Sunday, January 27, 2013

The midday sun doesn't stand still

John Goss

John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.

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Clues for understanding our place in the cosmos can be found all around us, even in this newspaper. Curious skywatchers may have noticed something odd when examining the sunrise and sunset times given each day on page A2 in The Roanoke Times: The midpoint of daylight hours isn't at noon.

Today the sun rises at 7:27 a.m. and sets at 5:39 p.m., making the time of midday occur at 12:33 p.m., not 12 noon. Why? Moreover, the time of midday a few weeks hence will be noticeably different. Again, why?

Our Earth rotates a full 360 degrees in 24 hours, giving us one day. Because each day consists of 24 hours, the Earth rotates 15 degrees in each hour. Therefore, each 15 degrees in longitude equals one hour in time. This forms the basis of our time zones.

By international agreement, the 0 degree longitude mark runs through Greenwich, England. Beginning at this line, each 15 degrees westward results in the time being one hour earlier. Because Roanoke's longitude is 80 degrees west, it is five full 15-degree increments (and then some), or five hours earlier than it is in London. So, when it tolls noon there, it is only 7 a.m. here. Because we lie another 5 degrees west of longitude 75 degrees, it takes another five-fifteenths of an hour -- i.e., 20 minutes -- for the sun to reach its halfway point in the day sky. That puts our midday time at 12:20, called local mean noon.

There still is a 13-minute discrepancy between 12:20 and today's midpoint time of 12:33 p.m. This is a direct result of Earth's orbit around the sun. Whether we realize it or not, our planet's elliptical path affects us every day, as exemplified on page A2.

One feature of elliptical orbits is that when the orbiting body, in this case the Earth, is closer to the sun, it moves faster, and when it is farther it moves more slowly. While its orbital speed changes throughout the year, the Earth's rotational rate of 24 hours per day remains constant. When the Earth is close to the sun, the relatively steep curvature of its orbital path and its slightly faster velocity causes the sun to appear a minute or so behind schedule in the sky, taking another minute or so to reach midday each day. (We reached our closest point to the sun on Jan. 2, so our planet is now moving relatively fast.)

As the days pass, the time discrepancy accumulates, so that today the sun requires another 13 minutes for it to reach the day's halfway point at 12:33 p.m. At this time, the sun shines directly south and reaches its highest point in its daily arc across the sky.

As time passes, the Earth continuously moves to a different part of its orbit, traveling at a slightly different speed and tracing a slightly different curvature. That means each day the time of midday slowly shifts, either earlier or later, depending where the Earth lies along its orbital path. By Feb. 11, midday advances only another minute to 12:34. But by April 15, it retreats to 12:20. Then, the sun is no longer slow but right on time.

All of this also results in the curious effect of the latest sunrise and earliest sunset not falling on Dec. 21, the shortest day of the year, nor the earliest sunrise and latest sunset falling on June 21, the longest day.

It's funny what an elliptical orbit brings. Follow the listed sunrise and sunset times and see for yourself.

Weather Journal

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