For some solstice excitement, or at least a little connection to how some ancient people focused on the sun by watching this special sunrise, turn into the live feed from Stonehenge. It’s the first sunrise after the Winter solstice.
It's official! Happy December Solstice! Seasons greetings! May all your days be sunny & bright!
It happened 15:58 GMT • 10:58 am EST • 9:58 am CST • 8:58 MST • 7:58 am PST! pic.twitter.com/ShtmJ4db5A
— C. Alex Young, Ph.D. (@TheSunToday) December 21, 2021
On the June solstice, the Sun reaches its highest point in the sky for an observer at the North Pole. On the December solstice, the Sun reaches its highest point in the sky for an observer at the South Pole. Solstices also mark the change from fall to winter or spring to summer.
The solstices happen because of the tilt of Earth’s axis of rotation with respect to its orbital path around the Sun. The axis of rotation is an imaginary line drawn straight through Earth connecting the North Pole to the South Pole. This line is tilted 23.5° from Earth’s orbital path around the Sun. This means that the Sun’s light does not hit the Earth evenly at a particular latitude above and below the equator. This tilt is the cause of Earth’s seasons.
The equator is an imaginary line drawn right around Earth’s middle, like a belt. It divides Earth into the Northern and the Southern Hemispheres.
The imaginary lines around Earth that are parallel to the Equator are called lines of latitude. They are numbered from 0° to 90°. The one at 0° is the equator itself. The higher the number, the farther north (if it’s a + number) or south (if it’s a − number).
Because of Earth’s tilt is 23.5° the latitudes of +23.5° and −23.5° are special and have special names. The one in the Northern Hemisphere, +23.5° latitude, is called the Tropic of Cancer. The one in the Southern Hemisphere, − 23.5° latitude, is called the Tropic of Capricorn.
These are the latitudes where the Sun is directly overhead at noon once a year. In the Northern hemisphere, on the Tropic of Cancer, that is the June Solstice. In the Southern Hemisphere, on the Tropic of Capricorn, that is the December Solstice. These solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year.
Science communicator Dr. James O’Donoghue creates amazing visuals. He created two visuals to illustrate the reasons for the seasons, solstices, and equinoxes.
Here is an animation of the Sun’s seasonal progression through the sky created by Sam Cornwell and shared at the NASA Astronomy Picture of the Day. It was made using solargraph images from a Solarcan camera. The first frame was recorded around December 21, 2018 and the last around June 21, 2019. The gif shows 27 frames.
The most important aspect of the seasons and the light (and heat) received by different parts of Earth at different times of the year all comes down to geometry. That is the geometry of a sphere.
Another thing to think about for this geometry is the location of the objects that we can see in the night sky (or the sky near sunset and sunrise).
Something that is a bit counter to our perspective and our senses is that we are all standing on a globe and our perspective looking to the sky depending upon where we are on the globe. A great way to see this is to follow the planets visible at night.
If you have a chance, maybe even view them from the Northern and Southern hemispheres, and compare what you see. That second part can be a bit difficult but we can use tools to help us see the differences.
The stars which a super far away rotate with us through the night and the planets follow the ecliptic (some planets are slight off the ecliptic but they basically follow it).
The moon’s motion in the sky is even more complex because the moon itself has a separate motion above and below Earth’s rotation of 5 degrees or more than 10 moons in size above and below the rotation plane of Earth (the moon is about a half a degree in size so 5 degrees is 10 moons).
We can take a look at where in the sky planets are visible from anywhere on Earth using the TimeandDate.com Night Sky Tool.
During the beginning of December 2021 three, very bright planets—Venus, Saturn, and Jupiter—were visible in the sky shortly after sunset. These are still visible for a few more weeks, right after sunset, along with Comet Leonard – Don’t miss them!!
These planets are a great reference point for this geometry, which we will be talking about in some future posts. (NOTE: The Moon moves every night, so it won’t be in this location – use the TimeandDate.com Night Sky Tool or an app like Star Walk to figure out what you will see.
Here is what those planets looked like from the Northern Hemisphere near Baltimore, MD, the USA on December 7, 2021, 1 hour, 37 minutes after sunset. Venus was visible just below and to the right of the Moon while Saturn was just above the Moon and Jupiter was above and to the left. The line in the image is the ecliptic. This is exactly what people saw that night in the northern hemisphere. Here’s how the map was made.
Here is what it looked like in real life. It’s dark outside (and mostly cloudy). The sunset was at 4:42 pm (the earliest of the year)! This photo was taken at 6:19 PM. The photo lighting has been tweaked to better show the planets.
If we look on the same day just after sunset in the Southern Hemisphere between the tip of South America and Antarctica, the sky would appear different, flipped. Now Venus is below and to the left of the Moon. Saturn is just below the Moon and Jupiter is below and to the right. We can also see the constellations are flipped with respect to the ecliptic line. Here’s how the map was made.
We can see this exact alignment if we look at this photo taken from a ship returning from the December 2021 Antarctic eclipse. This image was taken at the exact time as the picture above from a similar location. Notice that the sunset was at 9:59 pm (the latest of the year)! It’s not completely dark outside, even though it’s 11:03 PM. A future post will explore the geometry of this, comparing locations. Stay tuned.
And now for the professional version, so much better than ours, taken with iPhones…
Explanation: It is not a coincidence that planets line up. That’s because all of the planets orbit the Sun in (nearly) a single sheet called the plane of the ecliptic. When viewed from inside that plane — as Earth dwellers are likely to do — the planets all appear confined to a single band. It is a coincidence, though, when three of the brightest planets all appear in nearly the same direction. Such a coincidence was captured earlier this month. Featured above (right to left), Venus, Saturn, and Jupiter were all imaged together in a line just after sunset, from the San Fermo Hills, Bergamo, Italy. Joining the alignment are Earth’s Moon, and the position of the more distant Uranus. Bands of clouds streak across the sky toward the setting Sun. As Comet Leonard fades, this planetary alignment — absent the Moon — should persist for the rest of the month. Learn more at APOD »
Find ISS viewing times here – you can get alerts for when it’s above the tree line. However, on this night of looking for the planets, it completely surprised me, a minute after taking some photos. I only had about 3 minutes total with no clouds! A neighbor & I were looking at this and we noticed a small bright dot about the size of the planets moving towards the left, right through Saturn, then into the clouds. My Star Walk app confirmed it was the ISS!! What a fun surprise!!