AkaSci 🛰️Dec 21

Happy Winter Solstice, which will occur at 15:03 UTC (10:03 a.m. EST) today.

Today, earth's axis, which is currently tilted 23.44° from the plane of its orbit around the Sun (the ecliptic), will point away from the Sun.

It is the shortest day of the year in the northern hemisphere; which means that days will keep getting longer starting tomorrow 🥳

https://earthsky.org/astronomy-essentials/everything-you-need-to-know-december-solstice/
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Show threadAkaSci 🛰️

Below are images of Earth taken 6 days ago (Dec 21, 2025) and on June 19, 2025 near the last Summer Solstice, from the direction of the Sun.

The tilt of earth's axis at these two extreme points can be seen clearly.

These images were taken by the NASA DSCOVR satellite, which is located 1.5 million km away at the Sun-Earth L1 Lagrange point.

https://epic.gsfc.nasa.gov/?date=2025-12-15
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Dec 21 at 1:06pmWeb
Show threadAkaSci 🛰️Dec 21

This image of Earth posted by ESA was taken by the Meteosat MTG‑I1 weather satellite 2 days ago on 19 December 2025 at 06:00 GMT. It vividly shows Earth’s day–night terminator line tilted near its annual maximum of about 23.5°.

MTG-I1 (Meteosat Third Generation Imager-1) is located in a geostationary orbit (35,786 kilometers) at 0 degrees longitude, directly above the Prime Meridian.

https://bsky.app/profile/esa.int/post/3maimej6le22v
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Show threadAkaSci 🛰️Dec 21

The next (northern hemisphere) winter solstice on Mars will occur on April 25, 2026.

The axial tilt of Mars at 25.2° is close to that of Earth. The tilt causes seasons similar to earth.
A year on Mars is 687 Earth days long.
Seasons are not of equal length on Mars due to its orbital eccentricity.
The New Year starts on the day of the Spring Equinox.
Year 1 was designated to start on Apr 11, 1955.
This is year 38.

https://www.planetary.org/articles/mars-calendar
https://www.instagram.com/popular/james-o%27donoghue/
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Show threadAkaSci 🛰️Dec 21

An analemma is a plot of the position of the Sun as viewed from a fixed position on Earth at the same clock time every day for an entire year.

The lowest point of the analemma occurs close to the winter solstice in the northern hemisphere.

http://www.star.ucl.ac.uk/~apod/apod/ap130922.html
https://en.wikipedia.org/wiki/Analemma
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Show threadAkaSci 🛰️Dec 21
Also, see https://fosstodon.org/@AkaSci/115688248995273358 for
why the date of the earliest sunset of the year does not coincide with the winter solstice.
@rozeboosje
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Show threadAkaSci 🛰️Dec 22

Beautiful pic of the sunset at Stonehenge taken on December 20, 2025.

"There, through stones precisely placed 4,500 years ago, a 4.5 billion year old large glowing orb is seen setting.

Even given the precession of the Earth's rotational axis over the millennia, the Sun continues to set over Stonehenge in an astronomically significant way."

https://apod.nasa.gov/apod/ap251222.html
Image Credit & Copyright: English Heritage, Josh Dury
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Show threadPino CarafaDec 21
@AkaSci it's fascinating
Show threadPino CarafaDec 21
@AkaSci Also note that thanks to that "fat end" of the Analemma we've already started seeing the "grand aul stretch" in the evening happening since the 14th of December. Even though the days kept getting shorter, "solar noon" was moving to a later time on the clock so that the Sun had already started setting later.
Show threadMidgePhotoDec 21

@AkaSci
#NealStephenson the #SciFi author makes some use of them in his large interesting novel #Anathem

Which also has some echoes of what, perhaps, scientists might consider around this period of Administration of the USA.

#Analemma #Solstice

Show threadDolsonDec 21
@AkaSci are the “north” poles for other planets just chosen as the ones pointing roughly the same way as earth’s, or is there a more scientific way they designate the North Pole
Show threadaprilfolliesDec 21
@bcoffy @AkaSci The first one. We use Earth’s North Pole as the frame of reference. Of course you have planets like Uranus, tilted more than 90 degrees (“on its side”), where that becomes… tricky.
Show threadAkaSci 🛰️Dec 21
@aprilfollies @bcoffy
To add to the accurate explanation by aprilfollies - since 1982, the International Astronomical Union has defined the north pole of a planet to be the pole that lies north of the ecliptic plane (the plane of the solar system).
https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/why-dont-we-say-it-has-a-direct-spin-with-the-axis-tilted-82176-instead-of-98176/
https://www.astronomy.com/science/ask-astro-how-do-we-distinguish-north-and-south-poles-on-other-planets/
What defines a planet's north pole?

Uranus is often said to have a retrograde rotation with its axis tilted 98°. Why don’t we say it has a direct spin with the axis tilted 82°? Since 1982, […]

Sky & Telescope
Show threadStephanDec 22
@AkaSci These two pictures prove it: the equator can be seen from space!
🤗