Expert: Tom Whiting - 1/15/2008

Question
QUESTION: Mr Whiting
After Dec 21, why is the extension of daylight in the eve not equal to the ext of daylight in the morning. I understand why we gain 60 secs of daylight in the eve. My question is why is the morning daylight only extended by 20 seconds. Thank you in advance for your time on this.
Pete Miller

ANSWER: Hi Pete,
If we were in a perfectly circular orbit around the sun,
then earliest sunset/latest sunrise would occur on December 21 or the December solstice.  But we're not.
(Earth is at perihelion on or about January 2nd at 91.4 million miles, and aphelion on or about July 4th at 94.5
million miles.)

And since we are in a slightly elliptical orbit, the earliest sunset is on or around December 7th and the latest sunrise is on or around January 5th  (for latitude 40 north).  So we are not gaining 20 seconds from Dec 21
until January 5th, we are still losing daylight
(or gaining night-time for us astronomers, depends on how you look at it)
in the morning twilight. Since all this is occuring at the
bottom of a sine wave, the time element is a matter of a
few seconds, not the rounded off value of one minute that
you see on the "local news" or weather show.
And December 21, or the solstice, is roughly the mid-point between the other two dates, when the sun is at it's lowest
point in our N. Hemisphere sky.

The same happens on the June solstice, but reversed.
Earliest sunrise this year is June 14 and latest sunset is
June 27th, for the same reason, namely an elliptical orbit.

The Earth is not unique; all planets are in elliptical
orbits around the sun (Kepler's first law, all planets orbit the sun on ellipses).
And this is also why Mars closest approach to the Earth in 2007 was on December 18, but the opposition day (Sun-Earth-Mars straight line) was on December 24, because Mars also follows an ellipse, and not a perfect circle.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA

FOLLOW UP:
Oh, so to answer your question for right NOW, January
15th, we've been gaining on the bottom of the sine wave
for sunset since December 7th, for over a month, thus the "60 second" gain on sunset, (we're further up the curve)- - but for the sunrise sine wave, we've only been gaining on it since January 5th, only 10 days up the curve, so that's why, right now, we're only gaining a few seconds every 24 hours, on sunrise times.
That's why the difference.
Clear skies,
Tom


---------- FOLLOW-UP ----------

QUESTION: Tom
thanks again for your time.
what I don't understand is why the change in sunset time day to day is not equal to the change in sunrise time day to day. if the sunset is 60 secs later over a 24 hr period it would seem the sunrise would be 60 secs later as well. it is the disparity between the two that has me perplexed.
thnks again
Pete

Answer
Hi Pete,
It doesn't work that way...like 60 seconds on each end AT the same time....
The disparity is caused by the two separate sine waves governing each, times of sunset and sunrise, they are slightly out of phase, not in synch (by about an average of 3 weeks), due to our elliptical orbit.  So we've been "climbing" up the sine wave for earliest sunset since December 7th, actually slightly gaining daylight since that date on the evening end, whereas we've only been climbing up the sine wave for lastest sunrise since January 5th (at latitude 40 north)...we are actually still losing daylight on that end, the sunrise end,  until it becomes zero on January 5th, the date of latest sunrise.  As I said earlier, the winter solstice, our "shortest" day...is the mid-point between those two sine waves....and it's all mainly due to our elliptical (non-circular) orbit around the sun.
   
Just like in June, our earliest sunrise (at 40 N. latitude)
is on or about June 14th; we actually start losing morning daylight (just a few seconds initially) from June 15th onward, but we're still gaining evening daylight until latest sunset on or about June 27th... so the first day of summer, our "longest day", is the midpoint between those two events, or two sine waves,  on or about, June 21st.

(Sorry, but I just can't think of another, easier analogy, to make it an easier explanation). The only way to really understand it, is a study of sine waves and how sine wave motion works, which I believe they cover in Plane Geometry, generally in 7th or 8th grade.  So go back and read up again on sine wave motion, and then I'm sure you can understand it. That's just the way it happens with elliptical orbits, and it's been doing it that way for over 4 billion years now.

So all I can say, for an answer is, the two sine wave events are not in exact synchronization because we're not in a perfectly circular orbit around the sun. And that's the "why or how come"... the difference that you noted.
Hope this helps,
Clear Skies,
Tom
PS...I cannot supply the understanding, I can only supply
the reasoning, and explanation of same.