writes:

Jupiter, Saturn, and Venus are visible to the naked eye in broad
daylight at sea level, if you know exactly where to look.

Are these consistently above the horizon at all locations?

From what I've read, the ANS looked specifically at stars, not
planets, but I may be wrong.

I suppose that technically stars are visible, too, it's just that the
difference isn't perceptible to the naked eye.

I've
seen them all, at times when I knew their approximate position
relative to the moon. You must look in exactly the right place --
an error of a half degree or so puts them out of the central field
of view of your eyes and renders them invisible against the
glare of daylight.

Hmm ... I'll have to try it sometime.

The brighter stars are just outside the ability of most people's
naked eyes to see in broad daylight, but a small pair of
binoculars will pick a few out nicely. Again, proper aim is
crucial. Only a handful of the brightest stars will typically be
visible at any given time, but you don't need very many for
navigation.

The ANS required three, supposedly. It also required a very accurate
clock. With the clock setting, it could find its way. I guess it
just looked around at the sky until it found a few stars, then checked
it against its own computed ephemeris based on the time of day.

It sounds like the optics of the SR-71's nav system are at
least as good as a standard issue Mk 1 pair of eyeballs
coupled with a 7x35 pair of binoculars. It may be that, for
rapid startup, the system needs some hint of the location
and time of day to figure out where to begin its search for
stars, but I'm just guessing on that point.

It definitely needs very accurate time of day, but I don't think it
needs a hint of its location--although may it does, or maybe a general
hint speeds up acquisition (which usually took a matter of minutes, I
think).

--
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