APOD: Lyrids in Southern Skies (2017 Apr 27)

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APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by APOD Robot » Thu Apr 27, 2017 4:10 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. Seen from the high, dark, and dry Atacama desert a waning crescent Moon and brilliant Venus join Lyrid meteor streaks in this composited view. Captured over 5 hours on the night of April 21/22, the meteors stream away from the shower's radiant, a point not very far on the sky from 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. In the foreground are domes of the Las Campanas Observatory housing (left to right) the 2.5 meter du Pont Telescope and the 1.3 meter Optical Gravitational Lensing Experiment (OGLE) telescope.

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by tennisjazz » Thu Apr 27, 2017 5:23 am

"The radiant effect is due to perspective as the parallel meteor tracks appear to converge in the distance." ... well in fact, unlike the railroad tracks in the image at http://en.es-static.us/upl/2010/12/rail ... 337388.jpg hyperlinked from "tracks appear", these meteor streaks do converge at their source, comet Thatcher, don't they?

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Nitpicker » Thu Apr 27, 2017 6:12 am

tennisjazz wrote:"The radiant effect is due to perspective as the parallel meteor tracks appear to converge in the distance." ... well in fact, unlike the railroad tracks in the image at http://en.es-static.us/upl/2010/12/rail ... 337388.jpg hyperlinked from "tracks appear", these meteor streaks do converge at their source, comet Thatcher, don't they?
Comet Thatcher is somewhere in the outer solar system at the moment. Last seen at its perihelion, just inside Earth's orbit, in 1861.

The parallel meteor paths come from the remnants left behind by Thatcher, which locally, all travel with about the same speed and direction (following the path of the comet's orbit, but broadening over time). The small subset of these remnants, that happen to strike Earth each year, would form a cross-sectional area exactly Earth-sized, so they don't really converge, either. They just appear to. The region of the comet's meteoroid trail that crosses Earth's orbit, is much larger than the Earth. (After all, the Earth travels on its orbit each day, a distance ~200 times its diameter, and the Lyrid shower lasts for up to 10 days each year.)

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by JohnD » Thu Apr 27, 2017 12:53 pm

Declared as a "composite picture", is there no limit to the manipulation of images that APOD publishes?
The sky had to have been taken by a moving camera, unless the meteors all occurred within a second or two, then the equally long exposure image (taken by moonlight?) of the ground and buildings was from a fixed camera, then each image carefully dissected and put together. All very artificial - or am I being picky?
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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Chris Peterson » Thu Apr 27, 2017 2:03 pm

JohnD wrote:Declared as a "composite picture", is there no limit to the manipulation of images that APOD publishes?
The sky had to have been taken by a moving camera, unless the meteors all occurred within a second or two, then the equally long exposure image (taken by moonlight?) of the ground and buildings was from a fixed camera, then each image carefully dissected and put together. All very artificial - or am I being picky?
APOD is a science-oriented site, and many of the images here are intended by the editors to get across some scientific concept. I think radiant images like this serve that purpose.

Certainly you could make a conventional astronomical image by mechanically tracking the radiant for several hours and stacking the resulting images. However, such images lack a sense of scale. By compositing in a single frame that contains the local landscape, it allows us to see the sky in a way that we're much more familiar with, especially when we're talking about a visual phenomenon like a meteor shower.

Do you have a suggestion for a better way to image a meteor shower that allows people to visually grasp the idea of a radiant?
Chris

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Catalina » Thu Apr 27, 2017 4:51 pm

The caption states that this image displays a "waning crescent moon". The moon appears quite bright and more toward full to me as portrayed. Is this due to the relative brightness of the "crescent" moon to the rest of the elements in the composition photograph?

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Chris Peterson » Fri Apr 28, 2017 2:03 am

Catalina wrote:The caption states that this image displays a "waning crescent moon". The moon appears quite bright and more toward full to me as portrayed. Is this due to the relative brightness of the "crescent" moon to the rest of the elements in the composition photograph?
Yes, it's just massively overexposed. At this scale the Moon should appear less than 4 pixels across; it actually appears some six times larger than that. The actual phase is totally lost in all that light.
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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Boomer12k » Fri Apr 28, 2017 3:22 am

"Going like a shooting star...in the sky...." er...something, something... "Ok, I'll work on it...
Going to be a BIG HIT... you can tell.... Hey, I was a third year Music Major....

Great Pic...

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Nitpicker » Fri Apr 28, 2017 3:55 am

Having never given much thought to meteor showers, I've never previously considered or attempted the following calculation, so I might rely on someone like Chris (or someone exactly like Chris) to check my method ...

Around April 22, Earth was hurtling through the solar system at ~30 km/s, in the direction of Capricornus, about 90 degrees west of the Sun (in Aries) along the ecliptic. As shown in the APOD (and Stellarium), the apparent radiant of the Lyrids was in Hercules (close to its border with Lyra), forming an angle of about 62-63 degrees in the sky, from the position of Earth's velocity vector in Capricornus.

From the semi-major axis of C/1861 G1 (Thatcher), determined in 1861 to be ~55.7 AU, I calculated the true orbital speed of the comet's meteoroid trail, to be ~42 km/s, when crossing Earth's orbit.

From this and a bit of vector arithmetic, I calculated the true radiant of the Lyrids (that is, the apparent radiant if the Earth was motionless in the solar system at this point in its orbit) to be about 101-102 degrees from the position of Earth's velocity vector, measured through the point of the apparent radiant. This places the true radiant in Draco, roughly 20 degrees from the ecliptic north pole.

I've attempted to display this graphically (using Stellarium) against an ecliptic grid. The blue arc shows the true sky angle measurement shown in a stereographic projection, from the position of Earth's velocity vector (bottom), to the true radiant of the Lyrids (top), passing through the apparent radiant (nicely annotated by the [brilliant] software):
lyrids.jpg
I calculate the relative, or geocentric velocity of the Lyrids, prior to striking Earth's atmosphere (and ignoring the localised gravitational effects on their speed), to be ~46 km/s. This seems to match pretty closely to the number I just found on the web.

Is my method sound and are my numbers close? Is it possible to determine these numbers without knowing the semi-major axis of the meteoroid trail? Interesting stuff ... I may just have found yet another new way to enjoy my astronomy hobby.
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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by BillBixby » Fri Apr 28, 2017 5:29 pm

Boomer12k wrote:"Going like a shooting star...in the sky...." er...something, something... "Ok, I'll work on it...
Going to be a BIG HIT... you can tell.... Hey, I was a third year Music Major....

Great Pic...

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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Chris Peterson » Fri Apr 28, 2017 5:54 pm

Nitpicker wrote:Is my method sound and are my numbers close? Is it possible to determine these numbers without knowing the semi-major axis of the meteoroid trail? Interesting stuff ... I may just have found yet another new way to enjoy my astronomy hobby.
Your methodology is sound, although I didn't check your actual results. They might be different depending on the inputs I chose. The most recent measurements for Lyrid bodies- these are direct measurements from multiple station cameras and from radar- give a semimajor axis of 10.8 AU. All of the orbital parameters can be obtained here, along with an accurate solar longitude and radiant position (note that meteoriticists were early adopters of the use of degrees for RA, so watch that if you're looking for hours).

The IAU shower database is the best source for meteor data.
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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by neufer » Fri Apr 28, 2017 9:07 pm

http://www.etymonline.com/index.php?allowed_in_frame=0&search=lurid wrote:
<<lurid (adj.) 1650s, "pale, wan," from Latin luridus "pale yellow, ghastly, the color of bruises," a word of uncertain origin and etymology, perhaps cognate with Greek khloros "pale green, greenish-yellow", or connected to Latin lividus (see livid).

It suggests a combination of light and gloom; "Said, e.g. of the sickly pallor of the skin in disease, or of the aspect of things when the sky is overcast" [OED]; "having the character of a light which does not show the colors of objects". Meaning "glowing in the darkness" is from 1727 ("of the color or appearance of dull smoky flames." The figurative sense of "sensational" is first attested 1850, via the notion of "ominous" (if from the flames sense) or "ghastly" (if from the older sense).>>
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Re: APOD: Lyrids in Southern Skies (2017 Apr 27)

Post by Nitpicker » Sat Apr 29, 2017 5:24 am

Chris Peterson wrote:Your methodology is sound, although I didn't check your actual results. They might be different depending on the inputs I chose. The most recent measurements for Lyrid bodies- these are direct measurements from multiple station cameras and from radar- give a semimajor axis of 10.8 AU. All of the orbital parameters can be obtained here, along with an accurate solar longitude and radiant position (note that meteoriticists were early adopters of the use of degrees for RA, so watch that if you're looking for hours).

The IAU shower database is the best source for meteor data.
Thanks for that Chris. Is the large variation in the derived semi-major axis values (listed in your first link), due to the fact that different parts of the trail get perturbed by different amounts? Or is it also that some of the observations are more reliable than others? I'm surprised such a big variation in a-value makes so little difference to the geocentric velocities (VG) and the other orbital elements derived from each observation. (I was pretty darn close with my back-of-envelope calculation, but it seems almost any "ball-park" a-value will give a reasonably close VG.)

I take it that the "true radiant" isn't a parameter that is considered too important. Nor, seemingly, the direction of Earth's velocity vector (does this have a better name?) Or maybe they are, but as intermediate parameters in the derivation of the orbital elements, and so not as well documented. Not sure.

With Earth's very slightly elliptical orbit, by how much does Earth's velocity vector vary annually, from 90 degrees west of the Sun in ecliptic longitude?

(Sorry for bombarding you with questions.)