23.5 cheers for daylight

The sun returns! We know it’s always been there, but soon we’ll really start to feel its presence. Photo: Bob King

I know it tends to cut into nighttime sky watching, but humans like daylight. Heck, even I do. Every January there comes a time when we start to notice the days getting longer. This happened to me two evenings ago when I got off work around 6 p.m. under a twilit sky. A month ago that same sky would have been twinkling with stars.

A quick check on sunset and sunrise times reveals that we’ve gained a full 40 minutes of daylight since December 21, the shortest day. The earliest sunset occurred around Dec. 8 (4:20 p.m. here in  Duluth, Minn.) and the latest sunrise (7:53 a.m.) around Jan. 3.  Those times are now 5 p.m. and 7:42 a.m. The sunrise lags behind sunset due to a combination of factors involving the angle of the sun’s path in winter and Earth’s orbital speed.

40 minutes is nothing to sniff at, but we’re just getting going. Each day, we add an additional 1 to 2 minutes of evening sunshine and 1 minute of morning light. What began as a trickle is rapidly becoming a cascade as we set our sights on March 20, the first day of spring.

The 23.5 degree tip of our planet, probably imparted by some long-ago impact in its formative years, is responsible for the ever-changing length of daylight during the year. Credit: Tau’olunga with my annotations

Those extra minutes are doled out day by day as the sun climbs ever northward on its yearly path around the sky. Its low position (along with short days) happens in winter, because that’s when the Earth’s northern hemisphere is tipped away from the sun. Summer happens when we’re tipped toward the sun.

The full range of the sun’s north to south movement in the sky is 47 degrees or nearly five fists held vertically against the sky. Why 47? If you divide it in half you get 23.5 degrees, and that’s the angle at which our planet’s axis is tipped. You can see that the difference between the high summer sun and low winter sun is simply a reflection of our planet’s tilt.

The sun’s position is shown at noon on the first days of winter, spring and summer 2012-2013. The sun’s path – known as the ecliptic – climbs up or northward starting on the first day of winter. The separation between each position is 23.5 degrees, equal to the tilt of Earth’s axis. Created with Stellarium

Astronomers divide the sky in half with an imaginary circle called the celestial equator. This is really nothing more than a projection of Earth’s actual equator into the sky above. For an observer on the equator, the celestial equator begins at the due-east horizon point, passes directly overhead and ends at the due-west point. For mid-northern latitudes, the celestial equator starts and ends at the same points but passes midway between the southern horizon and the zenith (overhead point). At the north pole the celestial equator runs right along the horizon from due east to due west.

The sun spends 23.5 degrees below the celestial equator from the first day of fall until the first day of spring and 23.5 degrees above the equator from the first day of spring until the first day of fall. At the equinoxes, the sun’s path momentarily crosses the celestial equator as it moves north or south.

If you’re looking for something to thank for providing the precious daylight you’ve been looking forward to for months, give a nod to Earth’s tilt.

14 thoughts on “23.5 cheers for daylight

  1. Hi Bob
    Thanks for that blog tonight, for as much as I love to look at the stars, I really love when the sun is out as I think it makes people so much happier and it puts a smile on my face :-)

  2. Hey Bobby boy, It is “latest sunrise” (on Jan. 3) I believe. Your blog is great. Always looking for the “magic” in life, at least from my perspective. Rick

  3. Bob, I grew up in Hudson. I check the news on the Star Observer’s page daily. I’m now a resident of Superior, Wi. Just wanted to let you know that I really enjoy reading your blogs. They keep me looking up! :)

    • Jason,
      I’m very happy to know you enjoy the blog. Looking up — yep, that’s what we’re about. If you’re interested, consider coming to one of the meetings of the Arrowhead Astronomical Society here in Duluth. We meet at 8 p.m. the second Wednesday of every month at the planetarium at UMD.

  4. Sorry, I dont have a website.
    But I happened to look out the window,after the the beautiful sight of the moon was mentioed on TV Winnipeg Mb and indeed it was truly amazing even
    without telescope. How I found your site was, I too was noticing the lengthening daylight as I take my late afternoon walk and decided to surf the net for an explanation. This phenomenon is midwy between solstice and “imbloc” is there a name for it
    Looking forward to more interesting info,

    A northerner!!!

  5. I wake up at 3 AM for work, but I will not mind having to go out a little later in the March evenings to see Panstaars as the later evenings will be getting warmer than in Winter.

  6. I too had this “aha” moment a couple of days ago as I was getting ready to bring my daughter to her piano lessons. I suddenly realized that it was still light out, when just last month in December, we were already looking at stars when I dropped her off! Keep up the good work Bob, enjoy your blog.
    Dave (your neighbour to the north in Thunder Bay)

    PS – our 2013 RASC General Assembly website will soon be up – when it is I will send you info and invites to our event this summer.

  7. Aloha Astro Bob!

    As usual, very good blog! It took me back to those days living in Minnesota and having those winter days of going to work in the dark and going home after work in the dark…virtually missing ALL the daylight! As much as I did love those “upper midwest” winters, I sure don’t miss those odd days of not experiencing sunlight.

    For some reason, I seem to forget that the earth has this 23.5 degree tilt instead of the earth actually tilting back and forth, which is something it would have to do to create “seasons” if it weren’t in an orbit around the sun (correct?). I’ve always had a real hard time not only grasping that fact but remembering it. Somehow, I doubt if I’m alone with that problem. It’s amazing how I can understand the “theory of relativity” and remember it with more ease! I believe I must have had a science teacher in my early years that didn’t make certain points very clear in his lectures or something, I don’t know.

    Thanks to people like you who have sites such as these, I get “reminded” of some important facts lest I turn and teach my children erroneous information. So thank you for the re-teaching I seem to need from time to time.

    Even though this is the “coldest” time of year for me in Hawaii (it gets all the way down to 58 degrees F.!), I know this is still considered very “warm” for those of you who live in Minnesota. Sure wish I could send you some of this ‘warmth’ to use when you have to venture outside on those bitterly cold evenings…

    Aloha For Now!

    • Hi Wayne,
      That tipping thing, which confuses many folks, is the reason I put the orbital diagram in the blog, so you can see that the tip maintains the same direction a full 360 around the sun.

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