Starship Asterisk*

A Fading Moonset Over Hong Kong

Explanation: Do stars appear dimmer when nearer the horizon? Yes -- atmospheric air absorbs and reradiates light, so that the greater the airmass through which one peers, the fainter an object will appear. Pictured above in a multi-frame image, stars, the planet Jupiter, and even the Moon show the horizon-dimming effects of Earth's nearly-transparent atmosphere. The image was taken in the evening about three weeks ago over Hong Kong, China. The brightest streak near the center is the setting Moon, while intermittent thin clouds sometimes dispersed moonlight into a larger halo. Jupiter sets just to the Moon's right. The dim steaks cutting across the image horizontally were caused by passing airplanes. The bright strange multi-pronged streak over the house is a helicopter taking off. An astute observer will also notice faint rays emanating from near the horizon. Their cause is unknown, but may be crepuscular rays caused by the Sun shining through gaps in thick clouds.

An astute observer will also notice faint rays emanating from near the horizon. Their cause is unknown, but may be crepuscular rays caused by the Sun shining through gaps in thick clouds.

It looks like this image was started about three hours after sunset- far too late for any solar crepuscular rays. But they could very reasonably be lunar crepuscular rays (that's a lousy term, since "crepuscular" means around sunrise or sunset, not moonrise or moonset). We might have seen those before, in the 2009 May 18 APOD (in the related Asterisk discussion, it was never settled for sure if that image actually shows lunar crepuscular rays). Certainly, given the length of the Hong Kong exposure, and the noted presence of clouds near the horizon, lunar crepuscular rays seem entirely plausible.

The idea that they were moon rays intrigued me, Chris. Visually, they look like they line up with where the setting moon would end up somewhere behind the clouds. I decided to trace along the edges of the rays as closely as I could to see where they converge. I know there is a moderate degree of inaccuracy given the faintness of the rays. I equalized the area where the rays were to try to increase the contrast and discovered a very faint one emanating from a significantly different angle from the rest of them, too.

I'm disappointed that they seem to converge at the wrong point for moon rays. Maybe I went about it wrong. Or maybe they aren't moon rays.

geckzilla wrote:The idea that they were moon rays intrigued me, Chris. Visually, they look like they line up with where the setting moon would end up somewhere behind the clouds. I decided to trace along the edges of the rays as closely as I could to see where they converge. I know there is a moderate degree of inaccuracy given the faintness of the rays. I equalized the area where the rays were to try to increase the contrast and discovered a very faint one emanating from a significantly different angle from the rest of them, too.

Well, there is quite a lot of distortion in the image, as evidenced by the fact that the star trails all have different apparent centers of rotation. The rays also appear to show curvature, so using straight lines to identify an intersection is probably introducing some error. Also, this exposure doesn't capture a single moment, but a long period, so there's no way to tell just when during the exposure the rays were captured. If different rays were produced at somewhat different times, they would naturally show different angles.

I still think that lunar crepuscular rays are the most likely explanation.

Chris Peterson wrote:

geckzilla wrote:The idea that they were moon rays intrigued me, Chris. Visually, they look like they line up with where the setting moon would end up somewhere behind the clouds. I decided to trace along the edges of the rays as closely as I could to see where they converge. I know there is a moderate degree of inaccuracy given the faintness of the rays. I equalized the area where the rays were to try to increase the contrast and discovered a very faint one emanating from a significantly different angle from the rest of them, too.

Well, there is quite a lot of distortion in the image, as evidenced by the fact that the star trails all have different apparent centers of rotation. The rays also appear to show curvature, so using straight lines to identify an intersection is probably introducing some error. Also, this exposure doesn't capture a single moment, but a long period, so there's no way to tell just when during the exposure the rays were captured. If different rays were produced at somewhat different times, they would naturally show different angles.

I still think that lunar crepuscular rays are the most likely explanation.

I don't think it is nearly cloudy enough for crepuscular rays caused by clouds.
These rays must be caused by the mountain ridge itself, IMO.

While they might be moonlight induced I would think it more likely that they
are a full time exposure of rays from a brighter section of Hong Kong itself.

Looking at the enlarged image: what is the streak just to the right of Jupiter? It seems to get brighter as it gets closer to the horizon instead of getting fainter, as the description states,

"Do stars appear dimmer when nearer the horizon? Yes -- atmospheric air absorbs and reradiates light, so that the greater the airmass through which one peers, the fainter an object will appear."

Why would this particular star trail (if that is what it is) appear to get stronger/brighter?
(I'm not referring to the 'multi pronged streak' that is over top of the house, as I understand that is a helicopter from the description, but the short streak just to the right of the Jupiter streak.)
Could it have something to do with the distortion that Chris Peterson referred to?
Additionally, the streak I'm asking about seems to split, which could very well indicate distortion.....


TIA

Indigo_Sunrise wrote:Looking at the enlarged image: what is the streak just to the right of Jupiter? It seems to get brighter as it gets closer to the horizon instead of getting fainter, as the description states,

Why would this particular star trail (if that is what it is) appear to get stronger/brighter?

Additionally, the streak I'm asking about seems to split, which could very well indicate distortion....

I'm guessing an airplane landing at HKG, TIA.

Three weeks ago, 2009 Nov 24, in Hong Kong, the moon was almost at first quarter phase. Sunset was at 17:39 and moonset at 23:57, over six hours later. (USNO)

neufer wrote:I'm guessing an airplane landing at HKG, TIA.

Multiple planes

Indigo_Sunrise wrote:Looking at the enlarged image: what is the streak just to the right of Jupiter? It seems to get brighter as it gets closer to the horizon instead of getting fainter, as the description states,

"Do stars appear dimmer when nearer the horizon? Yes -- atmospheric air absorbs and reradiates light, so that the greater the airmass through which one peers, the fainter an object will appear."

Why would this particular star trail (if that is what it is) appear to get stronger/brighter?
(I'm not referring to the 'multi pronged streak' that is over top of the house, as I understand that is a helicopter from the description, but the short streak just to the right of the Jupiter streak.)
Could it have something to do with the distortion that Chris Peterson referred to?
Additionally, the streak I'm asking about seems to split, which could very well indicate distortion.....

TIA

I think the one you are talking about is the helicopter, even though it appears to have the trajectory of a sky object, ending beyond the horizon. The splitting you mention is the "multi-pronged" nature Bob mentions, though he misleadingly describes it as "over the house". The near-horizontal curve, which could not be described as "multi-pronged", over the house is something else. I think it is the wire attached to the telephone pole in the foreground.

Three weeks ago Jupiter set before the moon (and south of it as well). But the positions for the streak just to the right of the Moon being Jupiter would be right for about the 20th of November.

"The bright strange multi-pronged streak over the house is a helicopter taking off."

Sorry, but I can't see where this is. Can someone point it out to me?

Thanks

Thanks, Bystander. That's what I thought was being referred to. However, it doesn't look "multi-pronged", just a line of dots.

Maybe the short streak beside Jupiter is the helicopter after all. Again, it's not multi-pronged, just two-pronged! The description in the original blurb is a tad misleading.

bystander wrote:

I don't think these could be described as "multi-pronged", but they are more accurately than anything else described as "over the house". We need Bob to tell us what he meant. I think these are antennae warning lights, on the ridge, or maybe extremely tall buildings far away; I'm not clear on where downtown Hong Kong is, though there are clearly distant city lights (on shorter buildings) further to the right.

Notice that the house, or whatever it is, appears to have solar panels installed on its roof! Good to see that in Hong Kong
Certainly "multi-pronged" and not a sky object, but hardly "over the house", except in the most general sense.

Chris Peterson wrote: Well, there is quite a lot of distortion in the image, as evidenced by the fact that the star trails all have different apparent centers of rotation. The rays also appear to show curvature, so using straight lines to identify an intersection is probably introducing some error. Also, this exposure doesn't capture a single moment, but a long period, so there's no way to tell just when during the exposure the rays were captured. If different rays were produced at somewhat different times, they would naturally show different angles.

I still think that lunar crepuscular rays are the most likely explanation.

Yes, I think you are right. Why are they curved, though? They do look straight but I looked at the image again and when I tried to trace the brightest one as accurately as possible, it did seem to have a very slight curve. I definitely did not think of that when I traced them the first time. And that faint ray near the center could have come from much earlier, maybe when the moon went behind a cloud rather than the horizon.

geckzilla wrote:Why are they curved, though?

Wide angled lense?

Ok, I was going to ask if it was from the lens but somehow it didn't make it into my post.

A likely story, Geck.

geckzilla wrote:Yes, I think you are right. Why are they curved, though?

As noted, the use of a wide angle lens introduces field distortion. But there's also the fact that you're trying to map a sphere to a plane. This was very obvious in an earlier APOD, where a lot of curvature is seen because the image is centered between the north and south celestial poles, so we basically see northern stars circling one and southern stars circling the other. Since the current APOD is looking west, there must be some of that going on as well.

The curved crepuscular rays, of course, have nothing to do with the Earth's rotation! That's got to be a purely optical effect.

As far as the Star Trail curvature goes, Stars to the right of the moon (north) curve around the north polar axis and those left (south) of the moon curve around the south polar axis. like they do here http://antwrp.gsfc.nasa.gov/apod/ap090314.html