Happy Equinox! Welcome To The First Day Of Fall

A maple tree sheds piles of colorful leaves. Credit: Bob King

A red leaf spirals to the forest floor. Frost lingers on shaded green grass after sunrise. So it begins. Small changes soon add up to a brand new season, one that starts today Sept. 22 at 3:44 p.m. (CDT). Is fall here already?

Also called the autumnal equinox, September 22 is one of two times a year when the sun’s path crosses the celestial equator, an extension of Earth’s equator onto the sky.

Picture the celestial equator as half of an imaginary hula hoop tilting up from the eastern horizon, crossing the southern sky and then arcing back down to the western horizon. The hoop’s other half continues below the horizon and beneath our feet, circling over the opposite hemisphere.

On the first day of fall the sun crosses the celestial equator moving south. That means a higher sun for southern hemisphere observers – and more daylight – and a lower sun (more night) for folks living in the northern hemisphere. Six months from now the sun re-crosses the equator moving north, and the day-night balance reverses.

Two views of the sun’s travels along the celestial equator on the first day of fall. The left side shows the view from the equator where the sun passes overhead. The right shows the view from 50 degrees north latitude (S. Canada, Europe). At the north pole, the sun would follow a path along the horizon. Credit: Tau’olunga

If you live on the equator, the celestial equator begins at the due east point, passes overhead and meets the horizon again at the due west point. With the sun sitting squarely on the celestial equator tomorrow, it will beam down from directly overhead at local noon.

Today in Quito, Ecuador the forecast is for partly sunny skies and a high of 79 degrees F. Since Quito’s on the equator, residents there will only see their shadows at noon if they make a point of staring straight down at their feet. At the north pole, where the celestial equator rings the entire horizon, the sun circles along the horizon for 24 hours, never setting. Shadows stretch like taffy into the distance. At mid-northern latitudes, the sun shines halfway up in the southern sky at noon, and no one will pay attention to their shadow.

Because the sun continues moving south of the celestial equator, it quickly drops below the horizon at the north pole and doesn’t return for another six months. Naturally, the situation is reversed for the south pole; there old Sol finally climbs out of his dark cave and stays up 24 hours a day for next six months. For those of us living between those extremes, the sun’s southward trek means it drops ever lower in the sky (for the northern hemisphere), bringing shorter daylight hours, longer nights and the end of life as we know it. Just kidding.

We can tip our hats to the 23.5 degree tilt of the Earth’s axis for making the four seasons possible. During a year’s time as we orbit the sun, the height of the noonday sun changes because of the changing orientation of our axis. When the north polar axis is pointed toward the sun, it reaches its most northerly point in the sky. Summer begins, and the combined effects of the sun’s steeper slant on our backs and longer daylight hours make for longer, hotter days.

During northern hemisphere winter, the axis points away from the sun and our star is southernmost and lowest in the sky. Shorter days and a low sun bring cold weather.

Fall and spring are in-between times when neither hemisphere is tipped toward the sun. We all face the our star broadside, and day and night lengths are nearly equal across the globe. That’s the meaning of equinox – a single word that combines the Latin words  ‘equal’ and ‘night’.

Fall’s first day brings with it another bit of symmetry. Since the sun sits directly on the celestial equator it will rise due east and set due west tomorrow – everywhere except the north pole.

Everyone has favorite things they like about autumn. For me it’s the colors, the way the air smells, earlier nights, cooler temperatures and not having to mow the lawn. I try to be grateful for the simple yet profound fact that were it not for the tilt of Earth’s axis, there would be no looking forward to the change of seasons.

Note: I apologize for being unable to include several helpful illustrations in today’s blog. I’ve apparently run out of space! Hopefully that will be corrected tomorrow. Click the links below to see the images.

* Shadows at noon at the equator

* Sun crossing the celestial equator on the first day of fall

* Earth’s tipped axis and the reason for the seasons


11 Responses

  1. Edward M. Boll

    So, if I take a walk at 3, this will be my last Summer walk of 2013. Beautiful afternoon. Last night on the way home from work it looked weird to see Saturn to the right of Venus but higher. Uranus faintly shines almost all night. Still visible with binoculars 35 minutes before sunrise, that is my question.

  2. Edward Scherrer

    thank you for the clear description of the fall equinox.

    On saturday i will be in Eagan at my daughter’s home and we plan to view the sky with her telescope shortly after sunset. Where might I find information about which planets, if any, might be viewable in the early evening? Of course, we will need a clear sky. I think I have already learned the moon will not rise until about 1:39 AM the next morning. So I am hoping we may find one of the planets. It has been a long time since I have viewed the might sky.

    Thanks, Ed

    1. astrobob

      Hi Edward,
      Your best easy planets are Venus and Saturn. Venus will be the brilliant “star” low in the southwestern sky about 40 minutes after sunset. Saturn will be trickier to see. It’s the “star” about 5 degrees to the upper right of Venus.

      1. Edward Scherrer

        Thanks for your help! I think I glimpsed Venus with binoculars about 7:45 PM last night low in the southwestern sky. If the weather is clear Saturday evening we should be able to find Saturn also.

        1. astrobob

          Glad you found it. Don’t wait too long to look at Saturn – it’s getting lower in the west every night. Good luck! Later this week I’ll have a map you can use to find the planet Uranus.

  3. Edward M. Boll

    With ISON at magnitude 11. 1 on Sept. 16, it could be 10 by Sept. 30. Encke should be a little brighter with Lovejoy probably around magnitude 11 by month’s end.

  4. Sean

    Hate to quibble, but i guess i don’t hate it that much since i guess i’m about to. If u are referring to “astronomical” seasons – as they’re often referred to – then spring and summer have equal amounts of sun, as do fall and winter. only that in spring it increases, and summer it decreases, and vice-versa vis-a-vis fall and winter. i do find usefulness in the concept of “solar” seasons, i.e. solar summer being the 3 months centered on the summer solstice, etc. in which case the seasonal info would b accurate. this concept, if more widespread, would also help people when it comes to more practical things like applying sunscreen. most people have this unfounded belief that this should somehow be based on the temperature. for example, parents will apply sunscreen to the kids in my class when there’s a 90-degree day in September, but not on a 50-dgree day in April. but the sun is actually stronger in April (assuming equal cloudiness) since the only real factor in the sun’s differing ability to burn is its height in the sky, and whether there is anything blocking its rays such as clouds or your roof or whatever. if u are applying in September u should be applying in April, and if you’re not in April u shouldn’t be in September, for consistency’s sake. It’s the little things that bug me sometimes. ok end of rant.

    1. astrobob

      Good point about the “solar” seasons, but the astronomical version rules I think because of its precision and scientific usefulness.

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