Astrologists say that today's winter solstice could be the worst day astrologically, as the sun passes over Capricorn, causing easy tasks to feel harder and annoyances to seem larger.
The organisers of the event write on their website: "The Winter Solstice is the most important day of the year at Stonehenge and a truly magical time to be there". The 21st is the shortest day of the year with only 9 hours, 12 minutes and 52 seconds of daylight.
Each hemisphere will experience winter when they are tilted away from the sun and summer when it is tilted towards the sun.
The date where Earth is the nearest to the star is marked by the summer solstice.
Exactly How Short Will The Day Be?O'Kelly stood inside the famous passage tomb as the light from the rising sun travelled 19m up the corridor to illuminate the central chamber.
Each year, the OPW runs a Winter Solstice Lottery - in 2017, it says, over 33,000 people applied from as far afield as Austria, Italy and the US.
You would think our temperatures will be at the coldest on the Winter Equinox but the truth is that there is a lag time in the cold.
Fáilte Ireland and the Office of Public Works have teamed up for the broadcast on the shortest day of the year. In other words, the sun reaches its southern-most position for those of us north of the equator. If you're farther north, your earliest sunsets are around December 7. Let us know how you'll celebrate sailing next June and share the anticipation for Summer Sailstice 2018 with sailors across the globe.
The sunrise on the day is at 8.04am.