Expert: Steven LJ Russo - 12/21/2009

Question
QUESTION: Today, December 21, 2009, is the Winter Solstice for the northern hemisphere, the day when the Earth reaches the point it is furthest from the sun. This is typically also the shortest day of the year, and it is here in NYC.

A co-worker asked when is the longest night. A search online brings up the answer that "the shortest day is followed by the longest night." However, I don't think that's always true.

At first I thought that the nights both before and after the shortest day would be equally long, but then I figured that this would only be true if the Solstice point was reached exactly at solar noon.

So now I'm thinking that, for a given spot on the Earth, that the longest night would occur before the shortest day if Solstice is reached before solar noon, and conversely that the longest night would come after the shortest day if Solstice is reached after solar noon.

Am I correct in my thinking, or is the shortest day always followed by the longest night?

ANSWER: It is more complicated than you think.

First of all, we are closest to the Sun in Winter, 3 million miles closer than any other time of the year.  Actually around January 2nd.

Believe it or not, Dec 21 is NOT the shortest day of the year, as all the dates between the 16th and 25 are all the same, approximately 9 hrs and 18 minutes of daylight.

So all the nights in those dates are the same.

Steve


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

QUESTION: That was my mistake, I meant the Earth's northern axial tilt is furthest away from the sun on this day, not that the Earth itself is furthest away.

However, while all the days in that time span are approximately the same, according to this calendar, each day length is several seconds different from each other. At my location the day lengths range from 9h 15m 23s on Dec 18, to 9h 15m 04s for Dec 21, to 9h 15m 36s on Dec 25. Today is 2 seconds shorter than yesterday, and 1 second shorter than tomorrow.
http://www.timeanddate.com/worldclock/astronomy.html?month=12&year=2009&obj=sun&

I realize that a difference of less than a minute is really an academic point, but if the days differ in lengths, then the nights must too.  

Answer
And that's where more technicalities come in.

Some Astronomers and almanacs consider "night", to start when the lower limb of the Sun hits the horizon.  Others when the Middle of the Sun hits the Horizon, and others when the top of the Sun hits the horizon.

Still others say its "Night" when twilight ends.  Ohhh, but us astronomers can never do anything easy.  Which twilight?  civil, nautical, or astronomical.

I guess to "sum" it all up, you are correct that if the days are different, so are the nights.

Steve