Sunday, December 28, 2014

How Much Longer Does Each Day Get Over The Course Of Winter?

Tahoe is at 39 degrees of latitude, so our winter days aren’t nearly as short as in places farther north like Seattle or Minneapolis or “the Portlands,” either Oregon or Maine. Even so, I love longer daylight, and I wait for the days to start stretching out again after the Winter Solstice, the shortest day of the year. But how much is the increase in daylight every day?

The answer would seem to be easy to calculate. One could take the difference in day length between the Winter Solstice (Dec 21st) and the Summer Solstice (June 21st) and divide by the number of days in that period (about 182).

In Tahoe, our shortest day is about 9 hours and 28 minutes. Come the Summer Solstice, our longest day is about 14 hours and 52 minutes. If you divide the difference (5 hours and 24 minutes) by 182 days, you’d think that our day length increases by about 1 minute and 46 seconds each day.

But that isn’t the case! In fact, the amount of the day length increase changes dramatically depending on the time of the year. For example, in the days right after the Winter Solstice, the day length increases by only a few seconds with each passing day. But come the Vernal Equinox (Mar 21st), the day length increases by 2 minutes and 31 seconds with each passing day. Why the disparity?

I did a little research, and here’s what I learned.

To help illustrate why, visualize a clockface.

Pardon my scratchy, hard-to-read printing!
(You can see why my wife is the artist in the family!)

The Earth is tilted about 23 degrees. In December, when the Northern Hemisphere tilts away from the sun, we get a shorter day. In June, the opposite is true. The Northern Hemisphere tilts toward the sun, and we get a longer day.

As the Earth moves, it travels counter-clockwise when viewed from above. The closer the Earth is to the bottom of this sketch, the shorter the day in the Northern Hemisphere. In contrast, the closer the Earth is to the top of the sketch, the longer day.

But consider this. As the Earth moves from December 21st to January 21st, it's mostly just moved to the right on the sketch. It's gone very little toward the top. So the day length has increased just a smidgen in an entire month. 

But when the Earth gets to the part of its orbit from mid-February to mid-April, a month's worth of movement takes it much farther toward the top of its orbit, thus increasing the day length a lot!

Here's an example of what a difference that makes. In the ten days after the Winter Solstice, the total day length increase is only 2.5 minutes. But in the ten days after the Equinox, the day length increases about 25 minutes. Ten times as much! As represented on the sketch, that's because the Earth has moved ten times as much toward the place where we have the Summer Solstice.

So the next time you wonder how much day length increases or decreases, remember that it's completely dependent on the time of the year. At the Equinoxes, day length changes a lot every day. At the Solstices, day length barely budges.

Whew! Glad we figured that out, huh?!

P.S. I always wait for Owen McKenna's Ten/Ten Rule Of Sunlight to kick in on January 21st. Here's a blog about that.


  1. Replies
    1. You're welcome! Now that it is the end of January, our day length at 39 degrees of latitude will be ten hours or more for the next ten months.
      Love that sunshine!

  2. Great post! Isn't this discrepancy predominantly due to the fact that the Earth's orbit around the Sun is not circular, but elliptical?

    1. Thanks for your interest!
      The Earth's orbit is elliptical but only by a tiny bit. In the world of day length changes, pretty much all of the changes are determined by how much we move from one side of our orbit toward the other. Viewing our orbit as a clock face with the summer solstice at noon and the winter solstice at six o'clock, it is the relative movement from one to the other that increases or decreases our day length. If you're at noon, a fair amount of movement counter-clockwise along the perimeter of the dial doesn't get you much closer toward six o'clock. But if you're at nine o'clock, the same amount of movement moves you much closer toward six o'clock. It's how much you go from noon to six or vice versa that determines how much day length changes.
      I love that today (February 29th) we are moving into that part of the year where every day changes day length a lot!

    2. Thanks Todd, that really helped a lot. I would like to see a line graph too, hint, hint.