Venus whips the stars

Posted by Rich Talcott
on Wednesday, June 13, 2007

That bright point of light you see hanging in the western sky after sunset is none other than Venus. Aim your telescope at it, and you'll find it's more than a point. The "evening star" currently displays a disk some 25" across and just under half-lit.

Sure, Venus appears bright. Even a casual stargazer can see it beats Jupiter, the second-brightest light in the evening sky. (Jupiter now stands low in the southeast during the early-evening hours.) Venus seems to shine even brighter as it sinks lower and the sky grows dark. It doesn't set until around 11:30 P.M. daylight time, well after astronomical twilight ends at mid-northern latitudes.

But just how bright is Venus? To quote Maxwell Smart: "Would you believe it's brighter than all the visible stars combined?" If you don't buy that statement (and who among you KAOS aficionados would), let me show you.

Let's calculate how bright all the naked-eye stars would appear if they were combined into a single point. To do this, we need to make some assumptions. First, we'll assume the faintest naked-eye stars glow at magnitude 6.5 — a good estimate for middle-aged eyes from a dark site. We'll also assume the atmosphere does not dim stars, so a star just above the horizon appears as bright as it would at the zenith. Finally, we'll calculate the combined brightness of all the naked-eye stars and divide the result by two to get the answer for one hemisphere.

If we add the 21 stars brighter than magnitude 1.5 (from Sirius at —1.46 to Regulus at 1.35), they combine to a –3.09-magnitude star. If we then assume all 71 2nd-magnitude stars (those between magnitudes 1.50 and 2.49) can be approximated by 71 stars of magnitude 2.00, they sum to magnitude –2.63. This should yield a good estimate because, even though the brighter stars would have a disproportionate effect, there are more fainter ones.

Adopting the same approach for the 192 3rd-magnitude stars, 625 4th-magnitude stars, 1,963 5th-magnitude stars, and 5,606 6th-magnitude stars gives a total brightness for the nearly 8,500 stars of magnitude –4.98. If we then restrict this to one hemisphere of the sky, the brightness falls to magnitude –4.23. Throw in Jupiter for good measure, and you reach magnitude –4.45.

That's bright, to be sure, but Venus currently tips the scales at magnitude –4.5 — and it will brighten another few-tenths of a magnitude by July. So, as you watch Venus gleaming one of these balmy evenings, take a minute to realize how special our earthly neighbor is.

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