"Vision is not enough, it must be combined with venture. It is not enough to stare up the steps, we must step up the stairs." -Vaclav Havel

It's arguable that vision is the most important tool that an astronomer can have. This was particularly important back before the invention of the telescope, as the greatest astronomers of their time (like Tycho Brahe) were renowned for their visual acuity.

How do you stack up? You were curious about learning how good your hearing was, but what about your vision? Well, you can always get your vision tested using a Snellen eye chart, and read the bottom-most line that you can at the appropriate distance.

But where's the fun in that? And who wants to bother with an eyechart and measuring the proper distance?

Why not just use the astronomical objects available in the sky? You know, like this guy.

The Moon takes up about half-a-degree on the sky. In astronomy, we divide up each degree into 60 minutes of arc (known as arc-minutes), and we divide each minute up into 60 seconds of arc (known as arc-seconds).

Visible during the day or at night, the Moon is always -- except to those with awful vision -- very clearly a disc. A typical human being with "normal" vision (called 20/20 in the US) can see all the way down to resolutions of one arc minute, whereas the Moon, even at its smallest, appears to take up a whopping 29 arc minutes on the sky. Which, of course, is why we can see features on the Moon -- such as its phase and its great light-and-dark spots -- with our naked eyes!

But some people can see even better than average.

Laser interferometry -- despite that this runs contrary to what wikipedia tells you -- places a limit on what the perfect human eye (but still with rods & cones, sorry, Geordi La Forge) can see: down to a resolution of just 24 arc seconds, or 0.4 arc minutes. (This is the equivalent of 20/8 vision.)

It turns out that, for someone of just average vision, everything in the night sky that isn't the Moon will look like a single bright point of light.

But is your vision better than average? (For those of you with eyeglasses/contacts, you can ask whether your corrected vision is better than average!) You see, there are two planets in our Solar System that -- during the right time of the year -- you could see as a disc instead of as a point, if your vision is good enough! How does this work?

Planets -- although far away -- also have significant diameters. So long as they aren't too far away for how large they are, it should be possible for your eye to resolve that a planet is a disc, rather than a single point of light. In astronomy, we refer to this as the angular diameter of an object.

So which are the planets that we can do this with?

Say hi to Venus. At its dimmest, Venus appears brighter than even the brightest star. As far as distances go, Venus gets closer to Earth than any other planet; she sometimes passes within 40 million kilometers of Earth! When Venus is this close, someone with normal vision can just barely see Venus appear as a disc, rather than as a point. (It reaches a maximum angular size of 1 arc-minute and 6 arc-seconds.)

But Venus also gets far away from the Earth, falling all the way down to a minimum angular diameter of just over 9 arc-seconds, impossible to resolve for even a perfect human. Still, if you're meticulous, and you can see Venus as a disc on some nights but not on others, you can figure out what the limit of your vision is.

But there's another planet that's even better for this.

Also visible to the naked eye -- but not to everyone -- is the planet Jupiter, shown here as it was in 2008, just 2 degrees away from Venus! (Venus is the larger one.) Jupiter, when it's closest to Earth, appears to take up 50 arc-seconds on the sky. In other words, you can see it as a disc if your vision is slightly above average (about 20/16 vision), but not if your vision is only average!

But Jupiter also gets more than 300 million kilometers farther away from Earth, and when it's at its farthest it only takes up 29.8 arc-seconds, meaning that someone with 20/9 vision can see it as a disc, but one with 20/10 or worse will see it as a mere point!

Why do I bring this up now?

Because we are approaching "closest approach" to Jupiter! As the summer wears on, Jupiter will appear to get larger and larger, reaching its maximum size of 50 arc-seconds in September of this year (2010).

So if you can't see it as more than a point now, don't stop looking up! You could only be a few days away from seeing something that most people don't even know is possible! (Just make sure to wear your glasses, if you need them!)