The 2020 winter solstice occurs on Monday, December 21st, at 2:02am pacific time. It happens, of course, at the exact same moment around the world, and you can see what time it will occur in your neck of the woods here. The solstice and the seasons occur because the Earth is tilted 23.5 degrees away from the sun, and as it becomes cooler in the north, the Earth tilts gradually away from the sun. This means that the southern hemisphere becomes warmer and experiences summer when in the north hemisphere experiences winter. The solstice happens when the tilt is at its most extreme angle. (For an easy-to-follow explanation of the seasons, click here.)

The solstice is a rather unimpressive event, as opposed to a lunar or solar eclipse. You probably won’t see any dramatic shifts in the skies, but at noon, shadows made by the sun will be the longest of the year, because the sun’s arc in the sky has been dropping lower and becoming shorter since June. You may also notice that the sun seems to be rising and setting in the same place for the same reason. This is actually how the word “solstice” came to be – it means “sun stands still” in Latin. National Geographic has a great explanation of how a solstice occurs here. 

In the northern parts of Europe, Christianity became increasingly popular starting around the 2nd century CE, and the pagans (non-Christians) converted or were forced to convert to Christianity. Historian David Gwynn of the University of London thinks it’s no coincidence that Christmas began to be celebrated on December 25th, as it was closest to the winter solstice, which the ancient pagans would celebrate.

But who figured out what the solstice actually is? The answer to that question is not an easy one, but we do know that in about 830 CE, Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity (tilt) of the Earth, and the result was used in the Arab world for many years. In 1437 CE, Ulugh Beg determined the Earth's axial tilt as 23°30′17″ (23.5047°), and as mentioned before, the winter solstice occurs as the Earth tilts further away from the sun. Without the sun, there would be no solstice, and centuries later, a notable woman in science, Cecilia Payne (1900-1979), made a very important discovery about just what the sun - and therefore, other stars - are primarily made of: hydrogen and helium. She made this discovery while in graduate school at Harvard University in Cambridge, Massachusetts. She sent her thesis on this topic to a Professor Russell at Princeton, who said that her result was "clearly impossible," but she persisted and turned her thesis into a book called Stellar Atmospheres, which was well-received by astronomers, and within years, it was determined her results were fundamental and correct. She wrote her doctoral dissertation at Harvard, and was the first person to receive a PhD from Radcliffe College at Harvard. Without Cecilia Payne, we would not have some of the most important information about the foundations of the stars in the universe, and we would not be able to appreciate the sun in all of its glory, including at the solstice!