Happy Equinox! Time To Tip Your Hat To Earth’s Tipped Axis

One of the earliest of spring flowers, a crocus blooms on a sunny spring afternoon. Credit: Bob King

Such a beautiful flower. Look as hard as you like and you won’t find a single one in my town where more than two feet of snow still blankets the good brown earth. I’m not worried. Two weeks from now, the spring sun will reduce it all to puddles.

Today’s the vernal equinox, the start of spring in the northern hemisphere. It began at 11:57 a.m. CDT, the instant the center of the sun’s blazing disk crossed the imaginary circle in the sky called the celestial equator. If you live on the real equator, the celestial version passes directly overhead. That means no shadows at noon today for residents of places like Quito, Ecuador and Kampala, Uganda.

North Pole webcam 2013

Travel north of the equator and the celestial equator drops lower and lower in the southern sky. At the north pole, it sits exactly on the horizon 360 degrees all around. If you could stand there today, you’d be seeing your first sunrise since the autumnal equinox last September. It would also be the start of six months of uninterrupted daylight. By the way, the weather’s fantastic there today – sunny with a high of 24 degrees!

Because of the 23.5 tilt of Earth’s axis, the altitude of the sun varies cyclically across a year. In winter it’s 23.5 degrees below the celestial equator, while in summer it’s 23.5 degrees above. At the equinoxes, it straddles the equator. Created with Stellarium

Most of live between the pole and equator, where the sun stands roughly halfway up in the southern sky at local noon. That’s a far cry from winter, when the sun stood 23.5 degrees (a little more than two fists held at arm’s length) below the equator. Its rays were less direct and intense, and the time it spent above the horizon relatively brief, the two key factors that make a winter.

In summer, we experience just the opposite. The sun stands 23.5 degrees above the celestial equator; its rays are more direct and it spends many more hours above the horizon. Long days and short nights are a delight for many  … including the bugs.

The sun’s cyclic journey above and below the celestial equator all goes back to Earth’s tipped axis. As Earth travels around the sun in a year, the north polar axis tilts toward the sun in summer, taking it 23.5 degrees above the equator, and away from the sun in winter for a ride 23.5 degrees below the equator.

The tip of Earth on its axis causes the seasons. On the first day of spring or vernal equinox, the axis is perpendicular to the sun and days and nights are equally long in both northern and southern hemispheres. Notice the axis doesn’t “flip-flop” but remains pointed in the same direction. It’s the Earth’s orbital travel that causes it to point toward and away from the sun. Credit: Tao-olunga with my own additions

On the first days of spring and fall, the axis is oriented neither toward nor away from the sun. Day and night across the planet are paired up at 12 hours apiece. After today, daylight slowly gains the upper hand by 2-4 minutes a day. Doesn’t sound like much, but like snow, it quickly adds up. By June the mid-latitudes will have gained some four additional hours of solar photons.

What spring looks like where in Duluth this season – a high sun but plenty of snow to go around. Photo taken March 16, 2014. Credit: Bob King

You’ve probably heard that you can balance an egg on its end on the first days of spring and fall. Like water going down the bathtub drain in different directions depending on your hemispher this is an urban myth. It’s hard to balance an egg ANY time of year. Just try it.

I think we all relate to the new season for the same reasons generations of humans before us have. Rebirth, renewal and the return of warmth and light capture the essence of spring. We tip our hats to the random impact at the dawn of the solar system that set Earth’s axis askew.

10 Responses

  1. Sean

    well, technically, the South Pole would have seen its 1st sunrise days ago since the “center” of the sun is at the horizon there (assuming a flat horizon) at the equinox, but its limb crossed previously, just like our equilux comes a few days b4 the equinox.

    1. astrobob

      I used the center of the sun’s disk as sunrise but, you’re right, technically it’s the upper edge.

  2. Terry

    Hi Bob,

    I love reading your articles. Appreciate your time and effort into it.

    I have a question about the North Celestial Pole

    I understand that the Pole star will change during the long period of precession of the equinoxes, but my question is about the North Pole fixed at different seasons.

    1) Does the earth remain pointing to the Pole Star (Let say for now, Polaris) at different seasons?

    If yes, how could it be when the earth remain titled at the same degree, circling around the SUN and yet the earth Pole Axis remain pointing at the same Pole Star?

    I understand that the North star remain fixed while other stars are seen from moving from east to west from earth from a night view or within a few days.

    But does the earth still points to this specific Pole Star location at Polaris at different seasons since the position of the earth around the sun is already different?

    Because at different seasons, the position of the earth is at different location/position on the ecliptic.

    If the position of the earth is at different locations around the sun, how does the earth fix its North Pole to a specific star of the North celestial pole?

    Do reply. I am a beginner. Just curious. Thanks.

    Have a nice day


    1. astrobob

      Thanks! To answer your questions:
      Yes, the Earth’s axis points toward Polaris year-round at both ends of
      its orbit because Polaris is so incredibly far away – 434 light years
      – compared to the minuscule length of Earth’s orbit. Picture it from
      Polaris’ point of view, where the Earth and sun would be too close to
      see as separate bodies even with a large telescope. If Polaris were
      only as far as say, Neptune, our axis would point at the star at one
      of its orbit but would “miss it” on the other end. Does this help?

      1. Terry

        Hi Bob,

        I kept asking myself and until I search how far these stars are in Wiki. lol. It is incredibly far and that is why it appears to goes to the same directions despite the different locations of the earth revolving the Sun.

        Sorry for asking a silly question. I am just starting learn.

        Yes, you are absolutely correct. I came back to confirm my own reasoning if it matches yours.

        Thank you so much for replying.

        Have a good day.

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