[X] My Assistant

[TheAnt]

@Stu: Light gathering binocs are almost like made for comets but for astronomical purposes not much else. Mine do show Andromeda, the Orion nebula, the Moons of Jupiter and sing of Saturn but that's about it (Don't even consider aiming such at the Moon, else not just the night vision might be ruined.)
That was a nifty solution, and it do address one of the problems for larger telescopes. That they are a pain to move to the observation site.
The one depicted almost made me rant about the advantage of using both eyes, but that's exactly what he had there!

[AndyG]

I love the picture, Stu, but I suspect you'll need to keep four jacks in the boot to steady the "observatory" when you get up there.

Andy

[JRehling]

I thought I would add: The total brightness of the Milky Way corresponds to an apparent magnitude of -5.0, brighter than Venus. However, Venus is visible in dusk, dawn, city lights, and even in direct daylight, whereas the Milky Way, being more diffuse, is totally invisible unless the sky is reasonably dark. And likewise, a comet which is visible in daytime is still in large part lost as the luminosity of the more diffuse tail, which may sum to something rather considerable, will be effectively zero against its background. For a comet to appear brighter than the Moon during the day, it would have to be both large and extremely close to Earth.

[Stu]

So much utter, utter crap being talked about online now re ISON I was moved to make this...



If you want a real laugh/want to despair for future of our species, check out J7409 on YouTube....

[jasedm]

There's a lovely crisp animation of the comet's trajectory here

It's worth watching in 720p and full screen as the poster advises.

For a while it looks as though the comet passes very close to Mercury, but no less than 50 million km apparently.

Jase

[Explorer1]

It'll still be close enough for MESSENGER to attempt some snapshots (pending mission extension approval).

See here: http://messenger.jhuapl.edu/qa/?faq=1&ca=36#qn466

[djellison]

If you turn on the comet layer in 'Eyes...' - then ISON is there as well. http://eyes.nasa.gov

Shortcut URL - http://1.usa.gov/133yxyt (Java enabled browser on Mac or PC required)

D

[jasedm]

[quote name='Explorer1' date='May 29 2013, 07:24 PM' post='200545']
"It'll still be close enough for MESSENGER to attempt some snapshots (pending mission extension approval)."

Here's hoping the mission extension gets the nod.

Devil's advocacy: I wonder what MESSENGER's cameras could achieve at that range, that would usefully add to our knowledge of cometary dynamics.
We are en-route to Churyumov-Gerasimenko with Rosetta, and HiRise will in all probability swing around to have a look at PANSTARRS as it passes Mars, is ISON a distraction from precious time at Mercury when we probably won't get back there again for several decades?

Having said that, Phoenix over Heimdall and shots of Curiosity descending into Gale were priceless in their publicity value.

Just saying

[Explorer1]

Bepi-Colombo is not 'several decades' away from Mercury....

[jasedm]

Of course! So much going on, had quite forgotten about Bepi-Colombo...

[JRehling]

A more general reflection on my earlier comment: The notion of apparent magnitude is almost meaningless to apply to daytime observations of non-pointlike objects.

The daytime sky is highly variable in its own brightness as a function of angular distance from the Sun, humidity, etc. It has an approximate brightness, per unit area, in the ballpark of 40% of the Moon's areal brightness.

The perception of contrast (object vs. background) is quite different than the usual astronomical domain of seeing against a black background. The sensitivity of the eye to such contrast is also a function of the location in the visual field, and of the presence of edges. So a bright object with a diffuse border (like a comet) might be undetectable in circumstances where the same object, no brighter, might be visible if only it had a sharp boundary.

All told, a bright daytime comet is almost impossible to rate as brighter than or less bright than some other object of comparison: It's highly contextual. So is a -15 comet brighter than the Moon? In an absolute sense, surely. But it may be much harder to notice. Potentially impossible. Or impossible from a humid meadow in Florida, and possible from a dry plain in Arizona.

Magnitude just doesn't say very much in this context.

[djellison]

99% of the public are not going to know that. They're going to hear the media repeat, ignorantly, things like 'as bright as the full moon' and wonder why they can't see it.

For the VAST majority of people - the use of 'magnitude' is enormously unintuitive, misleading and typically leads to disappointment. They are not to be blamed either. It's not 'on them'. Not even slightly. The way astronomers notate brightness is misleading to the layperson - period.

[dvandorn]

Yep -- a comet's apparent brightness (and especially daytime visibility) is very dependent on a lot of factors, many of them atmospheric.

The only comet I ever recall seeing in a pretty bright daytime sky was Hale-Bopp. Ikeya-Seki in 1965 was not visible in the northern hemisphere before it broke up at perihelion, so I never managed to catch it. But I do recall seeing Hale-Bopp before sunset (both from here in Minnesota and also from England, where I visited in April of '97). Only a bit of a stub of a tail was visible in daytime, but the coma was brighter than Venus usually gets at its brightest.

Of course, the most striking memory I have (and will ever have) of a comet was Hale-Bopp from the airliner I took to England. It was nighttime along the arctic great-circle route, Hale-Bopp rising above the curtains of the aurora borealis, the tail appearing first like a ghostly spotlight rising straight up out of the aurorae. In contrast to the pink of the aurorae, the tail looked almost greenish-bluish.

It's one of my most cherished memories. I do hope we get another great comet in my lifetime.

-the other Doug

[tedstryk]

The way astronomers notate brightness is misleading to the layperson - period.

More precisely, it is designed for stars - point sources. It is problematic for the non-expert when used for anything else.

[scalbers]

Yep -- a comet's apparent brightness (and especially daytime visibility) is very dependent on a lot of factors, many of them atmospheric.

Ikeya-Seki in 1965 was not visible in the northern hemisphere before it broke up at perihelion, ...

-the other Doug

A relatively distant comet should have a better chance of being visible in the daytime in the sense that it's brightness is better concentrated. One reference I came across suggests that the dimming due to diffuseness takes more noticeable effect when the apparent angular diameter (counting magnification of any optical aid) exceeds 15 arc minutes.

While Ikeya-Seki would have been better positioned in the southern hemisphere weeks before perihelion, it should have been visible in binoculars from mid-northern latitudes. There are of course reports Ikeya-Seki was visible at perihelion from Japan. Its visibility then was mainly a matter of longitude because the enhanced perihelion visibility was limited to just a few hours. My sub-hourly updated visibility ephemeris for this comet helps to illustrate this right around perihelion:

http://laps.noaa.gov/albers/ast/eph/ikeya_...965S1-A.co.html

Note that last column called "VIS" that takes into account various visibility factors, and the explanation link for the various parameters. The hour by hour interplay of calculated magnitude and angular distance from the sun is quite interesting. Only certain longitudes had the sun up at the right time.

I have ISON and other comets calculated at this link: http://laps.noaa.gov/cgi/albers.homepage.cgi

ISON is geometrically similar to Ikeya-Seki, though it seems less bright from what I've recently heard. Unless the brightening trend improves I'm skeptical that it will become visible naked eye. We should be able to get updated observations as the comet pulls away from the Sun's glare in August.

Incidentally Hale-Bopp rates as the most prominently visible comet I've seen (from the U.S. since the mid-1960s). However it was just around magnitude -0.5 while Venus can get close to -5 magnitude.