Starship Asterisk*

Chris Peterson wrote: ↑Sun Jan 14, 2024 3:53 pm

johnnydeep wrote: ↑Sun Jan 14, 2024 3:48 pm

Chris Peterson wrote: ↑Sun Jan 14, 2024 3:19 pm
Sure, it's perfectly correct to say that the radius is 22°. And in general, radius is more commonly used for most things. It's generally a better metric than diameter. We usually understand "radius" as a distance from a center that doesn't have to imply a circular structure. And it makes sense when there is no matching part of the structure on the opposite side. And in terms of halo optics, almost everything is understood as an angle with respect to the light source. So radius is all that makes sense.

The radius of the Moon is 0.25° and the radius of the halo is 22°.
The diameter of the Moon is 0.5° and the diameter of the halo is 44°.

And the FOV of an astronomical image could be expressed as a radius in degrees or as a total X° x Y° areal extent. I've seen both used in various APODs.

I'm simply talking about what is logical. It is not logical in most cases to talk about the diameter of a halo, since its structure is intrinsically radial in nature. That is not true for the Sun or the Moon or a field of view, except in special cases.

johnnydeep wrote: ↑Sun Jan 14, 2024 3:48 pm

Chris Peterson wrote: ↑Sun Jan 14, 2024 3:19 pm

johnnydeep wrote: ↑Sun Jan 14, 2024 11:32 am

Well, no, since it’s the angular extent (or diameter) of the Moon that’s 0.5°. But, yes, calling it a 22°halo is confusing, but that seems to be the convention. And the APOD is still wrong to call that a diameter. The total angular extent (diameter) of the halo is 44°, but I guess you could say that the extent of the halo away from the Sun is 22° all around it.

Sure, it's perfectly correct to say that the radius is 22°. And in general, radius is more commonly used for most things. It's generally a better metric than diameter. We usually understand "radius" as a distance from a center that doesn't have to imply a circular structure. And it makes sense when there is no matching part of the structure on the opposite side. And in terms of halo optics, almost everything is understood as an angle with respect to the light source. So radius is all that makes sense.

The radius of the Moon is 0.25° and the radius of the halo is 22°.
The diameter of the Moon is 0.5° and the diameter of the halo is 44°.

And the FOV of an astronomical image could be expressed as a radius in degrees or as a total X° x Y° areal extent. I've seen both used in various APODs.

Chris Peterson wrote: ↑Sun Jan 14, 2024 3:19 pm

johnnydeep wrote: ↑Sun Jan 14, 2024 11:32 am

Ann wrote: ↑Sun Jan 14, 2024 4:42 am

Diameter, schmiameter!!! You's expect a 22°halo to be 22°, wouldn't you?

Anyway, if a a 22°halo is really 44°, then I take it that our 0.5° Moon should really be called a 0.25°Moon, right?

Ann

Well, no, since it’s the angular extent (or diameter) of the Moon that’s 0.5°. But, yes, calling it a 22°halo is confusing, but that seems to be the convention. And the APOD is still wrong to call that a diameter. The total angular extent (diameter) of the halo is 44°, but I guess you could say that the extent of the halo away from the Sun is 22° all around it.

Sure, it's perfectly correct to say that the radius is 22°. And in general, radius is more commonly used for most things. It's generally a better metric than diameter. We usually understand "radius" as a distance from a center that doesn't have to imply a circular structure. And it makes sense when there is no matching part of the structure on the opposite side. And in terms of halo optics, almost everything is understood as an angle with respect to the light source. So radius is all that makes sense.

johnnydeep wrote: ↑Sun Jan 14, 2024 11:32 am

Ann wrote: ↑Sun Jan 14, 2024 4:42 am

johnnydeep wrote: ↑Sat Jan 13, 2024 8:58 pm

Well, 44°(diameter) is 88 times 0.5° (diameter).

Diameter, schmiameter!!! You's expect a 22°halo to be 22°, wouldn't you?

Anyway, if a a 22°halo is really 44°, then I take it that our 0.5° Moon should really be called a 0.25°Moon, right?

Ann

Well, no, since it’s the angular extent (or diameter) of the Moon that’s 0.5°. But, yes, calling it a 22°halo is confusing, but that seems to be the convention. And the APOD is still wrong to call that a diameter. The total angular extent (diameter) of the halo is 44°, but I guess you could say that the extent of the halo away from the Sun is 22° all around it.

Ann wrote: ↑Sun Jan 14, 2024 4:42 am

johnnydeep wrote: ↑Sat Jan 13, 2024 8:58 pm

Ann wrote: ↑Sat Jan 13, 2024 8:49 pm Since I was the one who asked about the 22°halo, thanks for answering! I get it now. It has to do with the size of the halo in the sky. I do understand what it means that the apparent size of the full Moon in the sky is 0.5°, so that means that the size of the halo is 44 times the apparent size of the Moon?
...
Ann

Well, 44°(diameter) is 88 times 0.5° (diameter).

Diameter, schmiameter!!! You's expect a 22°halo to be 22°, wouldn't you?

Anyway, if a a 22°halo is really 44°, then I take it that our 0.5° Moon should really be called a 0.25°Moon, right?

Ann

johnnydeep wrote: ↑Sat Jan 13, 2024 8:58 pm

Ann wrote: ↑Sat Jan 13, 2024 8:49 pm Since I was the one who asked about the 22°halo, thanks for answering! I get it now. It has to do with the size of the halo in the sky. I do understand what it means that the apparent size of the full Moon in the sky is 0.5°, so that means that the size of the halo is 44 times the apparent size of the Moon?
...
Ann

Well, 44°(diameter) is 88 times 0.5° (diameter).

Chris Peterson wrote: ↑Sat Jan 13, 2024 9:30 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 8:07 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 6:51 pm

A million here, a billion there... in any case, a transient coincidence over the existence of the Solar System.

I suppose. But assuming a steady rate of 3.8 cm/yr, that would be about 38,000 km further in 1 Gy time. Then it definitely wouldn't cover the Sun as much, but it's still "only" 10% farther away.

But that's more than enough to make total solar eclipses impossible.

johnnydeep wrote: ↑Sat Jan 13, 2024 8:07 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 6:51 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 6:47 pm

About that. All the references I find say the Earth-Moon distance is increasing at 3.8 cm/year (the same rate as fingernails grow). So, in 5 My, it will only be 190 km further away. I don't think that would affect the size much! (unless my math is way off?)

A million here, a billion there... in any case, a transient coincidence over the existence of the Solar System.

I suppose. But assuming a steady rate of 3.8 cm/yr, that would be about 38,000 km further in 1 Gy time. Then it definitely wouldn't cover the Sun as much, but it's still "only" 10% farther away.

Ann wrote: ↑Sat Jan 13, 2024 8:49 pm Since I was the one who asked about the 22°halo, thanks for answering! I get it now. It has to do with the size of the halo in the sky. I do understand what it means that the apparent size of the full Moon in the sky is 0.5°, so that means that the size of the halo is 44 times the apparent size of the Moon?
...
Ann

Chris Peterson wrote: ↑Sat Jan 13, 2024 6:51 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 6:47 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 5:50 pm
Enjoy it while you can. It's only been that way for a few million years, and will only be so for a few million more!

About that. All the references I find say the Earth-Moon distance is increasing at 3.8 cm/year (the same rate as fingernails grow). So, in 5 My, it will only be 190 km further away. I don't think that would affect the size much! (unless my math is way off?)

A million here, a billion there... in any case, a transient coincidence over the existence of the Solar System.

johnnydeep wrote: ↑Sat Jan 13, 2024 6:47 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 5:50 pm

Christian G. wrote: ↑Sat Jan 13, 2024 5:31 pm
I've always found this coincidence incredible: the Sun is both 400 times larger and 400 times farther than the Moon! I mean, what are the odds?… It sure makes total eclipses all the more special.

Enjoy it while you can. It's only been that way for a few million years, and will only be so for a few million more!

About that. All the references I find say the Earth-Moon distance is increasing at 3.8 cm/year (the same rate as fingernails grow). So, in 5 My, it will only be 190 km further away. I don't think that would affect the size much! (unless my math is way off?)

Chris Peterson wrote: ↑Sat Jan 13, 2024 5:50 pm

Christian G. wrote: ↑Sat Jan 13, 2024 5:31 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm
That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

I've always found this coincidence incredible: the Sun is both 400 times larger and 400 times farther than the Moon! I mean, what are the odds?… It sure makes total eclipses all the more special.

Enjoy it while you can. It's only been that way for a few million years, and will only be so for a few million more!

Christian G. wrote: ↑Sat Jan 13, 2024 5:31 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

I've always found this coincidence incredible: the Sun is both 400 times larger and 400 times farther than the Moon! I mean, what are the odds?… It sure makes total eclipses all the more special.

Christian G. wrote: ↑Sat Jan 13, 2024 5:31 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

I've always found this coincidence incredible: the Sun is both 400 times larger and 400 times farther than the Moon! I mean, what are the odds?… It sure makes total eclipses all the more special.

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

Chris Peterson wrote: ↑Sat Jan 13, 2024 4:47 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 4:34 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm
That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

That's interesting, but it makes perfect sense! (And just to clarify, the Sun's angular diameter is about 0.5°, just like the moon, not 44 degrees as I first thought you were saying!)

Our local temperature has been in the single digits (F) for over 24 hours. I'm quite sure that wouldn't be the case if the Sun were 44 degrees in diameter!

johnnydeep wrote: ↑Sat Jan 13, 2024 4:34 pm

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

That's interesting, but it makes perfect sense! (And just to clarify, the Sun's angular diameter is about 0.5°, just like the moon, not 44 degrees as I first thought you were saying!)

Chris Peterson wrote: ↑Sat Jan 13, 2024 3:24 pm

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

That's the angular size of the Sun, too. So we could say that if the Sun were 88 times closer (so about a million miles away) it would look the same size in the sky as this halo.

johnnydeep wrote: ↑Sat Jan 13, 2024 2:48 pm

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann

It's just an angular measurement that we use all the time. The well-known angular diameter (extent) of the Moon in the sky is about 0.5 degrees. Similarly, the angular diameter of the halo is 44 degrees.

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann

Knight of Clear Skies wrote: ↑Sat Jan 13, 2024 9:59 am

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann

It's the angle that the high altitude ice crystals reflect the sunlight at. Like a rainbow, at observer at a particular point sees a circle at the light rays converge.



Hope that's some help.

Ann wrote: ↑Sat Jan 13, 2024 7:50 am Okay, Chris (and Donald). What the heck is a 22^o halo?

The halo is just a circle. How can it be 22^o?

Bear in mind that I'm unbelievably thick when it comes to math, so if you have to show me some math formulas, you may as well not bother and I'll just accept that the circle is 22^o and give the whole thing a 360^o shrug!

Ann