September 23rd is the first day of the fall season, beginning with the autumnal equinox at 4:21 AM. There are a few ways of looking at this day. First, it has been celebrated around the world from harvest time to ancient pyramid shadow alignments. This day, the sun is at the halfway point between the summer and winter solstice. For the northern hemisphere it marks the first day of fall, and an increasing drop in daily temperature as the days get shorter. But is there equal daylight? It is equal gravity? Can you stand an egg on end? Here is a look at everything I felt relevant for fellow science geeks to eat up and teachers to share with their students. This includes a few videos as well.
If we plot the angle of the mid-day sun (as seen here), the Autumnal Equinox is a day the sun’s ray’s are focused directly on the equator. With respect to the Northern Hemisphere, this angle is halfway between the highest point during the summer solstice (June 21), and the lowest point during the winter solstice (December 21). So if you were to plot how high the sun gets in the sky all year long, this day is half-way, but going lower. This change is due to the tilt of Earth on it’s axis at 23.5 degrees. So as we orbit the sun, the Northern Hemisphere either points towards or away from the sun, resulting in a higher or lower sun angle as seen in the image at the top of this post.
Why Do We Call It Fall?
This is a question now both of my boys are old enough to ask. I like to keep the options open for the young kids, so to them and during my school assemblies I say:
- The leaves begin to fall
- The temperature begins to fall
- The night begins to fall, earlier with speed this time of year.
Note: I give a similar answer for the reasons behind the name Nor’Easter: A coastal storm that
- moves towards the northeast
- heads into the Northeastern US
- and sends cool, damp northeasterly winds inland.
Sometimes more than one answer fits.
Temperature Fall
As the days get shorter and the nights get longer, the average temperature begins a drop of about one degree every two days for the rest of this month.
The weather almanac for Baltimore shows the normal high temperature today at 75F. If you were to compare that to the the other ‘equinox’ in March, the average temperature is 51F. Much cooler despite the same sun angle. This is due to seasonal lags of temperature. On any given day, the sun is highest in the sky between noon and 1 PM, but the hottest daily high would be between 3 to 5 PM. Our strongest solar rays are on June 21, but the hottest temperatures are the middle of July, weeks later. In short, there is a delayed response of warming, but also cooling. Since the ground and nearby water ways are still warm from summer, that residual heat helps delay our cooling. The upcoming longer nights will change that quickly.
Over the next 30 days, we will lose 10 degrees and expect a high of 65F by the third week of October. The cooling rate slows down a little and averages out to one degree cooler every three days.
Fall Begins: Equal Is Not Equal Sunlight
The word equinox sounds a lot like ‘equal’, and that is how it is often introduced in an earth science class. I was originally taught that it was the day when the entire planet also had equal time of daylight and darkness as well. That is not entirely true.
In Baltimore:
- Sunrise at 6:55 am, Sunset at 7:02 pm
- 12 hours and seven minutes of daylight.
Sure, the nights are getting longer. In fact this is the fastest change we can see in our region all year, as we are now losing about 3 minutes of daylight each day. But, you have to consider that light bends in our atmosphere. This image shows sunrise as seen from the International Space Station. The glow of light appearing above the sun shows how light bends around our atmosphere before you see the actual sun. Think about the red and orange sky in the morning and evening, that is a demonstration of how light bends in our atmosphere, especially at dawn and dusk. The view of the sun itself is also seen as the light bends around the horizon. So while the sun itself should be in our view for 12 hours today (splitting the time with darkness), the light of the sunlight bends in the atmosphere and gives us an extra few minutes of daylight on each end of equinox day.
Push forward three days to September 26
- Sunrise at 6:58 am, Sunset at 6:57 pm
- 11 hours and 59 minutes of sunlight, 12 hours and 1 minute of darkness. Nighttime will win.
The coolest thing seen in space: Sunrise from NASA and ISS
Stand An Egg On End? Some teachers in my High School tried to convince the class that the equal force of the sun’s gravitational pull on the equator would allow an egg to stand on it’s end during an equinox. Simply put, that is not true! It’s a demonstration I continue to see in both autumn and spring. I actually tried this many years ago on TV in Binghamton, NY. Since I could not duplicate it, I used double sided scotch tape to keep the egg upright during the broadcast. The truth: If you have the right egg, a flat spot to sit it on… just add patience and a steady hand will do the balancing any day of the year. I’m not the only one to debunk this. Check out this old video I found online showing a similar demonstration.
Polar seasons: Today, technically the sun will set on the north pole to begin 6 months of darkness. It will look more like twilight for about a month. On the flip side, the sun will rise on the south pole. Side note: Considering the heat and Arctic melt that was discussed at great length this summer, there has been record ice building and expansion in the Antarctic. Like last winter, we do expect a rapid recovery of rebuilding ice around the North Pole in the next few months, thanks to the darkness and building chill. Then the fun part begins, how it may relate to our winter and snowfall.
Video: National Geographic Describes Equinox Around The Globe
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