Daylight Growing Time

I always greet Daylight Savings Time with mixed feelings. On the one hand, I love having light at 7 pm, but oh, waking up in the dark again for a while is so gloomy after having had some time awakening to daylight. At 7 am this morning I noticed it was still pretty dark. I tried to take a picture but it was so dark it wasn’t worth saving. By 7:15, however, it was fairly light. It won’t be too long before I’m arising to daylight again.

I just recently learned that we gain 90 minutes of daylight in March at 45°N latitude. In grade school science, when we learned about why the days get longer and shorter, we never got down to the details of the rate of change of the daylight hours. Although I hadn’t given it a great deal of thought, I always assumed it was linear and every day increased or decreased by the same amount. I did a little research and found the following graph of the daylight hours in St. Paul, MN. The blue line shows the number of daylight hours based on time of year, and it’s clearly not linear. I’ve marked off in green the segment of the year we’re in now, and it’s definitely the steepest part of the slope, indicating the fastest rate of change. How exciting! It’s just a few more days to the equinox.


Note: if I had visualized the change as a graph I would never have assumed linearity. If it was linear, the graph would be a straight line to the solstice, then immediately turn and be a straight line downward to the next solstice, which would be ridiculous. I’m also conveniently ignoring the fact that this part of the year is the steepest upward slope. There is, sadly, an equally steep downward slope in the fall.



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