Friday, September 21, 2018

Equinox

Today is the last day of 2018 with more daytime than nighttime in the northern hemisphere. Tomorrow is the autumn equinox. There are two of these each year, one in the spring, one in the fall.

The equinox is a date with equal parts day and night. Sunrise and sunset are separated by 12 hours.

A few years back I realized this is true everywhere in the world. Even Alaska, the land of the midnight sun during the summertime, has 12 hours of sunlight and 12 hours of darkness on the equinox.

If one looks at a live map of the ISS orbit, the shadow shows where the sun is shining and where it is not. At the moment of the equinox, in theory, the lines should be completely vertical. I've never seen it that way, and I've always missed the moment when all daylight at the top flips from top to bottom, or vice versa.

According to Google the equinox is at precisely 9:54 PM tonight. I'm not sure how often Heavens Above actually updates. It may never actually have the bars exactly vertical if that only happens in between its update intervals. Maybe some other equinox will happen at a more normal waking hour. Feel free to check for me and let me know.

If one looks at the contours of where light is on the earth during a solstice, those lines have shifted from nearly vertical to diagonal with long curves at the top and bottom. The same thing happens with how fast daylight increases or decreases at various times of the year. It's not as though various places in the northern hemisphere simply add daylight at consistent increments until the summer solstice and then shift to subtracting until the winter solstice. Around the times of either solstice, the amount of daylight change is much less per day than it is around the time of the equinox.

That means that around this time of year, the amount of daylight change per day is at its fastest peak, and for the fall in the northern hemisphere that means daylight diminishing. Ours is a late culture which tends to notice the sunset more than the sunrise. Change in the time of the sunrise noticed much less.

Throw in the resetting of the clocks back to their true time at the end of Daylight Saving Time, and you complete the trifecta of why darkness coming early can seem sudden to so many. If we didn't have DST now, then sunrise would be at 6 AM and sunset at 6 PM. That would already feel early, but we don't feel it being that early because of DST. Instead, depending on where one is in their time zone, we have sunrise at 7 AM, solar noon at 1 PM, and sunset at 7 PM on the equinox.

I've been enjoying the Sundial app in recent weeks. With the iOS 12 new Screentime feature, it's currently one of my most active notifying apps. I have alerts set for two twilights, sunrise, mid-morning, solar noon, mid-afternoon, sunset, two more twilights, moonrise and moonset. It's like a no-data app for helping indoor life maintain a sense of outdoor life.

While some people may think that sunrise and sunset are obvious, especially if one is outdoors, that's not always true. If a day starts or finishes cloudy, it's not easy to tell when exactly the sun is rising or setting, so it can be useful to have the notification for those times. Cloudy fall weather days are another way the changing of the sunset time can be masked for people and then later seem to suddenly have changed.

For today, enjoy the daylight and know that we're only 90 days away from the winter solstice when days start getting longer again.

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