This Week's Sky at a Glance, September 29 – October 8

FRIDAY, SEPTEMBER 29

■ Even as the stars come out in late twilight, Cassiopeia is already now higher in the northeast than the sinking Big Dipper is in the northwest. Later in the evening, Cassiopeia's broad W pattern wheels higher and stands on end (its fainter end).

■ Also in early evening, look above the bright Moon to see if you can make out the Great Square of Pegasus through the moonlight. It's balancing on one corner. . . the Great Diamond of Pegasus? The line from its top corner through its bottom corner points down at the Moon.

SATURDAY, SEPTEMBER 30

■ Vega is the brightest star very high in the west after nightfall. Arcturus, equally bright, is getting low in the west-northwest.

The brightest star in the vast expanse between them, about a third of the way from Vega down toward Arcturus, is Alphecca, magnitude 2.2 — the crown jewel of Corona Borealis.

SUNDAY, OCTOBER 1

■ The waning gibbous Moon rises in the east-northeast around the end of twilight. Ten or fifteen minutes later, Jupiter rises just 2° or 3° to the Moon's lower right. Later in the evening the two shine high, as shown below.

These are currently the two brightest objects in the entire evening sky, from horizon to horizon. What are the odds that the two brightest objects in that whole expanse would appear so close together? Hint: think carefully, it's tricky. And the trickiness leads down some fascinating and fundamental rabbit holes.¹

MONDAY, OCTOBER 2

■ Now the Moon shines only about 2° from the Pleiades (for evening in North America) as shown above. The moonlight may nearly wipe out the Pleiades for the unaided eye. But in a binoculars' field of view, where both objects fit at once, the Pleiades are much easier. Of course it will help to move the Moon out of the field's edge.

TUESDAY, OCTOBER 3

■ After dark, Vega is the brightest star very high west of the zenith after dark. Face west and look up at it. While you're facing west, look to Vega's lower right by 14° (nearly a fist and a half at arm's length) for Eltanin, the nose of Draco the Dragon. The rest of Draco's fainter, lozenge-shaped head is a little farther behind. Draco always eyes Vega as they wheel around the sky.

The main stars of Vega's own constellation, Lyra — also pretty faint — extend 7° from Vega on the side opposite Draco's head.

■ Once the waning gibbous Moon is up in good view in late evening, you'll notice that it's on the line from Aldebaran at its lower right to Capella more than twice as far to the Moon's upper left.

Also spaced along that line, between the Moon and Capella, are first, Iota Aurigae, magnitude 2.7, and then closer to Capella, Eta Aurigae at magnitude 3.2.

WEDNESDAY, OCTOBER 4

■ This is the time of year when the rich Cygnus Milky Way crosses the zenith right after nightfall is done (for skywatchers at mid-northern latitudes). The Milky Way extends straight up from Sagittarius in the low southwest, passes overhead, and runs straight down through Cassiopeia and Perseus in the northeast.

THURSDAY, OCTOBER 5

■ Last-quarter Moon tonight (exactly so at 9:48 a.m. on the morning of the 6th). The Moon rises around 11 p.m., and by two hours later on the morning of the 6th it's shining well up just above the stick figure of Castor as shown below.

FRIDAY, OCTOBER 6

■ Arcturus shines in the west as twilight fades away. Capella, equally bright, is rising in the north-northeast (depending on your latitude; the farther north you live the higher it will be.) They're both magnitude 0.

Later in the evening around 8 or 9, Arcturus and Capella shine at the same height. When will this happen? That depends on both your latitude and longitude.

When it does, turn around and look low in the south-southeast. There's 1st-magnitude Fomalhaut at about the same height — exactly so if you're at latitude 43° north (Boston, Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude, Fomalhaut will appear higher than Capella and Arcturus are. Seen from north of there, it will be lower.

That bright light more than a third of the way from Capella to Fomalhaut is, of course, Jupiter.

Higher above Fomalhaut glows Saturn, pale yellow and steady.

SATURDAY, OCTOBER 7

■ The Great Square of Pegasus balances on its corner high in the east at nightfall. For your location, when will it be exactly balanced? That is, when will the Square's top corner be exactly above its bottom corner? This will be sometime in the evening depending on your latitude. Try lining up the stars with the vertical edge of a building as a measuring tool. The line's tilt changes very slowly.

SUNDAY, OCTOBER 8

■ Cygnus the Swan, with Deneb as its tail, floats straight overhead after nightfall. Its brightest stars form the big Northern Cross. When you face southwest and crane your head up, the cross appears to stand upright. It's about two fists at arm's length tall, with Deneb as its top. Or to put it another way, when you face that direction the Swan appears to be diving straight down. Migrating away for fall?

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This Week's Planet Roundup

Mercury gets lower in the dawn this week. About 30 or 40 minutes before sunrise, look for it due east three fists at arm's length or more to the lower left of high Venus, as shown above. Mercury remains a bright magnitude –1.0.

Venus, brilliant at magnitude –4.7 in Leo, is as high as it's going to get as the "Morning Star." Look east before and during dawn. Venus rises more than two hours before dawn's first light — a weird UFO on the horizon, very far below Castor and Pollux.

Watch Regulus, only 1 percent as bright, close in toward Venus from the lower left. They're going to pass 2.3° from each other on the morning of October 9th.

In a telescope, Venus is a thick crescent waxing on its way to its dichotomy (half-lit phase) in late October.

Mars is out of sight behind the glare of the Sun.

Jupiter (magnitude –2.8, in Aries) rises in the east-northeast around the end of twilight. Watch for it to come up about 13° below the brightest stars of Aries. Jupiter dominates the eastern sky in late evening and shines highest in the south during the early-morning hours.

Saturn (magnitude +0.6, in dim Aquarius) is the brightest "star" in the southeast in twilight. A month past opposition, it shines at a good height for telescopic viewing as early as 8 or 9 p.m. Fomalhaut twinkles two fists at arm's length below it, and Altair shines four fists to Saturn's upper right. Saturn stands highest in the south around 10 or 11.

Uranus, magnitude 5.6 in Aries, is 9° east of Jupiter. Add it to your late-night list.

Neptune, magnitude 7.8 at the Aquarius-Pisces border, is nice and high by mid-evening 24° east of Saturn.

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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 Daylight Time (EDT) is Universal Time minus 4 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 be sure to 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.

Do 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 getting to know 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."

But finding faint objects 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 1,050 chance of being within 2.5° 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 36, of landing within 2.5° 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 a lot less.) So, do you calculate using the area of a band around the sky 10° wide? 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 look up further subtleties that tweak the odds further.

So then, what is the real, exact 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 instances and count up their frequency. Any probability that you can calculate in advance 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 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.

Neuroscientists are discovering that the neural networks in brains, human and animal, are running the Bayes formula constantly to test inputs from the senses against priors in memory to make tentative predictions of reality. 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 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 real world around us.

Even individual cortical neurons, when put next to each other in a petri 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. You actually live in your mental model of the world, which exists inside of 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 uselessly distracted by observing the vast internal machinery that's constantly creating our model of the world in real time.

Some scientists in this field even think that the process of modeling by Bayesian feedback loops may create 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 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 and it feels like the real self. The you that you experience yourself to be, the you that you are so intimately aware of, is another shortcut to avoid getting bogged down in awareness of the complicated, underlying machinery creating the 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 as you, not as the interlocking network of Bayesian inference loops that is actually creating it. The sense of you is just another "hallucinated" shortcut to guide the body in operating effectively enough in the world to survive and reproduce.

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

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