sunrise, sunset times: not what you might think.

Many people are aware of the winter solstice which is the shortest day of the year, which last occurred on December 22nd. However this date is not the latest sunrise nor earliest sunset for the year. Keeping this discussion practical, rather than technical, let’s ask the question, “So what does this mean to me as a cyclist?”

A good web based tool to look at for data like this.

Wikipedia has an excellent semi-technical description.

AxialTiltObliquity

AxialTiltObliquity

Using the data from the time-and-date site I found that the earliest sunsets actually occur in early December and the latest sunrises occur in early January. So, here we are in mid-January, how does this effect our cycling or other outdoor activities?

Data for Atlanta, Ga:
Jan 16, 2011
Sunrise: 7:42 AM Sunset: 5:53 PM
Daylight hours: 10h 10m 37s
Daylight hour change: + 1m 10s

Since the Winter Solstice occurred, our daylight hours have increased 16 minutes, but sunset times are actually 24 minutes later from early December. On the other hand, sunrise times are only 3 minutes earlier since the solstice.

Another thing to keep in mind is that the rate of change for these processes is highest near the equinoxes (equal day/night hours). During mid-March the days are getting longer at a rate of 126sec/day. During early October the days are getting shorter at a rate of 124sec/day.

So if it seemed to you, that the celestial events like sunrise and sunset weren’t in sync with the solstices, it wasn’t your imagination.

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