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I have a basic understanding of the sinusoidal relationship of the rate of change of daylight as the year goes on. But it seems intuitive that the changes should be equally split between sunrise and sunset.

For my location, Bend Or., todays sunrise/sunset is: 07:40 and 16:43 And for January 31st it is: 07:24 and 17:14

So sunrise will be 16 minutes earlier and sunset will be 31 minutes later. It appears that there is twice the added daylight to sunset than to sunrise. Why aren't the differences equivalent?

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  • $\begingroup$ If you look that the table here, timeanddate.com/sun/usa/bend, you'll see that the time of solar noon has also drifted ahead minutes. Factoring this in, sunrise has been shifted 16+8=24 minutes earlier and sunset has been shifted 31-8=23 minutes later. So they are equivalent. ;) $\endgroup$ – David H Jan 6 '15 at 10:54
  • $\begingroup$ Also, you may be interested in this physics.SE response: physics.stackexchange.com/questions/38270/… $\endgroup$ – David H Jan 6 '15 at 10:55
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It's a consequence of the equation of time, which represents the time difference between apparent solar time (time as measured by a sundial) and mean solar time (a fictitious device; essentially mean solar time is time as measured by a clock). Below are two plots from the referenced website.


Figure 1: Equation of time. Currently (early January), a sundial will be slightly behind a clock and is getting slower.



Figure 2: Time derivative of the equation of time. Apparent solar time changes most rapidly from early December to early January, with the maximum change occurring on December 25.


Days are currently getting longer in the northern hemisphere, but noon is advancing by 20 to 25 seconds per day. Sunset advances by half of the increase in the length of day plus that 20 to 25 second advance in noon. Sunrise advances by half the increase in length of day less that 20 to 25 second advance in noon.

The increase in the length of day (time from sunrise to sunset) depends on latitude. In Bogota Columbia (4°35′53″N latitude), length of day increases by only 3 to 10 seconds per day during January, growing throughout the month. That tiny change in length of day means sunrise continues to advance throughout the month of January in Bogota. On the other hand, in Reykjavík, Iceland (64°08′N latitude), length of day increases drastically from day to day, even at this time of year. Sunrise started becoming earlier in Reykjavík on Christmas day.

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  • $\begingroup$ Thank you David. It was your short paragraph about solar noon shifting that allowed me to groc this. The graphic of the analemma (which shows there is some lateral (east/west) motion of the sun throughout the year rather than just the north south motion associated with the seasons) was particularly helpful. $\endgroup$ – Steve H Jan 7 '15 at 14:22

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