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Sunset visibility of comet Lovejoy  @psychohistorian.org

Sunset visibility of comet Lovejoy

posted: 2954 days ago, on Sunday, 2011 Dec 25 at 16:46
tags: astronomy, comets, Ed Foster, Kos Coronaios.

[update Jan 04, 10:45] First evening pics of Comet Lovejoy? Detailed finder chart for Comet Lovejoy, Jan 04-10.

[update Jan 03, 19:30] New evening sky finder chart for Comet Lovejoy.

Comet Lovejoy is currently a fading object in the morning sky. (For images from around South Africa, visit the Comet Lovejoy gallery.)

Until yesterday, I wondered: "Has anyone seen it in the evening sky yet?" Now methinks a probable "Yes!" can be given.

Last night (Tuesday, January 3) Ed Foster may very well have imaged the comet in the early evening sky. Scroll down for his images.

The finder chart above shows the position of the comet in the early-evening sky. By all accounts, the comet has faded to below naked-eye visibility, so a pair of binoculars will be needed to give you an eye-full.

It should be relatively easy to find, above the obvious patten of Triangulum Australe.

The tail should be slanted to the left and upwards, and it's anybody's guess how long or visible it will be.

The diagram on the right shows the path of Comet Lovejoy across the South Celestial Pole, January 04-10.

Download a detailed finder chart for Comet Lovejoy (466 kB PDF) for January 04 - 10.

The bright star at the bottom of the chart is alpha TrA. The triangle of stars that makes up Apus will be very handy when searching for the comet. Today (Jan 04) Kos wrote to say: "To confirm comet is definitly not naked eye and I battled see it in bins 12x50."

Your best best to see it now would be to use the finder chart, a pair of binoculars, and a bit of patience.

The comet is dim, so you'll need to dark adapt somewhat to get a good chance of seeing it.

PHOTOS: Tuesday evening, January 03

Last night (Tue, Jan 03) Ed and Lynnette Foster and myself set out to find the comet after a break from the last few day's cloudy weather.

By 21:15 we were set up along the side of the road in Technopark, Stellenbosch. In retrospect, a stupid choice.

Light pollution, exacerbated by thin low cloud, is never a pretty sight. Worse still, the bright lights from the surrounding techno-buildings meant that the only degree of dark adaption possible was zero. And worst of all, nobody brought pizza.

Despite these serious lack of victuals we doggedly imaged the bright sky, secure in our faith that something would be the result. The tiny aperture of my Canon Powershot wasn't up to the task, it turned out. But Ed's 300mm telephotolens did a lot better.

The leftmost image is a crop from a stack of five 15-second exposures (ISO 320, f/5.6, 300mm EFL) that shows alpha TrA at the bottom. At the top-left of the frame is a 90-degree triangle of stars; the uppermost is HD 151442, 7.6V. The comet is a 45-arcmin extension to the bottom-right (PA about 40).

The top-centre image is from a stack of 30-second exposures; the next image circles the comet's position.

The bottom-centre image (ISO 320, f/5.6, 27mm EFL, 15-seconds) shows the horrible conditions. The mountain sloping in on the left is the Helderberg, and the brightest star at top-right is alpha TrA. The photographer and assistant appear at bottom-right.

So the lesson is: avoid brightly-lit places with cloud, dark adapt if you want to see it, and remember the pizza.

Friday evening, December 30

Gerrit Penning writes: "No luck seeing the comet from Witsand, Southern Cape, through a 12x60 binoculars (30/12/2011 21h15). Although the brightest stars of Ara is visible, the usual fogginess of an ocean sky is present and there is still a bit of dusk around."

Gerrit was lucky - I couldn't even find the two bright Ara stars from Stellenbosch this evening -- the low band of cloud that has blanketed the horizon the last two days, persists (see pic above).

Thursday evening, December 29

Willie Koorts and Johan le Roux again attempted to find the comet from Gansbaai. Willie said that despite very good weather, there was again a low-lying bank of clouds over the ocean. They searched for the comet with 8x50 and 20x80 binoculars from a spot near the Danger Point light house, but to no avail. Willie took a series of 15-second exposures which he processed with Rot'n'Stack, hoping that the comet's tell-tale tail would be visible. Unfortunately, the image shows no sign of the tail, although the last photo of the series (see inset image) shows an elongated brightening which could just be it!

Wednesday evening, December 28

Willie Koorts and Johan le Roux (observing from Gansbaai) and myself (from Stellenbosch) couldn't find the comet because of low-lying cloud over the ocean.

Earlier, Nigel Wakefield of Durban, who photographed the comet on December 21, wrote to say:

"I have just been checking my software and tomorrow Lovejoy becomes an evening as well as a morning object! So we will have two chances to see it provided it doesn't fade too much. By the 31st. it passes -60 dec to become circumpolar for us. I haven't seen this forecast anywhere else."

I speculated about when the comet will first become visible. The pair of graphs below compare the comet's morning and evening apparitions. Each graph plots the difference in azimuth between the comet and the Sun, versus the altitude of the comet, at the moment of sunrise and sunset, respectively.

Both the azimuth difference, and the altitude, influence the visibility of the comet. The comet's apparent magnitude, of course, also determines if it will be seen or not, but that's another story. The comet became a naked-eye object on December 21, with just a 2 azimuth difference but at an altitude of 14 (at sunrise). Using just the altitude as a predictor suggests, according to the second graph, that Lovejoy should be visibile in the evening sky no earlier than 2012 January 01. However, its azimuth difference is then already substantial (about 42).

On the evening of December 25, the comet is already 26 away from the Sun - but then its altitude is only about 2.

To try and account for the combined role of altitude and azimuth, I dreamed up a weighted measure of its position, as follows:

weighted apparent position = altitude x altitude x azimuth difference.

The change in this weighted measure is plotted below.

When the comet became a morning-sky object, its weighted apparent position was a little less than 500 units. This suggests that the earliest evening visibility will be on December 27.

So, let's see what happens!

nothing more to see. please move along.

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