Friday, December 22, 2017

Sunrise to sunset, Pittsburgh to Cairo

There is a myth that the winter solstice is the shortest day of the year. I called it a myth because according to the sunrise and sunset tables posted by the U.S. Naval Observatory, the shortest day as measured by the time between sunrise and sunset actually occurs a few days before the solstice.

Having said that, I was looking at some factoids about Lake Superior recently. One factoid said in summer, the sun sets 35 minutes later on the western edge of the lake than it does on the eastern edge. So naturally the thought occurs, what about the Ohio River?  And what happens this time of year?

So, according to the Naval Observatory, today, Dec. 22, the sun will rise in Pittsburgh at 7:40 a.m. Eastern Standard Time and set at 4:57 p.m. At Cairo, the sun will rise at 7:07 Central Standard Time and set at 4:44 p.m. Add one hour for the difference between Eastern and Central time, and you see that the sun will rise 27 minutes later in Cairo than in Pittsburgh and set 47 minutes later, giving Cairo 20 more minutes of daylight today.

The Point at Pittsburgh, as seen from the M/V Tommy H. The boat is on the Monongahela River. The Allegheny River is on the other side of the fountain, and the Ohio River is on the left. That's Heinz Field in the background. PNC Park is up the Allegheny and out of the picture to the right.

The difference in daylight probably comes from the fact that Cairo is farther south than Pittsburgh. Pittsburgh is about 40 degrees 26 minutes north latitude, while Cairo is about 37 degrees 1 minute. That three and a half degrees of latitude gives Cairo more daylight this time of year.

However — there's always a however — in summer things are a bit different. On June 22, 2018, Pittsburgh will have 15 hours and 4 minutes of daylight while Cairo will have only 14 hours and 42 minutes, a difference of 22 minutes. The guess here is that's because the Northern Hemisphere is tilted away from the sun this time of year and toward it in summer. Higher latitudes get more sunlight than lower latitudes in summer and less in winter.

P.S. In case you're wondering about the longitude of the two cities, the Naval Observatory calculates Pittsburgh as being at 79 degrees 55 minutes longitude. Cairo is 89 degrees 11 minutes. That's a difference of about 9 degrees and 13 minutes, or about one fortieth of the way around the world.