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DR. JERRY WILSON


QUESTION:There seems to be a discrepancy between the time of sunrise given in the paper and my actual observations. Do the papers print one time for the Eastern Time Zone, or do papers in Charleston, Greenwood and Atlanta report times for their specific locations? (Asked by a curious early-riser.)

REPLY: From what I have learned, the U.S. Naval Observatory is responsible for sunrise and sunset times. It is quite a complicated program, and only an excerpt is given on their homepage. They state: “This program represents many months of painful development and results from a great deal of work by many people (who prefer to remain nameless).”

The program used does involve the local longitude and latitude for a desired location, so I would surmise the sunrise and sunset times are computed for National Weather Service offices in major cities. Computing these for a time zone would not be precise enough. The Eastern Standard Time Zone runs from Maine to Illinois, and it takes the Sun an hour to traverse this distance.

An interesting consideration in sunrise and sunset is refraction. This is the bending of light as it passes through a medium. You see this when you put a spoon or pencil in a glass of water -- it looks offset because of refraction. The denser air near the Earth’s surface bends the sunlight when the sun rises and sets, and as a result we are able to see part of the sun before it actually rises and after it actually sets.

The program takes this into account and defines sunrise, for example, as the time when the apparent altitude (distance above the horizon) of the upper limb of the sun will be -50 arc minutes (which takes into account 34 arc minutes of refraction).

This is a bit technical, but here are a couple of interesting facts as a bonus:

-- The length of the day (between sunrise and sunset) does not increase or decrease by a constant amount each day. Between the winter solstice (Dec. 21) and the summer solstice (June 21), the days lengthen from one to four hours as the sun rises earlier and sets later. This is because the Earth’s orbit is elliptical and moves at different speeds at different times of the year. So, each day, the Earth moves differently relative to the sun, and we view this as changes in the motion of the sun in the sky.

-- We say June 21 and December 21, respectively, are the longest and shortest days of the year. However, these are not the days with the earliest and latest sunrises. These occur about a week earlier and later. The reason for this is our use of clocks, which run at the same steady rate. If we used sundials to measure time, things would be in order. Sundials do not indicate time at a steady rate because of the Sun’s irregular motions in the sky.

So, anybody with a sundial, get up and check it out.

C.P.S. (Curious Postscript): “There are a terrible lot of lies going about the world, and the worst of it is that half of them are true.” – Winston Churchill

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or e-mail jerry@curiosity-corner.net. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to www.curiosity-corner.net.