• The Christmas Star of 2020

An Astronomical Coincidence

December 21, 2020, was a special day when it comes to the planets and the sun.  It was the winter solstice on the same day as a “Great Conjunction.”

  • Conjunctions occur when two astronomical bodies appear close together.
  • A conjunction of Jupiter and Saturn is called a “Great Conjunction.”  On average they occur every ~ 20 years.
  • Every year on December 21-22, the winter (summer) solstice occurs in the Northern (Southern) Hemisphere.
  • Solstices are due to the tilt of the Earth’s axis of rotation relative to the plane of its orbit revolving around the Sun.
  • The Great Conjunction and Winter Solstice are unrelated to each other.

It was an astronomical coincidence that the 2020 Great Conjunction occurred on the same day as the Winter Solstice.

Diagram of the Great Conjunction of Jupiter and Saturn

During a Great Conjunction, the angle between Jupiter and Saturn as seen from Earth usual range between [0.5 deg, 1.3 deg].  The 2020 Great Conjunction was the smallest visible separation angle, with a separation of 0.10 degrees, since the year 1226 in the Middle Ages!

Sun setting on the horizon on Winter Solstice 2020 by Cerro Pedernal, New Mexico

Winter Solstice Sunset

December 21, 2020, 4:54 PM local time, Cerro Pedernal, New Mexico

Great conjunction Jupiter-Saturn setting on top of Pedernal, New Mexico

“Christmas Star on Pedernal”

December 21, 2020, 7:11 PM local time, Cerro Pedernal, New Mexico

It is all about angles

Although Jupiter and Saturn appeared close to each other during the Great Conjunction, I could still see a definite separation of the two planets, even with the naked eye. This was predicted.

  • The 2020 Great Conjunction was the smallest angular separation (6 arcminutes) of Jupiter and Saturn visible since 1226 (2.1 arcminutes).
  • There are 60 arcminutes per degree, so 6 arcminutes are equivalent to 0.10 degrees.
  • The 2020 Great Conjunction angular separation of Jupiter and Saturn was approximately 1/5 of the moon’s angular diameter as seen from Earth.
  • Human eyes can resolve stars separated by 1 – 4 arcminutes apart.
  • As the Great Conjunction appeared lower on the horizon the planet’s separation appears larger due to the Moon Illusion effect.

The tilt of the Earth’s axis results in solstices, equinoxes, and seasons.

Composite photograph of the Moon and the Great Conjunction I took on December 22, 2020.  This composite photo (not a real scene that one would actually see!) is helpful to see the angular size of the conjunction. Jupiter and its moons are on the left, Saturn on the right.

The Christmas Star

Scientists have researched whether or not the Star of Bethlehem was actually a Great Conjunction.  Conjunctions of Jupiter with the bright star Regulus in the years 2 BC and 3 BC have been investigated in the context of the Gospel of Matthew and the Star of Bethlehem.

Looking through the mosquito netting on my backpacking tent in the night sky, I notice that bright stars have a “cross” appearance to them.  The screen causes the star’s light to diffract (bend) around the screen’s material and causes diffraction spikes.   You can also see diffraction spikes when looking at stars with your naked eye and squinting — the result of diffraction due to your eyelashes.

It is perfectly reasonable that historic viewers of the Great Conjunction saw diffraction spikes.  Historical renditions of Star of Bethlehem depict the conjunction with what looks like diffraction spikes.

To demonstrate this effect, I include a photograph I took with a Bahtinov Mask which creates diffraction spikes to help photographers manually focus on distant stars.

Image of Jupiter (left) and Saturn (right) on December 22, 2020, barely shows The Rings of Saturn.  To see Saturn’s rings, I had to limit the amount of light reaching the sensor, using a faster shutter speed (1/60 sec, f/5.6, ISO 1000, 1344 mm lens).   At this lower exposure, the moons of Jupiter are not visible.

Photo of the Great Conjunction looking through a Bahtinov Mask to demonstrate an extreme diffraction star effect.

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