APOD: Circling the Sun (2024 Jan 13)

[APOD Robot]

Circling the Sun

Explanation: Earth's orbit around the Sun is not a circle, it's an ellipse. The point along its elliptical orbit where our fair planet is closest to the Sun is called perihelion. This year, perihelion was on January 2 at 01:00 UTC, with the Earth about 3 million miles closer to the Sun than it was at aphelion (last July 6), the farthest point in its elliptical orbit. Of course, distance from the Sun doesn't determine the seasons, and it doesn't the determine size of Sun halos. Easier to see with the Sun hidden behind a tall tree trunk, this beautiful ice halo forms a 22 degree-wide circle around the Sun, recorded while strolling through the countryside near Heroldstatt, Germany. The Sun halo's 22 degree angular diameter is determined by the six-sided geometry of water ice crystals drifting high in planet Earth's atmosphere.

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[Donald Pelletier]

22° is not the diameter, it is the radius : https://en.wikipedia.org/wiki/22%C2%B0_halo (https://fr.wikipedia.org/wiki/Halo_de_22%C2%B0)

[Ann]

22° is not the diameter, it is the radius : https://en.wikipedia.org/wiki/22%C2%B0_halo (https://fr.wikipedia.org/wiki/Halo_de_22%C2%B0)

Good to see you here, Donald! I have enjoyed, and posted, many of the astro images that you have uploaded to Wikimedia Commons!

Ann

[Chris Peterson]

This year, perihelion was on January 2 at 01:00 UTC, with the Earth about 3 million miles closer to the Sun than it was at aphelion (last July 6), the farthest point in its elliptical orbit. Of course, distance from the Sun doesn't determine the seasons..

While that is certainly true, our distance from the Sun does influence the impact of the seasons and global climate in general.

[Ann]

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]

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]

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.

Thanks! I think I get it (mostly)!

Ann

[Robert Arnold]

And... Johannes Kepler not only discovered the elliptical nature of earth's orbit, he also proved that snowflakes have six and only six sides.

[johnnydeep]

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.

[Chris Peterson]

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]

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.

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]

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!)

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]

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!

Yup. But I still did a double-take on your wording.

[Christian G.]

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.

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]

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.

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!

[johnnydeep]

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.

And it hasn't even always been this way. about 2.5 billion years ago the Moon was 60,000 km closer the to Earth than it is now, and so would have appeared larger in the sky than the Sun. From https://www.astronomy.com/science/our-m ... ion-years/

Hmm, has the Sun's diameter or the Earth's orbital distance from it changed much over the past 2.5 Gy?

[johnnydeep]

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!

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?)

But I can't square the 3.8 cm/year rate this this chart from https://www.researchgate.net/figure/Evo ... _313849245, which seems to be saying that the moon is only receding at a rate of 0.13 cm/year currently?

Click to view full size image

Unless this is an old paper with no longer valid data...

[Chris Peterson]

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?)

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

[johnnydeep]

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.

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]

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?

Unless there is another nasty mathematical trick that I have to take into account?

Ann

[johnnydeep]

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]

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.

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

[johnnydeep]

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.

True, but I'd still call it close enough to be described as very unusual. I wonder what other planets' moons are currently within 10% of the size of the Sun - and eclipse capable - when observed from the planet "surface" (to include the four gas giants).

[Ann]

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]

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

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.