Here is something that has bothered me for many years, and since the BA has graciously provided this soapbox and the subject is appropriate for this bulletin board, here goes.

Some years ago, there used to be public service announcements during the summer warning people to stay out of the sun during the middle of the day because of the dangers of ultraviolet, ranging from the short-term risk of sunburn to the long-term risk of melanoma (skin cancer). The announcement would usually suggest avoiding the sun between the hours of 10am and 2pm. There are three things wrong with the time given:

1. Most of the continental United States and a good portion of temperate zone industrial countries are on some form of daylight saving time.
2. Most of the population of the continental United States lives well to the west of the prime meridian of their time zone.
3. In July and especially August the equation of time ranges from sundial 8 to 16 minutes slow.

As a result of the three effects listed above, the hours of strongest UV can be quite a bit later than you would think.

For example: Norman, Oklahoma is at longitude 97° 27? West, almost halfway between the prime meridians for the Central and Mountain time zones. As a result of 1. and 2. above, the mean time of high noon in the summer is at 1:30pm, and adding in the equation of time means that high noon is at 1:45pm. Someone going out at 2pm thinking it is safe is actually walking out into the strongest sunlight of the day!

By the way, the May 1980 issue of Scientific American had an article on daylight saving time that contained a most interesting map--it was a superposition of the four time zones onto a common prime meridian. Generally most cities are well to the west of their prime meridian, Milwaukee, Chicago, and Boston being among the few cities east of their prime meridian.

In conclusion, remember to take daylight time, your longitude (4 minutes later for every degree west of the prime meridian, 4 minutes earlier for every degree east of it), and the equation of time (15 minutes more) into account. Learn when NOT to burn!

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<font size=-1>[ This Message was edited by: Celestial Mechanic on 2002-07-01 13:57 ]</font>

<font size=-1>[ This Message was edited by: Celestial Mechanic on 2002-07-02 00:11 ]</font>

Interesting. I say hats and sunscreen most of the time anyway, but it is hard to be consistent. Sounds like using the degree the Sun is above the horizon might be a better choice. In winter, the sun sets here at 1600,(4PM). You'd hardly risk a sunburn at noon then.

It should then state "between the hours of 10am and 2pm local apparent time" With a compass local apparent noon can be deduced. When the shadow of a vertical pole points north, that's noon. For most of the US and lower regions of Canada mag north and geo north would only be a few minutes apart. Right ? or am I missing something?

I think the idea is more of a general guideline than a specific range of times. The idea is to avoid the times when the Sun is highest in the sky and therefore at it's strongest. So 10-2 is not a hard and fast time limit, just a general time range to avoid. Anyone who thinks "Oh, it's 2:01, I'm ok then" doesn't have a lot of sense.

But then again, there seem to be a lot of people without much sense, so maybe they should be a bit clearer. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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