Fall is one of my favorite times of year! I love the warm, spice filled recipes and Fall foliage. It brings me so much joy!
I decided to make this week about Fall because I saw an opportunity to bring out my Astronomy background on this blog. I get asked a lot why is there an official first day of Fall, Winter, Spring, Summer and why we have seasons. This past Thursday, September 22nd was the first day of Fall. This marks an astronomical event which is the leading role in why we experience seasons. What does that really mean though? How do we determine this day?
Astronomers call the first day of Fall the Autumnal Equinox. Both at the Autumnal Equinox and the Vernal Equinox solar energy is equally distributed between the northern and southern hemispheres of the Earth because of its location in Earth’s orbit around the Sun. Looking the picture below, you can see that due to the tilt of Earth in its orbit the Sun’s energy coverage between the northern and southern hemispheres changes throughout its orbit. This is why we have seasons!
In June, the solar energy has a higher distribution over the northern hemisphere, while in December the distribution is higher in the southern hemisphere. Hence, the seasons are opposite between the northern and southern hemispheres. Also, something to note in the image above, in Summer time for that hemisphere the pole has 24hr daylight and the closer you are to the pole the longer your day is in Summer. Then in Winter, the pole has 24hr darkness and therefore the closer you are to the pole the shorter your day is.
The Solstices are when we begin to experience a reverse in how the length of daytime changes. The Summer Solstice marks the longest day of the year for the northern hemisphere and the Winter Solstice marks the shortest day of the year for the northern hemisphere. Now I also get questions about why the hottest days of the year do not coincide with the longest days of the year, this is because the land and oceans have not built up heat yet from these longer days to play their role in warmer temperatures. That is why peak temperatures do not match up with the longest days.
Hope you enjoyed my Astronomical post this week! Let me know if you have any questions and if you liked this post.