A Report on Expedition One to Comet C/2011 L4 (PANSTARRS)

Last night (8 March) one of my photography students and I gave it a very good try, but the clouds didn’t let us actually see Comet C/2011 L4 (PANSTARRS). We met at the parking lot of Canadian Warplane Heritage, beside Hamilton’s international airport,at 6 PM. Despite repeated forecasts for a sunny afternoon – and the Hamilton clear sky chart showing fairly good observing conditions at the time – the cloud deck was 50% complete and 90% to the west: just 2° or less of clear sky in the west.

We decided not to give up, though, and drove like crazy toward the gap in the clouds. We ended up droving west past Brantford and got a little more clear sky, but ultimately, we missed it… there was a little gap in the clouds along the horizon, and we were hopeful. We saw aircraft in there but no comet. Too late, too many clouds. Also, we were depending on a generalized chart of the sky as to where the comet should be. I suspect that by the time we got to our spot and stopped the comet has set.

The horizon west of Brantford, Ontario, at 7 PM on 8 March 2013: too late to see PANSTARRS. Did see lots of planes, though.

The western horizon west of Brantford, Ontario, at 7 PM on 8 March 2013: too late to see Comet C/2011 L4 (PANSTARRS). Did see lots of planes, though, like the one in this photo.

The forecast for Saturday 9 March is for good weather; some cloud in the afternoon, but hopefully the little part of the sky we need to see the comet has a chance of being clear. The clear sky chart suggests that by 6 PM (18:00) we might have clear skies: http://cleardarksky.com/c/Hamiltonkey.html.

To reduce the uncertainty in where to look, I loaded the orbital elements for the comet into Stellarium, a sky chart program I use, and plotted the expected locations for Comet C/2011 L4 (PANSTARRS) for three specific times tonight as viewed from Dundas Ontario. Here are the expected bearings for the comet tonight (relative to the horizon and north), and taking into account the bending effects of the atmosphere:

At 6:30 PM: azimuth 255° 26′, altitude 7° 18′

At 6:40 PM: azimuth 257° 13′, altitude 5° 35′

At 6:50 PM: azimuth 258° 58′, altitude 3° 40′

Azimuth is “compass direction.” 255° is 15° south of west. The sun will set at 265° on the 9th, at about 6:18 PM. So, the comet should be a bit further south than the place the sun sets.

Altitude is elevation above the horizon. The width of a thumb held at arm’s length is about 2°; the width of a fist at arm’s length is about 10°. So, the comet should be closer to the horizon than the width of a fist at arm’s length at 6:30. It’ll take only a little in the way of stuff on the horizon to lose it.

The upshot of all of this is that it’s a good idea to get as high as possible before looking for this comet 🙂

Another entry will be filed when the next expedition reports back.

© David Allan Galbraith 2013
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