[mdda]

One thing that surprised me was the statement about a sun that would soon engulf the nearby planet - and being so relatively large that the planet was 'more than half' in sunlight (~ the sun's disc shines 'around the corners').

But isn't that true on earth too (to a much smaller degree)? As long as the sun's radius is larger than the earth, then sunlight will fall simultaneously on more than a half of the earth's surface, no?

[dredmorbius]

It is true for the Earth and Sun, yes, but the amount of overfill is small.

I don't know the apparent size of the star in question here -- ah, the paper (http://arxiv.org/pdf/1312.3943v1.pdf) gives it: 46.5%. Compare that with the apparent size, or angular diameter, of the Sun as viewed from Earth: 2 degrees. About the size of your thumb at arm's length.

Your mileage may vary, but if I extend both my hands out, touch the thumbs together, and spread my thumbs and pinkies as wide as I can, that's roughly 20x wider than my thumb. Imagine that as the Sun looming in your sky.

[SiVal]

I believe the angular diameter of both sun and moon are roughly half a degree, not two degrees. You remembered the two correctly, but put two degrees in one sun instead of two suns in one degree. That's how our brains work, isn't it?

[dredmorbius]

Thanks for that.

I managed to mis-read from Wikipedia: "The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2″ (two arcseconds)." Somewhat lower on the page we find: "The table shows that the angular diameter of Sun, when seen from Earth is approximately 32 arcminutes (1920 arcseconds or 0.53 degrees), as illustrated above."

So, if my initial estimate of "thumb at arm's length is the size of the Sun", then my "two hands spanned" is about 4x too small ... woah. Hold your arms out at a 45° angle. That's about the size of it.

That would be ... a lot of star in the sky ...

I also should've trusted my first impression on the 0.5 degree as well. I believe I knew that. Hasty verification fouls memory.

[Dylan16807]

Interesting that your first comment explicitly stated 46 degrees but then you somehow logiced yourself out of that.

[jibsen]

I believe you actually see the sun "around the corner", due to atmospheric refraction [1].

[1]: http://en.wikipedia.org/wiki/Atmospheric_refraction

[dredmorbius]

That accounts for part of it, but light from different spots on the Sun traces slightly different paths to the Earth. One of the results of that is that shadows cast long distances don't have distinct edges, because there's a zone in which the Sun is partially, but not completely, obscured.

https://en.wikipedia.org/wiki/Umbra

The dark region of the shadow is called the umbra, the partially shaded region is the penumbra. This effect is probably most widely associated with solar eclipses, where the Moon passes in front of the Sun. Where the Sun is only partially obscured, the Earth is in the penumbra. Totality exists within the umbra.

In some orientations of the Moon's orbit, it's further from the Earth (and hence smaller relative the much more distant and constant-sized Sun), and despite the fact that an observer is directly behind the Moon relative to the Sun, totallity isn't observed. This is an annular eclipse, where the Sun forms a ring around the Moon.

https://en.wikipedia.org/wiki/Annular_eclipse#Types

[JoeAltmaier]

Sure the sun isn't a point source in the sky; as it sets and you can still see it past the half-set point, you're experiencing the 'extra light'.

[lostlogin]

No, because of distance. I'm sure there is a simple equation that shows this but I don't know it. Presumably most stars we see are larger than earth, yet we can only just see them and they are basically point sources.

[baddox]

There is no distance at which a larger spherical star won't shine on over 50% of a smaller sphere's surface area. That includes distant stars and Earth.

Here's a rough diagram I sketched up in Autodesk Inventor:

http://i.imgur.com/u9Xjqsa.png

We have a "planet" with diameter 0.275 unit and a "star" with diameter 1.0 unit. If their centers are 1.0 unit apart, then you can clearly see the extra part of the planet's circumference that is covered by the star's light. If you increase their distance to 10 units, the two lines connecting the circles get closer and closer to becoming parallel, but they will never become parallel because the star is larger than the planet.

[vacri]

This being said, the earth isn't a perfect sphere. There will be a distance where the topology of the earth will block distant stars from shining on >50% of the planet... for some of the day.

[lostlogin]

I stand corrected.