So what would Proxima Centauri look like from Proxima b?

[major_clanger]

The announcement of the discovery of a planet orbiting Proxima Centauri is absolutely fascinating; we finally have a (roughly) Earth-size world orbiting in the (notionally) habitable zone of a (by astronomical standards) close star. There's been a lot of speculation about what Proxima b might be like, but one description I saw didn't quite ring

Comments

[ffutures]

It's amazing how these calculations work out sometimes. When I was writing my Weinbaum RPG I wanted to know the brightness of the sun on Pluto, since Weinbaum described it as being like a moonlit night. Except that the maths really didn't support that, and I ended having to hand-wave it away:

Pluto's daylight actually ranges from two hundred to six hundred times brighter than moonlight on Earth, comparable to a badly overcast day on Earth or the lighting in an average home. It generally appears to be much darker because the Sun is perceived as a bright star, there is no atmospheric scattering of light, shadows are completely dark, and most of the surface materials absorb light with minimal reflections.

[major_clanger]

Whereas of course in reality most of Pluto's surface is icy, and thus very bright.

Unless of course you're standing in whatever Cthulhu Regio is made of.

(Snoozing Shoggoths, presumably).

[a_cubed]

Really nice analysis. I love the final piece about photography settings. That gives a really useful real world distinction with a flavour of humour. You should try to work that into a story at some point.

[history_monk]

Good stuff!

[davidwake]

This is an excellent addition to last night's discussions.

Can you mock up a picture: Earth daytime, Earth moonlight and Proximity's daytime?

There's an artist's impression on the Wikipedia entry, but that 'sun' looks the same size as ours. There's a comparison further down, so I guess that does the trick.

What difference does Alpha Centauri A and B make to all this? Are they bright enough to make a sunny day? One thought, if Proximity is tidally locked, then Alpha Centauri A and B will rise and fall every 11 days.

[major_clanger]

Proxima Centauri is about 0.21 light years from Alpha Centauri, which is close-by in stellar terms but a long way for a companion star (the jury is still out on whether Proxima Centauri is in orbit around Alpha Centauri AB or just formed with them and is thus on a similar path through space).

From Earth, at a distance of 4.37 light years, Alpha Centauri A has a magnitude of 0.01 and Alpha Centauri B a magnitude of 1.33. The brightness increase from Proxima Centauri will be

2.5 log (4.37 / 0.21)2 = 6.6 magnitudes

So, Alpha Centauri A would be Mag -6.6 and Alpha Centauri B would be Mag -5.3. That's about three times brighter and about half as bright again than Venus at its brightest respectively.

From Proxima Centauri, the maximum separation of Alpha Centauri A and B would be about 0.15 degrees at most. Much of the time they would probably look like one bright star perhaps four times brighter than Venus. In other words, not enough to affect how bright daylight seemed on Proxima b.

[davidwake]

With Proxima being bigger, but fainter, would you see Alpha Centauri A and B during the day? Would you see other stars in the sky?

Of course, dependent upon any atmosphere, which I'd have thought likely given that Proximity is slightly bigger than the Earth. If Proxima gives a dull daylight day, then on a dull day, it would be very gloomy.

[major_clanger]

Probably, yes. At its brightest, you can see Venus during the day if you know where to look. In a dimmer sky, I'd expect Alpha Centauri A/B to be very obvious.

In terms of the sky, this is my guess at Earth (top) and Proxima B. It's hard to do a real comparison as of course your eyes would adjust in dimmer light, but it would seem dimmer and yellower.

[davidwake]

So, it's sort of...

[motodraconis]

Love it!