Q&A: Why the Equinox Isn’t

Question: I live in Bethesda Maryland. Today is March 17. The forecast shows sunrise at 7:16 AM and sunset at 7:18 PM. Yesterday it was 7:17 AM to 7:17 PM. But the equinox is still three days away. How is this possible? I thought the word “equinox” literally meant “equal night.” —Matthew Koll, Bethesda, MD

Answer: I’ve gotten this question before, so I figured it was about time to do a post explaining the Equinox in more detail. The word “equinox” does indeed mean “equal night (and day)”. See the etymology here. And this would seem to require sunrise at exactly X:00 am and sunset at exactly X:00 pm. Obviously, that doesn’t quite happen. It’s close, but no cigar. There’s a lot of misconceptions about the Equinox, not the least of which is that myth about balancing an egg.

The graphic above was sent with this week’s question. It’s the kind of printout provided by many websites and mobile apps, and it’s what prompted Matthew’s question. It shows the data for March 17, three days before the official Equinox date.

The first thing to know is: Although the Equinox happens on a specific calendar date (March 20), it actually occurs at a specific time on that date (04:30 UT this year). Astronomically speaking, the equinox is the point in time when the Sun crosses the celestial equator. Put another way, it’s the point in time when Earth’s rotational axis (the line between the North and South Poles) is at a 90° angle to the line between the Earth and Sun.

Second, your longitude makes a difference. The clocks in a given time zone are set (astronomically) for the approximate longitudinal center of the time zone. So there’s some shift in sunrise and sunset times as you move away from that center. Bethesda is close to the center of EST, so you won’t see much of this effect at your location. Time zones at your latitude (39° N) have a width of about 1300 km (800 miles). Every clock in that time zone is set to the same time, so if you’re on the eastern edge of EST (say, in Portland ME) sunrise will obviously happen earlier than on the western edge.

Your latitude also has an effect. See this excellent explanation at TimeAndDate.com. The 23.5° tilt of Earth’s rotation axis relative to the plane of its orbit around the Sun generates a latitude-dependent variation in sunrise and sunset times. Close to the equator, for example, day and night are never of equal length.

The closer you get to the poles, the more variable the day/night ratio. North of the arctic circle, you can get zero hours of day and 24 hours of night. Near the poles, discrepancies in sunrise and sunset times from other factors are “magnified.”

Finally, if you define “sunrise” as the moment when the the top edge of the Sun breaks the horizon, atmospheric refraction will cause the Sun to “appear” up to as much as 6 minutes early. The same effect will cause “sunset” to be delayed. There’s a good diagram showing the relevant geometry here.

Reasoning further from that definition, there’s the effect of topography along your horizon. The mountain just east of my home can delay sunrise by up to 2 hours. The more-distant mountain range to my west makes sunset happen around 6.5 minutes early on an Equinox (see my Equinox Sunset time-lapse), and around 20 minutes early near the Winter Solstice. Of course, topography isn’t a problem out on the ocean, or the Great Plains, but even there refraction still has its effect.

From the chart on Timeanddate.com you can see that, wherever you are on the planet (except for the equator), there’s always two dates each year “near” the Spring and Fall Equinoxes when day and night are within seconds of being equal. They could be exactly equal, but that would only happen at specific locations on the planet.

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2 comments on “Q&A: Why the Equinox Isn’t”

  1. Also has to do with the refraction by atmosphere. More than half the globe is always in daylight, therefore the equal daylight/night occurs on the winter side of both vernal and autumnal equinox.

    1. Indeed, and I talk about refraction in the 3rd-last paragraph. You are correct that (all things equal) refraction lengthens every day with an earlier sunrise and delayed sunset. You can see the “winter-side bias” you speak of in the chart at TimeAndDate.com. An excellent insight, thanks!

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