This Week's Sky at a Glance, January 12 – 21

FRIDAY, JANUARY 12

■ In this coldest time of the year, the dim Little Dipper (Ursa Minor) hangs straight down from Polaris after dinnertime, as if, per Leslie Peltier, from a nail on the cold north wall of the sky.

The Big Dipper, meanwhile, is creeping up low in the north-northeast. Its handle is very low, but its bowl is nosing up toward the upper right.

SATURDAY, JANUARY 13

■ The waxing crescent Moon forms a long triangle with Saturn and Fomalhaut in late dusk, as shown above.

SUNDAY, JANUARY 14

■ The Moon, Saturn, and Fomalhaut now form a similar triangle to the one they did yesterday, but mirror-flipped on the axis from the planet to the star as shown above.

■ The Gemini twins lie on their sides these January evenings, left of Orion. Their head stars, Castor and Pollux, are farthest from Orion, one over the other. Castor is the top one. The feet of the Castor stick figure are just left of the top of Orion's (very dim) Club.

MONDAY, JANUARY 15

■ A fast-creeping red dwarf. Have you ever seen a red dwarf star? These are the most common stars in space, but they're so intrinsically dim that not one is among the 6,000 stars visible to the naked eye on even the darkest night.

One of the nearest lies just 3° west of Procyon, beautifully placed late these January evenings. It's Luyten's Star, also known as GJ 273, and at visual magnitude 9.9 it's in range of small telescopes. Use the finder charts with Bob King's article Catch Luyten's Star.

This humble object is only 12.3 light-years away, so it is also a high proper motion star; it creeps across its starry backdrop by 3.7 arcseconds per year. This means that a careful amateur telescope user may detect its motion in as little as about three years, writes King, "depending on its proximity to field stars and the making and breaking of distinctive alignments with them." He suggests, "Make an initial observation, note the position in a sketch, map or photo, and then return a couple years later. Hey, no hurry."

To locate and identify Luyten's Star with King's charts you'll need to be fairly good at telescopic star-hopping. This is an essential skill for every amateur astronomer to master. See How to Use a Star Chart with a Telescope, and do be ready for a certain amount of frustration at first before you get the hang of it; everyone goes through this. Don't give up.

TUESDAY, JANUARY 16

■ The big Northern Cross in Cygnus, topped by Deneb, is nearly upright in the west-northwest right after full darkness falls. Another hour or so and it's standing on the horizon. How straight up it stands depends on your latitude.

WEDNESDAY, JANUARY 17

■ First-quarter Moon (exact at 10:52 p.m. EST). The Moon shines high in the south at dusk in early evening, with Jupiter at first about a fist to its left, then later to its upper left.

THURSDAY, JANUARY 18

■ The Moon, a day past first quarter, shines close to Jupiter. These are currently the two brightest things in the evening sky, and they are only 3° or 4° apart (for the Americas).

What are the odds that those two brightest objects in that whole expanse, from horizon to horizon, will appear so close together? Hint: Think carefully, it's tricky. And the trickiness of the question leads down a fascinating, and fundamental, rabbit hole.¹

FRIDAY, JANUARY 19

■ Zero-magnitude Capella high overhead, and equally bright Rigel in Orion's foot, have almost the same right ascension. This means they cross your sky’s meridian at almost exactly the same time: around 9 or 10 p.m. now, depending on how far east or west you live in your time zone. (Capella goes exactly through your zenith if you're at latitude 46° north: Portland, Oregon; Portland, Maine; Montreal; central France.) So, whenever Capella passes its very highest, Rigel always marks true south over your landscape, and vice versa.

SATURDAY, JANUARY 20

■ Sirius twinkles brightly after dinnertime below Orion in the southeast. Around 8 or 9 p.m., depending on your location, Sirius shines precisely below fiery Betelgeuse in Orion's shoulder. How accurately can you time this event for your location, perhaps judging against the vertical edge of a building? Of the two, Sirius leads the race early in the evening; Betelgeuse leads later.

SUNDAY, JANUARY 21

■ Right after dark, face east and look very high. The bright star there is Capella, the Goat Star. To the right of it, by a couple of finger-widths at arm's length, is a small, narrow triangle of 3rd and 4th magnitude stars known as "The Kids." Though they're not exactly eye-grabbing, they form a never-forgotten asterism with Capella.

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This Week's Planet Roundup

Mercury remains 11° lower left of bright Venus low in the dawn all this week, as shown above, and remains at magnitude –0.2.

Venus, magnitude –4.0, shines as the bright "Morning Star" in the southeast during dawn. It's getting lower every week. Nearby is not only Mercury but also sparkly orange Antares, magnitude +1.0. Look for Antares 10° to the right of Venus on the morning of January 13th. By the 20th, Antares is 17° to Venus's right or upper right, as shown above.

Mars, lower left of Mercury in bright dawn and a mere magnitude 1.4, is probably out of reach even with binoculars. Mars will creep up very slowly in the dawn for the next five months.

Jupiter, magnitude –2.5 in Aries, is the bright white dot very high to the south in early evening, less high in the southwest later at night. It sets around 1 a.m.

Saturn, magnitude +1.0 in Aquarius, sinks lower in the west-southwest during and after dusk and sets around 7 or 8 p.m. In late twilight, look for Fomalhaut twinkling nearly two fists lower to Saturn's lower left.

Uranus, magnitude 5.6 in Aries, awaits your binoculars in the darkness 13° east (left) of Jupiter in early evening. In a telescope at high power Uranus is a tiny but distinctly nonstellar ball, 3.8 arcseconds in diameter. Locate and identify it using the finder charts in the November Sky & Telescope, pages 48-49.

Neptune, fainter at magnitude 7.9, is at the Aquarius-Pisces border 21° east of Saturn. It's still moderately high in the southwest after dark. Neptune is only 2.3 arcseconds wide: harder to resolve as a ball than Uranus, but nonstellar at high power in good seeing.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time minus 5 hours. UT is also known as UTC, GMT, or Z time.

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For a more detailed constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a much more detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows all stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to mag 9.75). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.

Can computerized telescopes replace charts? Not for beginners I don't think, especially not on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning the sky. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

If you do get a computerized scope, make sure the drives can be disengaged so you can swing it around and point it readily by hand rather than only slowly by the electric motors.

However, finding a faint telescopic object the old-fashioned way with charts isn't simple either. Learn the tricks at How to Use a Star Chart with a Telescope. 

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Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the naked-eye heavens above. It's free.

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"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things."
            — John Adams, 1770

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1 . Okay, why tricky? Well, consider your "priors."

The whole celestial hemisphere above your horizon, 180° wide, has a surface area of 20,626 square degrees. So any two random points on it would have only a 1 in 400 chance of being within 4° of each other, this evening's separation of Jupiter and the Moon.

However, you come to this question with additional prior knowledge (or you should). For instance, the Moon and Jupiter don't roam all over the celestial sphere but stay close to the ecliptic line, which is 180° long from horizon to horizon. Two random points on that line have a much more likely chance, 1 in 22.5, of landing within 4° of each other.

But oops! The Moon and Jupiter are not right on the ecliptic. The Moon roams as much as 5° north and south of it. (Jupiter departs from the ecliptic much less.) So, do you calculate using the area of a band around the sky 10° wide, rather than a thin line? This makes a close positioning of the two bodies a good deal less likely.

But, oops again! Maybe you know that the Moon spends more of its monthly orbit near its greatest divergences from the ecliptic (and from Jupiter's path) than close to the ecliptic. (Why? Hint: think sine wave.) That makes the odds of a Moon-Jupiter coincidence a little less likely. Time to recalculate!

And you can find further subtleties that change the odds further.

So then, what is the actual, correct chance that you'll see them at least that close together when they're both in view?

There's really no such thing! Not unless you actually observe a very large number of randomly chosen, widely divergent instances and count up the frequency of different separations. Any probability that you can calculate in advance of doing such observations depends on the completeness, or rather the incompleteness, of your known priors.

It gets worse. Some priors we know accurately for a fact, such as that 180° of the ecliptic is always above the horizon. But in real life, most priors are not known exactly but are themselves judgment calls –– and those depend on other, prior judgment calls, and so on.

So how do we know the advance odds of anything happening, at least well enough for us to navigate the world in real life? How does any creature make its best judgments for living and surviving when information is incomplete?

Well, ever hear of the Bayes Theorem?

Worked out by the 17th-century mathematician Thomas Bayes, it's the mathematical formula for finding the probability of A given B, if you know the probability of A by itself, of B by itself, and the probability of B given A. Or in math-speak,

Neuroscientists are discovering that the neural networks in brains, human and animal, run the Bayes formula constantly to test inputs from the senses against assumed priors in memory to make tentative predictions of reality, which then become new priors. This is happening at every scale from the small and simple to the large and complex: from judging the reality of the simplest little sensory detections, such as edges and areas seen by the eye, all the way up to judging the reality of possible large patterns, abstractions, and systems -- in order to create a most-likely model of the outside world around us.

Even individual cortical neurons, when put next to each other in a dish, will find each other and start implementing the Bayes formula right there in the dish. It's what cortical neurons do, from mice to humans.

Now things get really interesting. We think that we live and move around in the real world directly, but this feeling is an illusion. It's a shortcut to simplify what is actually happening, to save brainspace. You actually live in your mental model of the world, which exists inside your skull. Neural networks implementing the Bayes formula are always building and adjusting this best-guess model from signals coming into your skull by way of nerves: from your eyes, ears, and other sense organs. The illusion that our minds live directly in the outer physical world is a short-cut simplification -- a "hallucination" as neural scientists call it -- that saves us from being needlessly distracted by observing the vast internal machinery that's constantly creating that real-seeming, 3-D model of the world around us.

Some neurobiologists working in this field even think that the process of model-building by Bayesian feedback loops also creates consciousness itself. Just as we use perception to create and live in an internal model of the outer world, we also use "interoception," perceptions of our own body and self, to create in the brain a neural model of our body and self. This internal model is, again, what you experience as the real thing. But just as with the outer world, your brain makes a "hallucinated" model of yourself as an organism, which feels like the real self. The you that you experience yourself to be, the you that you're so intimately aware of, is another shortcut to avoid getting bogged down in awareness of the complicated, underlying machinery creating that model.

If this idea turns out to be correct, your self-awareness, your sense of your own personhood, is an emergent phenomenon arising from Bayesian testing, adjusting, and model-building going on below consciousness, at all scales of complexity from little to big. This model experiences itself simply as you, not as a vast interlocking network of Bayesian inference loops. The sense of you is just another "hallucinated" shortcut, shaped by evolution to guide the body in operating effectively enough through the real world to survive and reproduce.

In a lot of the sciences, we live in interesting times. Who knew where the Moon and Jupiter could lead you?

Further reading: Being You by neurobiologist Anil Seth (2021). Or just google "Bayesian brain".

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