The Winter Solstice, 2021 as it appears (without the Earth’s atmosphere). Note the striking planetary line-up with the Milky Way’s galactic center as a backdrop, 25,000 light-years distant. Note the separation between the Ecliptic and Celestial Equator, an angular separation exactly equal to the Earth’s axial tilt. Venus, Saturn and Jupiter are well placed at twilight, following the sun after sunset. Image via Stellarium.
This Year’s Winter Solstice Occurs on Tuesday, December 21, 2021 at 10:59 am EST
The sun is at its lowest point on the sky at mid-day during the third week in December in the northern hemisphere and represents the ‘beginning of winter’. The two Equinoxes (Vernal or ‘Spring’, 3rd week in March and Autumnal, 3rd week in September) occur when the sun crosses the Celestial Equator at the midpoint between these highest and lowest points of the sun’s apparent travel across the sky (the two solstices). The Celestial Equator represents the projection of the Earth’s equator on the sky. It should be noted that the southern hemisphere experiences the same seasons but at opposite times during the year. Summer in Australia occurs during December and January while Winter occurs during June and July.
A Striking Planetary Alignment
If we could remove the earth’s atmosphere and look due south on the date of this year’s Winter Solstice, we would see five of the eight major planets all lined up along the ecliptic. Three of them, Venus, Saturn and Jupiter are still prominently placed at astronomical twilight (~60 minutes after sunset) with the other, Mars, preceding the sun before sunrise. The waning gibbous moon rises at 7:00 PM, EST with Orion, the mythical centurion, to the south.
With Orion, the celestial hunter to the south, the waning gibbous moon rises at 7:00 PM, EST on the night of this year’s Winter Solstice, December 21, 2021. Image via Stellarium.
The Angle of Insolation and the Shortest Day and Longest Night
Often referred to as the ‘beginning of winter‘, a generally subjective term, the Winter Solstice in the northern hemisphere is the day the sun is at its lowest point above the southern horizon at at mid-day. In practical terms, this is the moment during the year when northern hemisphere inhabitants receive the least amount of energy from the sun. This angle above the horizon, known as the ‘Angle of Insolation‘, determines how much energy is received from the sun at a given location. This angle of insolation derives from the earth’s axial tilt of 23.5 degrees from the vertical. With the axis tilted and always pointing towards the same point on the sky, the energy from the sun each hemisphere receives changes during the year. The Winter Solstice also marks the calendar as the shortest day and, correspondingly, the longest night.
Distance From the Sun
The earth’s distance from the sun does not play a major role in seasonal climactic change. The Earth is at it’s perigee during January and at its apogee during July. All orbits are elliptical with a closest point and most distant point; the closest point is the perigee or perihelion and the most distant point, the apogee or aphelion when it pertains to the sun.
December 25
It’s generally accepted that December 25th was not the day Jesus was born but the day that closely corresponded with the ancient Roman celebration of Saturnalia, the festival and holiday honoring the Roman god Saturn. All sanctions were lifted during this festival, permitting the Christians to likewise celebrate the birth of Christ unbeknownst to the Romans. That this day also corresponds with the Winter Solstice is no coincidence. The Romans observed the sun’s elevation at mid-day slowly increase with each day after the winter solstice; the sun, the bringer of life and warmth, was returning and thus, a cause for celebration.
It is also generally accepted that Jesus was born in the early Spring of 7 BC. Please see this article for all the details and full account of what ‘The Star of Bethlehem‘ actually was.
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