# What is actually meant by 'sun set' and 'sun rise' times, when taking into account the mirage due to light bending in the atmosphere

I’ve heard from the likes of Brian Cox that what we see of the sun during a sunset and sun rise is actually the mirage of the sun. The Sun has actually set/risen and we see it due to the way light is bent across the atmosphere. Apparently due to coincidence of the size and distance of the sun, its exactly the same size - so if we see 50% of the sun, the sun is 50% below the horizon. So, I understand all this, so here is my question :

If this is the case, then when we read things like what time sun sets and rises on websites, books, calendars, other official times, et al… does that mean when we see for example ‘sun set at 18:35’ is the time denoting the actual sun set taking into account of the mirage or what is visible to us. If I were to know the time and watched the sun against an accurate clock, would the sun not be visible before the actual sun set time?

I would also like to know how this affects things like iphone apps that tells us what time sun sets are and where it is in relation to the horizon at a given time. So is the set/rise time taking into account of the mirage and the time is recalculated to give us when it has ‘actually’ set/risen or when the mirage has set/risen.

Thanks

-
Is that the right link? I get a completely different question about luminosity and stellar mass when I click that. – Tim Seguine Nov 25 '14 at 9:09

See http://en.wikipedia.org/wiki/Sunset. The sunset is defined when the trailing edge of the sun disappears below the horizon, even though refraction means the sun actually went below the horizon some minutes before. Likewise sunrise is defined as the point when the leading edge of the sun is first visible even though this is some minutes before the sun actually rises above the horizon.

You ask where the sun really is: when the sun sets and rises it is about one solar diameter below where it appears to be, i.e. about half a degree. See http://en.wikipedia.org/wiki/Atmospheric_refraction for the details.

-