Starship Asterisk*

Posted: **Tue Jul 14, 2020 4:07 am**

Comet NEOWISE over Stonehenge

Explanation: Have you ever seen a comet? Tonight -- and likely the next few nights -- should be a good chance. Go outside just at sunset and look to your northwest. The lower your horizon, the better. Binoculars may help, but if your sky is cloudless and dark, all you should need is your unaided eyes and patience. As the Sun sets, the sky will darken, and there will be an unusual faint streak pointing diagonally near the horizon. That is Comet NEOWISE. It is a 5-kilometer-wide evaporating dirty iceberg visiting from -- and returning to -- the outer Solar System. As the Earth turns, the comet will soon set, so you might want to take a picture. In the featured image, Comet C/2020 F3 (NEOWISE) was captured two mornings ago rising over Stonehenge in the UK. Discovered with the NASA satellite NEOWISE toward the end of March, Comet NEOWISE has surprised many by surviving its closest approach to the Sun, brightening dramatically, and developing impressive (blue) ion and (white) dust tails.

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Posted: **Tue Jul 14, 2020 7:05 am**

Should that be North East???

Posted: **Tue Jul 14, 2020 10:37 am**

Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???

Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.

Posted: **Tue Jul 14, 2020 11:10 am**

Stonehenge makes a great foreground setting for Neowise!

Posted: **Tue Jul 14, 2020 11:28 am**

neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.

Actually the comet is circumpolar for many of us far enough north.

Posted: **Tue Jul 14, 2020 12:44 pm**

hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???
Comet NEOWISE has been rising before Sunrise in the Northeast.
Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.

https://en.wikipedia.org/wiki/Archaeoastronomy_and_Stonehenge wrote:
Computer rendering of the overall site
<<The prehistoric monument of Stonehenge has long been studied for its possible connections with ancient astronomy. The site is aligned in the direction of the sunrise of the summer solstice and the sunset of the winter solstice. Stonehenge has an opening in the henge earthwork facing northeast, and suggestions that particular significance was placed by its builders on the solstice and equinox points have followed. For example, the summer solstice Sun rose close to the Heel Stone, and the Sun's first rays shone into the centre of the monument between the horseshoe arrangement. While it is possible that such an alignment could be coincidental, this astronomical orientation had been acknowledged since William Stukeley drew the site and first identified its axis along the midsummer sunrise in 1720.

Stukeley noticed that the Heel Stone was not precisely aligned on the sunrise. The drifting of the position of the sunrise due to the change in the obliquity of the ecliptic since the monument's erection does not account for this imprecision. Recently, evidence has been found for a neighbour to the Heel Stone, no longer extant. The second stone may have instead been one side of a ‘solar corridor’ used to frame the sunrise. Stukeley and the renowned astronomer Edmund Halley attempted what amounted to the first scientific attempt to date a prehistoric monument. Stukeley concluded the Stonehenge had been set up “by the use of a magnetic compass to lay out the works, the needle varying so much, at that time, from true north.” He attempted to calculate the change in magnetic variation between the observed and theoretical (ideal) Stonehenge sunrise, which he imagined would relate to the date of construction. Their calculations returned three dates, the earliest of which, 460 BC, was accepted by Stukeley. That was incorrect, but this early exercise in dating is a landmark in field archaeology.>>

https://simpsonswiki.com/wiki/Springfield_Observatory wrote:

<<The Springfield Observatory came to prominence when it was alerted to a comet (discovered by and named for Bart) hurtling towards Springfield. After the disaster was averted, Moe and a large group of survivors decided to burn down the observatory, so that "This'll never happen again!".>>

Posted: **Tue Jul 14, 2020 1:29 pm**

hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.

Of course, even circumpolar objects "rise" and "set" in this context.

Posted: **Tue Jul 14, 2020 2:42 pm**

hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.

So is the Sun if you're above the Arctic Circle in the summer. No view of Neowise if you live too far north.

Posted: **Tue Jul 14, 2020 3:12 pm**

APOD Robot wrote: ↑Tue Jul 14, 2020 4:07 am Comet NEOWISE over Stonehenge

Explanation: Have you ever seen a comet? Tonight -- and likely the next few nights -- should be a good chance. Go outside just at sunset and look to your northwest. The lower your horizon, the better. Binoculars may help, but if your sky is cloudless and dark, all you should need is your unaided eyes and patience. As the Sun sets, the sky will darken, and there will be an unusual faint streak pointing diagonally near the horizon. That is Comet NEOWISE. It is a 5-kilometer-wide evaporating dirty iceberg visiting from -- and returning to -- the outer Solar System. As the Earth turns, the comet will soon set, so you might want to take a picture. In the featured image, Comet C/2020 F3 (NEOWISE) was captured two mornings ago rising over Stonehenge in the UK. Discovered with the NASA satellite NEOWISE toward the end of March, Comet NEOWISE has surprised many by surviving its closest approach to the Sun, brightening dramatically, and developing impressive (blue) ion and (white) dust tails.

Another impressive comet pic, but, as usual, one that belies what you can see with your own eyes. The image details reveal:

Camera: @Canonuk EOS Ra
Lens: @SigmaPhoto 150-600mm f/5-6.3 DG HSM
Focal length: 1500mm
Aperture: f/5
Iso: 1600
Shutter speed: 30s for sky; 15s for foreground.

That lens is a zoom lens, and is no doubt zoomed in, but I'm not sure what the magnification is. Is it calculable from the image specs given? (I never did have a good grasp of optics

)

Posted: **Tue Jul 14, 2020 3:40 pm**

johnnydeep wrote: ↑Tue Jul 14, 2020 3:12 pm
APOD Robot wrote: ↑Tue Jul 14, 2020 4:07 am Comet NEOWISE over Stonehenge

Explanation: Have you ever seen a comet? Tonight -- and likely the next few nights -- should be a good chance. Go outside just at sunset and look to your northwest. The lower your horizon, the better. Binoculars may help, but if your sky is cloudless and dark, all you should need is your unaided eyes and patience. As the Sun sets, the sky will darken, and there will be an unusual faint streak pointing diagonally near the horizon. That is Comet NEOWISE. It is a 5-kilometer-wide evaporating dirty iceberg visiting from -- and returning to -- the outer Solar System. As the Earth turns, the comet will soon set, so you might want to take a picture. In the featured image, Comet C/2020 F3 (NEOWISE) was captured two mornings ago rising over Stonehenge in the UK. Discovered with the NASA satellite NEOWISE toward the end of March, Comet NEOWISE has surprised many by surviving its closest approach to the Sun, brightening dramatically, and developing impressive (blue) ion and (white) dust tails.
Another impressive comet pic, but, as usual, one that belies what you can see with your own eyes. The image details reveal:

Camera: @Canonuk EOS Ra
Lens: @SigmaPhoto 150-600mm f/5-6.3 DG HSM
Focal length: 1500mm
Aperture: f/5
Iso: 1600
Shutter speed: 30s for sky; 15s for foreground.
That lens is a zoom lens, and is no doubt zoomed in, but I'm not sure what the magnification is. Is it calculable from the image specs given? (I never did have a good grasp of optics :()

"Magnification" is a meaningless concept photographically. The image information doesn't make much sense. How is the focal length 1500 mm given a lens that ranges in focal length from 150 to 600 mm?

Posted: **Tue Jul 14, 2020 4:24 pm**

Chris Peterson wrote: ↑Tue Jul 14, 2020 1:29 pm
hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.
Of course, even circumpolar objects "rise" and "set" in this context.

I don't think that is the context; 'ascend' and 'descend' maybe, but not 'rise' and 'set'.

Posted: **Tue Jul 14, 2020 4:30 pm**

The caption states "two mornings ago". Unfortunately such inprecise statements are common on APOD. Using "Starry Night" I find, that the image was taken on June 11 at around 3:00 GMT.

Posted: **Tue Jul 14, 2020 5:23 pm**

hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Prof Greg Parker wrote: ↑Tue Jul 14, 2020 7:05 am
Should that be North East???
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.

It also never sets from view at my summer home on the crater Plato.

Posted: **Tue Jul 14, 2020 6:22 pm**

MarkBour wrote: ↑Tue Jul 14, 2020 5:23 pm
hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
neufer wrote: ↑Tue Jul 14, 2020 10:37 am
Comet NEOWISE has been rising before Sunrise in the Northeast.

Comet NEOWISE will now be setting after Sunset in the Northwest.
Actually the comet is circumpolar for many of us far enough north.
It also never sets from view at my summer home on the crater Plato. :-)

You sure about that?

Posted: **Tue Jul 14, 2020 7:47 pm**

Chris Peterson wrote: ↑Tue Jul 14, 2020 3:40 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 3:12 pm Another impressive comet pic, but, as usual, one that belies what you can see with your own eyes. The image details reveal:

Camera: @Canonuk EOS Ra
Lens: @SigmaPhoto 150-600mm f/5-6.3 DG HSM
Focal length: 1500mm
Aperture: f/5
Iso: 1600
Shutter speed: 30s for sky; 15s for foreground.
That lens is a zoom lens, and is no doubt zoomed in, but I'm not sure what the magnification is. Is it calculable from the image specs given? (I never did have a good grasp of optics )
"Magnification" is a meaningless concept photographically.

Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?

The image information doesn't make much sense. How is the focal length 1500 mm given a lens that ranges in focal length from 150 to 600 mm?

Yeah, that is odd. Maybe it's a copy/paste error from a different pic taken with a telescope... The specs on that SigmaPhoto lens (from https://www.sigmaphoto.com/150-600mm-f5-63-dg-os-hsm-c) say that the maximum magnification is "1:5" which I suppose means it's a 5x zoom lens.

Posted: **Tue Jul 14, 2020 7:56 pm**

johnnydeep wrote: ↑Tue Jul 14, 2020 7:47 pm
Chris Peterson wrote: ↑Tue Jul 14, 2020 3:40 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 3:12 pm Another impressive comet pic, but, as usual, one that belies what you can see with your own eyes. The image details reveal:

That lens is a zoom lens, and is no doubt zoomed in, but I'm not sure what the magnification is. Is it calculable from the image specs given? (I never did have a good grasp of optics :()
"Magnification" is a meaningless concept photographically.
Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?

Magnification is essentially an angular measurement, one that involves telescopic optics- that is, an optical system with both an objective and an ocular (eyepiece). A single objective (like a telephoto lens, or a "telescope" used photographically) doesn't operate the same way. You simply end up with an image of some size at the focal plane (telescopic optics are afocal- there is no focal plane).

Posted: **Tue Jul 14, 2020 8:48 pm**

Nice shot.its a keeper.

Posted: **Tue Jul 14, 2020 9:37 pm**

Chris Peterson wrote: ↑Tue Jul 14, 2020 6:22 pm
MarkBour wrote: ↑Tue Jul 14, 2020 5:23 pm
hamilton1 wrote: ↑Tue Jul 14, 2020 11:28 am
Actually the comet is circumpolar for many of us far enough north.
It also never sets from view at my summer home on the crater Plato.
You sure about that?

It gets low, but never out of view (same as Capella).
If you want to come visit, I live on the northern rim, a ways out of the outskirts of Hong Kong Luna.
It'd be great if you could bring one of your telescopes.

Posted: **Tue Jul 14, 2020 9:37 pm**

Chris Peterson wrote: ↑Tue Jul 14, 2020 7:56 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 7:47 pm
Chris Peterson wrote: ↑Tue Jul 14, 2020 3:40 pm

"Magnification" is a meaningless concept photographically.
Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?
Magnification is essentially an angular measurement, one that involves telescopic optics- that is, an optical system with both an objective and an ocular (eyepiece). A single objective (like a telephoto lens, or a "telescope" used photographically) doesn't operate the same way. You simply end up with an image of some size at the focal plane (telescopic optics are afocal- there is no focal plane).

Ok. Still trying to understand this. With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I really need to read more about optics, cameras, telescopes and photography...)

Posted: **Tue Jul 14, 2020 9:54 pm**

johnnydeep wrote: ↑Tue Jul 14, 2020 9:37 pm
Chris Peterson wrote: ↑Tue Jul 14, 2020 7:56 pm
johnnydeep wrote: ↑Tue Jul 14, 2020 7:47 pm

Hmm. Is that because you can blow up a photograph after taking it to simulate any magnification you want?
Magnification is essentially an angular measurement, one that involves telescopic optics- that is, an optical system with both an objective and an ocular (eyepiece). A single objective (like a telephoto lens, or a "telescope" used photographically) doesn't operate the same way. You simply end up with an image of some size at the focal plane (telescopic optics are afocal- there is no focal plane).
Ok. Still trying to understand this. With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I really need to read more about optics, cameras, telescopes and photography...)

Telescopes are afocal. They have no focal plane- you can't hold a screen behind the eyepiece and see an image. What is coming out of the eyepiece is a fairly collimated bundle of rays. This enters your eye, and the lens in your eye brings the image to a focus on your retina. Afocal photography works the same way, with the camera lens being a required element of the optics to create a focus at the film or imager. There should be no focusing involved, except to the extent that the eye or the camera is focused at infinity- just the same focus they'd be using if you pulled the telescope out of the way. And from this, the concept of magnification makes sense. Divide the size of the object with the telescope by the size without it, and you have the magnification. You can't do that with a focal system, however, like the usual optics used for astrophotography or for ordinary photography.

FWIW: the image scale of the full-sized picture is 25.2 arcsec/pixel. Given that the E-W diameter of Stonehenge is about 35 m, this means the image was made 75 m south of the center of the circle, which is right on the big semicircular path that is around the stones. It is also consistent with a lens focal length of 150 mm.

Posted: **Tue Jul 14, 2020 10:36 pm**

johnnydeep wrote: ↑Tue Jul 14, 2020 9:37 pm
With a telescope there's an objective lens and an eyepiece, and dividing the focal length of the former by the focal length of the later gives you the "magnification", and you also have to "focus" the image in order to see it clearly. Yet I see articles about the "afocal method" of astrophotography that merely uses a camera in place of the human eye. But there's still focusing involved and also magnification, right? (As I said, I really need to read more about optics, cameras, telescopes and photography...)

An objective lens of focal length F turns a scene of angular width θ into an image of physical width Δ:

Δ = (θ x F)

An eyepiece lens of focal length f turns an image of physical width Δ into a scene of angular width ϴ:

ϴ = Δ / f = (θ x F)/f

Hence, an angular magnification M of (F/f)

Images often provide a scale Δ = so many degrees/minutes/seconds of the original scene

but "magnification" (in the common sense) depends upon how closely you, yourself, observe that image

(e.g., are you right up front in the movie theater or sitting way at the back of the balcony?)

so as to provide a scale Δ = so many degrees/minutes/seconds of the viewers experience.
.................................................................................
P.S., My 65" TV screen often shows people more or less "life sized"
...if you wish to define that as a "magnification of one" then that's a whole other matter.

Posted: **Tue Jul 14, 2020 11:44 pm**

Just think, if Neowise's orbit is just under 6,800 years, the last time around when it appeared in that sky, Stonehenge wasn't built yet! In fact the Stonehenge builders wouldn't be born for another 1,800 years or so!

Posted: **Tue Jul 14, 2020 11:57 pm**

De58te wrote: ↑Tue Jul 14, 2020 11:44 pm Just think, if Neowise's orbit is just under 6,800 years, the last time around when it appeared in that sky, Stonehenge wasn't built yet! In fact the Stonehenge builders wouldn't be born for another 1,800 years or so!

Actually, its orbital period before coming into the inner system was about 4500 years. It has been perturbed into a new orbit with a period of 6800 years. So what that actually means is that last time around was right around the time that key parts of Stonehenge were under construction. Its builders may have seen this comet.

Posted: **Wed Jul 15, 2020 10:56 am**

Hello all,

I took this image, the 1500mm focal length can easily be explained by me pressing the 0 one too many times. It's 150mm focal length.

If anyone wants to see where it was taken from, it was the south side of the A303 south and slightly west of Stonehenge.

Posted: **Wed Jul 15, 2020 12:39 pm**

Chris Peterson wrote: ↑Tue Jul 14, 2020 11:57 pm
De58te wrote: ↑Tue Jul 14, 2020 11:44 pm
Just think, if Neowise's orbit is just under 6,800 years, the last time around when it appeared in that sky, Stonehenge wasn't built yet! In fact the Stonehenge builders wouldn't be born for another 1,800 years or so!
Actually, its orbital period before coming into the inner system was about 4500 years. It has been perturbed into a new orbit with a period of 6800 years. So what that actually means is that last time around was right around the time that key parts of Stonehenge were under construction. Its builders may have seen this comet.

They probably gave the comet a different name back then, however.

Starship Asterisk*

APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

APOD: Comet NEOWISE over Stonehenge (2020 Jul 14)

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