Since the summer solstice in June, the days have been gradually shortening and the nights lengthening in the Northern Hemisphere. However, this trend is about to change. On Saturday, December 21st, we will welcome the winter solstice, the day with the least daylight of the year and the official start of winter for the majority of the world's population.

The phenomenon of solstices, along with the vernal and autumnal equinoxes, has captivated curious minds for millennia. It is a day where scientific principles blend with ancient customs across the globe. The winter solstice signifies the shortest day and the longest night in the Northern Hemisphere, with the sun positioned at its southernmost point directly above the Tropic of Capricorn. Conversely, in the Southern Hemisphere, where only about 10% of the global population resides, the December solstice ushers in the longest day of the year and the onset of summer in countries such as Australia, Botswana, and Chile.

The solstice typically occurs on December 21st, but it can vary by a day due to the discrepancy between the solar year and our calendar year. For precise observation, the exact time of the 2024 winter solstice will be 9:21 Coordinated Universal Time (UTC) on Saturday, as reported by Earthsky.org and the Royal Museums Greenwich. Here are some examples of the local times corresponding to 9:21 UTC in various locations across the Northern Hemisphere:

• Yokohama, Japan: 6:21 p.m. Saturday  
• Bangkok, Thailand: 4:21 p.m. Saturday  
• New Delhi: 2:51 p.m. Saturday
• Baghdad, Iraq: 12:21 p.m. Saturday
• Helsinki, Finland: 11:21 a.m. Saturday
• Lisbon, Portugal: 9:21 a.m. Saturday  
• Halifax, Nova Scotia: 5:21 a.m. Saturday    
• Philadelphia: 4:21 a.m. Saturday  
• Mexico City: 3:21 a.m. Saturday
• Portland, Oregon: 1:21 a.m. Saturday
• Honolulu: 11:21 p.m. Friday

To determine the timing for your specific location, EarthSky provides a convenient conversion table, or you can enter your city in the “Converted Time” box on their website. Please note that some websites may post times that differ by a minute or two from the official time set by the Royal Museums Greenwich in England.

The effects of the winter solstice are most pronounced closer to the North Pole, where daylight hours are significantly reduced. In Singapore, which is only 85 miles north of the equator, the difference in daylight is minimal, with approximately nine fewer minutes compared to the summer solstice. The day remains nearly 12 hours long throughout the year. In London, despite the cold, there are still over seven hours of daylight for a stroll along the Thames. The contrast is more dramatic in Oslo, Norway, where the sun rises at 9:18 a.m. and sets at 3:12 p.m., providing just under six hours of weak daylight. In Nome, Alaska, residents will experience only three hours and 54 minutes of very weak daylight. However, this is quite generous compared to Prudhoe Bay, Alaska, which lies within the Arctic Circle and will not receive any sunlight at all.

The Earth's tilt on its rotational axis is responsible for the changing seasons. As the planet orbits the sun, each hemisphere experiences winter when it is tilted away from the sun and summer when it is tilted towards the sun. The exact cause of this tilt is not fully understood, but it is believed that violent collisions billions of years ago, as the solar system was forming, may have caused the Earth's axis to tilt.

It is no coincidence that many cultures and religions celebrate holidays around the time of the winter solstice, such as Christmas, Hanukkah, Kwanzaa, and pagan festivals, marking the return of longer days. Ancient peoples, whose survival depended on a precise understanding of seasonal cycles, commemorated the first day of winter with elaborate ceremonies and celebrations.

These festivities symbolize renewal and are deeply rooted in spiritual significance. "Christmas has adopted many customs and likely its date from the pagan Roman festivals of Saturnalia and Kalends," says Maria Kennedy, an assistant teaching professor in the Department of American Studies at Rutgers University, who specializes in Christmas studies. Saturnalia began on December 17th, and Kalends began on January 1st. Early Christian church founders condemned these holiday practices, but their popularity persisted. The Christian observance of Christmas eventually aligned with the same time of year, despite the absence of a specific birth date for Jesus in the Gospels.

Here are more details on some of these ancient customs:

In Welsh, "Alban Arthan" translates to "Light of Winter," and it may be the oldest seasonal festival known to humanity. As part of Druidic traditions, the winter solstice is a time associated with death and rebirth. Newgrange, a prehistoric monument in Ireland built around 3200 BC, is connected to the Alban Arthan festival.

In Ancient Rome, Saturnalia lasted for seven days and honored Saturn, the god of agriculture. The festivities resembled modern Mardi Gras celebrations, with a temporary lifting of moral restrictions and even a cessation of war-making. Slaves were granted a brief period of freedom during this time. Saturnalia continued until the third and fourth centuries AD.

The marking of the annual occasion was not limited to ancient Europeans. The Dongzhi Winter Solstice Festival has its roots in ancient Chinese culture, with the name meaning "extreme of winter." This festival is considered the peak of yin, representing darkness, cold, and stillness, thus coinciding with the longest day of winter. Dongzhi signifies the return of yang and the gradual increase of light and warmth. In some East Asian cultures, dumplings are traditionally eaten to celebrate this festival.