We are now in the historically hottest time of year here in the Reno area and this next week will certainly go along with that, and then some. High temperatures will rise to the upper 90s over the weekend, with sunny skies in the morning giving way to a few afternoon clouds (and a slight chance of isolated thundershowers.) As we head into next week, the ridge of high pressure strengthens, and triple digit temperatures will be with us through most of the week.

Sunrise water

Yesterday, we talked about how they come up with official sunrise and sunset times. Now if you have ever stayed up late at night wondering just who comes up with the “official” (or who the ubiquitous “they” are), might I prescribe some warm milk and read the following: The U.S. Naval Observatory has been commissioned with the responsibility of computing sunrise/sunset times worldwide. And it’s not really as tough a job as you might think. Since a flat horizon is always assumed, sunrise/sunset times can be calculated at any point on the earth if you know the latitude and longitude. Employ a little astronomy and trigonometry, and any personal computer could handle the task without straining its processors.

Sunrise diagram

But there’s one additional factor they have to take into account, because we see the sun before we geometrically ought to be able. That’s because the atmosphere acts like a lens, and bends (refracts) the light coming in from the sun, allowing us to see it before it would be visible if we didn’t have an atmosphere messing with the optics. While it may not seem that it could make a huge difference, on average it makes sunrise about two minutes earlier, and sunset the same amount later. If you wondered why the days seem longer, now you know.

Sharp eyed observers (with a near obsessive-compulsive eye for detail) will notice that the times are only given to the nearest minute. In an era where we can make astronomical calculations down to the second, why aren’t they any more precise?

Naval observatory

US Naval Observatory

The Naval Observatory will be the first to admit there is a degree of inaccuracy to their figures. Changing atmospheric conditions will affect the amount of refraction, which will affect when you can see the sun. And since there are very few places where you have a perfectly flat horizon, it’s unlikely you’ll ever see the sun appear on time anyway.

It’s a perfectly appropriate example of “close enough for government work.”

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