Question on sunrise?

Question

Has the time of sunrise changed over the centuries? There are those who say that the time of sunrise 150 years ago was some 3 minutes earlier than it is now.

If the sunrise time did change, have the clocks been adjusted to make up for this so our clocks will show the same time of sunrise today as they did then?

Answer 1

Sunrtise and sunset times are often a part of weather forcasts; meaning it is varying everyday. Clocks does not have to be changed.

Answer 2

First off, the length of a day changes all the time.  The moon is stealing rotational energy from the Earth through the tidal process, so today is longer than yesterday was.

Precessional effects change the tilt of the Earth.  So, that changes how strong summer and winter are, but that's in a cycle that varies and repeats over 26,000 years.

Answer 3

It's possible that sunrise has changed slightly over the centuries.  If we were to take the sunrise time for today, July 21, 2008 and compared it to sunrise on July 21, 1858 there may be some differences.  

However, I would think that the major difference would be based on how people measured time 150 years ago and depending on where the observations of sunrise occured, there may be a difference of up to an hour.  Before the concept of "standard time" was adopted by the United States in 1883, most time was measured by local sun time.  That time was calibrated by checking to see when the sun was at its highest peak during the day (noon), and setting a clock such as the town clock to match that time.  Of course, across a current time zone such as the Eastern Time Zone, there would be several local sun times that would vary by an hour or so from the eastern border to the western border of the time zone.  Since there were no "standard" times to calibrate, it is very difficult to put any confidence in the measurement systems at that time and compare it to today.  

It's a long way around the block to explain that there is little confidence that a difference that small can be reasonably and accurately detected.

Hope this helps!

Answer 4

Clocks are periodically adjusted by the insertion of a leap second.

As a matter of fact one will be inserted at the end of this year.