Christmas story, astronomy meet in Monday sky
from the December 21, 2020 Tallahassee Democrat
We’ll experience a rare astronomical trifecta tonight: The winter solstice, the peak of the Ursid meteor shower and the Great Conjunction of Jupiter and Saturn.
All on the same day, folks. For astronomers, this is a big deal.
First, let’s look at the solstice, the moment the sun reaches its southernmost point in the sky and winter begins: 8:30 a.m. Monday this year.
Sure, winter here in Florida isn’t the same thing as winter in, say, Chicago. While the occasional cold front forces us to trot out our seldom-used gloves and scarves, we spend most of our winters without bundling up.
Even though we won’t have Jack Frost nipping at our noses Monday, we can actually mark the change of seasons by simply watching the sunrise and sunset and looking at a clock.
On the winter solstice, the sun rises and sets at its southernmost points. Monday, you’ll see the sun rise at a compass heading of 116 degrees and set at 244 degrees; on the summer solstice, the run rises at 64 degrees and sets at 296 degrees.
The first day of winter is the shortest day of the year.
Monday, we will have 10 hours and 24 minutes of daylight, compared to 13 hours and 53 minutes of daylight on next year’s summer solstice on June 20.
If you ask astronomers to rate the Ursid meteor shower, which peaks Monday night and after midnight Tuesday, they’ll probably give it a solid “meh,” because the Ursids produce only 15 to 20 meteors per hour.
What’s interesting about this meteor shower is its radiant (the point in the sky from which the meteors appear to radiate), which is Ursa Minor, the Little Bear, also known as the Little Dipper.
And what’s interesting about Ursa Minor is that its brightest star is Polaris, the North Star, which, unlike all other stars in the sky, never appears to move (it’s always due north of wherever you are in the Northern Hemisphere) and is the traditional symbol of steadfastness.
In Shakespeare’s “Julius Caesar,” for example, Caesar tells Brutus and the other conspirators that he is “constant as the northern star,” at which
point the conspirators stab him to death.
A coming celestial event that falls outside our Monday trifecta window
is the Quadrantid meteor shower, which peaks the night of Jan. 2-3 and
can produce up to 200 meteors per hour under perfect conditions. Unfortunately, a large waning gibbous moon that night will block out all but
the brightest meteors.
Meteor showers are named for the constellations they appear to radiate
from, and the funny thing about the
Quadrantids is that they’re named for
a constellation that no longer exists.
In 1795, French astronomer Jerome
Lalande decided that a group of stars
between the constellations Draco and
Boötes looked like an astronomical
instrument called a quadrant, so he
named those stars Quadrans Muralis
(Latin for “mural quadrant” – quadrants
were often built into walls),
thus creating a new constellation.
The Quadrantids were first recorded
in 1825 and, because the shower’s
radiant was Quadrans Muralis, it was
named for that constellation. But
when members of the International
Astronomical Union made an official
list of constellations in 1922, they
booted Quadrans Muralis right out of
the sky, but the name of the meteor
shower remained the same.
Of course, the winter solstice and
Ursids happen every year, but the
Great Conjunction of Jupiter and Saturn
happens only once every 20
years.
In astronomy, a conjunction is
when two celestial objects temporarily
appear to be close together, and
Monday, Jupiter and Saturn will be
rubbing elbows, just 0.1 degree apart,
or the width of a toothpick when held
at arm’s length.
What makes this conjunction special
is that it will be the planets’ closest
approach to each other since 1623;
that’s 397 years ago, and, because the
two planets were only 13 degrees east
of the sun that time, it’s likely that nobody
even noticed.
But we will be able to see the conjunction
in the southwestern sky after
sunset Monday, and if the weather
cooperates it will be very easy to see,
with Jupiter shining at a magnitude
of -1.98 and Saturn at +0.63 (remember,
the smaller the magnitude, the
brighter the object). Go out to see it
early – the conjunction will disappear
below the horizon at around 7:15 p.m.
Since this is the holiday season,
here’s some Christmas-themed astronomy.
After observing the Great Conjunction
of Jupiter and Saturn in December
1603, German astronomer
and mathematician Johannes Kepler
did some impressive retro number
crunching and determined that a
Great Conjunction occurred in 7 B.C.
Then Kepler threw in some astrological
symbolism: Jupiter is the
planet of kings; Saturn is the protector
of the Jews – there’s a link between
Saturn, whose name gives us
the word Saturday, and the fact that
Saturday is the Sabbath (Sabbath in
Hebrew is Shabbat, and Saturn in Hebrew
is Shabbatai).
In addition, the 7 B.C. conjunction
was a rare triple conjunction, which is
when two planets come close together
and move apart three times over a
period of several months, and such an
event would have been noticeable to
serious sky watchers of the time.
Finally, all three of the conjunctions
took place in the constellation
Pisces the Fish. Fish are highly symbolic
in Judaism, and the Torah uses
them to connote fertility and abundance.
With all these things in mind, Kepler
proposed that the Great Conjunction
of 7 B.C. was the star that led the
Magi, who were trained in astronomy
and astrology, to Bethlehem to worship
the “king of the Jews.”
As we approach the end of this tumultuous
year, Monday’s astronomical
trifecta reminds us that, as unpredictable
and downright wonky as our
daily lives can be, the mechanics of
the universe are thoroughly predictable
and not at all wonky.
Seasons change on time; meteor
showers peak on their established
dates; and every 20 years, Jupiter and
Saturn come together in a Great Conjunction.
And that is a comforting thought.