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Lemmon's green halo set to dazzle Northern Hemisphere

Posted: 10 April 2013

2013 is set to be the year of the comets with 2011 L4 (Pan-STARRS) having already made a decent appearance and set to remain a fading circumpolar object during the spring months into summer, and the very exciting prospect of 2012 S1 (ISON) later in the year. Muscling in on the show is comet 2012 F6 (Lemmon), originally thought likely to be a modest, run-of-the-mill comet on show first in the Southern Hemisphere, it has delighted comet enthusiasts there by becoming far more impressive and it's now set to wow north of the equator.

Comet 2012 F6 (LEMMON) is a beautiful comet as this tremendous picture taken from Mercedes, close to Buenos Aires in Argentina on 6 February shows. Ignacio Diaz Bobillo used an AP130 EDF refractor and a moded Canon 1000D mounted on a Losmandy G11. The image is the sum of 37 three minute calibrated frames at setting ISO 1600. Check out his website.
Comet 2012 F6 (Lemmon) was discovered on 23 March 2012 by A. R. Gibbs using the 1.5-m reflector at the Mt. Lemmon Survey, in the Catalina mountains north of Tucson, Arizona, USA. It was a very faint 21st magnitude object lying at around five Astronomical Units (AU) away at discovery. Astronomers quickly calculated that this ancient, icy visitor from the Oort cloud had already invaded the inner Solar System but that was over 11,000 years ago. Its orbit is also steeply inclined to the ecliptic, similar to Pan-STARRS. At the start of 2013 it became clear that magnitude estimates for it were way-off; Lemmon appeared four magnitudes brighter at around seventh magnitude. Visual observers and imagers reported a great looking comet, shining with a lovely green glowing coma around eight to ten arcminutes across, with a beautiful, spiky tail. Lemmon passed through perihelion passage (closest to the Sun) on 24 March, passing within 109.4 million kilometres (0.7313 AU) and peaked at magnitude +5.5 in mid-March.

C/2012 F6 (Lemmon) follows a similar path through the sky to its predecessor C/2011 L4 (Pan-STARRS) earlier in the year. Credit: Greg Smye-Rumsby
LEMMON reappeared in the morning sky this month, still lying south of the celestial equator until 20 April when it crosses into the northern half of the sky. At this point Lemmon lies among the stars of Pisces and is still best placed for the Southern Hemisphere. From 15th to 17th April the comet passes a few degrees from Mercury; from Sydney, observers can look for the pairing 50 minutes or so before local sunrise (around 5.25am), as long as there is a clear eastern horizon available as they will be only 12 to 15 degrees up at the end of astronomical twilight (Sun 12 degrees below horizon). The latest observations (early April) show the comet is holding steady at magnitude +5.5 so it should be visible in binoculars despite the low altitude. As April progresses LEMMON will rise further out of the murk and muck as it heads north, skirting just to the east of the Great Square of Pegasus from early to late May.

Comet 2012 L4 (LEMMON) imaged on 13 February, again from Buenos Aires in Argentina. Credit: Luis Argerich
Observers in the overwhelming majority of the USA will have to wait until May to see the comet but those in the far southern states could pick it up in the last week of April given a good morning horizon. From the UK it will take a little longer for Lemmon to make an impact and hopefully it will be around mag. +6 to +7. Observers with dark skies and a good eastern horizon may spot it in binoculars in the second week of May but by mid-month it will be not far short of 15 degrees up 90 minutes before sunrise, which occurs at 5.08am BST in London. The sky will still be reasonably dark (at least from southern UK) and it should be possible to follow it in binoculars and small telescopes for a while longer in the gathering twilight. Come the month's end Lemmon will be almost 30 degrees up from London for the same degree of twilight at 3am, following closely in the footsteps of Pan-STARRS. In June it will pass five degrees west of the mighty Andromeda galaxy (M31) and be circumpolar, fading all the time as it heads back towards the icy depths of the Solar System.