Kelly Lepo
Happy #Solstice!
Now is a great time to remind everyone about the #ReasonForTheSeasons.
In other words, why is it cold in the winter and hot in the summer? (1/)
Image Credit: Eirik Solheim https://eirikso.com/2011/01/04/one-year-in-one-image/
#WinterSolstice #WinterSolstice2022
Kelly Lepo
It's not just that the days are longer in the summer and shorter in the winter. The angle of the sun is also important!
If you take a time-lapse photo of the sun over the course of the year, you will see that the sun is higher in the sky at noon in the summer and lower in the sky in the winter.
This is a Solargraph photo of Kanonia Street in Warsaw, a 129-day exposure. Lines are the path of the Sun, gaps are from clouds. (7/)
Credit: Sharkovitzz/Wikimedia Commons: https://commons.wikimedia.org/wiki/File:St._Kanonia_Bell.jpg
Kelly Lepo
Note the tilt does not bring the Earth significantly closer or further away from the Sun! The angle is the important part, not the distance.
The Earth's orbit is almost (but not quite) a perfect circle. It is only slightly elliptical.
The Earth is also closest to the Sun in January, so distance cannot be the reason it’s cold in the winter in the Northern Hemisphere. If the distance determined the seasons, we would also not have opposite seasons in the Northern and Southern Hemispheres. (6/)
Kelly Lepo
So why does this happen? The Earth's axis is tilted ~23 degrees relative to its orbit. The axis always points in the same direction, so as the Earth orbits around the Sun, sometimes the north pole is tilted towards the Sun and sometimes the south pole is.
(5/)
Credits: MIT/Alissa Earle
Kelly Lepo
Here is a time-lapse of the Earth, as seen from a geostationary satellite. Looking at the Northern Hemisphere, night is on the left and day is on the right.
This matches what we see from the ground. The northern Hemisphere gets more daylight hours for half the year and simultaneously the Southern Hemisphere gets fewer hours of daylight. This reverses after six months. On the equator, the days are always about the same length.
(4/)
Credit: NASA Goddard https://svs.gsfc.nasa.gov/11664
Kelly Lepo
Let's look at the length of the day in each location:
Dark is night, light is day
Looking at these graphs we notice:
1) When the days are longer in Montreal, they are shorter in Melbourne, and vice versa.
3) Quito has more or less the same amount of sunlight all year long
(3/)
Kelly Lepo
Here is a graph of the average temperatures in Montreal, Canada, in the Northern Hemisphere, Quito, Ecuador, which as the name suggests, is almost on the equator, and Melbourne, Australia, in the Southern Hemisphere.
You should notice a few things:
1) When it is hot in Montreal, it is cold in Melbourne, and vice versa.
2) The temperature at each location shows a predictable, yearly cycle
3) Quito is more or less the same temperature all year round
(2/)
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