In which I am geeky about day length

Somehow, I made it 44 years without every really paying attention to the day to day differences in the time the sun sets. My agrarian ancestors would be horrified, I’m sure, but for most of my working life the only real question was “is it going to be light outside when I leave the office?”

In the summer, the answer is yes.

In the winter, the answer is no.

And for most of my life, that’s all I noticed.

But a couple months ago, I started riding with a group at West End on Tuesdays and Thursdays; the ride starts at 6:30, which is much earlier than the rides I’d done in the past. This summer has been great for my cycling; I’ve lost weight and gotten stronger, both of which have made me faster. The route to the ride doesn’t change, so I could gauge my progress by what time I pass certain points, and especially what time I get home.

The first thing I noticed was that my “great ride” finish time got closer and closer to 8:00; a couple times, I even got home BEFORE 8.

The second thing I noticed was that not only did I never turn on my headlight, but most of the riders I was with didn’t even bother to bring one. Sure, if you took breaks and didn’t plan to finish until the bottom half of the 8:00 hour, you’d need one, but if you mashed hard and kept moving you really didn’t need to bother at all in June (earliest sunset: 8:17, on the first), or in July (8:15, on the 31st)

Of course, after July 1, days start getting shorter again, and by mid-August I was using my headlight on the last parts of the ride even though I was finishing at the same time.

I started to wonder — never having bothered to care before — about the rate at which the day was getting shorter. I’d blithely assumed the change was linear at first, but there’s every reason for it not to be. It’s just that I never had cause to notice it before because I’d never been outside consistently during the time in question.

Being a giant nerd, I went looking for data. It wasn’t hard to find. This month the changes from Tuesday to Thursday, and especially from Thursday to the following Tuesday, have been especially pronounced; the table quantifies it, but in a way I didn’t anticipate.

The surprise was that yes, the acceleration IS there in the last half of the year — but it had already happened. Each day in September is about 1:45 shorter than the previous one; there’s almost no variance at all. The first was 1:44 shorter than the last day of August, and the 30th will be 1:46 shorter than the 29th.

The giant jumps happened two months ago. July first was only 22 seconds shorter than the last day of June, but by the end of the month the delta had ballooned to 1:17. The change in August wasn’t as huge, but it’s still much bigger than September: 1:19 on August 1 to 1:42 on the 31st.

I just didn’t NOTICE until September, because (a) the average day-to-day change in July is so much smaller and (b) if I’m honest, I admit that the need to use my headlight made me much more conscious of the rate of change.

But those big bites in September really do drive the point home. There’s only about a nine minute difference in daylight between July 1 and July 31, but those 1:45 changes add up quick in September. The 30th is more than half an hour shorter than the first.

My ride on the 11th had three and a half minutes less sunlight than my ride on the 9th, just two days before. My ride tonight will have nearly NINE minutes less sunlight than my ride Thursday.

tl;dr: Orbits and seasons, man.

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