Summer northwest sky darker than southeast sky?

[Howard B]

I've noticed the last four or five years that the northwest sky during the summer appears darker to my eye than the southeast sky, with the Milky Way in between. I've measured the difference with my SQM and generally get about a 0.1 difference - for instance, if I get a reading of ~21.70 in the northwest sky I'll get ~21.60 in the southeast sky. I've seen this at the Golden State Star Party and the Oregon Star Party but not at sites with overall brighter skies. No doubt the summer sky has always been this way, and I just started to notice the difference a few years ago, but the more I see it the more I wonder what's going on.

Has anyone else noticed this difference and know what causes it?

[Ivan Maly]

I sort of noticed it - I was wondering just recently if a new source of light pollution appeared in the SE at my usual site. I left it at that then. Now that you asked, I think that - of course! - the brightening is general in the south because the sun is not very low under the horizon. The Milky Way, once it is out of the brightest part of the summer sky, i.e. has moved to the SW horizon, makes the brightening in the SE stand out and cause questions. Here (42 N) according to my (haphazard) measurements we lose (technically gain) almost half a mag/sq arcsec overhead in summer, undoubtedly because the sun is barely below the astronomical twilight "depth" all night. I am grateful; where I lived before it is dawn over the southern horizon all night in summer (and in some of the nicest places I traveled the sun simply did not set ;)

[Howard B]

Hi Ivan,

I notice the effect as soon as it gets fully dark and it lasts all night - the Milky Way looks like it divides the sky into two halves with the entire half of the sky to the west-northwest of the Milky Way noticeably "blacker" compared to the entire east-southeast half of the sky. So this doesn't appear to be caused by the low elevation of the sun under the northern horizon, which would keep the northern half of the sky brighter than the rest of the sky. I observe in different places between 40 and 45 degrees north in the summer so our latitudes are similar, and the effect looks the same from June through October, my main observing season. It may persist through the autumn, winter and spring, but the rainy season begins in October here so the only thing I see in the sky is the famous Oregon Nebula!

[Howard B]

Here's a crude all-sky diagram that may help show what I mean. The difference in brightness between the two halves of the sky is exaggerated but shows that the two sides of the summer sky do not look equally dark to my eyes. There is no significant light pollution at the observing sites that I've seen this effect.

838

[Ivan Maly]

Yep, you are right of course. Funny I had to concentrate now to remember which side of the sky was light all night in the North - the northern side of course! So this is not it. Well, since it's the opposite side... then... gegenschein?

EDIT: Not trusting my basic astronomy any more, I checked with my planetarium program. Yes, if you look where the ecliptic runs and the point opposite the Sun on it this time of the year, it is where we see the brightening. (32 degrees up exactly SE at midnight today - marked conveniently by the full Moon.) Interesting... I had long wanted to see gegenschein but when I did - as it now seems - I was rather upset thinking it's light pollution.

[Jim Chandler]

Hi Ivan,

I'm skeptical of the gegenschein being bright enough to be perceived as lightening up a large area of the sky. Even out here four miles from McDonald Observatory, it takes a night of above average transparency to be able to scan the sky and notice the gegenschein. It's a really subtle effect; much, much dimmer than the zodiacal light.

Jim

[Ivan Maly]

The brightening in the summer SE sky looked dimmer to me than the typical zodiacal light at the same site. The height of this unexplained "light dome" above the treeline from the horizon is ~30 degrees. There is some sporadic mining/exploration activity in the area where I observe, so light domes can indeed come and go theoretically ;) But I will assume for now that what I saw was what Howard saw at his sites.

[Don Pensack]

For nearly 5 decades I've thought the sky "following" the Milky Way to be "washed out" compared to the sky preceding it. The background sky in Capricornus, Aquarius, Pisces, etc. has always seemed brighter to me than the sky in the Hercules/Corona/Lyra area.
It's not just to the naked eye, either--it's through a telescope, looking at the background sky around galaxies and planetaries.
But this isn't something that has only been seen by me in the Summer. It shows up if I stay up till dawn much earlier in the year and watch this part of the sky rotate into view.

However, I also notice the same effect preceding and following the Winter Milky Way, with Orion, Taurus, Perseus,et.al. seemingly in a darker sky than Puppis, Cancer, Hydra, Lynx.

I have some ideas, and they may be worth exploring:
1) The presence of a lot of bright stars in the areas preceding the Milky Way, which occurs both times, followed by their general dearth following the Milky Way, which also occurs both times, may be key. When you first arrive at a dark site in the night, the sky appears black in between the stars. Slowly, as you dark adapt, the sky turns grey, and then silver-grey, as your sensitivity to light improves. is it possible that having a large number of bright stars makes the sky appear darker in the areas preceding the Milky Way? is it possible, therefore, the sky appears greyer and less dark in the areas where there are no bright stars?
2) Is it possible that the sky is not uniformly bright in every direction due to galactic cloudiness that reduces the transparency of the nearby Universe? I recall see very long exposures in many areas of the sky that showed a substantial amount of "Galactic Cirrus" lies in particular directions.
3) Add to the Summer observation the placement of the Gegenschein which, though only dimly visible, does affect the greyness of the sky in an otherwise somewhat dimmer area anyway. This would affect the seeming transparency, but i don't think a brightness difference would be noted, as in the observation.
4) Using a computer program with plotted stars to mag.16+, and turning off all DSOs and shrinking the stars to points reveals that there doesn't appear to be a significant difference in the number of stars preceding and following the Milky Way. If anything, there might be a slightly larger number of faint stars preceding. That would have the opposite effect than what is noted.

This is an interesting thread. I would want to hear more ideas.

[Howard B]

Hi Don,

Excellent, you've noticed this too! I've posted this same question in Cloudy Nights in the "General Observing and Astronomy" forum and so far the ideas put forward are similar - the contribution of the ecliptic/Gegenschein to the east and galactic dust to the west are the most popular ideas but no one really knows. There was a suggestion that perhaps star density to the east is great because of our position in the Milky Way, but as you pointed out that doesn't really pan out. I think your idea #1 is really interesting - it makes sense that the perceived sky background in areas with brighter stars would seem to be blacker than a part of the sky with dimmer stars. That would make this effect, in part anyway, an illusion, but with the addition of the ecliptic/Gegenschein to the east and galactic dust to the west could be partially real as well.

Sadly, I've seen the winter sky from a true dark sky site much too infrequently so I don't have a memory of the eastern sky looking washed out like in the summer but I hope to remedy that situation this winter and will be looking for this effect then.

[Don Pensack]

The very first year I observed with a telescope was 1963, starting in the summer. As I sat up with a telescope and planisphere to learn the constellations and seek the Messier objects, I was always disappointed with how washed out the southern sky looked after Sagittarius moved to the southwest. At first I thought it was just high haze, but it repeated night after night. When, later in the same year, I noticed it again after the winter Milky Way had passed, I knew it was something else, but I hadn't though of it again until now.
Thanks for bringing back the memory.

A good way to test the theory is to see if constellations with all bright stars seem to have darker backgrounds than those with all faint stars.

[Howard B]

That's interesting and reminds me of something else too. Every year at the GSSP and OSP I overhear observers around me commenting around 2 am-ish how washed out the sky looks. Comments like "We're losing the sky now" or "man, look how washed out the sky looks now, and it was so good earlier!" The Milky Way is more or less stretching east to west at this point of the night, and since it seems most people are looking toward the southern half of the sky, whatever is causing this effect explains this nightly comment as well.

[Marko]

Where is that huge tidal tail that is off of sag? I don't recall what it was called. Does it cross that span close over our area of the galaxy and appears then to 'follow' the main plane of our galaxy? I have no idea and this is a wild guess.

[Ivan Maly]

I did see it last night as Howard depicted it: the contrast between the entire NW half and the entire SE half separated by the Milky Way. By SQM-L the difference seemed to be 0.1 mag/sq arcsec, but the contrast is huge given the angular size. This will account for my repeated observation this season that the best transparency is just after astronomical twilight, with no connection to the CSC forecast.

This contrast between essentially two halves of the sky is different from the ecliptic-high glow in the SE. I did not see that glow last night - either they stopped torching gas there or the overall sky conditions (dewpoint 17C) prevented visibility if it is in fact celestial.

[Howard B]

Some local observers have it seen the effect I described the past few nights here in Oregon too Ivan. Just as you noted, it's more obvious visually than the SQM readings would suggest.

[Marko]

I'm going to throw another wild idea out there on this one. I doubt that Earth is located dead center of the galaxtic plane and frankly I don't know its location in terms of what side of the center of the plane of our galaxy. I am sure somebody knows and it is published somewhere. So if this is the case then would it perhaps be true that we may from our location be looking through more of the loosely packed stars cloud around our galaxy if we look at one side vs the other side of our galaxy. These stars being far more sparse than the main arms of the milky way but still unresolvable to our eyes may give a brighter glow to the sky or lower SQM if you measured it. Perhaps careful SQM measurements with the very same instrument on both sides on the same night with no light domes in either side may add to this theory.

[Ivan Maly]

One of the best views (maybe the best) I had from Down Under was Milky Way stretching across the sky with the Galactic Center high near culmination. The reason I liked it so much was the asymmetric appearance of our galaxy, with respect to the perceived major axis, i.e. approximately the Galactic equator in reality. I understood from Glenn LeDrew's explanation that was given to me that this is an illusion created by nearby dust clouds. Yet to me it looked like a galaxy viewed from slightly above the plane of its disk, the direction up being indicated by a line drawn from the Galactic Center approximately to Antares (i.e. NW). In this direction there were dust lanes and the bulge, and the opposite (SE) edge of the galaxy was extremely diffuse, losing itself imperceptibly far from the perceived major axis.

[Don Pensack]

One of the best views (maybe the best) I had from Down Under was Milky Way stretching across the sky with the Galactic Center high near culmination. The reason I liked it so much was the asymmetric appearance of our galaxy, with respect to the perceived major axis, i.e. approximately the Galactic equator in reality. I understood from Glenn LeDrew's explanation that was given to me that this is an illusion created by nearby dust clouds. Yet to me it looked like a galaxy viewed from slightly above the plane of its disk, the direction up being indicated by a line drawn from the Galactic Center approximately to Antares (i.e. NW). In this direction there were dust lanes and the bulge, and the opposite (SE) edge of the galaxy was extremely diffuse, losing itself imperceptibly far from the perceived major axis.
I've noticed this, too.
At Mauna Kea, where the galaxy rises nearly parallel to the horizon, the "northern bulge" (toward Antares and Libra) is large and well rounded, while the "southern bulge" (toward Capricornus, Corona Australis, et.al) seems flattened and much less bulging. Mellinger's Milky Way panorama shows this quite well, too.
However, this really doesn't explain why the sky preceding the Milky Way appears darker and less washed out than the sky following.
Plus, my SQM measurements always show my local mountain site to reach peak darkness around 2am, and galaxy and planetary views corroborate that.
Yet, this time of year that means the sky appears hazier and more "washed out" at the time the SQM indicates the darkest sky.

Since the SQM measures overall sky brightness, which, of course, includes stars, when the Milky Way moves away from the zenith the sky gets darker.
Could it be that we finally dark adapt fully and notice the true brightness of the background sky? Could it be that, as I mentioned in my earlier post, the absence of really bright stars allows us to see the sky brightness as it really is? Because even in clear skies the SQM measures that part of the sky as darker (fewer bright stars, no Milky Way). The Gegenschein only explains a small area of the sky, and it is nowhere near here at other times of the year when the same phenomenon shows up.
A mystery.

[Ivan Maly]

... while the "southern bulge" (toward Capricornus, Corona Australis, et.al) seems flattened and much less bulging.

I thought that it actually extended, in the very diffuse and imperceptible manner, farther from the Galactic equator than the northern bulge did - and hence could contribute to the phenomenon at hand. But probably its larger extent that I seem to remember was merely an illusion, one to which you did not fall prey. Fading imperceptibly indeed does not mean extending indefinitely. It just seemed so to me.

[Don Pensack]

Look at this photo:
http://www.google.com/imgres?imgurl=http://jila.colorado.edu/~ajsh/insidebh/mellinger.jpg&imgrefurl=http://jila.colorado.edu/~ajsh/insidebh/schw.html&h=480&w=640&sz=309&tbnid=2IahXL7B9c4z1M:&tbnh=90&tbnw=120&zoom=1&usg=__0Q3LL0Fuv6PvOWv_uV8DyPXwYGc=&docid=c5i9Ledno22LvM&sa=X&ei=d9s5Up-5CqSMyAHqh4HQAQ&ved=0CD0Q9QEwAg
and notice how much flatter the Milky Way is on the left (east) side than it is on the right (west).
It's real. What that means about the sun's place in the disc, I don't know.