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View Full Version : Bad APOD? Jun 10, 2006



Irishman
2006-Jun-12, 03:07 PM
http://antwrp.gsfc.nasa.gov/apod/ap060610.html


Explanation: A waxing crescent Moon shines over the caldera of dormant volcano Haleakala and observatory domes in this dramatic view from above the clouds.

Now I'm not certain, but that picture doesn't appear to be a crescent moon, but a gibbous one.

HenrikOlsen
2006-Jun-12, 03:23 PM
If the date for the picture is correct, the moon was a waxing crescent, with new moon on 27th May and first quarter on 4th June.

The moon is severely overexposed in the picture which is probably what cause its appearance.

tony873004
2006-Jun-18, 01:21 AM
The glow of sunset (or sunrise) is below the Moon. So it has to be crescent: waxing if sunset, waning if sunrise.

HenrikOlsen
2006-Jun-18, 01:55 AM
The glow is light pollution from Honolulu, not the sun. :)

tony873004
2006-Jun-18, 03:40 AM
It does look more like city light pollution than sunset afterglow :)

However, Honolulu is on Oahu and Haleakala is on Maui, 120 miles away to the southeast. Maui has no cities large enough to produce that kind of light pollution.

At least I think I have my Hawaii geography correct ;)

HenrikOlsen
2006-Jun-18, 12:37 PM
If you read the text accompanying the picture, you'll see that they say it's Honolulu.

tony873004
2006-Jun-19, 03:06 AM
I didn't read the text. You're right. It does say Honolulu.

But at 10,000 feet elevation, this observatory would have a horizon 100 miles away. So Honolulu, 120 miles distant, is below the horizon, seriously diminishing its light pollution. But I think I can see it on the horizon, a little to the right of the Moon's azimuth. It's directly above the blurry dome that was probably moving during this long-exposure picture. Its a reddish glow, and spans about 10% of the horizon.

But what I was talking about is the glow that exists across the entire horizon. The curved horizon suggests that this is a wide-angle photo. This glow is way too strong and too spread out to be produced by distant Honolulu. I tried setting up this sky in Stellarium and when I advanced time to get the same stars on the horizon as in the photo, it was just moments after Stellarium's sky became pitch black, and the last afterglow of sunset disappeared. But Stellarium gives me a 0 degree horizon, and at 10,000 feet, your horizon will be below 0 degrees altitude, perhaps showing you some lingering sunset glow.

But it is a bit strange that the sunset afterglow would be below Honolulu's glow. So this photo-spanning glow is probably reflected moonlight off the white cloud tops, rather than sunset afterglow as I had suggested.

Just my guess :) ...

hhEb09'1
2006-Jun-19, 02:25 PM
But at 10,000 feet elevation, this observatory would have a horizon 100 miles away. So Honolulu, 120 miles distant, is below the horizon, seriously diminishing its light pollution.I get over 122 miles, instead of 100 miles for that calculation. Plus, some of the lights of Honolulu are above the sea level.

tony873004
2006-Jun-19, 04:58 PM
Your right. I got a littly sloppy. I use the formula Horizon in miles = 1.17 * sqr (height in feet) which gives 117 miles, in rough agreement with your figure. A light on top of a 200 ft hotel would add an additional 1.17 * sqr(200) = 16.5 miles to the range 132.5 (a little more with your formula). And a more careful measurement of the distance shows it to be 110 miles, not 120.

So I guess Honolulu's lights would be directly visible.

hhEb09'1
2006-Jun-19, 05:36 PM
Your right. I got a littly sloppy. I use the formula Horizon in miles = 1.17 * sqr (height in feet) which gives 117 miles, in rough agreement with your figure.Weird. I did a little googling, and I came up with horizon distance formulas with the constant anywhere from 1.17 (http://www.boatsafe.com/kids/distance.htm) to 1.32 (http://science.howstuffworks.com/question198.htm) (I may have made a mistake with this last one, but I think that that is the equivalent). What I did was use 1.22--I derived it similarly* to how it's done on this wiki page (http://en.wikipedia.org/wiki/Horizon) where the formula is actually given as sqrt(1.5 * height in feet). They say "the Imperial one is actually quite precise"

*The square of the distance (straight line, not along surface, but that's close) to the tangent is equal to the height times (twice the radius plus the height, but the height is negligble in this sum)

Jeff Root
2006-Jun-28, 05:09 PM
The photographer who took the June 10 APOD answered my e-mail
question about the big yellow glow in the clouds below Haleakala:



The glow closer to the scopes (Haleakala summit) is from the
town/city of Kihei/Wailea. This is a popular long strip along the
southern west coast that is visited frequently by visitors. Lots of
light from that spot. We frequently have thick cloud cover to 8,000
ft and those lights are blocked making the summit quite dark, this is
our best observing time and sometimes we have humidity in 2-4% range
with hardly any particulates in the air. Those are the exceptional
nights and they are frequent!! Haleakala is a spectacular place to
observe and soon we will begin our imaging program.
-- Jeff, in Minneapolis

Moose
2006-Jun-28, 05:18 PM
So I guess Honolulu's lights would be directly visible.

And what we're seeing, in any case, (unless I'm mis-seeing the details), is urban light pollution reflecting off of clouds, which are significantly over sea level.

[Edit: Second look, this time at the detailed shot] Oh! The clouds are below the observatory, so what we're seeing in the distance is indeed direct (though mostly masked/diffused) lighting. Never mind. I'll go back to sleep now. ;)

Pretty shot though.

tony873004
2006-Jun-28, 05:37 PM
Jeff's explanation makes sense. The glow close to the scopes is just too bright to be Honolulu.

Interesting side note: The October 2005 issue of Astronomy Magazine shows a 25 minute full-sky exposure. Along the horizon in this picture taken near Desert Center, California is glow from El Centro, California, Mexicali, Mexico, Palm Springs, Califorina and Las Vegas, Nevada, all of which are well below the horizon.