APOD: Lyrids in Southern Skies (2014 Apr 24)

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APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by APOD Robot » Thu Apr 24, 2014 4:06 am

Image Lyrids in Southern Skies

Explanation: Earth's annual Lyrid meteor shower peaked before dawn on April 22nd, as our fair planet plowed through dust from the tail of long-period comet Thatcher. Even in the dry and dark Atacama desert along Chile's Pacific coast, light from a last quarter Moon made the night sky bright, washing out fainter meteor streaks. But brighter Lyrid meteors still put on a show. Captured in this composited earth-and-sky view recorded during early morning hours, the meteors stream away from the shower's radiant near Vega, alpha star of the constellation Lyra. The radiant effect is due to perspective as the parallel meteor tracks appear to converge in the distance. Rich starfields and dust clouds of our own Milky Way galaxy stretch across the background.

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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Nitpicker » Thu Apr 24, 2014 10:11 am

Very interesting and pretty pretty. Do meteor showers tend to be observed as fairly localised phenomena? Or, if they put on a good show from one place, are they generally good from other places at the same time? I can't help thinking that Canada, or some northern part of the US might have been a better place to observe this shower on this morning, before the Moon rose south of the celestial equator.

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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 1:19 pm

Nitpicker wrote:Very interesting and pretty pretty. Do meteor showers tend to be observed as fairly localised phenomena? Or, if they put on a good show from one place, are they generally good from other places at the same time? I can't help thinking that Canada, or some northern part of the US might have been a better place to observe this shower on this morning, before the Moon rose south of the celestial equator.
Showers aren't local, but they are regional. The best viewing is in places where the radiant is higher in the sky. Most showers are broad enough to be seen over at least a full day, meaning longitude isn't an issue, but some are very narrow, peaking over just a few hours, in which case you need to be in the right place. The phase of the Moon is important, but changing your location can't help much when it comes to lunar interference.

There is a northern hemisphere bias to meteor shower activity.
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by BDanielMayfield » Thu Apr 24, 2014 1:41 pm

Chris Peterson wrote:There is a northern hemisphere bias to meteor shower activity.
That is surpising. How strong is the bias, and is the cause known?
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 2:32 pm

BDanielMayfield wrote:
Chris Peterson wrote:There is a northern hemisphere bias to meteor shower activity.
That is surpising. How strong is the bias, and is the cause known?
I don't have the numbers at hand. The bias, however, is statistically significant. The cause is unknown. It could be some sort of random fluctuation, given that meteor debris streams are short lived (usually not more than a few thousand years), so the pattern of the streams in the inner system is always shifting. Or, there could be some perturbation pattern (caused, say, by the particular inclinations of some planets) that biases the path of comets. I'm expecting at least one paper on the matter to show up in the next few months. It will just describe the statistics, but that may drive some more theoretical research with simulations.
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by LocalColor » Thu Apr 24, 2014 4:30 pm

Thank you APOD for showing us what we miss when our skies are cloudy.

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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by MarkBour » Thu Apr 24, 2014 5:44 pm

Are most meteor showers the result of earth passing through comet dust?
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Nitpicker » Thu Apr 24, 2014 6:08 pm

Chris Peterson wrote:
Nitpicker wrote:Very interesting and pretty pretty. Do meteor showers tend to be observed as fairly localised phenomena? Or, if they put on a good show from one place, are they generally good from other places at the same time? I can't help thinking that Canada, or some northern part of the US might have been a better place to observe this shower on this morning, before the Moon rose south of the celestial equator.
Showers aren't local, but they are regional. The best viewing is in places where the radiant is higher in the sky. Most showers are broad enough to be seen over at least a full day, meaning longitude isn't an issue, but some are very narrow, peaking over just a few hours, in which case you need to be in the right place. The phase of the Moon is important, but changing your location can't help much when it comes to lunar interference.

There is a northern hemisphere bias to meteor shower activity.
In this case, though, changing location to Western Canada, say, at the same UT period of this APOD, would mean that the you could avoid the Moon being in the sky (and the radiant would be higher). So, I suppose what I'm really asking is how regional is "regional"?

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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 8:24 pm

Nitpicker wrote:
Chris Peterson wrote:Showers aren't local, but they are regional. The best viewing is in places where the radiant is higher in the sky. Most showers are broad enough to be seen over at least a full day, meaning longitude isn't an issue, but some are very narrow, peaking over just a few hours, in which case you need to be in the right place. The phase of the Moon is important, but changing your location can't help much when it comes to lunar interference.

There is a northern hemisphere bias to meteor shower activity.
In this case, though, changing location to Western Canada, say, at the same UT period of this APOD, would mean that the you could avoid the Moon being in the sky (and the radiant would be higher). So, I suppose what I'm really asking is how regional is "regional"?
As noted, "regional" means that there's no advantage to changing your longitude to observe (except for the fairly rare cases where the shower peak is very narrow), and there's always an advantage to changing your latitude to get the radiant higher in the sky. The Lyrid shower is going to be best just before dawn from locations around 34°N. It will also be best just before dawn at Las Campanas, but since the radiant will only reach an altitude of about 28°, the ZHR will be quite a bit lower.

The advantage to being at the latitude of western Canada (but you could be at any longitude) is that you have many more hours with the radiant above the horizon, which this year meant more hours without interference from the Moon.

While you have the greatest ZHR when the radiant is high in the sky, you often have the most spectacular meteors when the radiant is low, even just below the horizon. A low radiant is when you see the most Earth-grazers.
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 8:30 pm

APOD Robot wrote:Captured in this composited earth-and-sky view recorded during early morning hours, the meteors stream away from the shower's radiant near Vega, alpha star of the constellation Lyra. The radiant effect is due to perspective as the parallel meteor tracks appear to converge in the distance.
I'd say that the tracks diverge from a point in the distance. When the radiant is low like this, you can aim a camera 180° away in azimuth, and you'll see the same effect, but in this case the meteors will be converging on the shower's anti-radiant (although this point will be below the horizon, so the effect isn't quite as strong).
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Nitpicker » Thu Apr 24, 2014 9:02 pm

Chris Peterson wrote:
Nitpicker wrote:
Chris Peterson wrote:Showers aren't local, but they are regional. The best viewing is in places where the radiant is higher in the sky. Most showers are broad enough to be seen over at least a full day, meaning longitude isn't an issue, but some are very narrow, peaking over just a few hours, in which case you need to be in the right place. The phase of the Moon is important, but changing your location can't help much when it comes to lunar interference.

There is a northern hemisphere bias to meteor shower activity.
In this case, though, changing location to Western Canada, say, at the same UT period of this APOD, would mean that the you could avoid the Moon being in the sky (and the radiant would be higher). So, I suppose what I'm really asking is how regional is "regional"?
As noted, "regional" means that there's no advantage to changing your longitude to observe (except for the fairly rare cases where the shower peak is very narrow), and there's always an advantage to changing your latitude to get the radiant higher in the sky. The Lyrid shower is going to be best just before dawn from locations around 34°N. It will also be best just before dawn at Las Campanas, but since the radiant will only reach an altitude of about 28°, the ZHR will be quite a bit lower.

The advantage to being at the latitude of western Canada (but you could be at any longitude) is that you have many more hours with the radiant above the horizon, which this year meant more hours without interference from the Moon.

While you have the greatest ZHR when the radiant is high in the sky, you often have the most spectacular meteors when the radiant is low, even just below the horizon. A low radiant is when you see the most Earth-grazers.
Thanks Chris. A very informative answer. I nominated western Canada only because over the same time period this APOD was taken in Las Campanas, the time before sunrise was about the same, and Canada has a high enough latitude to have had a late moonrise on this morning. But I take it the Lyrid shower does not typically have a rare, narrow peak, if longitude is not an issue.

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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 9:53 pm

MarkBour wrote:Are most meteor showers the result of earth passing through comet dust?
Yes.
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Thu Apr 24, 2014 11:54 pm

2014-Lyrids.jpg
Here's what I got from Colorado (lat 39° N). 34 Lyrids on the evening of April 21/22.

In regards to the comments above about radiant position, here's the graph of activity. The radiant rose at UT 03:00, and where the rate takes off at about 08:00 the radiant was at 50° altitude. It was almost straight overhead as twilight kicked in at the end. It's possible that the increase after 08:00 was the result of a real increase in activity, but this shower is pretty broad, so I think most of what we're seeing is just the effect of a higher radiant.
2014_lyrids_freq.gif
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by MarkBour » Fri May 02, 2014 3:58 pm

Hmmm ... interesting. Warning to those who don't know me: The following is the musing of a rank amateur, so as you read this, be warned that it is not the least bit authoritative. I'm just thinking it through, wondering if I can make some correct statements about what is going on.

I would assume meteor strikes have the same bias as meteor showers, and I wonder if a similar bias can be found in some way on the Moon, though it might be a bias that you could plot in spacetime, but that would not show up simply in space (as in the distribution of craters on the surface of the Moon). That is, if the Moon is picking up more meteors the same times the Earth is, that effect would get scattered about as the Moon would be "leading" with a different part of its face at different times, when the Earth was "leading" with its northern hemisphere.

I assume that this concept is the thing that is being measured by all of this: Which part of the Earth is "leading" through space as it encounters a cometary dust trail. But this would mean "leading" in the sense of motion with respect to the dust trail, not any more absolute notion. And that dust trail is also in solar orbit, so I'm not at all sure I have a decent mental picture of the motion of the Earth through a trail of debris.

Meanwhile, for the Moon, I imagine it would have its own (stronger?) bias of motion. It would seem that it would probably have the highest number of collisions when it is outside of the Earth's orbit, coming around us, as it would be travelling fastest then.

I know that Earth is approaching perihelion when the southern hemisphere is leading along the path, and approaching aphelion when the northern hemisphere is leading, if I have that correct, but I don't see how that could account for the bias. I'm guessing that most comets orbit the Sun in the same direction as the Earth does, and most of them are going to experience most damage at closest approach, and leave a lot more debris on the way out, but that still doesn't "ring a bell" for me as to why the northern hemisphere would be collecting more dust than the southern.

Of course a really good simulation would be a great way to work some of this out.
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Re: APOD: Lyrids in Southern Skies (2014 Apr 24)

Post by Chris Peterson » Fri May 02, 2014 5:04 pm

MarkBour wrote:I would assume meteor strikes have the same bias as meteor showers, and I wonder if a similar bias can be found in some way on the Moon, though it might be a bias that you could plot in spacetime, but that would not show up simply in space (as in the distribution of craters on the surface of the Moon). That is, if the Moon is picking up more meteors the same times the Earth is, that effect would get scattered about as the Moon would be "leading" with a different part of its face at different times, when the Earth was "leading" with its northern hemisphere.
Meteor strikes (really, meteorites) on the Earth are unrelated to showers. There is no observed geographical bias in the distribution of meteorites. On the Earth, all meteor strikes are produced by sporadics- bodies not associated with showers. On the Moon, nearly all strikes are probably shower members. However, almost all meteorites on Earth had parent bodies in low inclinations, and there is theoretical support that this leads to a polar bias for those with higher initial velocities.

There is definitely a bias with both showers and sporadics favoring the leading orbital edge of the Earth or the Moon. But since that edge rotates- once every 24 hours for the Earth, once every 29 days for the Moon, there is no overall geographic bias on either body created by this effect.
Meanwhile, for the Moon, I imagine it would have its own (stronger?) bias of motion. It would seem that it would probably have the highest number of collisions when it is outside of the Earth's orbit, coming around us, as it would be travelling fastest then.
The difference in velocity with lunar phase is insignificant. For all practical purposes, the Moon is in a circular orbit around the Sun at the same velocity as the Earth. That is, each travels around the Sun at 30 km/s, with the Moon's orbit around Earth changing its own solar orbital speed by ±1 km/s.
I know that Earth is approaching perihelion when the southern hemisphere is leading along the path, and approaching aphelion when the northern hemisphere is leading, if I have that correct, but I don't see how that could account for the bias. I'm guessing that most comets orbit the Sun in the same direction as the Earth does, and most of them are going to experience most damage at closest approach, and leave a lot more debris on the way out, but that still doesn't "ring a bell" for me as to why the northern hemisphere would be collecting more dust than the southern.
New and long period comets have a uniform distribution of inclinations- that is, as many are in retrograde orbits as prograde. This is part of the strong evidence for a spherical Oort Cloud. Short period comets, however, have a strong prograde bias (80%), and a bias towards lower (more ecliptic) inclinations. This may be explained by a more toroidal structure of the inner Oort Cloud.
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