.
 "C2011L4" "Sol"

Class "comet" # Just copying the data for Halley
Mesh "halley.cmod"
Texture "asteroid.jpg"
Radius 5 # best guess at maximum semi-axis
MeshCenter [ -0.338 1.303 0.230 ]

Epoch 2456361.66972 #  2013 Mar. 10.1697 TT
Period 1117595.564 # (q/(e-1))^1.5 hyperbolic orbit
SemiMajorAxis -30303.0303 #Hyperbolic orbit
PericenterDistance   0.3015421      
Eccentricity 1.000028
Inclination 84.2072
AscendingNode 65.6658
ArgOfPericenter 333.6517
MeanAnomaly 0.0  

# Again, this data is copied straight from the ssc files for Halleys’ Comet
# chaotic rotation, imperfectly defined:
# this version from "The New Solar System", 4th Edition; Eds.
# JK Beatty, CC Petersen, A Chaikin
Period 170 # 7.1 day axial rotation period
Inclination 66
PrecessionPeriod 3457004.12 # 3.7 day precession period

Albedo 0.8

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Thank you

Am off to find a a stellarium file as well.

Unfortunately been clouded out for weeks here. Might get some clear skies if Sandra doesn't blot us out soon.
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