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Winter stars vs Summer stars

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#1 mathteacher

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Posted 21 June 2010 - 11:44 PM

I just spent 2 nights camping at Canyonlands NP in southern Utah. I think it was in the black zone. However, I was not impressed by the number of stars I saw. I remember being in the Sierra Nevada Mtns. in the winter, and being overwhelmed by so many stars that recognizing constellations was difficult. This was not the case 2 nights ago, when I saw well defined constellations and not many other stars.

My question: are there fewer stars to be seen in summer? Other possible explanations: my eyes were over-exposed to sun that day, poor transparency?

Let me know your thoughts.

Thanks!

#2 Spaced

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Posted 22 June 2010 - 12:01 AM

No moon in sky? Low humidity?

I was down there about two weeks earlier a couple of years ago. There was quite a bit of humidity in the air, reducing transparency. Clouds would build up in the afternoon and linger to evening. I'd get up around 2 or 3 a.m. and it would be better.

#3 Dain

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Posted 22 June 2010 - 12:22 AM

Had to have had bad seeing/transparency or something related. Too much humidity?

I was out in the middle of the Mexico/Arizona deserts last summer coming through Rt. 40 and the skies were excellent. The Milky Way was pretty darn bright, especially through Sagittarius. It truly was hard to connect the constellations.

#4 Tony Flanders

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Posted 22 June 2010 - 05:24 AM

I just spent 2 nights camping at Canyonlands NP in southern Utah. I think it was in the black zone. However, I was not impressed by the number of stars I saw. I remember being in the Sierra Nevada Mtns. in the winter, and being overwhelmed by so many stars that recognizing constellations was difficult. This was not the case 2 nights ago ...


A few things. First, it may be a matter of familiarity. I bet if you looked at the winter sky now, you'd have no trouble recognizing constellations. After all, in addition to having by far the brightest stars, the winter sky has the most recognizable constellations.

Second, when were you observing? Remember, the sky doesn't get fully dark now until around 11 p.m. Many if not most campers are in bed by then. In winter, you're up after dark whether you like it or not.

But above all, the Northern Hemisphere's evening sky really does have fewer stars in spring than at any other time of year. That's because you're looking out at right angles to the plane of the Milky Way, into the depths of intergalactic space. That's true in autumn too, but there happen to be a few bright stragglers that perk up the autumn sky.

If you'd stayed up past midnight, you would have seen the bright stars of the summer Milky Way climbing high in the east. In full summer, the sky has almost as many stars as in winter. And what it lacks in super-bright stars, it makes up in the mass of faint stars that compose the Milky Way.

Down along the southern horizon, Scorpius is a super-bright constellation even now in early evening.

#5 Hrundi

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Posted 22 June 2010 - 06:31 AM

The winter sky has a stronger presence of bright stars in my opinion.
The various high mag stars near orion, such as betelgeuse, rigel, sirius, procyon etc.
Then you've got the coma berenices star cluster, hyades star cluster, pleiades, alpha persei star cluster.

#6 Tony Flanders

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Posted 22 June 2010 - 08:03 AM

The winter sky has a stronger presence of bright stars in my opinion.


Opinion has nothing to do with it -- this is a demonstrable fact. I actually wrote a computer program to test this, just in case anybody doubts the evidence of their own eyes.

Of the ten brightest stars visible from latitude 45N, five (Sirius, Capella, Rigel, Procyon, and Betelgeuse) are crammed into less than 1/10th of the sky, from Rigel (RA 5:15) to Procyon (RA 7:39). This is precisely the area that's visible during mid- to late winter.

You will get similar statistics, though not quite as dramatic, if you extend this to the 50 or 100 brightest stars. This is largely because the closest really big star-forming region is in Orion. Other bright winter stars lie in the nearby Perseus Arm of our galaxy.

Try a web search for "Gould's Belt" for more information.

#7 mathteacher

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Posted 22 June 2010 - 07:55 PM

Thanks for replies, everyone. I was observing around 2am, after the moon set. Deneb was near zenith. The milky way spanned from sagittarius in the west to cassiopeia in the east. It was nice, but maybe I was just expecting too much.

#8 skysurfer

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Posted 23 June 2010 - 06:13 AM

Of the ten brightest stars visible from latitude 45N, five (Sirius, Capella, Rigel, Procyon, and Betelgeuse) are crammed into less than 1/10th of the sky, from Rigel (RA 5:15) to Procyon (RA 7:39). This is precisely the area that's visible during mid- to late winter.

And you forgot Aldebaran (+0.9).
For most of the population (living south of about 34 N) , add Canopus (-0.7, the brightest of them all except Sirius) to it.

OTOH, in (northern) summer, there are more starfields, clussters and Milky Way, from Cyg to Sgr, much brighter than in the winter sky.

#9 Tony Flanders

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Posted 23 June 2010 - 09:47 AM

I just posted the December 2008 Hobby Q&A from Sky & Telescope on our website. It answers this question in a quantitative way, though with certain assumptions that aren't spelled out in complete detail. As I remember, I assumed 0.30 extinction, which is good but not great for a sea-level site.

If you ignore extinction, the line flattens a lot. After all, precisely half of the Milky Way is technically above the horizon at any moment. The difference in spring and fall is that the Milky Way is very low, so it suffers from heavy extinction.

Click here to read the article.

#10 dymy24

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Posted 23 June 2010 - 12:54 PM

Interesting article.

For some reason, i don't recall ever having seen that section of S&T before.

#11 GlennLeDrew

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Posted 28 June 2010 - 08:06 PM

Tony,
Just a clarification; even at its closest to us in the direction of Cas/Per, the Perseus arm is a good 5,000 l-y distant. Out there even massive supergiants are just within naked-eye visibility, at ~5-6 magnitude. Any stars we'd call 'bright' must necessarily reside within our own Local Arm (or perhaps part of the way into the inter-arm region). This includes the Gould Belt (a fascinating structure I've studied in much detail.)

#12 Starman1

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Posted 29 June 2010 - 11:44 PM

I observe at a dark site at 8350' which has an average sky brightness of 21.4 mpsas. I have another site, at 3135' that averages 21.6 mpsas. And another site, at 850' that averages 21.75 mpsas.
The most impressive skies? the brighter ones at 8350'.
Why? Much less extinction. I've seen as low as 0.15mag/atmosphere. The largest portion of the sky is the band that surrounds you from the horizon up to 45 degrees altitude. If the stars are faded out in that sector due to high extinction, the skies appear washed out. When faint stars are visible near the horizon, the whole sky appears more impressive.
The most impressive skies I've seen were one night at the 8350' site that reached 21.85 mpsas. Orion had literally hundreds of stars visible and the Gegenschein looked like a large oval cloud in the sky.
So, either you were at low altitude or the sky wasn't very transparent. It happens. I've seen clear skies look silver due to an incredible amount of high altitude water vapor (mostly in the form of ice crystals)--not a good night for observing faint fuzzies.

#13 gnowellsct

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Posted 01 July 2010 - 10:02 PM

cirrus clouds can't live with 'em can't live without 'em

#14 Cotts

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Posted 02 July 2010 - 07:44 AM

Actually, we juggle three 'observing parameters' (if I may coin a term here): Sky darkness, Sky Transparency and Seeing. How rare is it to get all three at the 'excellent' level? As we all know, it is exceedingly rare, indeed.

Also, Don, what are the extinction units you mentioned (mag/Atmosphere)? And how could an observer measure this quantitatively at an observing session?

Dave

#15 Starman1

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Posted 02 July 2010 - 10:42 AM

Actually, we juggle three 'observing parameters' (if I may coin a term here): Sky darkness, Sky Transparency and Seeing. How rare is it to get all three at the 'excellent' level? As we all know, it is exceedingly rare, indeed.

Also, Don, what are the extinction units you mentioned (mag/Atmosphere)? And how could an observer measure this quantitatively at an observing session?

Dave

I have seen this as low as 0.15mag/atmosphere in dry air at 8350'.
Here is how I measured it, and i will agree with anyone who criticizes the method as flawed, but the method has served me well to compare sites and directional sky brightnesses:

I have an SQM (Sky Quality Meter) with the 80 degree measurement field. It was pointed out, in early discussions, that having a field-narrowing tube in front of the sensor could produce consistent results if a compensatory factor were built in. In other words, say I get a zenith measurement of mag.21.4 with the sensor at full aperture. I then add a field-narrowing tube in front of the sensor (reducing the field to, say, 10 degrees) and get a zenithal measurement of 23.4. That says that the tube will reduce the brightness of the measurement by 2 magnitudes, and it will do so consistently. If I now point the SQM, with the tube, to a point 30 degrees above the western horizon and get a measurement of 23.0 (which is really 21.0), it means that at an airmass of 2 I have a sky that is brighter by 0.4 magnitude.

It should rightly be pointed out that increasing sky brightness by 0.4 magnitudes does not necessarily mean that the dimmest star visible will now be 0.4 magnitudes brighter. There is not a one-to-one match between sky brightness and dimmest star seen. So extinction is likely to be less than the increase in brightness noted.

So I supplement my extinction estimate with the visibility of faint stars at low points in the sky. If I can see magnitude 5+ stars at the horizon, then the sky is much clearer than when I can barely see magnitude 3, for example.

I have measured sky brightnesses at a near pristine site of around magnitude 21.85. On that night, the adjusted brightness at 30 degrees in a direction with absolutely no extraneous light in the sky, I measured 21.7. That's an increase of 0.15 magnitude per atmosphere. Now, as I said, that may not correspond to the actual extinction, which might have been better than that. But, having spent thousands of nights under the stars, I can fairly easily tell when the extinction factor is low because of the appearance of the sky in the horizon-to 45 degree altitude band. In the Summer, the "dark horse" (or Barnard E) dark lane structure in the bulge of the Milky Way is a good indicator. When it is stark and easy to see and there are complicated structures visible, the air is very clear. When the bulge of the Milky Way extends to the brightest stars of Libra, the air is very clear. Low extinction is one of the reasons I favor high altitude sites over slightly darker lower altitude sites. After all, not everything you view is at the zenith.

But it is consistent with models of extinction for high altitude sites in the southwest, and this model:
http://www.orion-dru.../extinction.htm

As for your 3 parameters, I pointed this out in an article a few years back:
http://www.cloudynig...hp?item_id=1252
A real good example is being completely clouded over at a pristine site with no lights to illuminate the clouds. That is the darkest measurement you can get under the sky, but it is not clear.

At my site, only 100 miles from LA, the clarity of the clear sky DOES correspond well to the darkness measurement, however, and clarity seems to be the most important factor at that site for the spotting of the faintest details.

#16 Tony Flanders

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Posted 04 July 2010 - 06:09 AM

I have seen this as low as 0.15mag/atmosphere in dry air at 8350'.
Here is how I measured it, and i will agree with anyone who criticizes the method as flawed, but the method has served me well to compare sites and directional sky brightnesses ...


I'd say your method is probably good enough to rate extinction crudely on a better-to-worse scale. But to distinguish between 0.1, 0.15, and 0.2 mag per airmass? I have my doubts. Too many variables, including the somewhat crude nature of an SQM and different inherent sky brightnesses in different parts of the sky.

The way the pros do this, of course, is to measure the apparent brightness of known reference stars at different altitudes above the horizon. But that requires highly accurate photometry equipment, and can't be done by eye alone.

#17 Starman1

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Posted 04 July 2010 - 11:12 AM

Tony,
No disagreement.
I learned to do this is so I could measure differential brightnesses at different compass points so I could actually measure the effects of light pollution.






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