APOD: Circling the Sun (2024 Jan 13)

[Chris Peterson]

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.

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]

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.

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]

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.

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]

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.

Ok, got it. Halos are different!