A Few Caveats regarding Day Length

I have to admit I have spring fever. I seem to have moved to a part of the country where the seasons actually follow the solstices and equinoxes, and politely divide themselves into 4 fairly equal parts. My crocuses are blooming, shorts are starting to appear on some of the more robust males on campus, the Canadian geese have paired off (which is actually very freaky), and tonight the Sun crosses the equinox at 7 minutes after midnight in Greenwich (that’s GMT-0). The Sun, when it rises over Edwardsville and campus tomorrow, will be hanging out over the Northern Hemisphere.

When I teach about equinoxes and solstices in class, the observant student may notice a slight discrepancies between what is taught and what they find in their newspaper. For instance, I’ll say that on the Equinox, the Sun rises exactly in the East and sets exactly in the West (totally true). I’ll say the Sun passes directly over the Equator at the Equinox (also, totally true). And, I’ll say the day and the night are equal on the Equinox (kinda sorta true). What I don’t do is define sunrise and sunset, and this is where that last “kinda sorta, huh?” moment comes into play.

In a logical world, we might say that when the mid-point of the Sun crosses horizon, the Sun (depending on direction of motion) has risen or set. Based on this definition, the equinoxes have equal amounts of day and night*.

But who said we live in a logical world? The technical definition of sunrise is more in line with a vampire’s definition: as soon as a single beam of sunlight creeps over the horizon we have sunrise. This moment of daylight occurs well before the midpoint rises, and makes the day a bit longer than my more geometry-minded definition.

Sunset seems to have been defined by an hopeful kid who has to come in at sunset: The Sun is considered up until the very last beam is blocked by the horizon. The Sun’s last edge, for obvious reasons, sets well after the midpoint, and this too makes the day a bit longer.
Put together this means, based on newspaper rise and set times based on the above definitions, the equinox is going to have more than 12 hours of Sun, and the date with equal amounts of day and night falls before the spring equinox.

Now, even by correctly defining sunrise and sunset, I still don’t get at the newspaper definition. The problem is the atmosphere. While good for life, the atmosphere is bad for astronomy. It has some bad habits like blurring stars and bending light. When light rays pass from one substance to another, they will bend if the substances have different refractive indexes. This is how glasses and magnifying lenses work. Air and glass (or plastic) have different refractive indexes, and as a light ray enters glass (or plastic) its path changes direction. Until within my lifetime, real glass (silicate-based stuff) was used to make prescription eyewear, and to bend the light enough to correct for really bad vision, really thick glasses were needed. Today, thanks to plastics, we can produce materials that bend light more effectively than glass, and make thinner, lighter weigh glasses for the vision impaired.

As sunlight passes from space (a vacuum) into the Earth’s atmosphere (not a vacuum) it sees a change in refractive index and its path changes direction. The direction of this change causes light that enters the atmosphere below the horizon to get bent such that the Sun will appear above the horizon. This works for both sunrise and sunset, and has the effect of making the Sun appear to speed up toward the zenith and slow as it nears the horizon (it doesn’t really change speeds) and to set after logic and the Earth’s rotation rate says it should have set. This means that our atmosphere makes our days longer by bending light toward us. This is probably a good thing for our sun addicted society, but it is a bad thing for vampires, astronomers, and people trying to figure out just when there are equal amounts of day and night.

So when, if you bring in all these extra definitions and facts, is there equal day and equal night? Sadly, there is no one answer. It depends on where you are on the planet. According to the US Naval Observatory, at mid-northern latitudes (around 40 degrees) the days of equal day and night are around March 17 and September 26. This would imply that for mid-southern latitudes, the dates of equal day and night are around March 24 and September 19.

So, go out and play in the flowers, any of you lucky enough to have flowers, and if you are in the Norht, know that day now out lasts the night.

(Oh, and if you’re curious, the longest and shortest days, even with all these extras, are still on the solstices).

*if you ignore the atmosphere


  1. Astrolink
    Mar 21, 2007

    Hi …

    Take a look at : Astronomy News , it’s also available via WAP , plus it’s available in 11 languages !

    Enjoy …

  2. Astrolink
    Mar 21, 2007

    by the way, we’ll be happy to have your suggestions and\or any idea which leads to improvement.

  3. Jorge Schrauwen
    Mar 21, 2007

    Bit off topic but…
    How does an american pronouce: crocuses?
    I know what it is in dutch but I have a hard time imagening an american say it like we do ^^

  4. Stuart
    Mar 22, 2007

    Pamela, you might want to change the spelling of “US Navel Observatory” as that implies a whole load of people sitting around staring at their navels 😉

    Jorge, I’ve never heard an American say it but in the UK we pronounce crocuses with hard c’s something like: kro-kus-is.

  5. Pamela
    Mar 22, 2007

    Hmmm… That Navel vs Naval thing has bitten me before. Thanks for the catch 🙂

    And yes, we say kro-kus-is here too.

  6. Ed Minchau
    Mar 25, 2007

    I have updated the Space Blogroll to point to your new blog URL, and have updated the Space Feeds aggregator to point to your new site feed. Nice new digs, by the way.


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