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Choreography of the Cosmos: Why the Sky Never Stands Still · Star Trails: From Backyard Astronomy to Cosmic Wonder

• 0:07
  Howdy Star Gazers and welcome to this episode
• 0:10
  of Star Trails. My name is Drew, and I'll be
• 0:13
  your guide to the night sky for the week of January
• 0:16
  the 18th through the 24th. This week we'll continue
• 0:21
  in our series aimed at new astronomers by exploring
• 0:24
  how the universe moves, it's stranger than you
• 0:27
  think, and we'll cover everything from the backward
• 0:30
  motion of planets to the wobble that lets us
• 0:33
  glimpse a bit of the far side of the moon from
• 0:35
  time to time. Later in the show we'll take a
• 0:39
  look at what you can expect to see in the night
• 0:41
  sky this week, and we'll officially kick off
• 0:43
  our Star Trails book club. Whether you're tuning
• 0:47
  in from the backyard or the balcony, I'm glad
• 0:50
  you're here. So grab a comfortable spot under
• 0:52
  the night sky and let's get started. Step outside
• 1:01
  on a clear night. Let the air settle. No wind,
• 1:05
  no sound, no sense of movement at all. You're
• 1:09
  standing perfectly still. But you're not. Because
• 1:13
  right now, at this second, You're moving at about
• 1:17
  a thousand miles per hour, because Earth is spinning.
• 1:21
  If you're anywhere near the equator, you're riding
• 1:24
  the fastest part of that spin. At the same time,
• 1:29
  Earth is racing around the sun at roughly 67
• 1:32
  ,000 miles per hour. That's one full orbit every
• 1:36
  year. An enormous curve traced through space
• 1:40
  that you never feel. And the sun itself, the
• 1:44
  anchor of our entire solar system, is orbiting
• 1:47
  the center of the Milky Way at around half a
• 1:50
  million miles per hour, carrying Earth, the moon,
• 1:54
  and every planet along for the ride. And even
• 1:57
  that isn't the end of it. The Milky Way itself
• 2:01
  is moving through intergalactic space at well
• 2:04
  over a million miles per hour, drawn by the gravity
• 2:08
  of nearby galaxy groups. So, depending on how
• 2:12
  you measure it, you can easily say you're moving
• 2:15
  at more than a million miles per hour. You're
• 2:18
  making Formula One champions like Lewis Hamilton
• 2:21
  and Max Verstappen look slow, just by standing
• 2:25
  still. Of course, it's all relative, because
• 2:29
  everything in the universe is moving. The universe
• 2:33
  doesn't feel fast, but it is. And once you understand
• 2:37
  that, the movement of the night sky starts to
• 2:40
  make a lot more sense. So let's start with the
• 2:44
  motion you can actually see. Over the course
• 2:47
  of an evening, stars rise in the east, arc across
• 2:51
  the sky, and set in the west. Orion climbs higher.
• 2:56
  Sirius flashes near the horizon. Everything seems
• 2:59
  to drift westward together. That isn't the sky
• 3:03
  moving, it's the earth turning. We're standing
• 3:07
  on a rotating planet, and the sky is fixed far
• 3:10
  beyond us. As Earth spins, our view changes.
• 3:15
  If you sit outside long enough, you can actually
• 3:17
  feel this motion intellectually. The stars aren't
• 3:21
  sliding across the sky. You're being carried
• 3:24
  beneath them. This is the most immediate motion
• 3:28
  of the universe, and it's the one backyard astronomers
• 3:31
  learn first. It's also the reason time matters
• 3:34
  when you observe. Where something is at 8 p .m.
• 3:38
  is not where it will be at midnight. We say this
• 3:42
  frequently, the sky is a clock and it never stops
• 3:46
  ticking. While the motion of the stars may seem
• 3:49
  imperceptible at a glance, if you've ever used
• 3:52
  a telescope, you've seen the motion firsthand.
• 3:56
  While you're magnifying the view of a planet,
• 3:58
  such as Jupiter, you're also magnifying the motion
• 4:01
  of the sky. If you're using a telescope without
• 4:04
  a motor drive, you've no doubt been frustrated
• 4:07
  as you've watched your target planet speed out
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  of view, forcing you to reframe it. Even the
• 4:20
  light from stars has to account for our motion.
• 4:24
  As Earth moves through space, the direction starlight
• 4:27
  appears to come from is shifted ever so slightly
• 4:30
  in the direction we're traveling. This effect
• 4:34
  is called aberration of starlight, and it's subtle,
• 4:38
  only a tiny fraction of a degree. A useful way
• 4:42
  to imagine it is walking through rain. Even if
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  the rain is falling straight down, you tilt your
• 4:48
  umbrella forward because you are moving. Starlight
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  works the same way. The stars aren't changing
• 4:55
  position, but because Earth is racing along its
• 4:58
  orbit, the light reaches us at a slight angle.
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  What's remarkable is that this effect was measured
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  centuries ago, long before spacecraft or satellites.
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  Astronomers noticed that star positions shifted
• 5:13
  in a way that could only be explained if Earth
• 5:16
  was moving through space. In other words, the
• 5:20
  sky isn't just telling us where things are, it's
• 5:23
  quietly revealing how fast we're going. If Earth's
• 5:28
  rotation explains the nightly motion, the moon
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  introduces complication. The moon doesn't rise
• 5:35
  at the same time each night. It shifts eastward
• 5:38
  by about 13 degrees every day, which means it
• 5:42
  rises roughly 50 minutes later from one night
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  to the next. That's why it sometimes appears
• 5:48
  in the evening sky, sometimes after midnight,
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  and sometimes not at all. Its phases are orbital
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  geometry. As the moon orbits Earth, we see different
• 6:00
  portions of its sunlit half. That slow, steady
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  orbit shapes the rhythm of the month. And the
• 6:08
  moon's orbit isn't perfectly circular. When a
• 6:11
  full moon happens near its closest point to Earth,
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  it appears slightly larger and brighter. A so
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  -called supermoon. When it's farther away, it
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  looks smaller and dimmer, and we call that a
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  micro moon. One of the most common things people
• 6:28
  hear about the moon is that we always see the
• 6:30
  same face, as if the moon has chosen a side and
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  stubbornly refuses to turn around. But what's
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  actually happening is subtler and more elegant.
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  The moon does rotate, it just rotates at exactly
• 6:45
  the same rate that it orbits Earth. This is called
• 6:50
  tidal locking, and it's the result of gravity
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  acting over enormous stretches of time. Early
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  in its history, the moon likely spun faster.
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  Earth's gravity raised tides in the moon's interior,
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  not ocean tides, but solid body distortions.
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  Those tidal bulges created friction, and friction
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  drains energy. Over millions of years, that process
• 7:15
  slowed the Moon's rotation until it reached a
• 7:18
  stable state, one rotation for every orbit. The
• 7:23
  result is synchronization. As the Moon goes once
• 7:27
  around Earth, it turns once on its axis. From
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  our point of view, that means the same hemisphere
• 7:33
  always faces us. And it's worth pausing on a
• 7:38
  detail that often gets missed. Tidal locking
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  doesn't mean the moon doesn't spin. If it didn't
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  rotate at all, we would see every side over the
• 7:47
  course of a month. The fact that we don't is
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  proof that it's rotating perfectly in step with
• 7:53
  its orbit. Even then, the story isn't perfectly
• 7:57
  tidy. Because the moon's orbit is slightly elliptical
• 8:01
  and tilted, its rotation speed doesn't match
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  its orbital speed exactly at every point. That
• 8:08
  mismatch produces Libration, a gentle rocking
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  that lets us glimpse a little beyond the Moon's
• 8:14
  edge over time. That's why across months we can
• 8:17
  see about 59 % of the Moon's surface, rather
• 8:21
  than the 50 you might expect. So the Moon keeps
• 8:27
  one face towards Earth, not because it's frozen
• 8:30
  in place, but because it's locked in a gravitational
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  rhythm. rotating, orbiting, and wobbling just
• 8:36
  enough to remind careful observers that even
• 8:39
  the most familiar object in the night sky is
• 8:42
  still very much in motion. Now let's zoom out.
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  Over weeks and months the entire night sky changes.
• 8:53
  Orion dominates winter evenings, but by late
• 8:56
  spring it slips away. Summer constellations take
• 9:00
  its place. By autumn, the sky feels unfamiliar
• 9:04
  again. The stars aren't leaving. We are. As Earth
• 9:08
  orbits the sun, our nighttime view points in
• 9:12
  different directions at different times of the
• 9:14
  year. The stars behind the sun in July are not
• 9:18
  the same stars behind it in January. Seasonal
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  skies are a consequence of perspective, not disappearance.
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  And once you see that pattern, the night sky
• 9:29
  stops feeling like a puzzle and starts feeling
• 9:31
  like a landscape. Some of the oddest behaviors
• 9:36
  come from relative motion. Planets, for example,
• 9:41
  usually drift eastward against the background
• 9:43
  stars. But every so often, they slow down, stop,
• 9:48
  and move backward for a time. This retrograde
• 9:52
  motion baffled astronomers for centuries. The
• 9:56
  planets aren't reversing course, we're just passing
• 9:59
  them. Earth moves faster on an inner track, like
• 10:03
  a car overtaking another on a highway. From our
• 10:07
  moving viewpoint, the slower planet appears to
• 10:09
  loop backward briefly before resuming its usual
• 10:13
  direction. It's simply a matter of perspective.
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  Up until now, everything we've talked about happens
• 10:20
  on human time scales. Hours, days, months, and
• 10:25
  years. But the universe has slower motions too,
• 10:29
  much slower. Earth's axis isn't fixed. It wobbles,
• 10:34
  tracing a slow circle over about 26 ,000 years.
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  This motion, called precession, means the identity
• 10:43
  of the North Star changes over time. Polaris
• 10:46
  is only temporarily special. In the distant past,
• 10:50
  other stars marked North. In the far future,
• 10:54
  different ones will again. Even Earth's axial
• 10:58
  wobble isn't perfectly smooth. On top of long
• 11:02
  -term precession is a smaller motion called nutation,
• 11:06
  a subtle nodding and jittering of Earth's axis
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  caused mostly by the Moon's changing gravitational
• 11:12
  pull. As the Moon's orbit tilts and shifts, Earth
• 11:16
  responds with a gentle shimmy layered over its
• 11:19
  larger wobble. The key idea here is Earth doesn't
• 11:24
  move in idealized curves, and it never has. Even
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  the Sun isn't standing still. Earth doesn't orbit
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  a perfectly fixed Sun. Instead, both Earth and
• 11:37
  the Sun orbit a shared center of mass called
• 11:40
  a barycenter, a point in space determined by
• 11:43
  their combined gravity. Most of the time, that
• 11:48
  point lies just outside the sun's surface, but
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  it shifts constantly as planets move. Add in
• 11:56
  massive worlds like Jupiter and Saturn, and the
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  sun begins to wobble, subtly but measurably.
• 12:03
  This means the solar system doesn't revolve around
• 12:06
  a motionless center. Everything is responding
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  to everything else. All the stars we see are
• 12:13
  also moving. They orbit the galaxy. drift relative
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  to one another, and slowly change position against
• 12:21
  the background sky. This is called proper motion,
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  and while it's almost invisible over a single
• 12:28
  lifetime, it becomes obvious across centuries
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  and millennia. If you want proof that stars themselves
• 12:36
  are moving and not just in some abstract, academic
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  way, There's a quiet little red star in the constellation
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  Ophiuchus that tells the story better than any
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  diagram. It's called Bernard's star, and it holds
• 12:51
  a remarkable record. It has the fastest known
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  proper motion of any star in the night sky. Bernard's
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  star drifts across the sky at about 10 arc seconds
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  per year. That doesn't sound like much, and visually
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  it isn't. But over a human lifetime, its position
• 13:11
  shifts enough to be measured easily with modest
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  equipment. Over a few centuries, it noticeably
• 13:18
  changes where it sits relative to neighboring
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  stars. Given enough time, it will wander completely
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  out of the familiar star patterns we use today.
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  What makes this especially striking is why it's
• 13:32
  happening. Bernard's star isn't unusually fast
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  in an absolute sense, it's just close, only about
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  six light years away. Its proximity makes its
• 13:44
  motion obvious. The stars are not fixed points,
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  they're travelers, and we're watching them in
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  the middle of their journeys. Constellations
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  feel eternal, but they aren't. They're temporary
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  patterns, snapshots taken during a brief moment
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  when star motions happen to line up in a familiar
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  way. The sky we know is not the sky our distant
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  descendants will see. All of this motion, spins,
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  orbits, wobbles, drifts, can feel overwhelming
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  when you list it out, but here's the quiet truth
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  at the center of it all. The sky looks stable
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  because we're moving with it. We share Earth's
• 14:25
  rotation. We share the Sun's galactic orbit.
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  We share our local neighborhood of stars. And
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  even at the largest scales, motion never disappears.
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  The Milky Way is not a rigid disk frozen in space.
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  It flexes and stretches under the gravitational
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  influence of nearby galaxies. especially the
• 14:47
  Andromeda galaxy, which is slowly approaching
• 14:50
  us. These galactic tides gently reshape stellar
• 14:55
  orbits over hundreds of millions of years. The
• 14:58
  same gravity that raises tides on Earth and locks
• 15:02
  the moon to our planet also sculpts galaxies.
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  There's one last wrinkle in this motion. When
• 15:10
  you look at the sky, you're never seeing things
• 15:13
  as they are right now. Moonlight is about one
• 15:17
  second old. Sunlight left eight minutes ago.
• 15:21
  The stars of Orion shine from hundreds or thousands
• 15:25
  of years in the past. So all the motion we're
• 15:28
  talking about, you're watching it slightly delayed,
• 15:32
  stitched together from different moments in time.
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  Astronomy isn't just the study of where things
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  are, it's the study of where things were and
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  how their motion reaches us across distance.
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  The night sky isn't still, and once you understand
• 15:48
  that, every observation starts to feel less like
• 15:51
  confusion and more like choreography. After a
• 16:03
  quick break, we'll return with this week's night
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  sky report and the first reading assignment for
• 16:08
  our book club. Stay with us. Welcome back! Tonight
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  and over the coming week, a nearly absent moon
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  will make for some of the best dark sky observing
• 16:31
  many of us get all winter. We reach New Moon
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  tonight, which means the moon will be nearly
• 16:37
  invisible in the sky for several evenings around
• 16:40
  this date. With virtually no lunar glow to wash
• 16:43
  out faint objects, this is an ideal time to explore
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  galaxies, nebula, and star clusters with binoculars
• 16:50
  or a telescope. With the moon out of the way
• 16:54
  early in the week, the planet Jupiter continues
• 16:56
  to dominate the night sky. Earlier in January,
• 17:00
  Jupiter reached opposition, meaning Earth passed
• 17:03
  directly between it and the Sun, and it will
• 17:06
  remain bright and prominent through late January.
• 17:10
  The gas giant rises around sunset and stays visible
• 17:13
  all night. Climbing into the south and shining
• 17:16
  is one of the brightest objects in the sky. Even
• 17:20
  small telescopes and good binoculars can reveal
• 17:22
  its four largest moons, and under stable seeing,
• 17:26
  hints of cloud bands on its disk. Not far behind
• 17:31
  Jupiter in the evening sky is Saturn, visible
• 17:34
  after sunset in the southwest. Through a small
• 17:37
  telescope, Saturn's rings continue to be a delightful
• 17:40
  sight, though it will be lower in the sky than
• 17:43
  Jupiter. On January 23, an attractive pairing
• 17:48
  awaits observers. The thin, waxing crescent moon
• 17:51
  will appear close to Saturn and Neptune in the
• 17:55
  western sky after sunset. Saturn is, of course,
• 17:59
  bright enough to see with the unaided eye, and
• 18:02
  Neptune, much fainter, can be pulled out with
• 18:05
  binoculars or a telescope if you know where to
• 18:08
  look. An app like Stellarium can help you locate
• 18:12
  it precisely. This grouping makes a nice target
• 18:15
  for early evening observation as the moon's crescent
• 18:19
  returns. This week also offers the chance to
• 18:23
  revisit some deep sky favorites. The Beehive
• 18:26
  Cluster in Cancer is well placed in the eastern
• 18:29
  sky after dusk. This open cluster, also cataloged
• 18:34
  as Messier 44, is a beautiful sight in binoculars
• 18:38
  or a wide field telescope. appearing as a misty
• 18:41
  patch of dozens of stars. Also this week, if
• 18:45
  you're using telescopes with tracking or a good
• 18:48
  star chart, you can enjoy fainter wanderers.
• 18:52
  Uranus remains a binocular object tucked near
• 18:55
  the Pleiades cluster, and Neptune will be easier
• 18:58
  to spot during the Moon -Saturn close approach
• 19:00
  on the 23rd, though both require optical aid
• 19:04
  and some patience. Finally, take a moment to
• 19:07
  step back from specific targets and enjoy the
• 19:10
  winter Milky Way sprawling across the sky from
• 19:13
  Orion through Gemini and into Auriga and Perseus.
• 19:18
  With few nights this dark all month, the winter
• 19:20
  constellations and the rich patches of interstellar
• 19:23
  dust and star -birth regions around them are
• 19:26
  especially satisfying even through binoculars.
• 19:30
  That's a snapshot of what's up this week. Bundle
• 19:34
  up, step outside early in the evening, and let
• 19:37
  your eyes adjust. The dark skies this week are
• 19:40
  a gift. Okay folks, the Star Trails book club
• 19:50
  is finally happening. If you want to follow along,
• 19:54
  you're going to need a copy of Night Watch by
• 19:56
  Terrence Dickinson. Make sure you get the 5th
• 20:00
  edition for the most up -to -date information,
• 20:02
  although I'm reading the 4th edition, which is
• 20:05
  accurate to 2025. I don't think the stars on
• 20:09
  the included charts will have moved that much
• 20:11
  to make a difference. In our first show in February,
• 20:15
  two weeks from now, I'm going to talk about my
• 20:18
  favorite portions of the first three chapters.
• 20:21
  Chapter 1, Discovering the Cosmos, is a broad
• 20:24
  survey of the goals of the book. Chapter 2, The
• 20:29
  Universe in 11 Steps, sets up a fascinating discussion
• 20:33
  on the scale of the universe. And Chapter 3,
• 20:37
  Backyard Astronomy, really matches up well with
• 20:40
  the Star Trails philosophy, and it's full of
• 20:42
  useful tips for navigating the sky. I'll offer
• 20:46
  up my thoughts in more detail on these sections,
• 20:49
  and if you've read Nightwatch, I'd love to hear
• 20:51
  your reflections also. Feel free to comment over
• 20:55
  at StarTrails .Show. That's going to do it for
• 21:02
  this week. If you found this episode interesting,
• 21:05
  please share it with a friend who might enjoy
• 21:07
  it. The easiest way to do that is by sending
• 21:10
  folks to our website StarTrails .Show. And if
• 21:14
  you want to support the show, use the link on
• 21:16
  the site to buy me a coffee. It really helps.
• 21:20
  Be sure to follow Star Trails on Blue Sky and
• 21:23
  YouTube. Links are in the show notes. Until we
• 21:27
  meet again beneath the stars, clear skies everyone!