The month of December always marks the beginning of winter in the northern hemisphere. This year that happens at 5:23 p.m. on Friday the 21st. That moment marks the lowest point the sun will reach in our sky for the whole year, giving us our longest night and shortest day.

If you photograph the sun every few days from the same location at noon for a year, you would see it traces a figure eight in the sky. That’s called the analemma. You could also create this shape by tracing the length of the shadow of a stick in the earth at consistent times.

The analemma is partly tilted on its side and the top part is narrower than the bottom part of this figure 8. The exact tilt and shape of this figure 8 conveys a lot of information about your latitude on the earth, our axial tilt of 23.5 degrees, and the fact we are orbiting the sun in ellipses, not circles. It even tells you we are moving faster around the sun in January when we are closest to it at perihelion and slower in July when we are the farthest away from it at aphelion. That part of it is called the equation of time.

There are many great highlights this month to make the long, cold nights more enjoyable. They include some very close planetary conjunctions, the best meteor shower of the year, a close approach of a comet at perihelion, another asteroid at opposition, and great new pictures and data from NASA’s Insight Mars mission after a scheduled landing in late November.

Just as we lost Venus and then Jupiter over the past two months, we will lose Saturn to the sun’s glare by the end of the first week of this month. That will leave only Mars, the last one in the great planetary lineup of the four brightest planets that we were able to see for most of last summer and into fall. Seeing those four bright planets for that long a time really gave us a better understanding of the relative motions both of our next-door neighbors in space, Mars and Venus, and a better idea of how much slower Jupiter and Saturn move and why that’s true.

Mars will be in the news quite a bit this month. The Insight lander will have just landed on the Martian surface and will be getting ready to start working for us to better our understanding of certain aspects of the planet. Insight is an acronym standing for Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport. That gives you a better sense of what it’s designed to discover about Mars. It should be able to reveal more about the inner, molten layers of Mars and figure out how fast the planet is losing heat to the coldness of space. Its seismometer will be able to record Marsquakes and asteroid impacts. They expect to detect about 50 quakes per year, which is about 100 times more activity than the moon has. They also expect about 10 asteroid impacts that will be powerful enough to measure.

Insight will be landing in a very flat and plain part of Mars near the equator to maximize the amount of sunlight it can receive, not far from where the Curiosity Rover landed over six years ago in August of 2012. Unlike the more exciting roving missions across parts of the Martian surface, the Insight mission will just sit in place for over a year and do its work. Insight will dig about 10 to 15 feet into the surface to get good data on heat flow in the soil. The Phoenix mission near the North Pole of Mars previously had dug only about one foot into the soil. So when you look at the orange light in our sky that is Mars, be sure to make the connection that we have just landed a new mission up there that will further our understanding of that planet, and in turn help us to appreciate our own planet even more.

Mars will race across the sky from Aquarius into Pisces this month. It will be setting around 11:30 each night. We are getting farther ahead of Mars in our orbits, but Mars is moving at about the same rate that the earth is orbiting the sun, one constellation per month, so the result will be that Mars always will appear to set at the same time even though everything is in constant motion.

Venus made a steep and dramatic climb into our morning sky last month as it reached its peak brightness. Venus will start fading a little this month as it races farther ahead of Earth. Look for two other planets to join Venus in the morning sky after the first week of this month. Mercury will show up first and then Jupiter. Keep watching as Jupiter will pass within less than one degree of Mercury on Friday the 21st, the day winter begins. After that Jupiter keeps climbing and Mercury sinks back into the horizon again.

The annual Geminid Meteor shower peaks this month on Thursday night the 13th into Friday morning the 14th. This is usually the best meteor shower of the year; we could get up to 100 meteors per hour this year from a dark sky site.

The moon will be near first quarter, so it will set by midnight and be out of the way by the time the shower really gets going as we pass through more debris from this rare type of asteroid called a rock comet named 3200 Phaethon. All the meteors will appear to radiate from a point in Gemini just above Castor, which is part of the most prominent group of stars in the winter sky called the winter hexagon.

Comet Wirtanen will be climbing into Eridanus the River, just below Taurus the Bull this month as it reaches perihelion on the 12th. It will be less than 7 million miles away at that point and may even become bright enough to see without any optical aid. After that it will keep climbing into Taurus, but it will keep fading out. It orbits the sun in just under six years.

There will also be another asteroid at opposition this month on the 27th. It will cross from Monoceros into Orion that evening. This is the fifth-brightest of all the millions of asteroids in the belt between Mars and Jupiter, with a diameter of 115 miles. It’s believed that this single asteroid could be the source of over one quarter of all of the meteorites that impact the earth. It is a chondrite with a high nickel-iron content of around 30 percent.

DECEMBER HIGHLIGHTS

Dec. 7: Gerard Kuiper was born on this day in 1905. The New Horizons spacecraft will fly by another Kuiper Belt object on the first day of 2019. New moon is at 2:22 a.m. Mars and Neptune will be much less than one degree apart in the evening sky.

Dec. 11: Annie Jump Canon was born on this day in 1863. An important member of the “Harvard Computers,” Canon helped establish a stellar classification system. A book by Dava Sobel called “The Glass Universe” tells this very interesting story of discovery.

Dec. 14: The Geminid Meteor Shower peaks this morning. The moon and Mars are less than 4 degrees apart.

Dec. 15: First-quarter moon is at 6:50 a.m.

Dec. 17: On this day in 1903 the Wright Brothers flew an airplane a few meters on a beach in North Carolina. Less than 66 years later we would fly two humans all the way to the moon.

Dec. 21: Winter starts at 5:23 p.m. Jupiter and Mercury will be less than one degree apart in the morning sky.

Dec. 22: Full moon is at 12:50 p.m. This is also known as the Cold, Long Night, or Moon-Before-Yule.

Dec. 25: Isaac Newton was born on this day in 1642.

Dec. 27: Johann Kepler was born on this day in 1571. He developed his three laws of planetary motion, working closely with Tycho Brahe.

Dec. 28: Sir Arthur Eddington was born on this day in 1882. Among many other accomplishments, he took a photograph of a star behind the sun during a total solar eclipse on May 29, 1919 to prove Einstein’s Theory of General Relativity to be exactly correct.

Dec. 29: Last-quarter moon is at 4:36 a.m.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.

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